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Sat Jul 20, 2013, 05:16 PM

Quantum Entanglement, Dark Counts, Coincidence Detection

A while ago, there was a discussion here during which I made the claim that the Copenhagen Interpretation of quantum mechanics sucks balls. Also as a consequence of that discussion, I started working on an experiment similar to that of Dr. John Cramer of the University of Washington that has the goals of first, demonstrating communication using entanglement, then extending it to demonstrate retrocausal effects.

Recently I e-mailed Cramer to see how his experiment was going, and he said that he was having trouble with the dark counts of his detectors. What this means is that the avalanche photon detectors register a number of false detections of photons, measured in counts per second. This, according to Cramer, was swamping the real detections of photons and burying the signal in noise.

Also, I read a book called Exploring Quantum Physics through Hands-on Projects which gets into entanglement near the back. I e-mail the authors about detection rates and such, and they said that if the optics were aligned just right, I could get around 200 coincidence detections per second. What's happening is that some small number of photons from the pump beam are converted to infrared photons and emitted in a cone of about 5 degrees around the pump. Some percentage of those are entangled pairs, on opposite sides of the cone, and by detecting two photons at the same time - coincidence - one is filtering out just entangled pairs.

What I'm trying to understand is which such a low coincidence count? With Cramer's setup, he should be generating around a million entangled photons per second. The detectors I want to get have a dark count of 250, although there are other versions with 1000, or 2000, and well as lower dark-count detectors. They have about 70% efficiency in detecting photons. According to the book, about 6*10^4 entangled pairs per mW of pump power are created per second. That means that 15 million pairs are created per second.

Photons will be picked off from opposing sides of the cone where the wavelengths of the two photons are equal (810nm with a 405nm pump). Assuming one degree is filtered out on each side, that's 41667 photons per second on each side. Forty friggin' thousand! I would think that the coincidence counts, even given the inefficiencies of the detectors, would be many times the 200 quoted by the authors that I e-mailed. So am I stupid? Is there something major I'm missing here? Hopefully in DU's userbase of 200k, someone will have the answer as to why I'm an idiot.


Now, to describe more about the experiment:

What Cramer is trying to do is kind of an extension of an experiment done by Birgit Dopfer or Austria in 1998. In that, two beams containing entangled photons were generated and one beam was sent through a classic double-slit, and the other beam went to a lens and detector:




The beam going through the double-slit creates an interference pattern on the other side: each photon goes through both slits since they are in a superposition state with respect to their momentum, and the quantum waves from the slits interfere with each other. However, is any measurement is made on which slit a each photon goes through, the interference pattern is destoryed - the superposition state is destroyed. Dopfer made a measurement on the other beam of photons that didn't go through the double slit but were entangled with the photons that did, and this also destroyed the interference pattern. The coincidence detector filters out only entangled pairs of photons, so that unentangled, single photons are not counted.

Instead of using a double slit, which sinply blocks the vast majority of photons, Cramer is using a Mach-Zehnder interferometer and so am I:



What happens here is that a laser beam is fed in at the left and hits a beam splitter. Each photon is BOTH reflected and transmitted (superposition again), hit both of the two mirrors which reflect it to the second beam splitter, or recombiner in this case. From that second beam splitter, the photons recombine with themselves, then go out of one of the two outputs of the beam splitter. Two interference patterns are generated. I've made a couple of these and am currently redesigning the mounts for the optics to make it easier to tune the interferometer to get the path lengths equal and to adjust the beam angles so that I get more or fewer interference fringes.

The outputs directly from the beam splitter look just like laser beams - they're tight, pencil-like threads of light. Only by spreading them out with lenses can I actually see the interference patterns. Without the lenses, the beam widths are like the original laser beam - about 1/8th inch diameter. The detectors have an active area of about 1/5th of a millimeter, so a good amount of light will miss the detector. There should still be a lot more than 200 per second. I plan to use a couple of galvanometers to adjust the beam angle in two dimensions so that I can scan out the interference pattern on the detectors. I have a pair of galvos and will get a couple more pairs. They're really cheap on Ebay and I got some a while ago to do laser animation.

The first beam splitter will be implemented as a mirror that reflects the bottom half of the beam. Since position of a photon within the laser beam (or downconverted, infrared beam) is in a state of superposition, again the photons will both be reflected and transmitted. The other beam with the other entangled photons can also be split this way, and the momentum/position entanglement of the two beams will them be maintained. By making the information as to the momentum of each photon available, even just in principle, the superposition state will be destroyed and thus, the interference patterns from the Mach-Zehnder interferometers will be altered. Only a percentage of photons in each beam are part of the entangled pairs (I think 20%-30%), and the unentangled photons will continue to make interference patterns. However, some parts of those patters should have very few photons hitting them, ideally zero, and when the entangled photons are knocked out of superposition, they won't make nice interference patterns but rather just blobs of light. So it should be possible to detect more photons in the normally dark parts of the patterns. Thus, it should be possible to send information using entanglement.

The really strange and cool part of this whole experiment is that it should be possible to delay the measurement of momentum of the entangled beam - the one not going through the interferometer - simply by moving the beam splitter for it and the detector further away from the BBO crystal that downconverts the pump beam. The decision to focus the two beams from the splitter to the same spot or not, which determines whether photons are allowed to stay in momentum-superposition or not, can be made after detection of the interference pattern. In this way, it should be possible to send information backwards in time slightly. An enhancement would be to split the entangled beam and send both halves into separate fiber optics, then focus the outputs onto a detector or not after the fibers, thus delaying the measurement by a lot more. This should allow for a delay of around 50 microseconds which isn't a lot, but it's enough that digital electronics can do something interesting with the data before using it to determine whether to maintain or destroy the interference pattern, thus sending the data back again 50uS.

Anyway, I'm just about to buy the BBO crystal set and the detectors. I really do want to know if somehow, I'm stupid in my calculations as to how many entangled photons should be detected. Also, I need to get to work on these optics mounts this weekend.

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Reply Quantum Entanglement, Dark Counts, Coincidence Detection (Original post)
mindwalker_i Jul 2013 OP
darkangel218 Jul 2013 #1
mindwalker_i Jul 2013 #2
darkangel218 Jul 2013 #3
cstanleytech May 2015 #91
nebenaube Jul 2013 #4
mindwalker_i Jul 2013 #5
nebenaube Jul 2013 #6
mindwalker_i Jul 2013 #7
nebenaube Jul 2013 #37
mindwalker_i Jul 2013 #39
napoleon_in_rags Jul 2013 #8
caraher Jul 2013 #9
napoleon_in_rags Jul 2013 #10
mindwalker_i Jul 2013 #17
caraher Jul 2013 #12
caraher Jul 2013 #13
napoleon_in_rags Jul 2013 #14
napoleon_in_rags Jul 2013 #15
caraher Jul 2013 #16
napoleon_in_rags Jul 2013 #18
mindwalker_i Jul 2013 #19
napoleon_in_rags Jul 2013 #21
mindwalker_i Jul 2013 #22
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mindwalker_i Jul 2013 #24
napoleon_in_rags Jul 2013 #25
mindwalker_i Jul 2013 #26
caraher Jul 2013 #28
mindwalker_i Jul 2013 #31
napoleon_in_rags Jul 2013 #30
mindwalker_i Jul 2013 #32
napoleon_in_rags Jul 2013 #33
caraher Jul 2013 #35
caraher Jul 2013 #27
mindwalker_i Jul 2013 #36
caraher Jul 2013 #41
mindwalker_i Jul 2013 #42
caraher Jul 2013 #29
caraher Jul 2013 #11
mindwalker_i Jul 2013 #20
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Response to mindwalker_i (Original post)

Sat Jul 20, 2013, 05:32 PM

1. This is me after reading your post :

 



I bookmarked it for another reading attempt in the morning.

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Response to darkangel218 (Reply #1)

Sat Jul 20, 2013, 05:41 PM

2. Oh, sorry about that


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Response to mindwalker_i (Reply #2)

Sat Jul 20, 2013, 05:45 PM

3. Nooo, its my fault.

 

And my physics teacher.

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Response to mindwalker_i (Original post)

Sat Jul 20, 2013, 06:19 PM

4. I understood every bit of it...

 

A few weeks in the Jesse Beams Laboratory helped. Thank you so much for introducing me to the the Mach-Zehnder interferometer. Reality will be changed because of it.

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Response to nebenaube (Reply #4)

Sat Jul 20, 2013, 07:16 PM

5. Mach-Zehnder interferometers are interesting

I think they've been around for a long time, about as long a the Michaelson-Morley interferometer, For this particular experiment, they're a really good improvement over double-slits.

Now, my #4 threader broke (fucker!) and I have to go get another one.

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Response to mindwalker_i (Reply #5)

Sat Jul 20, 2013, 08:04 PM

6. knowing they exist and know what everyone else calls them are two different things.

 

Regardless, I need some that are off-the-shelf.

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Response to nebenaube (Reply #6)

Sat Jul 20, 2013, 09:15 PM

7. Off the shelf?

There are some that are prebuilt, but they're really expensive, and I don't think you can modify them much if at all. I built mine on an optics bench that I got from Ebay. Posts from Ebay too, optics from surplus Shed, and I'm making my mounts out of aluminum from the hardware store.

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Response to mindwalker_i (Reply #7)

Mon Jul 22, 2013, 11:17 PM

37. I don't see why what I need would be any bigger then say a .22 slug (for lack of a better reference)

 

prism or slide slip, two photodiodes, bias circuit, comparator.

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Response to nebenaube (Reply #37)

Mon Jul 22, 2013, 11:31 PM

39. See my post currently at the bottom

for some good detectors for visible light. That was in the book, and I picked up a bunch from Digikey. They're about $3 each and convert light power to a voltage.

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Response to mindwalker_i (Original post)

Sun Jul 21, 2013, 01:18 PM

8. Hey, so are you doing this on your own dime?

If so, could you post a little more on the equipment you're using? (If its something the average person could buy) I love the idea of people doing these experiments in their homes, that's so cool.

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Response to napoleon_in_rags (Reply #8)

Sun Jul 21, 2013, 02:19 PM

9. Pick up that book he mentioned

I just saw it in Powell's and it includes information on where to look for the equipment, cheap.

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Response to caraher (Reply #9)

Sun Jul 21, 2013, 02:37 PM

10. You're at Powell's in Portland? I'm jealous.

Exploring Quantum Physics through Hands-on Projects...
http://www.amazon.com/Exploring-Quantum-Physics-through-Projects/dp/1118140664

Cool, will get.

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Response to napoleon_in_rags (Reply #10)

Sun Jul 21, 2013, 04:19 PM

17. Yeah me too

I grew up in Portland and seriously miss Powells.

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Response to caraher (Reply #9)

Sun Jul 21, 2013, 03:03 PM

12. I was on Tuesday

I was there for the American Association of Physics Teachers meeting. Meanwhile it's back to Indiana...

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Response to napoleon_in_rags (Reply #8)

Sun Jul 21, 2013, 03:10 PM

13. BTW, this is still pricey stuff

The detectors retail for over $3k each; you may be able get them for education for about half that. I have no idea what the Prutchis paid for theirs on eBay. The laser can be as cheap as tens of dollars for a blue laser pointer. The BBO crystal(s) will run you $500-1000.

I have a mad plan to set up a lab that will let people do their own experiments over the internet. (The idea would be to use a web browser as an interface that would control the hardware.) Not the same as hands-on, but not the same as simulation, either. I'm not sure whether to apply for grants or try crowdfunding, but I think there are plenty of people who would like to do some quantum experiments who don't have the money, time, space or expertise to set up the lasers and other optics, do the alignment, etc. I have all of those except the money...

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Response to caraher (Reply #13)

Sun Jul 21, 2013, 03:17 PM

14. Youre plan isn't mad, its pure genius. Set up a chemistry lab too.

I'm so in on that. I have some interest in this, but not a $3000 interest. Yet how awesome would it be to see the laws of nature on camera online, after enacting experiments of my own design?

The chemistry lab is a big thing too. Skill number 1 for curbing global warming in my opinion is actually petro chemistry, developing reactions that release carbon in forms other than C02. The chemicals for tests aren't much, but it puts you in the realm of online purchases that will get black helicopters over your house. Another thing you could do with that is synthetic chemistry, verified through statistical methods and a small number of tests. Eventually, you could answer a large class of chemistry experiments through computation alone, with almost complete certitude of correct answers.

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Response to caraher (Reply #13)

Sun Jul 21, 2013, 03:21 PM

15. I gotta say it again - genius.

Now I'm thinking of the educational angle. I'm taking online classes right now, and LOVING them. How awesome would it be to take chemistry physics and be able to do ALL my labs online?

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Response to napoleon_in_rags (Reply #15)

Sun Jul 21, 2013, 03:49 PM

16. In chemistry they worry a lot about "technique"

It all depends on your learning goals... if you're training a chemist a big part of the job is getting students to be good at handling chemicals, etc.

What's nice about the quantum optics labs for this is that a lot of the preparation time goes into aligning things or, if the count rates are low, simply collecting data. Neither of these time-sucks are terrifically educational in themselves, so if you can start with a pre-aligned system and you can just do something else while the counters are a-counting, experimenter interaction with the system can be more focused on things like setting polarized angles and analyzing data.

I think it's always best to be hands-on, live, touching the actual equipment. But doing things on the internet can really open up opportunities for schools with fewer resources and for amateur scientists, anywhere in the world.

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Response to caraher (Reply #16)

Sun Jul 21, 2013, 04:20 PM

18. That's how I would start if I were you.

Just touch bases with one of the schools that does classes online, and design all the labs per their spec. You don't even need to charge when you're just starting out. Then have a menu of labs ready, that you can send to all the online schools. Even if the students just paid, say a $90 lab fee for each quarter, that's 8k per class of 30 per year, and if you got a few different schools involved, (not to mention private individuals like myself) and got things automated, you're talking about a sustainable small business real quick, with low start up costs... Provide you have the quantum chops to start.

I would jump on this, put together a business plan, and for your marketing research contact a bunch of professors of online schools and see what they would be willing to have students pay. If the numbers are there, the loan will be no problem.

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Response to napoleon_in_rags (Reply #8)

Sun Jul 21, 2013, 04:29 PM

19. Yeah, I'm doing it on my own dime

Ebay is awesome for a lot of stuff: the optic breadboard, posts, translation and rotation stages, and of course blu-ray laser diodes are $50 for a quarter watt.

For making mounts and such, I have a milling machine from Sherline, and that thing is really helpful. I've had to learn machining on my own, so I can't make anything complicated, but it's enough.

The BBO will be two crystals oriented 90 degrees from each other, so the photons get entangled in polarization as well, although I don't need that for this experiment. It will be about $1000. The detectors, as mentioned before will be seriously expensive. Pe4rkin-Elmer is now Excelitas - might have the spelling wrong. For detectors with a dark count of 250, they're over $4k. The are versions with higher dark counts that are a lot less expensive. One reason I am asking about entanglement and detection rates is that I want to figure out how good/expensive a set of detectors I will need.

But yeah, Ebay seriously kicks ass for most of the stuff.

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Response to mindwalker_i (Reply #19)

Sun Jul 21, 2013, 05:16 PM

21. Okay, thanks for that info. Question 2:

Do you believe quantum non-local communication is possible? I mean, if that one experiment showed the interference pattern collapsing due to a measurement of the other beam, doesn't that mean its possible?

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Response to napoleon_in_rags (Reply #21)

Sun Jul 21, 2013, 06:26 PM

22. Yes, I think it's possible

The explanations for why entanglement can't be for communication rely on the fact that when you measure a property, you get a random value. When someone else measured their particle's property (usually Bob), they get the compliment of your random value. Nick Herbert had a scheme to get around it, but that required being able to clone a quantum particle so different properties could be measured. It resulted in the no cloning theorem.

However, this process relies on measuring the superposition state of many particles. That is a fundamentally different experiment, and it seems very logical to me that it would work. If it doesn't work, then something else has to break. It would be similar to being able to measure which slit a each photon went through in a double-slit experiment, yet still getting the interference pattern. Or it would be similar to measuring a particle's position and momentum with arbitrary precision.

So one way or another, an interesting result is coming out of this thing.

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Response to mindwalker_i (Reply #22)

Sun Jul 21, 2013, 06:54 PM

23. Interesting results are good. But be careful young mindwalker....

If you are correct, than 1 of 2 things is true:

A) You and that professor you are fond of saw something that Heisenberg, Bohr, Einstein and all the rest missed...or
B) You and that prof saw something that all these other brilliant men saw, but chose not to comment on due to their ties to the intelligence community.

The NSA has publicly stated that their intelligence gathering model is based on capturing signals in transit, not compromising individual computers. That means that if you were to discover a method of non-local communication and make it publicly available, that would the potential to shut down their whole model. Snowden warned of things they are doing, but yet they still persist in doing them, and probably will continue to. However, publicly available non-local communication would shut them down entirely. You would do the damage to the NSA of 10,000 Edward Snowdens if you were the one who released this info. That would not be without consequences.

I'm not trying to spook you, I am 1000% behind science as public endeavour, and I hope you will continue. Just be smart. Moving in undiscovered areas of science has big consequences, ever since a few nuclear physicists discovered some funny properties of some Uranium isotopes. I'm just asking you not to get so into the equations that you lose the context of your work...

PEace, and good luck with the experiments! I hope you will post more here as you learn more!
Nir

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Response to napoleon_in_rags (Reply #23)

Sun Jul 21, 2013, 09:41 PM

24. Interesting take

For A:

Einstein showed the possibility of entanglement to show that quantum mechanics couldn't be complete, so he already thought it could be use for communication. I think most physicists believe entanglement can't be used for communications. Their arguments make sense but don't address using the property of superposition itself for the communication. It will be interesting to see how this turns out, and what gets broken.

For B:

Could be. Cramer wasn't silent about it though, which means that either people hadn't thought of it or he has a bigger mouth than them I don't have any connection to Cramer, like having been a student or anything, except that he's a science fiction fan and author, so I met him at Norwescon. He came up with the transactional interpretation of QM, so I'm inclined to believe he knows some stuff.


As for the NSA aspect of all of this, keep in mind that quantum encryption exists. So it's possible to make communications between computers pretty safe, aside from bugs in the protocol. I think this would be even harder to implement in that two fibers would need to connect both ends - one for each of the "ways" after the first beam splitter. Beaming it in free space would be just about impossible.

There is, however, another aspect to be concerned with. If sending information retrocausally (backwards in time) even by microseconds is possible, and there are several experiments that have already been done that kind of show it, the potential to f*** things up is huge. I'm hoping to be able to write temporal while-wend loops, and that could break encryption in literally zero time.

Even with a single bit being transmitted, creating a paradox would be easy. I wonder what we would measure as the final result - received interference or not - or whether there's any way to get one value or the other. Bad as it might be to never have a result, what if one connected it to cause a macroscopic event, such as triggering Sendmail to e-mail Boehner and call him an asshole?

So, yeah, you're probably right that the NSA or some part of government would be interested if this succeeds.

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Response to mindwalker_i (Reply #24)

Sun Jul 21, 2013, 09:54 PM

25. "keep in mind that quantum encryption exists"

"So it's possible to make communications between computers pretty safe, aside from bugs in the protocol"

But is that what Snowden leaked about the NSA? That they are conspiring to make communications safe? Or did he leak that they are spying on all communications in the US, which would be impossible with non-local communications you are trying to create?

So yes, the I think the government would be very interested if this succeeds.

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Response to napoleon_in_rags (Reply #25)

Sun Jul 21, 2013, 10:30 PM

26. Snowden leaked that the govt. is tapping everything

Quantum encryption is available, although quite expensive. I think banks use it between themselves, but it definitely isn't used by end users like us. And like I say, this is going to be possibly more difficult to get to end users, and it's limited in distance to about 10km since light tends to lose entanglement in fiber after that. Having said that, it would be really cool to set up <10km links between people and places. One possibility would be to take communications from many sources (non-quantum, run of the mill network), bundle them all together and send that over a quantum link, then unbundle it and send each packet on its way. That or some variation might offer at least an unbreakable link.

But even with such a link, it's still impossible to avoid making a time machine - although it can be a forward-time machine or a backwards-time machine. If the interferometer is next to the source of entangled photons and the other photons are sent through fiber to a remote location, the decision to keep or destroy the interference will be made after said interference is detected (or not). This means that if a bunch of these links were used in the Internet, latency could actually be negative. That would be really interesting.

So yeah, I understand the mind-bending aspects that this experiment could open up. That's why it's come to the point where I can't not work on it - I'll go crazy if I don't even try to make it work. I was going crazy waiting for Cramer to say something!

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Response to mindwalker_i (Reply #26)

Sun Jul 21, 2013, 11:10 PM

28. What's available now is quantum-secured distribution of encryption keys

The thing to remember is that the encryption keys are themselves randomly generated, and thus exchange no information between Alice and Bob (the two people seeking to exchange encrypted information). What BB84 and other protocols do is give you a sort of "tamper evident" way of exchanging encryption keys, which are then used to encode and decode information sent over classical channels. Turning a stream of measurements on entangled photons into an encryption key always involves Alice and Bob comparing notes on the measurements they made and the results of the measurements over a classical channel. This means that you don't get superluminal communication, even though the measurements they make, individually, do involve evidently nonlocal interactions.

So I don't think you can use this to achieve negative latency, for instance, because the information doesn't lie in the entangled photons. Unless, of course, something radically different is going on. It's been a while since I've given much attention to Cramer's transactional interpretation, but it's very important to remember that to the extent it might allow for such communication it goes beyond (and contradicts) standard quantum mechanics. That doesn't mean it's wrong, of course; that's what experiments are for! (Or proofs...)

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Response to caraher (Reply #28)

Sun Jul 21, 2013, 11:47 PM

31. I think I switched topics without telling anyone

in the middle. Yeah, quantum encryption doesn't actually send information, exactly. For Cramer's scheme, however, the entanglement does carry information in terms of the state of superposition of groups of photons. That can also be used to get negative latency in that the decision whether to "read" the momentum of the photons can happen after the state of superposition has already been measured. Demonstrating that is a major goal for Cramer, and now me.

I mentioned experiments that have strongly hinted at this already. Some months ago an experiment was done where two pairs of photons were created. One photon from each pair was measured for polarization, and the other two went on for some distance before an entanglement "swapping" operation could be performed. If the operation was done, the two photons that HAD ALREADY BEEN DETECTED became retroactively entangled, and their polarizations matched. I think this was done by Zellinger. The was some stuff at the end of the article that explained why this wasn't time travel, but it didn't make sense to me - it sounded like Obi Wan: "So what I told you was true, from a certain point of view."

So my feeling is that entanglement and QM take an rather twisted view of time. Entanglement seems to ignore it. Granted, I'm a hard-core SF fan and can't separate that entirely from my opinions. Hence, I have to try it

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Response to mindwalker_i (Reply #26)

Sun Jul 21, 2013, 11:36 PM

30. Aw-ight, sir.

I think you know the score. I hope you'll ping me when you publish more of your results... And also, take a look at caraher's online lab idea. I think there's a lot of merit in that.

PEace!

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Response to napoleon_in_rags (Reply #30)

Sun Jul 21, 2013, 11:52 PM

32. That online lab is a really interesting idea

When I made the laser system, I hooked up a PIC microcontroller to it, learned to write firmware for it, and learned to write Linux drivers so I could control the system from the computer. Things like that would be really useful for the online lab. Also, a USB-based microscope has been useful for capturing interference patterns, and Saelig sells some really good USB-based oscilloscopes and logic analyzers - both of which would be really good for looking at the signals from the detectors. I actually just bought one that is both, and keeps the signals all in sync (2 scope, 8 logic, $323).

When I get the optics holder rebuilt on this interferometer and get some patterns, I'll post some pictures.

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Response to mindwalker_i (Reply #32)

Sun Jul 21, 2013, 11:55 PM

33. It would be a big money saver for all interested.

One person gets the equipment, one person gets it all right, and we can all learn from it. Makes a lot sense to me.

I think there's a big future in these online labs, especially for online courses!

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Response to mindwalker_i (Reply #32)

Mon Jul 22, 2013, 12:21 AM

35. Sounds like handy stuff

I've been thinking a lot about stuff like rotation stages. You can buy commercial ones for many hundreds of dollars a pop, but there's really not all that much to them. Ditto for linear stages.

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Response to mindwalker_i (Reply #22)

Sun Jul 21, 2013, 11:02 PM

27. I don't think this can work, but...

that's the beauty of experiment - nature will give the answer.

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Response to caraher (Reply #27)

Mon Jul 22, 2013, 11:16 AM

36. But discussing/arguing about it can be fun too

Why do you think it won't work? Note that I'm not being defensive, I want to understand what you are thinking.

There are two possibilities that I can come up with: if it's possible to use entanglement for communication, it opens up time travel due to relativity - In some frame of reference, effect will precede cause. More directly, delaying the choice to measure momentum affects events in the past. That's crazy! It open up the possibility of paradoxes and all manner of unpleasantness. The second possibility is to find flaws in my logic or assumptions. I can't think of any, but that doesn't mean there aren't flaws.

In the first case, this is crazy. Blame Dr. Cramer, he thought it up. I'm just the idiot following him I don't necessarily think that, because this would allow paradoxes and other stuff, it's necessarilty wrong. If the universe allows this to happen, has it already? Have paradoxes been created? Possibly, but if so, the effects haven't been to severe. Also, it seems like it would be very unlikely for this to occur naturally, and it would be even more unlikely if it did happen, for a paradox created in this way to control something macroscopic. Detecting whether sets of particles or photons are in superposition doesn't happen often by chance. Having that make a noticable change to the universe at large isn't that likel either.

As for my logic, there's a lot of room for error there. My understanding of QM and entanglement is mostly qualitative, but fortunately Cramer is a real phycists with real credentials (one of my majors was physics, the other two were computer engineering and electrical engineering, which is what I make a living in), so I can kind of trust him. Assuming that the logic is correct given what we know about QM and entanglement, if this experiment doesn't work, it will show us where our understanding is wrong. In itself, that would be very helpful!

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Response to mindwalker_i (Reply #36)

Tue Jul 23, 2013, 05:30 PM

41. Part of it is preserving causality

Nature seems to be pretty good about enforcing causality, so if it were possible to exploit quantum measurement to send signals between spacelike-separated events I'd be terribly disappointed that Nature was so sloppy! I also think a lot of theorists much smarter than I am have ruled out any way of doing this within the standard framework of quantum theory, which he stood up to decades of efforts to tear it down, so as a betting man I'd say the odds are against Cramer.

Neither of these arguments is really physics, of course. It really all boils down to experiments and their interpretation. In the case of sending signals "back" in time, it needs to be very clear what counts as a "signal" and a lot of loopholes need to be closed.

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Response to caraher (Reply #41)

Wed Jul 24, 2013, 12:19 AM

42. Nature seemed pretty good at enforcing Newton's laws

until we looked closer and it didn't. I would say that in our everyday perception, we don't see causality being violated, which is probably a good thing. However, we've seen that what we're used to seeing is definitely not the whole picture several times, like with relativity and QM.

At the end of 2011, Scientific American had a special issue on time. One of the main points was that we don't know what time is: Einstein said it was a very persistent illusion, and time tends to cancel itself out of equations. There's nothing that indicates why time exists except for our persistent perception of it passing.

Causality is deeply linked with time, so if we don't know what time is, it's at least reasonable to be suspicious of causality.

Another thought I've had on occasion is that special relativity talks about objects gaining mass as they move faster (relative to an observer - both the "fast" part and the "gaining mass" part). Because of this, getting over the speed of light would take more than infinite energy, but the case of quantum entanglement is very different. When two photons are entangled and go in separate directions, the little fuckers aren't passing a basketball back and forth or anything like that. They aren't exchanging virtual particles between each other, it's just a nonlocal aspect of the universe that they're part of the same thing.

Thus, it makes sense exchange of information between them would not be subject to special relativity - there's to stuff to gain mass. And it's already been demonstrated that the effects of collapsing into known states ignores relativity and time - I'm talking about the two sets of entangled photons with entanglement swapping. It seems to me that by measuring how often the two measured photons had the same polarization, one could very quickly determine whether the entanglement-swapping measure was going to take place in the future. Hence that already demonstrates retrocausality.

Of course, it want it to be that way because it would be cool

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Response to mindwalker_i (Reply #19)

Sun Jul 21, 2013, 11:18 PM

29. I think the Excelitas detectors for education have a 500 Hz dark count rate

Those are the ones that run ~$1500 or so to members of the ALPhA, the advanced lab group under the umbrella of the American Association of Physics Teachers.

How important the dark count rate is depends on your expected "true" count rate. If you need to know singles rates to great precision then you want to pay extra for low dark counts; but if you either expect a lot of counts or only really care about coincidences, you need not pay top dollar for low dark counts. Also bear in mind that the specs are upper limits, and the devices often outperform the specs substantially. I think they just build them all and sort them based on test results, so if they have a model promising 250 Hz and another at 500 Hz what that generally means is that the cheaper one will come in somewhere between 250 and 500 Hz. I think mine were spec'ed at 500 Hz and give me 350-450 Hz for dark counts

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Response to mindwalker_i (Original post)

Sun Jul 21, 2013, 03:02 PM

11. Coincidence count rates from downconversion vary a LOT

I've done experiments where I've had over 100,000 coincidences/second and ones with a few hundred per second. A lot depends on exactly how you get the photons to your detectors, wavelength, pump powers, and exactly what your experiment is.

What detectors are you using? I have some "bare" Perkin-Elmer/Pacer SPCMs and those can a pain to work with because the active area of the detector is a square region about 180 microns on a side. Sounds like the same detectors you're looking at. This means you need to use a short focal length lens and you should have some way of translating either the lens or the detector transverse to the beam. I'd say you'd be very luck to get 200 coincidences per second without lenses in place. Bear in mind that a downconverted beam is not going to look like a collimated laser. The usual BBO crystal pairs companies like Newlight sell give you same-wavelength pairs that come out along a 3 degree cone, and that cone spreads with distance. That can be another big source of variability in detection rates - whether you do anything to collimate that beam. For most simple Bell Inequality test experiments it's not necessary.

Especially if you're a beginner, it's better to pay extra for the fiber-coupled detectors. Also, the usual SPCMs have 70% efficiency at around 700 nm; at 810, you're going to be under 60%, which is still quite good.

Either way, it's very helpful to have a visible alignment laser to set things up. Kiko Galvez has some nice tricks and tips on doing alignment. His lab manual also includes some experiments using the Mach-Zehnder, and I think some tips on aligning it but especially finding the equal-delay position (if you have a grating spectrometer, like Ocean Optics sells, there's a neat trick using white light fringes that gets you close enough).

Back-calculating from your numbers, it sounds like you're planning to use a 250 mW diode laser at (nominally) 405 nm? Don't order your BBO until you have your laser, because anything with that much power is likely not to be single mode and probably doesn't operate at exactly 405. It's better to measure your laser's wavelength (and possibly its bandwidth) so they can cut your crystal at the right angle. It's not a huge problem if your laser is, say, 407 nm instead, because you can just tilt your crystals to achieve phase matching, but it just becomes one more thing to fiddle with, and it's probably more of a hassle if you're using the 2-crystal Type I phase matching scheme because your "tuning" tilt has to happen in the plane of the pump laser (which is typically set 45 degrees from vertical/horizontal).

One thing you haven't touched on much is the bandwidth of the downconverted photons. It's hard to assess guesstimates about how "bright" a downconversion source will be from rules-of-thumb like N pairs per second per mW pump power because how many of those you can "use" depends on the bandwidth of the pump laser, its spatial mode, what degree of entanglement you want (in the 2-crystal scheme, the degree of entanglement depends on how well the cone giving you pairs with one polarization overlaps the cone giving you pairs with the orthogonal polarization; you can make gobs of pairs and have poor entanglement!), and how much bandwidth you want in the downconverted beam. In the experiment I've been doing I have bandpass filters that introduce significant loss even within the band of wavelengths they pass (typically 70-80% transmission for a good filter) with passband widths of 10, 20 and 40 nm. It's pretty close to a factor of 10 loss in coincidence rate going from my 40 nm filter to the 10 nm filter, and some people use filters as tight as 1 nm depending on the experiment.

I'll have to come back to your post later to sift through more of the details. The main thing with count rates is to have some notion of what you really need them to be relative to the experimental noise. The good news is that your dark count rates probably won't cause a problem, because that won't really affect your coincidence rates. The rule of thumb for "random" coincidences is that they occur at a rate equal to the product of your single-detector rates times your coincidence window width. If the latter is 10 ns, that gives a randoms rate of 200 * 200 * 10^-8 = 4 x 10^-4, far less than one count per second. But you can get significant rates of accidental coincidences from detection of stray light, etc.

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Response to caraher (Reply #11)

Sun Jul 21, 2013, 05:10 PM

20. That's a log of really good information!

Thank you for your reply, this is exactly the kind of stuff I need to think about.

What Dr. Cramer is trying to do and I'm trying to follow is to eventually get to the point of not using a coincidence circuit. The idea is that normally, all photons going into the Mach-Zehnder interferometer generate nice interference patterns - some light spots, some dark spots. However, when the momentum-superposition of the entangled photons is destroyed by making a "measurement" on the beam not goign through the interferometer, the entangled photons no longer make an interference pattern. That means some of them will hit the parts of the pattern that were dark. Cramer says that the dark count is too much for him to be able to tell when the entangled photons are hitting those spots. Hence, I'm really worrying about dark count.

From what you said above, filter and such are big culprits in destroying photons prematurely. I imagine that by absorbing one photon of a pair, that's going to have an amplified effect of coincidence detection. I was kind of thinking that I would filter out the "degenerate" photons with a block of opaque material with holes drilled to just get those parts of the cones - with holes as big as the diameter of the pump beam. My thought process is that the pump goes into the BBOs (yes, I'll do a two-crystal type 1 SPDC), and all across the area of the cross-section of the pump beam, downconverted photons will be created and will exit with angles relative to the spot where they were downconverted. Actually, that beam should be slightly larger than the pump since downconversion can happen anywhere in the thickness of the crystals. However, once downconverted, they should all be going in the same direction - for all degenerate photons. Having another block-and-hole further away, or a tube, would filter out just those photons (like 99% would just be the degenerate photons), I would think.

As for fiber-coupling the detectors, I was thinking I wouldn't get that option but am rethinking it on your advice. The critical concern is injection into the fiber: the bare detectors have 180um, which is a good size relative to the beam to pick out just parts of the interference pattern. However, I hope to tune the interferometer so that there's just one interference fringe - on one port of the interferometer, it will be light in the middle, the other will be dark in the middle. If I could do that and inject most of the area from the dark-center output, then when the entangled photons are kicked out of superposition, I'll capture more of them if I can inject most of the beam into the fiber. Lenses would help, but do I also need to collimate it to get injection?

In order to measure wavelength of the laser, my first thought is to use a double-slit to make an interference pattern and measure the distance between fringes. Then wavelength can be back-calculated. I'm sure there's a better way, and probably more accurate. Cramer is temperature-controlling his laser to get it to exactly 405nm. That might be a possibility, or I could adjust the angle of the crystals as you say. I think maintaining a constant temperature on the diode would be critical.

I see experimental setups using a half-wave plate. What is this for? It adjusts the phase of the light, so is it necessary to have a certain phase when it hits the BBO?

Again, thank you very much for the information! It gives me a lot of ideas on how to filter, or not filter, and things that might help detection. The idea of being able to use a visible beam for alignment is really good - possibly I could use a visible laser and put a mirror in place to reflect that beam into the interferometer, or turn it to reflect the downconvered beam into the interferometer.

You rock!

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Response to mindwalker_i (Reply #20)

Mon Jul 22, 2013, 12:18 AM

34. I'm slowly figuring out what you're doing...

First, I should mention that the coincidence circuit isn't such a big deal these days. Mine cost a few hundred bucks; I'm using the FPGA setup Mark Beck describes based on the Altera DE2 development board. You can get a coincidence window under 10 ns wide with this unit. You probably don't want to use LabVIEW unless you have a ton of money or an educational affiliation (or, of course, already have it), but all you really need to use the DE2 is a serial connection and the correct command strings.

One downside of fiber is that coupling to the fiber tends to be lossy, so if you only want to sample a tiny part of your interference pattern the bare detector might be better. You really can't get around the need to filter, if only because your laser is bright and the detectors burn themselves out at count rates above 15 million counts per second - easily achieved with room light for a bare detector! (That's another advantage of the fiber - the small acceptance angle of the fiber tends to protect a bit better against such catastrophes.)

Injection into the fiber is mainly a matter of focusing the beam onto the fiber's entry port. How you focus will affect the efficiency of the coupling; you'll want a fiber coupler on a mirror mount to make small angle adjustments. If you establish the "flight path" of your photons with a regular laser beam you can do much of the adjustment "by eye" just looking at the light coming out of the fiber; when it looks good, you put the fiber on your detector.

Buy some green LED lighting. This will let you run your experiment with the detectors on and also be able to see what you're doing. You should have some filters to block anything under maybe 750 nm in any case, and since your eyes are most sensitive to green light you get the most ability to see with the fewest photons (and ~530 nm is below the detectors' peak sensitivity).

I just skimmed the progress reports on Cramer's web site as well as a few recent popular articles. It sounds like he's really made some things more difficult than he needed to by cobbling together his own detectors. You can buy APDs for much less money than SPCMs, but the latter are engineered to take out all the hassles he went through.

It's not clear what laser he's using now from what I've read; the Sacher system was probably overkill. You do want the wavelength to be stable, but it probably doesn't need to be 405 nm on the nose - there's no particular physics selecting that wavelength. The main reasons to have exactly that value is that the commercially-available filters will be centered on wavelengths like 800 nm, 810 nm, etc. Current, temperature and optical feedback all affect the lasing wavelength, and his Sacher system had all three. Usually grating feedback is important only if you're trying to excite a particular atomic transition, so all you really need is a steady current source and either a big heat sink or active temperature control (which is what you don't typically get with a $50 eBay laser pointer).

For wavelength measurement in a DIY experiment, I would recommend making a spectrometer with a diffraction grating and calibrating it by comparing with known reference lines from a mercury lamp (or a neon discharge tube). This could also be good practice using optical fibers; fiber-coupling the light to a spectrometer will help ensure a consistent illumination of your grating. Or you might be able to borrow one from a chemistry department somewhere for a one-time calibration of a homemade setup. You could use a double slit (or for that matter, a DVD or CD) to do the measurement, but you also need to estimate the uncertainty in your value. If you measure 407 +/- 5 nm is that good or bad?

Half-wave plates are generally used to rotate linear polarization. So if you want to pump a pair of BBO crystals with light polarized at 45 degrees, you set a half-wave plate at 22.5 degrees between the pump laser and the crystals. They work with birefringence - the polarization along a particular axis passes through the crystal faster than the perpendicular polarization. It's often important to use what's called a "zero order" waveplate, which generally have better performance, less wavelength sensitivity, and, most important of all, shift the two polarizations by exactly one-half wave relative to one another. ("Multiple order" waveplates will also rotate polarization, but advance one polarization relative to the other by n+1/2 waves, which can be a problem when you're trying to overlap single photons in time, for instance!)

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Response to caraher (Reply #34)

Mon Jul 22, 2013, 11:29 PM

38. Sir, you've just saved me a LOT of pain

and money! In fact, I'm saving this text just in case DU blows up (not as far fetched as one would think).

The green LEDs are a really good idea too. At least at the beginning, I'll use IR filters, but hopefully, later I can enclose the interferometer and detectors in a box, with just a hole and tube to pipe in the entangled beam. Possibly by firewalling the pump beam, I can just get the IR photons and eliminate lossy filters and other components.

Both NewLight and Excelitas have e-mailed me back and answered most of my questions. I'll probably order the BBO soon, and in the meantime continue to build up the parts for the interferometer, as well as a switching mechanism to guild either the entangled beam or the alignment beam into the interferometer. This would be a good place to put a galvanometer, actually. Or two since I'll probably make a second interferometer for the other beam.

Another thing I want to do is stick a set of galvos on the outputs of the interferometer, then have them guide the outputs through lenses and to two (cheap, visible) detectors. The book I mentioned talked about these: TSL254R-LF. I picked up 10 or so from Digikey a while back and haven't used them yet. This would allow me to validate the setup with visible light, develop a circuit to read the light intensity for a grid of positions, and send that to Linux through USB. Converting it to do the same for the detectors shouldn't be that hard. The scanners could also direct the output beams either to that, or to the real detectors later, so I can just switch between visible and entangled light (and stick a cork in the detectors when doing the visible stuff).

I just rebuilt the optic holders, so now I need to build the plates to bolt onto the rotating tables. The two beam splitters will go on those so their angles can be easily adjusted. That would be something to motorize for the online experiment, although those are really expensive.

Anyway, I'm rambling because I'm excited (hey, maybe I'll emit a big, fat photon ), so I'll shut up. But again, thank you so much for the information!

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Response to mindwalker_i (Reply #38)

Tue Jul 23, 2013, 05:21 PM

40. I do want to think a bit more about your experiment...

but I'm kind of thinking about my own more intensively right now

In my current experiment both detectors are inside a light-tight box whose only entry point is through a longpass filter. I'm using the ThorLabs FELH0750, which is $150 but has MUCH higher transmission than their regular filters (97% at 810 nm; you can download an Excel file with the specs from their site). I generally also have a bandpass filter in front of that for a variety of reasons, but I've run my system with just the longpass filter without harming anything. (I also have a blue-reflecting dichroic mirror at 45 degrees after my downconversion crystal, so there are two elements that provide big losses to my pump beam before the detectors. But my experiment is collinear Type II downconversion, so I need to work harder to eliminate the pump. If you're doing Type I with the 3-degree cone the pump beam is spatially separated from the downconverted photons.)

By extension, I'm thinking about what Cramer is up to as well. It seems to me that the kind of interferometry he is using (and perhaps you) makes the experiment harder than it needs to be. Cramer seems determined to work with photon momentum, which is nice for making the connection to the Young double slit experiment, but there are so many other properties to work with. The reason polarization is used so much is that it's comparatively easy to manipulate and you can often use "bucket" detectors rather than fiddle with anything highly position-sensitive. I also do now have a better appreciation of why Cramer is having so much trouble with dark counts - he's set up an experiment with a very small signal.

Schematically, if I understand what you (Cramer and you) want to do, it's essentially a more elaborate version of a delayed choice experiment. Your effort to send "retrocausal" signals rests on somehow using the choice of measurement to send a signal that affects a previous measurement (or at least a spacelike-separated measurement), right? If I were trying to do that I think I'd go for something like a variant of Aspect's implementation of delayed choice. I'm not sure what exactly the best thing to measure would be, but I'm thinking you'd want a setup where the choice of which measurement you make on one element of the biphoton system affects the behavior of the other element of the system. So, for instance, if you feed the signal into something like Aspect's setup, you'd need to set up a measurement of some property of the idler such that the subsequent measurement of the signal "tells" the idler what the choice was, but in a way that avoids the possibility of the causal "story" working in the other time direction (i.e. the measurement of the idler doesn't somehow "drive" the later choice of signal measurement). I'd be surprised if a two-slit welcher weg ("which path" or "which way" measurement had any unique properties that make it the measurement of choice here.

One more thing you should think about before paying Newlight for BBO - what kind of entanglement do you really need? The 2-crystal Type I setup is designed to create polarization entanglement, but it sounds like the Cramer Mach-Zehnder with the D-shaped mirror on the input is really all about momentum entanglement. If you really don't care about polarization, you might do better with a single BBO crystal (still cut for Type I phase matching). This also lets you filter out stray pump photons with a polarizer (since in Type I the downconverted photons have the same polarization, which is perpendicular to the polarization of the pump beam). You also don't need a phase shifter before the BBO if you're just pumping one crystal (that's necessary to get polarization entanglement).

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Response to caraher (Reply #40)

Wed Jul 24, 2013, 12:43 AM

43. I just added your message to my file :)

I'll definitely look into those IR filters - $150 isn't that much compared to the BBO or especially the detectors. I'm thinking that since I'm going to use type 1 downconversion in order to get a hight number of entangled photons, I can reflect the pump into a brick somewhere way outside the rest of the optics. And I can filter out just the degenerate photons with black plastic that has two hole in it. There will be a little bleed over, but it should be pretty minimal.

Here is a link to Cramer's initial article on the experiment. He talks about Birgit Dopfer's experiment, and how it seemed to show the interference getting destroyed by measuring the entangled photons. The coincidence detection was what people pointed to in order to argue that this couldn't be used to send information. He presented his experiment idea to a bunch of people (physicists) and nobody could give a good reason why his experiment wouldn't work in terms of physical laws. So there have been people who have looked at it and tried to poke holes in the idea.

The basic idea is to have some way of detecting when a stream of photons is still in superposition or not. Using momentum entanglement is one way of doing that - double slit or interferometer - but it would be really nice if there were an easier way, like using polarization. I'm going to have to read your post above a few times and follow the links before I understand what you are getting at.

As for the BBO crystal set, I'm aware that I don't need both for this experiment, but there may come a time when it would be useful. If, for instance, there's a way to detect when polarization is in a superposition state, these would come in handy. Who knows whether I'll get a bug up my backside to do some experiment with polarization in the future? Otherwise you are completely correct. Oh, and I wondered about the phase shift. It sounds like that isn't necessary for the 2-crystal setup, and that's good.

Anyway, you've given me a lot of good information to think about, again! I really appreciate you poking at the concepts and experimental setup because I've never done anything like this before. So thanks, again

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Response to mindwalker_i (Reply #43)

Wed Jul 24, 2013, 07:41 AM

44. Well, what I was getting at is only partially-baked

I guess I'm thinking about things like hyperentanglement experiments, where people created photons that are entangled in many degrees of freedom. One newly-popular one is spatial mode, using Laguerre-Gaussian beams of different orders (which are mutually orthogonal) to supplement other resources like polarization. Frequency and momentum are, of course, other degrees of freedom that are available, but there's some appeal to using discrete variables like polarization relative to a fixed basis or LG mode rather than a continuous variable because those things are readily interpreted as quibits. In the delayed choice experiment I linked, we're looking at the measurement of a single photon; but to do anything interesting that measurement on one photon must affect a photon with which it is entangled. It's a matter of how you extend the experiment. Cramer's setup looks like 2 MZ interferometers, one for each photon, while Aspect's experiment is a single MZ since there's just one photon.

With the BBO pair one thing you can do is set up your pump laser so it only drives downconversion in one crystal or the other (set the pump to 0 or 90 degrees relative to one of the BBO optical axes). You actually need the phase shifter to do the polarization entanglement for Type I; in other words, if you're just making one "cone" of one polarization it's unnecessary, but if you want to make 2 overlapped cones by pumping with a 45 degree polarization, you need the phase shifter to make the two cones indistinguishable in principle with the correct relative phase. You basically need to make the phase of the horizontal component of the blue pump light a bit different (I forget which way) from the vertical component to compensate for things like the birefringence of the BBO (one polarization propagates faster than the orthogonal polarization).

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Response to mindwalker_i (Original post)

Wed Jul 24, 2013, 08:47 AM

45. OK, I get it now

At least the original version of the experiment. It's very interesting and well worth doing, but I think Zeilinger's explanation is spot-on, and considering all the variations is a great exercise in thinking about causality.

First, I now understand why Cramer is looking at momentum - it's because the experiment needs to use two conjugate variables, of which one pair is position and momentum. When you get the interference pattern you're measuring momentum eigenstates, and when you get the 2 peaks (in the Dopfer experiment) you're measuring position eigenstates. To make this all work you always have to measure two things governed by an uncertainty relation.

Next comes the question of coincidence measurement. At first I was thinking about this experiment as involving spacelike separation of the measurement events, but I see that in order to make a time travel connection that's not the case - you need the measurement you are "choosing" to have timelike separation from the other measurement, otherwise all you're doing is repeating the coincidence measurement version and reconfirming the nonlocality of QM but without showing any backwards-in-time causality. Cramer's argument is very seductive - my initial reaction was that adding 10 km of fiber to the optical path shouldn't change the result. And if that's so, it's undeniable that the choice of which measurement to make 50 microseconds after the fact does indeed determine the measurement made first!

I also had some fuzzy notion that the need to sample many points to prove you have an interference pattern was somehow crucial, but by mentally extending that 10 km of fiber to an astronomical distance I convinced myself that this does not matter - you could have enough photons in transit to the distant collector that their partners could all be fully sampled and the interference pattern collected well before the first photon in the stream arrived at the distant detector.

I think where the Cramer experiment falls apart lies precisely in a genuine need to collect photons in coincidence. The issue is that we don't get to tell nature not to collapse the entangled wavefunction when it interacts with the near detector. In the Dopfer experiment, the near detector sits behind a double-slit, with the result that if you do not record only photons in coincidence you will always get a 2-slit pattern at the near detector. ("Likewise, registration of photon 2 behind its double slit destroys any path information it may carry and thus, by symmetry, a Fraunhofer double-slit pattern is obtained for the distribution of photon 1 in the focal plane behind its lens, even though that photon never passed a double slit (Fig. 4)!" You do, of course, get to choose which measurement you make at the far detector. If you choose to make the momentum eigenstate measurement, you will also get an interference pattern, because measuring the momentum eigenstate at the near detector (which is what's happened already at the near detector) means you wind up with a momentum eigenstate at the far detector.

If you choose to make the position eigenstate measurement at the far detector, you won't get two peaks - you'll get a smear that reflects the fact that you're measuring the momentum eigenstate that now describes the far photon. The welcher-weg information was erased when the initial measurement occurred at the near detector.

One could reverse the setup, making the path with the double slit longer. Then what happens is unsurprising - when you measure with the Heisenberg microscope set up with the detector at a distance f from the lens, you get an interference pattern; if you set it up at 2 f, you get welcher-weg information but no interference.

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Response to caraher (Reply #45)

Wed Jul 24, 2013, 09:44 AM

47. I think my explanation isn't quite right, but close

A better way to put it might be this: what you measure at the distant detector is determined by what measurement you've already made at the near detector (or system), when they have timelike separation. My answer presumed you measure the interference pattern up close, but you could also imagine instead doing a which-way measurement.

The quality of the measurement made second probably depends on things like the detection efficiency of the first measurement. That's because if you measure photons only half the time, you actually get a mix of eigenstates and the original wavefunction for the distant photon. Measuring in coincidence cleans up the signal. You can always gate the detection system for the second measurement with a suitable delay to clear out the cases where the first measurement did not detect a photon.

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Response to caraher (Reply #47)

Wed Jul 24, 2013, 12:55 PM

49. I'm going to have to read through this a few times

Information density is increasing, and edging to the limits of my knowledge, which is great! Now my mind is being stretched.

I want to, first, recreate Dopfer's experiment but with an MZ instead of double slits, since too many photons are lost hitting neither slit. Using a coincidence detector for the first stage is fine. You mentioned they were a couple hundred bucks, which is within my budget (better be if I'm getting BBO crystals and detectors). I was also going to look at the raw output of the detectors witha logic analyzer (and probably oscilloscope at some point) - see Sealig for USB logic analyzers. I want to se the interference pattern and see it get destroyed with the "long arm" detector at different distances behind the lens, just to make sure it all works with the MZ.

The next step will be to remove the coincidence detector and see whether the interference still changes. I need to understand your argument above to grasp why that might not work. If I understand correctly, this is all kind of like the quantum eraser, and by moving the lens and detector physically further from the BBO, it becomed a delayed-choice quantum eraser. If the delayed-choice quantum eraser worked, and removing the coincidence detector from my psuedo-Dopfer experiment works, I would thing the retrocausal signaling would work. There are limits to what is understood about QM and entanglement effect, then there are more limits to what I understand about them.

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Response to mindwalker_i (Reply #49)

Wed Jul 24, 2013, 09:21 PM

53. To be clear...

The FPGA that is a few hundred bucks is something you connect your single-photon counting modules to. The few hundred bucks does not include detectors, it just lets you monitor coincidences. (You could run up to 4 detectors into the FPGA, so you could also look for 3-way and 4-way coincidences if you like).

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Response to mindwalker_i (Reply #49)

Sun May 11, 2014, 05:25 AM

70. Any progress on the experiment?

Last edited Sun May 11, 2014, 02:00 PM - Edit history (1)

This is a fascinating and educational exchange - mindwalker_i and caraher, thank you! I especially congratulate you on discussing Dopfer/FTL without descending into preadolescent venom like many other threads.

I think caraher's explanation of the indispensability of the coincidence counter must be right, due to causality, the No-Communication Theorem, and most basically the fact the Zeilinger himself hasn't made any grand claims. Still, after reading about so many different ways of spawning entangled pairs and of detecting interference, it is counterintuitive in its own way that FTL communication via interference-checking wouldn't work. So I completely agree that mindwalker_i's (and Cramer's) experiment is worth attempting.

Aside from an update on that work, I was going to ask why a similar approach couldn't be applied to Walborn's quantum eraser experiment - using insertion of the linear polarizer at Dp to trigger an interference pattern at Ds. But it seems the diagrams there conceal the use of yet another coincidence counter. They're everywhere!

Finally, David Ellerman's paper on "why delayed choice experiments do NOT imply retrocausality" may be worth a look for its take on the key role of coincidence counters.

(Disclaimer: I'm not a physicist)

PS I updated the Ellerman link above to a more up-to-date revision of the same paper, hosted on his own site.

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Response to idkiigmy (Reply #70)

Sun May 11, 2014, 12:47 PM

71. I'm halfway through the Ellerman paper

It's good stuff and really calls attention to some important details I get sloppy about... This will be helpful in organizing my thoughts for teaching quantum next spring. Thanks!

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Response to idkiigmy (Reply #70)

Sun May 11, 2014, 01:53 PM

72. First of all, thanks for the links!

I'll go through those at some time soon.

I built a Mach-Zehnder interferometer and got that working, then added a couple of galvonometer set - they're used in laser animation to rapidly move the beam around and draw stuff - and sent the output beams to detectors. I'm still using visible light at intensities that can be detected by the three-terminal detectors from somewhere way above in this post. I could get a good pattern on an oscilloscope with this configuration.

The main problem was that I couldn't precisely adjust things. Every time I tried to move one part of the system, the others would get all screwed up, making it really difficult to carefully adjust mirrors and prisms to get solid interference patterns. So I started working on designing and building mounts for the optics. I've got pieces of a design that are starting to work, and am figuring out how to actually machine pieces. This is tough because I'm definitely not a machinist, and am learning everything as I go.

The interference pattern gets disturbed when my cats walk by. Even with the whole thing suspended on springs, it's incredibly sensitive to vibrations.

I have the BBO crystal (cut for colinear parametric downconversion) and the single photon detectors. The circuitry to count photons works, and interfaces to Linux through USB. I've made some progress building printed circuit boards - I really need to get this thing off of breadboards. However, I haven't actually done any downconversion yet. I need to build a mount for a dichroic mirror to reflect off the violet light after downconversion, and I need to build some blocks to only allow precise parts of the interference pattern to get to the detectors. More machining.

As for the ideas behind the experiment, those are still open. I have heard from Cramer that he is working with Zellinger, and he refers to some unpublished experimental results that Zellinger has that relate to this whole thing. I don't know what those are, but Cramer seems to be happy about it, and I doubt that it shows Cramer's experiment won't work. I still have this feeling that, if for some reason Cramer's experiment didn't work, one would be able to get an interference pattern AND measure the path of each photon. So something has to break - either you can actually get the which-way info and interference, or you can send information through entanglement. Give me FTL or Heisenberg's uncertainty principle gets its ass kicked.

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Response to mindwalker_i (Reply #72)

Sun May 11, 2014, 02:13 PM

73. Cool, keep us posted

I'm still really happy you're attempting this. I share many of your intuitions and you and Cramer seem to be the only ones putting them to the test. If funding becomes an issue this seems like just the sort of thing crowdfunding was meant for... BEST OF LUCK!

Btw, another guy who's expressed pretty knowledgeable interest in this work is Paul Friedlander.

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Response to idkiigmy (Reply #73)

Sun May 11, 2014, 02:25 PM

74. I've already bought the most expensive equipment (I hope)

The detectors were quite expensive, and hopefully nothing else will come along. The BBO crystal didn't even come close to those.

With a MZ interferometer, it should be possible to get it tuned just right so that all of the light comes out one port. It's when the beams are not exactly going in the same direction - diverging slightly - that you get interference patterns. If I can get my equipment up to snuff and get it so almost all the light does come out of one port, then sticking a finger in one path will result in half the light coming out of each port (by port, I mean the two outgoing faces of the final beam splitter). So if 20% of photons are from entangled pairs and I collapse their wave functions remotely, then the balance between light coming out of each port should be disturbed in a measurable way, like 10% will come out of one port as opposed to 0%.

But first, I need to get better mounts made

I think I've been to Paul's site - I remember the background from a page on Dopfer's experiment that used it. I'll have to delve deeper into his whole site.

And thanks for the note(s) of support! I keep trying to get myself to write a blog about this, but then I get busy with stuff (like machining).

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Response to mindwalker_i (Reply #72)

Mon May 12, 2014, 11:51 PM

77. Good mounting hardware helps immensely

Do you have an optical breadboard? (These typically have 1/4-20 holes drilled on a 1-inch square grid.) if you buy or make one that will let you use lots of standard mounting hardware. Machining might be cheaper for a lot of things, but don't forget to put a dollar value on the time you'd rather spend doing optics instead of machining!

I don't remember your optical setup, but optical mounting rails might be useful if you don't want to spring for a breadboard.

Some folks have developed optics mounts for 3D printers. I haven't tried any yet but I'm intrigued by the concept. Meanwhile I buy from eBay, ThorLabs and often from PhotoMachining's surplus pages. (This reminds me of a quip I read recently - theorists' shelves contain textbooks, experimentalists have catalogs - though these days so much is online I think the shelves have started to find other uses!)

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Response to caraher (Reply #77)

Wed May 14, 2014, 12:16 PM

79. Sorry for he late reply, bu yeah, I've been working with an optical breadboard

Ebay had it for ~$150, 2' x 3' or so. I got a set of the poles to mount on it and to mound stuff on, and some things to adjust position in one dimension. I think that there are some resonant vibrational modes with the long poles and breadboard, so vibrations of certain frequencies show up in the scans. Isolation is difficult, especially since there's a busy street outside.

For mounts, I need X and Z adjustment plus rotation, and I can keep using some sets of two aluminum plates, with three bolts and springs. I haven't seen mount with the two axes and rotation, but if they exist, they'd probably be around $500. for ~15 of them, it would hurt. Also, I need to make other things like the masks to limit what light gets to the detectors. I did something earlier that really sucked: a flat piece of plastic with a slot cut in it, then there were pieces of plastic that could slide in or out to widen or narrow a slit between them. Also did something like that for the height of the slit. It sort of worked, but I can do a much better job with a better design.

And there will undoubtedly be other things I need to build. So putting some effort into learning how to machine the mounts will pay off in terms of having a useful skill to build more stuff. About two years ago, I bould a milling machine from Sherline, so there's already some investment. That thing is cool Now I'm learning about lubrication and adjustments and stuff. I have a friend at work - old salt, kind of - who knows a bit about this, and he and I talk about machines. He gave me a really nice drill press, just because he has a better one and limited garage space

Another good reason to get a blog up would be that I could post some of these designs, and possibly get feedback on them. I could at least link to them from here.

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Response to mindwalker_i (Reply #79)

Wed May 14, 2014, 03:53 PM

80. I might get one of those Sherlines someday

We have an old Bridgeport in our department that's indifferently maintained, and it's major overkill in terms of size. Both that and our lathe are a bit frustrating to use (and my colleagues routinely use them for wood, which would have earned me a lifetime ban from the metal shop in grad school!).

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Response to caraher (Reply #80)

Wed May 14, 2014, 06:49 PM

81. I like their stuff a lot

Granted, I have extremely little experience with tools like this, but the mill has been easy enough to get started with. I'm kind of fighting for accuracy. There are some things to tighten, like the gibbs, and a hex bolt that keep a square-like interface between the axes from moving around, but if that too tight it becomes very hard to move the table in the Z (toward/away) direction. I also need to take the table pieces apart, clean them, and re-lube the tracks that they slide on.

I wouldn't mind getting a small lathe from ebay or something. That could be quite handy, but it would be another tool to learn. On the other hand, a 3D printer could be highly useful! Yet another tool to learn, but it would be worth it. I'm working almost entirely with plastic right now, although I might remake these mounts in aluminum once I've proven the design. It depends on how well the plastic works.

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Response to mindwalker_i (Reply #81)

Wed May 14, 2014, 06:57 PM

82. I actually find aluminum easier to machine than plastic

Though I suppose it depends on what kind of plastic...

Aluminum and brass, for me, are in my machining "sweet spot." Softer materials like plastic are more challenging for me to machine with any precision, and I'm rarely as happy as I'd like with the surface finish (too many ways to leave obvious tool marks).

Hands-down the worst material I've worked with: copper! Tools tend to "grab" it.

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Response to mindwalker_i (Original post)

Wed Jul 24, 2013, 09:01 AM

46. So... maybe you should use collinear Type II?

Since it's clear that this measurement is all about momentum and not polarization, and since I know you can get ginormous count rates using Type II collinear downconversion (note that this is what Cramer is doing, only in PPKTP rather than BBO!), you might be better off with Type II. You can separate the photons with polarizing beamsplitters, and they momenta carry the same correlations. And if you want to play with polarization entanglement at some later date, all you do is tilt the crystal a little bit (I'm not sure how many degrees that is at 405 nm to get, say, 3 degree separation of the entangled beams from the pump path, but it's reasonable, and you don't need that phase shifter, though you probably need to compensate for temporal walkoff inside the BBO, which is something you do with, say, quartz, after the crystal).

One handy feature of the collinear geometry is that you can do a lot of alignment using your pump beam (which you can see!). It's only when it comes to any focusing optics or other dispersing elements like prisms that the wavelength difference gives any trouble.

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Response to caraher (Reply #46)

Wed Jul 24, 2013, 11:20 AM

48. Type II Collinear?

I have not come across the term yet - my knowledge of nonlinear optics is a bit raw - but as I understand it, type I is the mode that gives off a cone, where photons on opposite sides are entangled. Type II is two sets of three concentric cones, and where the two middle cones intersect, there are entnagled pairs. I've read that type I has a lot more entangled pairs than type II. So what is this type II collinear? A quick google search gave me a bunch of PDFs that I can look through later. And it produces a lot of entangled pairs? That could be very useful, and separating with a polarizing beam splitter is not problem.

What I'm thinking is this: two beams are generated (or one beam, which is split via polarization), and one goes into the MZ interferometer. That forms two interference patterns, one at each output of the second beam splitter/recombiner (BS2), and those should be negative images of each other. In the interference pattern of the orthogonal output of BS2, there will be dark spots, but when the entangled photons in the beam input to the MZ are knocked output of superposition, some of them will land in those dark spots. Any photons that are not entangled will just keep doing what they've always done, forming the same patterns on both outputs of BS2. So the MZ kind of plays a similar filtering role as the coincidence detector in Dopfer's experiment.

So, the percentage of photons that are entangled with photons in the other beam shouldn't be too important. The raw number of entangled photons does matter, because that determined how many end up in the dark spots when they get knocked out of superposition (superposition in terms of momentum). From what I glean from your description of collinear mode above, there would be more entangled photons and they would already be in a beam rather than in a cone that has to be filtered (positionally, with bricks that have holes in them) to get beams. Am I on the right track?

My understanding of nonlinear optics is weak. My searches for information on google have turned up some stuff, but I think I need more depth. For my physics major, I didn't take a QM course but have learned just about everything on my own since school, which resulted in a qualitative understanding of a lot of concepts and ideas, but definitely a lack of understanding for the mathematics. Sadly, the book QWuantum Physics For Dummies kicked my ass on the second chapter and continues to do so occasionally when I get ambitious and pick it up again. There is, however, an absolutely awesome book, "The Quantum Universe" by Brian Cox and Jeff Forshaw that is really explaining some interesting things, mostly qualitatively, but it doesn't relate to entanglement (yet). Hopefully it will provide enough understanding so I can get through the Dummies book

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Response to mindwalker_i (Reply #48)

Wed Jul 24, 2013, 04:49 PM

50. Ok, I see basically what this is

Type II does have the two sets of concentric cones, and where the two middle cones intersect, there are entangled photons. For collinear type II, the angle between the two sets of cones is set so that there is only one point where the middle concentric cones intersect. Since in normla type II, the entangled photons in the two intersecting points (or lines, sort of) have opposite polarization, in the collinear type II, entangled photons are in the same beam but have opposite polarizations - or different, or 90 degrees off, or whatever.

The entangled photons will be in the same line as the pump beam that remains after downconversion, which is the overwhelming majority of the pump. Therefore that needs to be broken off with a dichroic mirror, prism, or something like that. The remaining IR photons can go through a polarizing beam splitter and sent in different direction to be messed with however necessary.

I see one big advantage would be that the entangled photons don't get spread out due to the location where downconversion happens being anywhere within the depth of the crystal - the violet photons downconvert into IR, and the IR photons continue going in exactly the same direction rather than deflecting at 3 degrees or so. That means the BBO can be arbitrarily deep without smearing out the downconverted photons. And therefore, lots of downconversion can be made to happen by extending the depth of the crystal.

This sounds like a much better way to go than the paired BBO crystals. Is there any reason not to go with this method? What should I look for when picking a crystal to buy (obviously type 2 collinear)?

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Response to mindwalker_i (Reply #50)

Wed Jul 24, 2013, 09:31 PM

54. Close

If you want polarization entangled beams with Type II, you basically have one cone of H polarization and another cone of V polarization, and along the lines of intersection a given photon is equally likely to "belong" to the H cone or the V cone. So the state you get is something like |HV> + |VH>, where the first letter is the polarization of the photon emitted along one intersection line and the second letter is the polarization of the photon emitted simultaneously along the other line. The pump beam, as always, is along a line halfway between them (for equal-wavelength photons).

If you tilt that crystal, for a given wavelength, the cones change opening angles and central directions. In Type II collinear downconversion, the cones that used to intersect along 2 lines now have just one line of intersection.

In general you can get more photons just by using a longer crystal, but there are limits. And there is what's known as temporal walkoff - one polarization moves ahead of the other polarization. One way to correct for that is to follow your BBO with a second BBO crystal, rotated 90 degrees, with half the length. This overlaps your H and V wave packets.

A typical temporal walkoff is about 100 fs for a crystal of about 1 mm length. That may or may not matter, depending on the experiments you want to do. For this experiment I don't think it matters at all, since you plan to have very different path lengths for the two photons anyway

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Response to caraher (Reply #54)

Wed Jul 24, 2013, 10:57 PM

55. Looking at NewLight's site

They do sell a type-II BBO, and it says there's a 5 degree separation I think from the pump, which sounds about right. So the cones come off at 5 degrees, but if the crystal is tilted, it will change that angle depending on which way it's tilted, correct? So at some point the cones just touch along that one line, and all the entangled photons are there. Which way should the tilt happen? Rotating it around the beam doesn't sound like it, so is it, say, moving that major optical axis closer to the beam in back and further away in the front? That seems like it would be reasonable.

Newlight's NCBBO5300-405(II)-HA5 is 3mm thick, 5mm x 5mm in the other two dimensions, cut for type II phase matched SPDC pumped by 405nm, with a half opening of 5 degrees. Mounted in a 1" holder. I could scan the two cones to a detector to get the positions of the cones, then keep doing that while rotating the crystal slowly. In contrast, the paired BBO crystals are .5mm thick. Would the type II work, rotating, reflecting out the pump and such, giving off more entangled pairs?

Unfortunately, I haven't found terribly good info on the net for showing how these work or estimating the brightness.

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Response to mindwalker_i (Reply #55)

Wed Jul 24, 2013, 11:50 PM

56. You should talk to their sales engineer

I'm not sure if there's more than one; every time I talk to them it's Jean. If you give the wavelength and tell them you want collinear Type II they'll know what you're talking about. It will take probably 7 weeks or so for them to make it for you (Incidentally, it's worth getting a coated crystal if only because BBO absorbs moisture and turns cloudy, and the coating helps protect against that! Their "p-coating" does that; I usually also get antireflection coatings, but you might actually want reflections to help you align it. In any event, you should store BBO in a sealed container with dessicant when not in use...)

I think the real issue with estimating brightness is that the full calculation is cumbersome and depends on knowing more details about your laser than most people have readily available.

Chapter 4 of this dissertation illustrates the effects of crystal tilt. The cones shrink as you tilt the BBO until they meet along a line, and if you keep tilting they eventually turn into non-overlapping cones and ultimately two beams. The 5-degree opening angle BBO cut for Type II can almost certainly be tilted to give collinear downconversion; on the other hand, a crystal made for collinear downconversion can surely be tilted to give rings with a few-degree opening angle.

(Incidentally, a beautifully-written dissertation on a topic similar to yours is David Branning's "Optical Tests of Complementarity and Nonlocality." Well worth reading... you probably won't follow all of it, but there are many passages accessible to the non-specialist reader.

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Response to caraher (Reply #56)

Thu Jul 25, 2013, 12:44 AM

57. Cool! Yeah, I've been e-mailing Jean as well

I'll definitely ask about type2 collinear and get a quote. She sent be some pictures of food jars the other day along with silica "do not eat" packets. I felt really stupid, but wasn't sure what a "desiccant jar" was and didn't want to screw up.

So I gather 3mm thickness should be sufficient? I'll e-mail Jean tomorrow and get a quote. Plus, I'll read those links (when I have a couple brain cells operating.

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Response to mindwalker_i (Reply #57)

Thu Jul 25, 2013, 11:47 PM

58. a factoid

Today I was getting roughly 2000 coincidence counts per second per milliwatt of pump power. That's pumping a 0.5 mm thick BBO crystal cut for Type II, with about 80 mW of blue light centered on 400 nm (I'm actually using frequency-doubled broadband light from an ultrafast laser running at around 800 nm), which was giving me up to 180,000 coincidence counts per second. I'm collecting pretty much all the downconverted light, not just the photons from the overlap region, but I also have a 20 nm bandpass filter and the 750 nm longpass filter I mentioned before, as well as a dichroic mirror at 45 degrees to reject the pump light; this is what's left over after those losses, losses due to other optics, and ~55-60% single detector efficiency at 800 nm.

I've pumped the same crystal with a 50 mW 407 nm diode laser and get very comparable count rates. The diode laser is a rather nice one made by Power Technology, but costs several thousand dollars. I think if I were doing your experiment I'd at least consider the ~$1k 405 nm laser system Roithner sells; it should give comparable performance. But I'd try with a cheap laser first, since if that fails you can always buy a pricier one, whereas if it works you've saved a bunch of money! 50 mW is plenty of power if you have a nice mode. It's also *relatively* safe, certainly much more so than a 250 mW unit!

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Response to caraher (Reply #58)

Fri Jul 26, 2013, 11:59 AM

59. 2000/milliwatt is really good

It sounds like I need to get the stuff - type-II crystal, detectors, polarizing beam splittler, dichroic mirror (I have a couple that I got from ebay), IR filters - and set up a basic system to get some initial coincidence counts. The oscilloscope/logic analyzer I mentioned earlier is arriving next week, and I'll be able to use that to get traces from both detectors to get an idea of how many photons total and how many entangled pairs are generated. I'll worry about a real coincidence deector later, but it doesn't sound like that will be too expensive - I could also build something simple, like triggering a couple 74LS121s with the detector outputs - they will output a pulse of a length determined by capacitors and other components after receiving an edge on their inputs - then feed those to an AND gate.

As for the pump, starting off with an ebay laser sounds good, but the one you linked to is really nice! The TEC will be really helpful to keep a constant temperature, so I'll probably upgrade to that at some point.

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Response to mindwalker_i (Reply #59)

Fri Jul 26, 2013, 02:14 PM

60. Since you're an electronics person...

the folks at Whitman also used homemade coincidence detectors. Because I'm a bit shakier on all that I was happy to sponge off their efforts and do nothing little more than build the little box that makes sure the signals are properly terminated and routed to the correct ribbon cable wires It also helps that I have a LabVIEW license; if I didn't I'd have had to work out how to communicate with their FPGA via RS-232, which isn't hard but which is another task.

The "traditional" approach to all this (for people who have all kinds of nuclear instrumentation lying about) is to use one detector to provide a "start" pulse to a time-to-amplitude converter (TAC) and use the second detector and a fixed delay to provide a stop pulse. They get a histogram of arrival times which helps them set a window they define as good for coincidence counting, then generally use a single-channel analyzer... I went through this once, but I can't say it really mattered for what I'm doing. As long as you don't do something silly like have one optical or signal path vastly longer than the other relative to the coincidence window width (remember that 1 foot = 1 nanosecond!), simple coincidence circuits should work just fine.

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Response to caraher (Reply #60)

Fri Jul 26, 2013, 04:10 PM

61. I've worked with LabView before

Long ago in school. Unfortunately I don't remember a log about it. Regardless, I kind of want to get three signals: one from each detector, and one from the coincidence circuit (two triggered pulse generators and an AND gate), and get a good feel for the percentage of enangled photons vs. non-entangled. I kind of wonder whether I can program up a PIC microcontroller to take a continuous stream from the detectors, give me total counts and percentages, and send it all to Linux. I think the pulses from the detectors are way to fast and short for a direct connection, but I could buffer it all with counters and such (as long as those are fast enough).

But after this great long discussion, I get the strong feeling that I just need to play with this stuff and screw around to get a feeling about how often there are coincindences, how filters change that, etc. Jumping right into sending beams into interferometers sounds like a recipe for frustration since it won't work, and I won't have the understanding to fix it.

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Response to mindwalker_i (Reply #61)

Fri Jul 26, 2013, 08:36 PM

62. I think that's right

The setup described at Whitman does exactly that - you get singles rates as well as the coincidences. The FPGA basically counts for as little as 100 ms, then sends a burst on the serial cable with I think up to 8 channels of count data, up to 4 channels of "raw" counts and up to 4 channels of user-selectable coincidence counts.

Your electronics don't really need to be heroically fast just to count singles - the SPCM pulses are, I think, a few tens of ns long, and you get something like 50 ns of deadtime following each count, and you'll never see a rate exceeding 10-15 MHz. As long as you can pick up the leading edge of a TTL pulse that wide you should be fine. So yeah, your microcontroller is probably too slow to actually do the counting directly, but some fairly simple digital electronics can feed it your counts at some manageable intervals.

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Response to mindwalker_i (Reply #61)

Fri Jul 26, 2013, 11:59 PM

68. One more thought...

One advantage of the TAC/MCA/SCA approach is that, armed with photon arrival time data, you can essentially look at whether coincidence measurements matter after the fact. Looking at this paper, they generate their interference patterns (I think) by post-selecting data: "Because we record the time interval distribution of coincidence counts, we can analyze two-photon interference visibility with delayed choice of the coincidence time window." In their case, they want to compare fringe visibility for coincidence windows larger and smaller than the path length delay difference. In the case of Cramer-like experiments, you can either use a coincidence window or ignore it entirely.

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Response to caraher (Reply #68)

Sat Jul 27, 2013, 01:10 AM

69. Interesting! This just further solidifies my determination

to get a hold of some stuff and try this all out.

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Response to caraher (Reply #56)

Fri Jul 26, 2013, 10:59 PM

63. The link to Chapter 4 of "this dissertation" does not work for me

I think it's because I don't have a login. Is there by chance an alternate link? It looks like something I need to read.

BTW, Newlight says they can cut what is normally a standard BBO to be collinear, and it's about have the price! A little over. I am so there!

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Response to mindwalker_i (Reply #63)

Fri Jul 26, 2013, 11:17 PM

64. The link is to the full dissertation

but there's no login. Here's an alternative (direct to the download):

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/78932/osuzer_1.pdf?sequence=1

I should really find another link with the same information. I just picked that because I'm doing closely related work, and knew the business about tilt was in there...

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Response to caraher (Reply #64)

Fri Jul 26, 2013, 11:26 PM

65. I found a much better piece

Google "generation of correlated photon pairs in type-ii parametric down conversion-revisited" and the first few hits will be .pdf files of the article, that very clearly lays out how Type II works and the role of tilt. This is from the Journal of Modern Optics in 1997

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Response to caraher (Reply #64)

Fri Jul 26, 2013, 11:27 PM

66. That workd, thanks again!!!

I'm slowly working through the links you've posted in this thread - there's enough to keep me busy for a while.



Braaaaains! Mine is melting, but it'll get better.

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Response to mindwalker_i (Reply #66)

Fri Jul 26, 2013, 11:37 PM

67. OK, but see my other post

The J. Mod. Optics article is a better reference than the dissertation

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Response to mindwalker_i (Original post)

Wed Jul 24, 2013, 05:40 PM

51. Have you checked the fuses?

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Response to Eddie Haskell (Reply #51)

Wed Jul 24, 2013, 06:27 PM

52. Uhm, what?

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Response to mindwalker_i (Original post)

Mon May 12, 2014, 07:11 PM

75. My intuition tells me the past and the future are rather symmetrical about any local present.

Humans tend to be uncomfortable with a "past" as insubstantial and as shifty as the "future" is.

This is an awesome DU thread.

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Response to hunter (Reply #75)

Mon May 12, 2014, 07:23 PM

76. Thank you! I'm a bit surprised at the attention it's gotten as of late

I think it's a year old, but in the last couple of days, there have been a couple new responses.

Physicists still don't have any real idea what time is. Their best guess is that spacetime is an unchanging "block" where everything is already laid out, and our impression just moves through this (static) landscape. That whole part about perception is dismissed as an illusion. My hope is that, if this experiment works, we'll have a tool with which to probe time and see what its properties are, like whether the past is a write-once ROM or can be overwritten.

And I kind of suck as a machinist

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Response to mindwalker_i (Reply #76)

Mon May 12, 2014, 11:58 PM

78. You should definitely read the Ellerman paper posted upthread

It argues pretty strongly (and I think correctly) that "delayed choice" experiments do not, in fact, imply any kind of retrocausality. The key is that all the effects of washing out and restoring interferences in quantum erasers, etc. are properly interpreted remembering that a beamsplitter doesn't actually measure anything. Instead, it passes a superposition state, and the crucial feature is whether we set up detectors to measure all the states in the superposition or only some smaller ensemble.

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Response to mindwalker_i (Original post)

Sun May 18, 2014, 12:01 AM

83. Fascinating read.

I actually more or less understood it. Physics is definitely not my field but I love to read about this stuff and I've always been fascinated by complementarity and the double slit experiment. I've even daydreamed about doing what you're doing and building some sort of test bed, but feared that any experiments on my part would amount to scientific quackery.

I'm going to bookmark this thread and come back occasionally hoping for updates.

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Response to drm604 (Reply #83)

Sun May 18, 2014, 01:44 AM

84. Thank you for your interest

To be perfectly truthful, I'm somewhat of a quack, or at least I feel like it. I've never built physics experiments or done anything like it. What's really cool is that I can apply some of the engineering-related things that I've done to this project: I built a simple laser animation system a while back, and learned how to get a Linux system to talk to a microcontroller (PIC18F2550/4550), to position galvanometers to draw things. Now I'm using that to move the interference pattern around over a light detector.

I did some work making pieces for my optical mounts today, and got better results than I have before. Earlier this week I cleaned up my milling machine, took it apart, got the crap out, and added synthetic lube. That helps a frigging lot! I need to make some more of these pieces, but hopefully by tomorrow I can get the first stage (Z direction) working, then do the next stage. It's tough, but I'm making slow progress.

Once I get the mounts built, I'll build up the interferometer and try to take some pictures to post.

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Response to mindwalker_i (Reply #84)

Sun May 18, 2014, 09:30 AM

85. To be clear, I wasn't suggesting that you are a quack, you obviously have some knowledge.

I was saying that I feared that that would be what I was doing.

It certainly would be fun to play with, strictly as a hobby. I'm a software developer but as a kid I enjoyed playing around with electronics hardware. Playing with lasers and optics combined with electronics and software would be a real kick.

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Response to drm604 (Reply #85)

Sun May 18, 2014, 05:57 PM

86. Oh I didn't think you were saying I was a quack

It never even occurred to me. However, I am kind of getting into stuff that is complicate and outside of my training or experience. It is, however, a lot of fun to bring in experience from other fields, like the microcontroller stuff.

Software is a big part of it too. I started programming on the Commodore VIC-20, rapidly moved to the '64, got into C on the Amiga, and so on for ~30 years. Microcontroller programming kind of reminds me of the C64 days a little. It also forced me to learn how to write basic drivers in Linux, which turned out to be rather simple.

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Response to mindwalker_i (Reply #86)

Sun May 18, 2014, 10:40 PM

87. I haven't programmed at the hardware level in decades.

I'm currently doing web development which is about as far from that as you can get. I'd love to do some low level C or even assembly programming, although I imagine even microcontroller programming may be done at a higher level these days.

Talking about programming, imagine if you could send bits back in time by even a tiny amount. If that could be shrunk down and replicated at the level we currently build circuitry, it would be an amazing advance in processing speeds. We could have essentially instantaneous computing.

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Response to drm604 (Reply #87)

Sun May 18, 2014, 10:51 PM

88. Oh now you've done it

Here is a piece I wrote a couple of years ago, talking about this very issue:

http://quantizedimagination.wordpress.com/

As a side note, I'm really bad at setting aside time and writing stuff up, which is why there's not much to that blog. I have another that has even less, but I want to get myself to the point where I post updates to it. That one is:

http://synapticdistress.wordpress.com/

Anyway, the idea of using short timelike loops is a very interesting one. I could imagine making a circuit that does division, leaving a remainder, that tries a number and if the remainder is nonzero, increments the divisor and send that back on the loop. RSA encryption could be wiped out this way. But also, doing short computations and sending partial results back could have wide ranging uses. The first to come to my mind is ray tracing.

Having two of these could allow someone to send messages back arbitrarily far in time if each loop was offset have a retrocausal period. That, of course, would lead to the effect being "macroscopic" in the sense that we could observe it. It would also lead to allowing for highly visible paradoxes (the original idea also would, but it's a little more hidden). That's where I start to wonder if this experiment could break the universe. That really seems unlikely, since if the universe had laws that allowed it to break itself, it would have happened.

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Response to mindwalker_i (Reply #88)

Mon May 19, 2014, 12:05 AM

89. Maybe it can break locally.

Maybe it would just collapse the local galaxy into a black hole or something. Maybe this explains the Fermi paradox. Any civilization that advances enough for interstellar travel or communication will be destroyed by some quack in his basement.

Or maybe it would break the universe, but we're the first civilization in this particular universe to discover the phenomena else we wouldn't still be here.

Or maybe it would just be destructive at a smaller (but still devastating for us) level, like a new type of nuclear bomb.

But yeah, it seems unlikely. In any case, if you don't do it, someone else will, so there's nothing to lose in trying and everything to gain.

We'll build it together and then use our subsequent lottery winnings to fund further research, lol.

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Response to mindwalker_i (Original post)

Tue May 26, 2015, 01:35 AM

90. entanglement project mindwalker

Hello Mindwalker_i ,

I am interested in your progress and your purchases of the BBO crystal and detectors. What is your pump source? I realize that the last posts on this was in May/???2014... and there have been a number of advances since that date. Are you continuing with this project?

Regards,

zoliver

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Response to xoliver (Reply #90)

Tue May 26, 2015, 01:22 PM

92. entanglement project

Yeah, I'm still working on it. The main problem is that I need to be able to finely position mirrors and laser beams in order to get the interference right. It's on'e thing to get an interference pattern with a Mach-Zehnder interferometer, but it's harder to get it interfering perfectly. So with that in mind, I am building nice little optic mounts with X, Y, and rotation adjustments. That's hard. The circuitry for interfacing a computer to the photon detectors was fairly easy, using some 74F chips for counters, and a PIC microcontroller for getting those counts back to the computer.

Once I can get some of these optic mounts done, I'll start working with the BBO to see how many entangled pairs it produces (pumped initially with a 405nm diode from ebay, but I may need to upgrade that to a lab laser). What I don't know is how coherent the downconverted photons will be, which will effect whether the interferometer works. Another possibility will be to use a double slit. I think that will be more immune to coherence issues, but a lot of photons will be lost between the slits.

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