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.
= new reply since forum marked as read
I bookmarked it for another reading attempt in the morning.
And my physics teacher. 
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.
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 
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!
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
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!
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).
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
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.
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).
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.
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
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.
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.
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
