Double slit experiment with detectors not recording

[lcdisplay]

As far a FTL communication is concerned, this is an area of intrigue for anybody with a pulse and as a Quantum Physics student with a pulse I am very intrigued.

I challenge the notion that a photon pair in a singlet state for example cannot transfer information FTL. Indeed it does transfer non usable information faster than light. That is not in question at this time (or is it).
So we develop the no signaling, no communication theorems which say no matter what you do you cannot use the wavefunction collapse to transmit information. And this today seems is correct but whether or not you can use the existence of an interference pattern or lack thereof as a way to transmit a 1 or 0 via a QE is a completely different phenomenon.
In this concept the no communication/no signaling theorem does not apply, at least not to me. And so I challenge the notion that FTL comm is not possible. As iron sharpens iron I would love to be challenged by you gentelemen into a discussion on this subject to see if sense can be made. I am fully open to the possibility that I am wrong as I make no special claim that my understanding is fool proof.

 

[lcdisplay]

If we take a Zeigler type of QE setup and somehow produce 100 nanoseconds worth of back to back to back photons so that there is thousands of them back to back in that 100 ns and send them off to NewYork and the entangled "pulse" to San Francisco (we produce them somewhere in the middle) we should be able to dicipher the inteference patterns (or lack of) quickly without the use of movable single detectors. Assuming little noise of course. This should be able to do the job but I could be wrong. What do you guys think?

 

[unusualname]

You can't know which photons contributed to the interference without measuring both the entangled photons.

What the photons do at the qm level is random as far as classical physics is concerned so there is no way of transmitting classical information.

An interesting question left unanswered by these experiments is how non-local the qm wave-function is, since the delays involved are rather small, and we can't do it via New York and San Francisco yet (or via alpha-centuri :) ) , Bell type results have been confirmed over a few kilometers in Switzerland, but that's about it.

If something like the Holographic Principle exists, then we may just be viewing a projection from another dimensional surface, which would help explain the non-locality. We simply don't know yet.

 

[lcdisplay]

You can't know which photons contributed to the interference without measuring both the entangled photons.

Please explain

 

[JesseM]

Please explain

There have been a few threads on the delayed choice quantum eraser (DCQE), see here for example. Here's what I said before about why you can't see an interference pattern without doing a coincidence count:

Even in the case of the normal delayed choice quantum eraser setup where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the setup in fig. 1 of this paper:

http://arxiv.org/abs/quant-ph/9903047

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.

 

[unusualname]

^^JesseM is referrring to a more complicated experimental setup than the one in the experiment I have been referring to (Walborn et al):

http://grad.physics.sunysb.edu/~amarch/

In this experiment you need to measure both entangled photons to ensure the which-path interference was erased, otherwise you are dealing with a set of photons with unpredictable properties, either because the entangled partner never made it through the eraser mechanism or you're detecting photons from laboratory noise.

The Walborn experiment makes this much more clear than other quantum eraser experiments, where phase matching etc might come into play.

 

[lcdisplay]

Jesse the quantum eraser experiment you have referenced is one of many possible ways to erase the which way information. In that experiment a bad design for FTL communication was chosen (good for simple path erasing). Look at Dopfer et al. The Heisenberg detector does not have this weakness.

Unusual, about the coincidence detector. Even if coincidence detection was necessary that alone would not prohibit FTL communication and that was proven by Weihs et al in the aforementioned refrence of my prev post. They just synchronized quantum clocks.

What about the "subset" of detected pairs. Again Wineland/NIST addressed that loophole and at most it would correspond to an SNR question. Additionally, spontaneous Parametric Down Converted photon pairs, among other entangled photon creation schemes, can be made to filter the entangled pairs from the non entangled pairs at the point of creation so this should not be a problem.

We know stray light is just an engineering challenge and finally the efficiency of pair creation is also just another engineering challenge which is starting to get solved.

So far I don't see a problem. What am I not seeing?

In my previous post I mentioned using a Zeigler type of QE but I meant Walborn

 

[unusualname]

I don't understand what your question is. Just becasue the interference seems to require ftl or non-local physics why do you think that implies ftl communication?

QM is random as far as classical physics is concerned, at least we have no known way of deterministically selecting a quantum state, so you can't transfer classical information ftl. There may be quantum processes operting ftl but that doesn't break einstein causality, in fact for all we know, the quantum processes themselves may be fundamentally random or acausal.

 

[JesseM]

Jesse the quantum eraser experiment you have referenced is one of many possible ways to erase the which way information. In that experiment a bad design for FTL communication was chosen (good for simple path erasing). Look at Dopfer et al. The Heisenberg detector does not have this weakness.

I suspect it probably does have the same weakness, i.e. there are multiple places the "idler" photon might be detected, and if you look at the subset of "signal" photons whose idler was detected in one specific position you could see an interference pattern, but if you add together all the subsets corresponding to different possible positions for the idler (weighted by the probability of the idler landing at each position), the sum is a non-interference pattern, so the total pattern of signal photons doesn't show interference. I don't know how to do the actual quantum optics calculations to show that this is true for the Dopfer experiment, so this is just a speculation, but I think it's a plausible one. See this thread and this one for more detailed speculations on just how things might work out in a few variations on the Dopfer experiment.

 

[lcdisplay]

Unusual, it's true the walborn experiment does seem to be similar to the zeilinger one looking at it more closely. I think I understand what you and Jesse are referring to as the subset of total photons now. The subset of entangled photons "selected" or measured by the polarizer means their entangled partners were polarized parallel to a primary axis of the QWP, so the QWPs act as retarders not "rotators" if you will, and thus we see the interference pattern. BUT only using a coincidence counter. Without the coincidence counter you will record a superposition of patterns which is the blob.

I will have to analyze the Dopfer experiment Jesse to see how the subset selection is done.

 

[Cthugha]

Sorry for the very late answer. I was attending a conference and did not have the time to post here.

Just sticking with the standard QE though it is not evident from the sources I have read that the coicidence counter is required in the experiments except for filtering and backup. These entangled photons come out like once every 100 pulses so they are rare enough that without the coincidence counter noise would dominate your results.

In the equations of quantum mechanics I don't know anybody (yet) who has shown that you must employ a coincidence counter.

It is true that you need one for rare single photon experiments in less than ideal light isolation conditions but if you had an ideal lab the need is not obvious.

In fact this is not true. Single photon interference patterns for downconverted light have been demonstrated without coincidence counting. The background noise is not necessarily that intense when using the right equipment. See for example
"Demonstration of the complementarity of one- and two-photon interference" by Abouraddy et al. (Phys. Rev. A 63, 063803 (2001) , also available on arxiv: http://arxiv.org/abs/quant-ph/0112065" )

Indeed it seems like in 1998 Weihs et al showed this. "G. Weihs, T. Jennewein, C. Simon, H. Weinfurter, A. Zeilinger, Phys. Rev. Lett. 81 5039 (1998).

That paper aims at the nonlocality issues. I am not sure what you think it means for this discussion.

I'm intrigued why though, Cthuga, you are so adamant that it is necessary. I'm not ridiculing you mind you as you sound quite well versed in QM. I am skeptical if anything like you as I have not performed the experiment myself but can only have faith that the data is correct. This is relatively cutting edge and if you have a different set of sources that point to a coincidence counter being needed I would love to read them.

My point is also contained in the paper I cited above using an even simpler experimental geometry. In fact it is the main point of that paper. It clearly shows that single photon interference patterns (without coincidence counting) and two-photon interference patterns (with coincidence counting) are complementary like position and momentum. If one thinks about that for a moment that issue is trivial. Single photon interference relies on spatial coherence of the single photon state. Therefore you can only see it under far-field conditions. That is, you have a narrow distribution of the wavevectors contributing to the interference pattern.
Two-photon interference relies on the coherence of the two-photon state. However, for entanglement to be meaningful you need near-field conditions resulting in a broad distribution of the wavevectors in your beam. As you see, the necessary conditions for single- and two-photon interference exclude each other and it is not possible to see a two-photon interference pattern without coincidence counting by any means.

That point has also been shown in Birgit Dopfers PhD thesis, but unfortunately it is written in German and seems to have vanished from the web. However, there are also several other papers on the complementarity between single- and two-photon interference which can be found quite easily.

By the way: testing the different explanations of DCQE presented in this thread is quite easy. Usually position information is easily dumped by imaging this part of the entangled photons into the Fourier plane and placing a detector there. This is enough to erase position information. In my explanation you now need a small detector on this part to filter out a narrow range of wavevectors in the Fourier plane. If destruction of position information alone was enough, you could also use a large bucket counter spanning the whole Fourier plane. Walborn for example uses a small detector at this position. I wonder whether someone actually compared these situations.

 

[lcdisplay]

Two-photon interference relies on the coherence of the two-photon state. However, for entanglement to be meaningful you need near-field conditions resulting in a broad distribution of the wavevectors in your beam. As you see, the necessary conditions for single- and two-photon interference exclude each other and it is not possible to see a two-photon interference pattern without coincidence counting by any means.

Cthugha thanks for your response, it's never late. I will need to read your reference. Also I'm not quite sure what you mean by single and two photon interference?

I'm pretty sure I understand now why the Walborn experiment must have coincidence counting and I'm relatively sure that same reasoning extends to the Dopfer Heisenberg detector setup where he spatially filters a subset of photons. I just haven't mathematically proven it to myself yet.

I'm actually writing a paper right now on why the Quantum Eraser experiments as they are today will not allow for FTL communication. So I'm looking at all the references.
Thanks

 

[Cthugha]

Also I'm not quite sure what you mean by single and two photon interference?

Single photon interference is what you see in a common double slit experiment. In this situation - as Dirac calls it - "every photon interferes only with itself". This is true even at low photon count rates when photons are arriving one at a time.

Two-photon interference results from two indistinguishable photons. The best known example is the Hong-Ou-Mandel effect (http://en.wikipedia.org/wiki/Hong–Ou–Mandel_effect" ), where two indistinguishable photons arriving at a beam splitter always take the same exit port. Here you superpose probability amplitudes of these two-photon processes which lead to the same result and can get constructive or destructive interference. Therefore, some of the processes possible for two distinguishable photons do not occur for two indistinguishable photons.

A more detailed discussion of what two-photon interference is can be found in the paper linked in the wikipedia article: http://physics.nist.gov/Divisions/Div844/publications/migdall/psm96_twophoton_interference.pdf" .

 

[plexel]

That point has also been shown in Birgit Dopfers PhD thesis, but unfortunately it is written in German and seems to have vanished from the web.

B. Dopfer, Ph.D. Thesis, U. Innsbrück 1998 http://web.archive.org/web/20070714025355/www.quantum.univie.ac.at/publications/thesis/bddiss.pdf