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Mandel & Faster than Light Communication ?

by ShalomShlomo
Tags: communication, faster, light, mandel
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vanesch
#19
May26-05, 01:53 AM
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Quote Quote by ShalomShlomo
See Fig 6 and the text there.
Ah, I didn't know that one. I have to say that I'm surprised about the result, and I have some difficulties with the explanation given in the paper, but you are right, this is one of the few quantum erasure experiments where there is NO subset selection.

I think that this is a very interesting experiment, and I don't understand it completely. However, I disagree with the explanation given in the paper as such; indeed, according to the explanation, it wouldn't make any difference if i1 and i2 were combined with i1 going AROUND NL2 or going through NL2. And if that were true, we could obtain paradoxial situations: we could in principle send i1 and i2 on long, long long optical fibers (say, a light minute long), register the interference pattern or not at Ds during a minute, and only decide then whether or not we mix i1 and i2. And according to whether we decide to mix them or not, we would see our results of the past minute CHANGE !
We could even make a paradoxial machine in the following way: if the computer controlling the experiment has statistical evidence for interference during the minute of Ds measuring time, he decides to BLOCK the output of the fiber of i1, in which case we shouldn't see an interference pattern (because we can find out which photons came from NL2, knowing the delay of 1 minute and what comes out of fiber 2). But we SAW an interference pattern. On the other hand, if the computer has statistical evidence for no interference, he can decide to mix the two fiber outputs in a beam splitter, and according to the paper, we now have erasure of the information which is at the origin of the interference pattern, so we should observe interference, but we didn't !

I will try to write down of what I can make up of the setup:

The fact that an interference pattern arises can be understood in a certain way because when a single photon of the argon laser is split at BS(pump), you have that the state of this pump photon is 1/sqrt(2)(|pump1> + |pump2>), and hence the two photon pairs from the NL1 and NL2 are entangled:

1/sqrt(2) Integral ( |pair1> + e^(i theta) |pair2> ) rho(theta) dtheta

Here, theta is the difference in optical path length between the point in NL1 where the conversion |pump1> -> |pair1> took place, and the point in NL2 where the conversion |pump2> -> |pair2> took place. But of course we have to (Feynman path integral) integrate over ALL possible thetas because that conversion point is "floating", within the xtal, and it is this smearing out over different positions which makes that there is no fixed phase relationship between pair1 and pair2 (and hence between a single component of each).

It is somehow my feeling that the fact that the idler photon of pair 1 goes through NL2 changes rho(theta) (gives preferred positions for conversion of |pump2> -> |pair2>. In a non-linear xtal, this is not unexpected. From the moment that rho(theta) is somehow peaked, there is a phase relationship between pair1 and pair2 and you can expect interference.
But I'd be highly surprised that this same experiment works if the combination of i1 and i2 is done without i1 going through NL2 ! A way to find out would also be to change the path length of i1 before it gets to NL2. I'm pretty sure that this SHIFTS the interference pattern at Ds, which means that i1 DID SOMETHING to NL2.

Do you know of more experiments of this kind ? It looks interesting in any case.

cheers,
Patrick.
werty
#20
May26-05, 02:32 AM
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Quote Quote by vanesch
we could obtain paradoxial situations: we could in principle send i1 and i2 on long, long long optical fibers (say, a light minute long), register the interference pattern or not at Ds during a minute, and only decide then whether or not we mix i1 and i2. And according to whether we decide to mix them or not, we would see our results of the past minute CHANGE !
No you cant do this because you have already made it possible to in principle decide what path the photon took. This is no different from throwing meassurering results in the trash, its not a quantum erasure.
vanesch
#21
May26-05, 03:16 AM
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Quote Quote by werty
No you cant do this because you have already made it possible to in principle decide what path the photon took. This is no different from throwing meassurering results in the trash, its not a quantum erasure.
I know. But can you explain me, according to the paper, what is the principal difference between combining i1 and i2, when i1 goes AROUND NL2, or when i1 goes THROUGH i1 ? According to the explanation, it is the fact of combining them that makes it in principle impossible to know if the light (after combination) came from i1 or i2. There is no special status given to the fact that i1 goes through or not, NL2. Do you agree with this ?

cheers,
Patrick.

EDIT: in fact, it is NOT throwing measurements in the trash: after combination of both beams at the output of the two fibers, it is again impossible in principle to find out from which NLx the photon came. I don't see the conceptual difference, in the explanation given, by combining the beams a) when i1 goes through NL2 b) when i1 goes on a short path around NL2 and is combined there c) when i1 goes on a short path around NL2, and i1 and i2 are then sent on long fibers of equal length before being combined.

I don't know enough of the physics of these parametric down converters, but a priori I cannot exclude a "phase locking" (classically speaking) of the conversion amplitude in NL2 by i1, and I would tend to think that this is the explanation ; in which case the setup has a completely classical explanation too. Phase locking is something which happens easily in non-linear systems. This would then also explain the necessity of i1 to have to go THROUGH NL2 of course.
werty
#22
May26-05, 03:58 AM
P: 36
Im starting to see the complexity of this problem. Its true what you say that it isnt the same thing as throwing the results in the thrash, but to remove the which-path information you need to combine the paths with a 50-50 beamsplitter and we would than have the standard qm-erasure/delayed choice setup. I now think I see the problem you probably noted previously, that in his setup a) (as you specified it) he dont use any such erasure of the which-path information. The information is simply removed in all cases!? But If he used b) instead he would have to use a beamsplitter and there would be no immidiate interference, he would have to use coincidences to pick out interference.

I now see what you hinted at in your previous post, if his reasoning is correct than it should be possible to construct your previously mentioned machine. Which I think is impossible :) Sorry for missunderstanding.

EDIT
Im also clueless how the downconverters work, dont know much about that phaselocking either.
vanesch
#23
May26-05, 04:31 AM
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Quote Quote by werty
Im also clueless how the downconverters work, dont know much about that phaselocking either.
I have more experience in electronics: a "free running oscillator" which contains the slightest non-linearity very easily locks in on a tiny tiny signal of the same oscillator frequency.

For instance, on a card I had 32 completely independent oscillating circuits (in fact they were amplifiers but due to some error in the amplifier feedback, they were actually oscillators running at 250 MHz). They were in principle totally independent one of the other except for the common power supply, but which was filtered independently for each oscillator. Guess what ? They all were oscillating IN PHASE !
Nevertheless, no signal was seen either on the power supply nor on anything else. So it was just their proximity on the same card, which was quite well done, with shielding ground planes all over the place except for some cm of tracks parallel before they went onto the output connector, which must have been responsible for the phase relationship. The inputs were left open.
So it must have been this small capacitive coupling between the outputs (a few picofarad at most) which was responsible for the lock-in.

A priori I could think of a similar mechanism where the i1 beam makes NL2 down conversion "lock in" on it. But, as I said, I don't know enough of the physics of down converters to know if this is a sensible thing to say.

cheers,
Patrick.
ppnl2
#24
May26-05, 01:26 PM
P: 28
The blasted links don't work for me. 404 error.
DrChinese
#25
May27-05, 12:29 AM
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Quote Quote by ShalomShlomo
Interesting analogy. But when blocking one of two slits, we understand that light already through the blocked slit continues to propagate to the detector, and interference-effects only disappear once the time taken for those waves to arrive have been reached. We have a well defined and well understood model for the disappearance of the interference pattern. ...

Classical !? Whilst we know the blocking of the idler beam propagates to the Signal detector, since we see the interference pattern disappear, we have no model to discover along which route or how soon the cause propagates to the effect ! And you are just guessing by your answers first of 2), and now of 3).

Can you come up with a model justifying answer 1, 2, 3 or 4 ?

ShalomShlomo
a. Just as in the double slit experiment, anything that gives us which-path information destroys the interference. I don't think anyone questions this.

b. An oddity of this experimental setup is that the signal and idler paths must match their lengths very closely BEFORE they arrive at NL2 for the interference to be exhibited without blocking of the idler. This may not be clear from the description of the setup, but I believe a careful examination of the setup will confirm this is required. I think this is what Vanesch and werty have been commenting upon in earlier posts.

c. So the question we are now down to is: could the idler be blocked anywhere else other than between NL1 and NL2 and still yield the effect? And I would say the answer must be no. Clearly, there is no difference between blocking the idler photon and placing a detector in the same spot that does the same thing (blocking). (And we already have a detector on the other side of NL2 for the idler anyway, so its path could be lengthened or shortened without changing the result.) So we can only block the idler to turn on/off the interference pattern within a certain limited length range.

d. When we block the idler photon, it instanteneously causes the signal photon to collapse from a superposition to a definite state. That collapse will occur between NL1 and NL2, and the signal photon must have travelled an equal distance from the source (as the idler has) at that point in time - just as in a double slit setup. This happens anytime one of an entangled pair is observed. I do not think anyone will question this.

e. Once the superposition is gone due to blocking of the idler photon between NL1 and NL2, the interference will also be gone when the associated signal photon arrives at the detector. Determine the remaining length for the signal photon to traverse before detection (as of the time when the idler is blocked, since the wavefunction collapse is instantaneous), divide by c, and you know how much longer before the interference pattern disappears. I do not think anyone will question this.

So I would say this answers the question of the timing posed by ShalomShlomo. I am still not sure which of his 4 answers this maps to, but I think it is 3.

f. So the last element is: could the idler photon be any farther away from the signal detector than the signal photon at the time of the collapse? I am assuming no in my answer. And I take it that way because of the oddity of the setup, requiring paths to overlap in such specific ways.

So are c. and f. fair assumptions? My argument would need them to be true, and I believe they are. But to be honest, I am not 100% sure. Of course, this answer can be determined experimentally.
DrChinese
#26
May27-05, 01:08 AM
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Quote Quote by ppnl2
The blasted links don't work for me. 404 error.
See if this helps:

Quantum Effects in Photon Interference

-DrC
ppnl2
#27
May27-05, 01:58 PM
P: 28
Quote Quote by vanesch
I have more experience in electronics: a "free running oscillator" which contains the slightest non-linearity very easily locks in on a tiny tiny signal of the same oscillator frequency.

For instance, on a card I had 32 completely independent oscillating circuits (in fact they were amplifiers but due to some error in the amplifier feedback, they were actually oscillators running at 250 MHz). They were in principle totally independent one of the other except for the common power supply, but which was filtered independently for each oscillator. Guess what ? They all were oscillating IN PHASE !
Nevertheless, no signal was seen either on the power supply nor on anything else. So it was just their proximity on the same card, which was quite well done, with shielding ground planes all over the place except for some cm of tracks parallel before they went onto the output connector, which must have been responsible for the phase relationship. The inputs were left open.
So it must have been this small capacitive coupling between the outputs (a few picofarad at most) which was responsible for the lock-in.

A priori I could think of a similar mechanism where the i1 beam makes NL2 down conversion "lock in" on it. But, as I said, I don't know enough of the physics of down converters to know if this is a sensible thing to say.

cheers,
Patrick.

I don't know much about phase locking so forgive what may be a dumb question.

Turn down the intensity on the thing so you only have one photon per year or so. Now when you have a photon in i1 you clearly do not have one in v2 and will not have one for a year or so. How can a photon in i1 cause NL2 to lock phase on a photon that will not arrive for a year?
ppnl2
#28
May27-05, 01:59 PM
P: 28
Quote Quote by DrChinese

Thanks dude!
ShalomShlomo
#29
May28-05, 04:33 PM
P: 15
Quantum Mechanics depends on the concept of simultaneous collapse of wavefunction of entangled particles at space-separated places.

Relativity depends on the concept of 'no such thing as simultaneous'.

In the experiment described in this thread, the conflict between the two Master Theories is brought to a head. Something has to give.
Our current understanding of them both is contradictory and cannot be complete.

Fortunately, the experiment described can through refinements itself reveal to us where our understanding is wrong.

If 1 turns out to be the right answer, then QM wins, and Relativity is in need of change (and causality is out of the window, and you can build yourself a communications-time-machine to receive messages from the future - terrifying, and therefore unlikely).

If 2 turns out to be the right answer, we have some hard questions to ask QM to understand what on earth is going on - what is the mechanism for propagation of QM information - something new has to be thought up.

If 3 or 4 turns out to be the right answer, we need to look at exactly how light propagates, and how light experiences time (quick answer - it doesn't, it is at a singularity - but there are different ways of existing inside a singularity and we need to know which one applies to light's propagation through space).

The role of the idler beam entering the NL2 DownConverter is I believe irrelevant, as it seems highly likely that the same results would be obtained if the idler beam was routed around NL2 as Vanesch has also pointed out (I am depending on the credibility of the experimenters - although I have no proof for this). This can be easily checked experimentally.

The described experiment was carried out and published in 1991, 14 years ago.

What has occurred since to further our understanding and resolve the contradictions between QM and Relativity ?

ShalomShlomo
DrChinese
#30
May28-05, 06:08 PM
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Quote Quote by ShalomShlomo
Quantum Mechanics depends on the concept of simultaneous collapse of wavefunction of entangled particles at space-separated places.

Relativity depends on the concept of 'no such thing as simultaneous'.

In the experiment described in this thread, the conflict between the two Master Theories is brought to a head. Something has to give.
Our current understanding of them both is contradictory and cannot be complete.

...

What has occurred since to further our understanding and resolve the contradictions between QM and Relativity ?

ShalomShlomo
This is an inaccurate portrayal. SR and QM are theories which cover different areas of physics. One does not yield one answer in contradiction to the other. Extension of each past its domain of applicability yields contradictory information, which tells us that we need to properly apply each theory.

There is substantial evidence that c is not the absolute upper speed limit in the sense that some would apply it*. And yet there is no specific equation of SR that is violated. And there is no actual evidence that QM is wrong. So the contradiction is strictly in the eye of the beholder.

This experiment is not now nor has it ever been considered evidence that SR and QM are in conflict. It is just another example of the quantum weirdness with entangled photons.

* Examples: a) Instanteous collapse of wavefunction, which still does not allow FTL signalling; and b) a few celestial objects are receding from us much faster than c, but we are not in the same inertial reference frames.
ShalomShlomo
#31
May28-05, 11:38 PM
P: 15
Quote Quote by DrChinese
This is an inaccurate portrayal. SR and QM are theories which cover different areas of physics. One does not yield one answer in contradiction to the other. Extension of each past its domain of applicability yields contradictory information, which tells us that we need to properly apply each theory.

There is substantial evidence that c is not the absolute upper speed limit in the sense that some would apply it*. And yet there is no specific equation of SR that is violated. And there is no actual evidence that QM is wrong. So the contradiction is strictly in the eye of the beholder.

This experiment is not now nor has it ever been considered evidence that SR and QM are in conflict. It is just another example of the quantum weirdness with entangled photons.
Everything you said above is correct, except the last paragraph.

This experiment does show that our understanding (probably of QM) is greatly lacking. We knew this before this experiment, thanks to Bell and Aspect, but this experiment adds to our lack of understanding greatly.

In the 14 years since the experiment described, has anyone attempted to resolve this hole in our understanding ?
werty
#32
May29-05, 10:39 AM
P: 36
I think Feynman said to his students during lecture "I dont know why nature behaves in this perculiar way, nobody does!".

And thats that with that, I think we will most probably never understand why nature behaves as it does and thats what I think bells theorem proved, theres no such thing as "local reality". Notice that "local reality" always stand together in these theorems, theres no evidence that theres a "non-local reality" neither that there is a "local-nonreality", (whatever that means). Also I think stringtheory or someother theory tries to explain entanglement as a consequence of tiny wormholes, then theres no need for FTL communication they have an alternative route! And wormholes arent forbidden in GR. So there you go, there are alternative explanations. And until someone shows that you can use Mandells setup to send information FTL, which I believe should have been done by now if the original experiment was done 14 years ago.
ShalomShlomo
#33
May29-05, 03:01 PM
P: 15
How can we experimentally measure the sought time delay between the blocking of ‎the Idler 1 beam, and the corresponding disappearance of the Signal interference ‎pattern (the cause/effect time delay) ?‎

Method 1:
Send the Signal beams along fibre-cables (maybe once or half way around the world) ‎before combining them to observe their interference (or non-interference) pattern, ‎depending on the blocking of the idler beam. This will enable confirmation or ‎elimination of answer 1.‎
Then send the Idler beams instead of the Signal beams along the fibre-cables before ‎combining/blocking them. The time delay from the journey along the cables help will ‎ascertain which answer from 2,3 or 4 is correct.‎

Method 2 (if Method 1 is experimentally unworkable - for example, if the ‎coherence of the light beams is not maintained during their journey through the ‎cables):‎
Create a feedback system between the Idler 1 blocker and the Signal Interference ‎pattern, by changing the Signal interference pattern from one detector, into 2 slits (one ‎fed by each of the Signal beams) - since the Signal beams are coherent, the two ‎separately fed slits will produce an interference pattern. Position a fast detector at one ‎of the peaks or troughs of the Signal Interference pattern, such that once a threshold ‎value close to the peak/trough value is detected, an electric (or light) signal is sent ‎through a flexible fixed length wire to the Idler-Blocker to block the Idler Beam. The ‎Idler Blocker blocks the beam (it also needs to be fast Ė along the timescales of ‎Aspectís experiment). Once the Idler beam is blocked, the Signal interference pattern ‎will automatically disappear, and when the Signal detector finds that the threshold ‎value has been breached it will send a signal along the wire to the Idler-blocker to ‎stop blocking. This will result in the Signal interference pattern reappearing, which ‎will cause the Signal detector to again reach its interference threshold value, and an ‎endless loop will commence, with the Idler-Blocker switching on and off.‎
From the frequency of the switching on and off of the Idler Blocker ‎it may be possible to determine the sought cause/effect delay (or non-delay) of the ‎time between blocking and interference disappearance (by changing the experimental ‎lengths of the beams and observing the differences in resonant frequency).‎
It may well be found, however, that the time delays in the experimental signal ‎processing drown out the desired cause/effect time delay which may well turn out to ‎be much smaller. If so, a further refinement can be made to discover the sought ‎cause/effect time delay regardless of the larger experimental delays.‎
Using identical equipment, set up an identical second experimental system, but the ‎blocker blocks the signal beam itself (there need be no idler beams, and the detector ‎will just react to the presence of the signal beam itself). As it is almost identical, the ‎resonant frequency of the second set of apparatus will be almost identical to the first ‎set of apparatus. By combining the on/off beat from the two resonant blockers in the ‎two sets of apparatus, in effect combining two almost identical signals with a very ‎slight difference in frequency, the sought cause/effect delay can be established by studying the harmonics of the resultant pattern, and ‎altering the light path lengths in the first apparatus and noting the change in ‎harmonics caused.‎

A picture is worth a thousand words, and if anyone would like a diagram of the above ‎experimental set-up clarifying the notes above, I will gladly send it (sorry, I donít know if it ‎is possible to post the diagram to the forum or not).‎

And so, using one of the two above methods, a startling experimental result can be ‎found that I think will shape QM for the next decade or so.‎

Who will be the first to carry out this or a similar experiment, which is crying-out to be completed ‎‎? Has it been carried out already ?‎

ShalomShlomo
DrChinese
#34
May29-05, 05:33 PM
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Quote Quote by werty
I think Feynman said to his students during lecture "I dont know why nature behaves in this perculiar way, nobody does!".

And thats that with that, I think we will most probably never understand why nature behaves as it does and thats what I think bells theorem proved, theres no such thing as "local reality". Notice that "local reality" always stand together in these theorems, theres no evidence that theres a "non-local reality" neither that there is a "local-nonreality", (whatever that means).
Quote Quote by ShalomShlomo
Everything you said above is correct, except the last paragraph.

This experiment does show that our understanding (probably of QM) is greatly lacking. We knew this before this experiment, thanks to Bell and Aspect, but this experiment adds to our lack of understanding greatly.
What werty says is accurate. Local non-reality is a possibility. Most interpretations of QM say that there is something special going on with the act of observation.

ShalomShlomo: QM provides a description that nicely matches experiment. At the current time there is no known hole, although the current description may be unsatisfying.
vanesch
#35
May30-05, 03:22 AM
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Quote Quote by ppnl2
Turn down the intensity on the thing so you only have one photon per year or so. Now when you have a photon in i1 you clearly do not have one in v2 and will not have one for a year or so. How can a photon in i1 cause NL2 to lock phase on a photon that will not arrive for a year?
I don't know, but I'd like to add 2 comments.
First of all, downconversion is a classically non-linear process, so you cannot turn down indefinitely the pump beam (and hence the production of pairs).
Second, the two conversion events are in fact in superposition: let us not forget that the pump photon that is to be converted (given a classical intensity of the pump beam!) is in a superposed state of going to NL1 and NL2. Of course, the exact location of that conversion is somehow a priori arbitrary within NL1 and NL2 (which makes that we don't see raw interference of the outgoing signal photons because they have random phase relations due to different optical paths before conversion). But nevertheless the two conversions are entangled (because originating from two terms in the single pump photon state). The paper seems to forget that: it says that these are "spontaneous conversions" ; if that were true, there wouldn't be any difference between using coherent pump beams (a split beam) or using two independent pump lasers. The very fact that coherence can be restored in certain circumstances between the two signal beams means that they were in an entangled state. This entanglement finds its origin in the superposition of the photon states pumping NL1 and NL2.
So you cannot say that "the photon came from NL1 so clearly it didn't come from NL2" in the same way as you cannot say, in a Young's experiment that the photon went through 1 slit so clearly it didn't come through slit 2.
The presence of one term in the superposition of the converted photon in NL1 could (I don't know how but the possibility is not excluded) induce some phase locking with the other term (the term, in the superposition, of the conversion in NL2), in the same way as stimulated emission does (although people checked that it is not exactly stimulated emission in that no MORE photons come out) ; but why cannot they "lock in" the existing emission, which, I argued, cannot really be "spontaneous" (otherwise there's no reason to use coherent pump beams for NL1 and NL2).


cheers,
Patrick.
ShalomShlomo
#36
Jun1-05, 12:16 PM
P: 15
I don't understand.

We're spending billions of dollars on super powerful particle accelerators, to try and understand the fundamental make up of our quantum mechanical particles.

And yet, the problem described here is an even more fundamental gap in our understanding, with the prize of a significantly deeper understanding of quantum mechanics, far beyond what we know today, and no-one is interested in discovering what is in front of our noses ?

Why ?


ShalomShlomo


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