1. Jan 11, 2005

### DrChinese

I have some questions about other possible tests of the EPR Paradox. The base of the paradox is using one entangled particle to gain information about the other. The usual setup discussed involves spin, often photon spin. But what about other measurable attributes of a particle?

If we take the same entangled photons and run them through 2 double slit setups on opposite sides of the source, they should presumably yield 2 interference patterns. Is this new setup clear? We are still observing particles in the singlet state, just not looking at the spin component.

Now, if we cover one of the slits on one side (say the Left), so we know which of the 2 slits the Left particle travels through, the interference pattern disappears on the left. Does this tell us anything about the particle on the Right? Is there some complimentary property that could collasped by such a measurement?

I am trying to imagine that the interference pattern seen on the Right would also disappear if one slit on the Left is covered. Have there been any actual experiments performed with this setup to see what would happen?

Thanks,

-DrChinese

2. Jan 11, 2005

### slyboy

To get such effects the particles would have to be entangled in position-momentum space rather than in their spins. Experiments on continuous-variable entanglement have been performed, but I am no expert on them. Perhaps somebody else knows?

3. Jan 11, 2005

### ppnl2

That would imply that you could pass real information between two points dispite the fact that there is no real object or energy traveling between the two points.

As far as I know one of the experiments here do this:

http://www.fortunecity.com/emachines/e11/86/qphil.html [Broken]

I'm refering to the experiment by Leonard Mandel. But I'm not a physicist and am not sure I'm understanding the thing correctly.

Last edited by a moderator: May 1, 2017
4. Jan 12, 2005

### DrChinese

Thanks, that is exactly in the spirit of what I am asking. I could not tell from this, but are the idler photons and the signal photons space-like separated? It seems that you could use this for FTL signalling if so. Packets of signal photons would either display intereference or not depending on the action taken at the idler photons - i.e. block or not block which would affect the interference pattern.

Anyone know about this? I'm sure Mandel must have thought of this but I did not see a specific in that source... (guess I should Google it and answer it myself :)

Last edited by a moderator: May 1, 2017
5. Jan 12, 2005

### vanesch

Staff Emeritus
The interference pattern of the set of photons A occurs, or not, according to a subset selection which is determined by a coincidence with a test on photon B. By changing the kind of test on photon B, you get different kinds of interference, or not. What you usually have in this kind of experiments is 2 interference patterns in the entire sample of A, which smoothen eachothers' fringes out. When you look at the pattern of a subset of photons A (coincident with something done on B), you suddenly see (one or the other) interference pattern. But in order to find that subset, you need the classically transmitted coincidence signal. If you only look at the entire population of photons A, you always get the same picture, no matter what you do with B.
There's no way, in standard quantum theory, to do FTL information transport using entangled systems. This can be demonstrated easily using the density matrix, and "tracing out" the remote system: this "local" density matrix contains all the available information (it gives you all the expectation values of observables which act only on the local system) that you can get out locally, no matter what you do on the other system.

I discussed a similar setup that was meant to serve as an FTL phone here a while ago:

" simple ftl setup using Stern-Gerlach"

cheers,
Patrick.

6. Jan 12, 2005

### dlgoff

Maybe this paper selfAdjoint referenced in his post yesterday has some relavence here:

http://www.arxiv.org/abs/quant-ph/0501034

Regards

7. Jan 12, 2005

### ppnl2

I'm not sure we are talking about the same experiment. Most of the experiments do require a coincidence detector but as far as I can tell this one does not.

The signal detector has photons from two different paths hitting it and each photon is in a superposition of being in both paths. On this bases I would predict an interference pattern even without knowing about the rest of the setup. What prevents the interference pattern here? What are ther subsets of photons?

I agree that you cannot use it for FTL communication but my (very limited!) understanding is that you cannot arrange a space like seperation between the blocker and the signal detector without some device (like a perfect one way mirror or something) forbiden in quantum mechanics. But I have never been able to work out the details or trace down a good explination of how this works. So I could easily be all wet.

This was published in SCIAM about ten years ago and I have been trying to get a handle on it ever since.

8. Jan 12, 2005

### Hans de Vries

I was reading this much more detailed description of a class-room
table-top quantum eraser here thinking the same:

http://people.whitman.edu/~beckmk/QM/qe/qe.pdf

It switches an Interference Pattern on and off at one place
by manipulating a $\lambda/2$ Half-Wave plate at another place,
in another beam, that went the other way, to never to
go back to the place were the interference happens.

So, if indeed it works this way then It seems almost unavoidable
that what you suggest should occur. It's a simple 1:1 relation with
a defined binary input at one place causing a defined binary output
(yes or no interference pattern) at another place.

Regards, Hans

9. Jan 12, 2005

### DrChinese

Thanks, I found it at https://www.physicsforums.com/archive/topic/t-46015_simple_ftl_setup_using_Stern-Gerlach.html [Broken].

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10. Jan 12, 2005

### DrChinese

From the reference: "Measurements made on one beam affect the
visibility of interference fringes in another, spatially separated, beam."

If I am understanding what they are doing (and it really is doing what they say), then the setup could be used as a building block to send a FTL signal. Clearly, a single signal photon in this case does not transmit enough information when it arrives at a detector to be a full bit of information. But a suffiiently large packet of them - even if from different source apparati - would contain enough information to constitute a bit.

The part I don't understand is that FTL is not mentioned in the reference, and you would expect them to have considered this already and commented upon it.

11. Jan 13, 2005

### Caroline Thompson

Am I right in thinking that this refers to a 1999 experiment by Kim et al?
Yoon-Ho Kim, R Yu, S P Kulik, Y H Shih and Marlan O Scully, “A delayed choice quantum eraser”, quant-ph/9903047 (1999)​
Sigh! It's amazing what paradoxes you can come up with if you insist on modelling light with photons! I maintain that experiments such as Kim's can all be explained once you recognise a few facts about the real, perfectly ordinary, correlations in the properties of the beams emitted simultaneously in the process of (degenerate) PDC. I have to admit that, despite some correspondence with Kim, I never quite got to the bottom of it -- I could not find out quite enough facts about, e.g. the orientation of the various bits of apparatus -- but I am confident that if I'd been there myself I'd have been able to give a complete explanation.
Caroline

12. Jan 13, 2005

### DrChinese

No, it is a different one: Gogo, Snyder and Beck, 2004. You might like this one, Caroline. Beck's interest is on tabletop experiments that can be done in an undergraduate setting. To me, that means a much lower cost to perform EPR tests. One thing that you and I agree on is that more tests should be done. I want to see all of the details of EPR tested! My questions are simply different than yours. So perhaps your ideas can be tested in the near future anyway if there are more labs doing this kind of research.

13. Jan 14, 2005

### Caroline Thompson

I haven't seen this particular article, but please do, if you've a moment, look at a letter I wrote relating to what must have been a similar paper back in 2002:

http://freespace.virgin.net/ch.thompson1/Letters/AJP.htm

which concerns:

C H Holbrow, E Galvez and M E Parks, "Photon quantum mechanics and beam splitters", Am. J. Phys. 70 (3), 260-265 (2002)

I wrote to Holbrow (and had a little correspondence) with CC to the journal.

I want to see both the double slit and Bell test investigated, but not by students supplied with ready-made apparatus and instructions on how to obtain the "correct" results! It is all too easy to produce the appearance of agreement with QM if you only look at a very limited range of the experimental parameters.

What I should like to see is comprehensive sets of experiments, investigating what happens if you vary such things as the make of beamsplitter and the make of photodetector. You can also investigate the effect of different detector settings, but some of the characteristics of the apparatus will have been "programmed" into it in the course of manufacture and calibration.

Caroline
http://freespace.virgin.net/ch.thompson1/

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14. Jan 14, 2005

### DrChinese

I received some clarification on the experiment. I didn't see it at first, but the interference is not the kind where it appears a la a double slit experiment. The pattern appears on a graph of coincidences, so that makes it take on the characteristics of a Bell test.

You might be interested to learn that it violated a Bell Inequality by over 20 standard deviations. It does use 2 channel detectors so it has some similarity to the experiments Caroline denies. But to the rest of us, it is another setup which yields a value which miraculously just happens to match the QM predicton.

Most importantly, this experimental setup is designed for operation at the undergraduate level. This is a great breakthrough; I couldn't understand previously why Aspect-like tests were not being used to probe many different permutations of EPR and generally provide a flood of confirming tests.

Aspect had to use time varying analyzers to rule out the possibility of some kind of signal being sent from one part of the measuring apparatus to another. The results were the same with or without the time varying enhancement to the experiment. A reasonable person would conclude this does not need to be a feature of every Bell test. That simplifies the development of EPR probes.

The Gogo/Snyder/Beck setup, like other quantum erasers, uses parametric down converters (PDCs) to create the source beams. This too should make further tests cheaper, easier such that they should proliferate.

15. Jan 14, 2005

### vanesch

Staff Emeritus
EXACTLY !! Hence my remark that "quantum eraser" is a strange name for it. These experiments are indeed CORRELATION MEASUREMENTS, a la Aspect.
Only, what's very nice is that instead of a single number, you get out a whole pattern of correlations which take on the aspect (:tongue:) of an interference pattern. That's much more difficult to explain away as the result of inefficient detectors and so on. How the hell can you, by coincidence, get an entire pattern with wobbles, if the two photons were not entangled ?
Some work for Caroline

cheers,
Patrick.

16. Jan 14, 2005

### DrChinese

We both know that it will be no difficulty for Caroline... just another wave of the hand .

I agree that "quantum eraser" is a strange name for it. It's snappy, but not very descriptive.

17. Jan 14, 2005

### Hans de Vries

It seems to be an ideal experiment in the sense that it has non of
the disadvantages of the usual EPR experiments:

- No math at all needed to convince, No dependence on the
interpretation of mathematical formula's. No distraction with
toy theories which seem arbitrary and possibly irrelevant.

- A very clear yes or no signal (yes or no interference) already at
this stage. No issues with efficiencies or accuracies.

- No unknowns with raw data or selected data.

access (and thus verification) of the experiment.

Regards, Hans

http://people.whitman.edu/~beckmk/QM/qe/qe.pdf

Last edited: Jan 14, 2005
18. Jan 14, 2005

### DrChinese

(OK, this probably makes no sense whatsoever, but...)

The EPR Paradox applies to any quantum properties which are measured on one particle of an entangled pair. That should apply to a position measurement as well. If you learn something about the position of one particle, say the Right, haven't learned something about the position of the one on the Left?

In our attached figure, one photon goes Left and the other Right. We measure the path/position of the photon on the Right by means of a detector. That leads to a loss of interference per the standard Double Slit experiment, and no interference is seen on the Right screen. We now know the which-path information of the Left photon as well. Should that affect what we see on the Left screen, so that its expected interference pattern is also erased?

-DrChinese

#### Attached Files:

• ###### EPR-DoubleSlit.jpg
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19. Jan 14, 2005

### Caroline Thompson

OK, I might take up the challenge! I've printed the paper out, had a quick read, and checked out the authors. It might be more worthwhile, though, to concentrate on some other similar experiment, since it does not look as if they intend submitting to a journal. If I'm going to analyse something like this in detail, I might as well produce a paper and try and publish.

Anyway, as I supected, it is the usual fun and games, where what is really going on is the use of one interference pattern to enable (by looking at coincidences) selection of another, the whole trick being shrouded in mysticism with stories of photons whose path you know, ones you could know if you tried, and extra information that washes out previous information!

There are a couple of basic facts that I challenge, in addition to the ludicrous way they word their "explanation". I strongly suspect that the true explanation will turn out to depend on the fact that they don't get either vertical or horizontal polarisation emitted from their pair of nonlinear crystals but (usually) both at once. [If they didn't, how would the two components interfere at the beamsplitters as they say they do?] The key "hidden variable" is likely to be, as in my "rotational invariance" paper (quant-ph/9912082), the phase difference between the two.

And those half wave plates don't just flip the polarisation direction, though sometimes this may be the net effect. If you look into how they work, you find that the name gives the game away: they delay one polarisation component by half a wave relative to the other. They only do this exactly, though, if the frequency of the light is exactly that for which the plate is designed. If the frequency can vary from one pulse to the next (as is apt to happen with PDC output) the delay will not always be exactly half a wavelength, with the effect that input polarisation at 45 deg is not exactly flipped to -45 deg. I don't know yet whether or not this will prove important here ...

Oh, and incidentally, they may have got the wrong idea about what polarisation directions they are really dealing with. They might be better off not assuming they know what effect half wave plates and the other apparatus has on polarisation direction and concentrating on their effects on relative phases of the two components.

That's a start. You may hear more from me on this in a week or two.

Caroline

20. Jan 14, 2005

### Hans de Vries

Maybe you should not be put of by terms like "measurement",
"knowledge of paths" et-cetera. These terms are indeed vague
and tend to lead to philosophers metaphysics. Concentrate on
the physics.

A defining moment is the absorbtion of the photon. The only thing
we could measure in the past, causing the term "measurement"
to be associated with the absorbtion of the photon.

A bit further in the future we may be able to do much more sensitive
measurements. For instance with nano-antennas that measure the
properties of the EM field of single photons passing by non-destructive.

The optical paths of the entangled photons in the quantum eraser
experiment have dozens of stages with different optical properties
between the interferometer and the detector of the idler photons.

The photons push billions of atoms and electrons around in the
optical dielectra without loosing their properties, the entangle-
ment or the superposition. So it doesn't seem to far fetched that
we will be able to make measurements as well without disturbing
these properties.

Regards, Hans