Entangled photons in bell experiment: transfer phase or angular momentum?

[Iforgot]

Regarding the polarization correlation studies generated using parametric down conversion. All the studies appear to be done correlating the polarization of linearly polarized photons.

Has any experiment been done showing the same effect with circularly polarized light?

1) If this experiment has been done with circular polarized light, then that would mean that quanta of angular momentum are being transferred faster light.

2) But if this experiment can only be done using linear polarized light I wouldn't find it as exciting. Because linear states are a super position of circularly polarized light. The orientation (s or p) depends on the phase between the different helicities. In which case, it's the phase that is propagating faster than light. Super luminal phase velocities aren't anything new or exciting to us.

 

[DrChinese]

Regarding the polarization correlation studies generated using parametric down conversion. All the studies appear to be done correlating the polarization of linearly polarized photons.

Has any experiment been done showing the same effect with circularly polarized light?

1) If this experiment has been done with circular polarized light, then that would mean that quanta of angular momentum are being transferred faster light.

2) But if this experiment can only be done using linear polarized light I wouldn't find it as exciting. Because linear states are a super position of circularly polarized light. The orientation (s or p) depends on the phase between the different helicities. In which case, it's the phase that is propagating faster than light. Super luminal phase velocities aren't anything new or exciting to us.

I can change linear polarization to circular and they will still be entangled. I can do entanglement experiments with things other than photons.

What effect do you think is happening faster than light? And why do you think that has anything at all to do with superluminal phase velocities? We are talking about a measurement on one photon which is outside the light cone of the other. There is nothing about phase velocities that has anything to do with the measurements.

 

[Iforgot]

As far as I know, no one has experimental demonstrated non-local phenomena arising from entanglement on anything other than linearly polarized photons. Can you refer me to the peer reviewed articles reporting otherwise? Otherwise I'm going to have to spend weeks sloging through the literature :P (see links below for where I'm starting)

http://iopscience.iop.org/0034-4885/41/12/002
http://prl.aps.org/abstract/PRL/v49/i2/p91_1
http://rmp.aps.org/abstract/RMP/v74/i1/p145_1

With regards to your questions: I would answer them, but I'm under the impression that they are completely rhetoric.

From your questions I'm assuming you don't realize that circularly polarized light is an eigenstate of angular momentum and that linearly polarized light is a superposition.

Furthermore it appears you don't realize that the phase of the coefficients in this superposition are what describe whether the polarization is linear vertical or horizontal.

 

[DrChinese]

As far as I know, no one has experimental demonstrated non-local phenomena arising from entanglement on anything other than linearly polarized photons.

I asked the questions to get better insight into what it is you are really asking. When you say "non-local phenomena", you could mean "violations of Bell inequalities". Or you could mean "violations of Bell inequalities under strict Einsteinian locality conditions". If you are saying the latter, then that has been done ONLY for photons. The issue of linear or circular is completely irrelevant to that experiment. I cannot assist you with that beyond what I say in the last paragraph.

http://arxiv.org/abs/quant-ph/9810080

Violation of Bell's inequality under strict Einstein locality conditions
Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, Anton Zeilinger (University of Innsbruck, Austria)
(Submitted on 26 Oct 1998)

"We observe strong violation of Bell's inequality in an Einstein, Podolsky and Rosen type experiment with independent observers. Our experiment definitely implements the ideas behind the well known work by Aspect et al. We for the first time fully enforce the condition of locality, a central assumption in the derivation of Bell's theorem. The necessary space-like separation of the observations is achieved by sufficient physical distance between the measurement stations, by ultra-fast and random setting of the analyzers, and by completely independent data registration."

If the former, this has been done for many quantum objects. Most scientists accept entanglement as such without having to demonstrate the higher standards of closing the locality loophole each and every time.

To demonstrate entanglement on circular polarized photons, you would need to derive a Bell Inequality for those. I don't know that is possible, not really sure, but I assume it would require there to be different flavors of circular polarization other than left or right. I don't believe such exist.

 

[Iforgot]

I'm feeling a little exasperated here for a number of reasons

1) I don't think you're making an effort to understand my question.

2) "Most scientists accept entanglement ..." I think you're missing the point of experimental validation.

3) It's bad practice to cite from ArXiv when a well known article is published in a reputable journal
http://prl.aps.org/abstract/PRL/v81/i23/p5039_1

The only reason I'm addressing your concerns is so that some one who knows a 3+ year grad student in quantum optics that happens to be reading this article doesn't think my question is sophomoric.

As implied in the sentence above, I believe a 3+ year grad student in quantum optics would have the answer to this question at his finger tips.

 

[DrChinese]

I'm feeling a little exasperated here for a number of reasons

1) I don't think you're making an effort to understand my question.

2) "Most scientists accept entanglement ..." I think you're missing the point of experimental validation.

3) It's bad practice to cite from ArXiv when a well known article is published in a reputable journal
http://prl.aps.org/abstract/PRL/v81/i23/p5039_1

The only reason I'm addressing your concerns is so that some one who knows a 3+ year grad student in quantum optics that happens to be reading this article doesn't think my question is sophomoric.

As implied in the sentence above, I believe a 3+ year grad student in quantum optics would have the answer to this question at his finger tips.

1. You have yet to make your question clear.

There are many examples of entanglement, however not everyone accepts those as proof of non-locality. Rather, it is considered proof of "quantum non-locality". Are you questioning that?

As I have said, I don't believe there are any experiments with circular polarized light violating a Bell Inequality, which was you original specific question. There are experiments involving circular polarized light and other quantum phenomena (GHZ comes to mind) but I don't think that really is the direction you are going. Again, I am trying to ferret out your question so it can be answered.2. I keep references handy to many experimental results. I pull those out as appropriate when there is some particular point that needs to be clarified. Your original citations were so old that it was hard to determine what level of discussion you want to start with.3. Around here, we usually cite the Arxiv version of articles as they are free to all readers. Your reference requires a subscription.

If you don't want my assistance, perhaps someone else can understand what you are seeking. If you want to continue, then that is fine too. Meanwhile...

As many times as I have read your OP, I keep returning to my basic point: linear vs circular in Bell tests does not make sense on any level I am aware of. Phase velocity is not relevant to discussions of quantum non-locality. And superluminal effects are not the overriding conclusion from Bell tests. Bell tests demonstrate that local realistic theories are not tenable. If you already reject local realistic theories because you believe QM is complete (answering the EPR question in the affirmative), then entanglement experiments will probably not be as exciting for you.

 

[Iforgot]

If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

Granted I may have a hard time making myself clear, but seeing as you haven't made any non-rhetorical or genuine requests to further explain my point, I see no point in continuing this discussion with you.

 

[Cthugha]

Three points:

Has any experiment been done showing the same effect with circularly polarized light?

Direct generation of photons in an entangled basis of left and right circularly polarized photons has been demonstrated for example in Nature 465, 594–597 (2010) by Salter et al. in terms of a cascaded decay. If I remember correctly the references inside also give hints at how to perform Bell measurements on such states.

If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

Annoying the science advisor who probably has the largest number of posts on entanglement in these forums is something only few people have achieved. Congratulations.

Granted I may have a hard time making myself clear, but seeing as you haven't made any non-rhetorical or genuine requests to further explain my point, I see no point in continuing this discussion with you.

The reason why you fail to make yourself clear is most likely that your claims 1) and 2) in your first post are simply and completely wrong (or if read in a benevolent way formulated in a very sloppy manner). Maybe it helps to read up on the meaning of linear polarization for single photons?

 

[Drakkith]

If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

This is an online forum. Requiring posters to have access to pay-for journals will substantially reduce the chances of actually having anyone answer your questions.

 

[Vanadium 50]

Furthermore, some of us do not always have access where we are posting. I usually post from home, for example, but my journal access is through work.

 

[San K]

I can change linear polarization to circular and they will still be entangled.

is there any energy loss (or energy required) to change the polarization?
(say from linear to circular)

 

[DrChinese]

Direct generation of photons in an entangled basis of left and right circularly polarized photons has been demonstrated for example in Nature 465, 594–597 (2010) by Salter et al. in terms of a cascaded decay. If I remember correctly the references inside also give hints at how to perform Bell measurements on such states.

The abstract from that reference:

"An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons1, 2. At present, entangled-light sources are optically driven with lasers3, 4, 5, 6, 7, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates8, 9. However, these sources are Poissonian4, 5 and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs10, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays11, in core components of quantum computing such as teleportation12, 13, 14, and in entanglement swapping15, 16. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications"

And a related article:

http://arxiv.org/abs/1103.2969

"A practical source of high fidelity entangled photons is desirable for quantum information applications and exploring quantum physics. Semiconductor quantum dots have recently been shown to conveniently emit entangled light when driven electrically, however the fidelity was not optimal. Here we show that the fidelity is not limited by decoherence, but by coherent interaction with nuclei. Furthermore we predict that on 100\mu s timescales, strongly enhanced fidelities could be achieved. This insight could allow tailoring of quantum logic to operate using quantum dots in the fault tolerant regime."

Of course, even with these articles it appears that the circular polarization vs linear is not really an important distinction (it's not mentioned). From the editor's summary of the 2010 article:

"For optical quantum computation and related information technologies to fulfil their promise, they will require a source of entangled photons that can be delivered efficiently on demand. Existing entangled-light sources are laser driven, and involve bulky and complicated optics. Salter et al. have now developed a compact light-emitting diode with an embedded quantum dot that can be driven electrically to generate entangled photon pairs. Much simpler than its laser-driven counterparts, this ELED (entangled-light-emitting diode) device, based on conventional semiconductor materials, is a promising start point for the development of an entangled light source for quantum information applications."

On demand entanglement! Cool. I presume that one would simply use a wave plate or similar if you specifically needed to have linear polarization for an application.

 

[DrChinese]

is there any energy loss (or energy required) to change the polarization?
(say from linear to circular)

Nope.

 

[Cthugha]

Of course, even with these articles it appears that the circular polarization vs linear is not really an important distinction (it's not mentioned).

You are of course right. It is absolutely not an important distinction. I mean: You could simply take light entangled in a linear basis and pass both beams through a quarter waveplate to get to a circular basis as well.

 

[San K]

You are of course right. It is absolutely not an important distinction. I mean: You could simply take light entangled in a linear basis and pass both beams through a quarter waveplate to get to a circular basis as well.

I read somewhere that solar sails use the energy of the photons hitting them.

here the photon is passing through (sort of striking) a quarter-wave-plate.

must it not lose some, however infinitesimally small, amount of energy?

...maybe a very small fraction of a quanta?

 

[Drakkith]

I read somewhere that solar sails use the energy of the photons hitting them.

here the photon is passing through (sort of striking) a quarter-wave-plate.

must it not lose some, however infinitesimally small, amount of energy?

...maybe a very small fraction of a quanta?

Solar sails work because upon absorption and reflection the photons impart momentum into the sail. The reflected photons lose this momentum and are redshifted slightly. (That's what I've been told here on PF at least)

 

[Cthugha]

must it not lose some, however infinitesimally small, amount of energy?

Why should it lose energy? It is not absorbed, not even scattered. It just travels through a birefringent medium. Photons can lose tiny amounts of energy in inelastic scattering processes such as Raman scattering, but these typically require absorption and reemission of that photon via a real or virtual intermediate state.

 

[San K]

Solar sails work because upon absorption and reflection the photons impart momentum into the sail. The reflected photons lose this momentum and are redshifted slightly. (That's what I've been told here on PF at least)

what you have said above is correct, i think.

however it's, perhaphs, interesting that energy can convert into a red-shift (phase change?) in time-space

 

[San K]

Why should it lose energy? It is not absorbed, not even scattered. It just travels through a birefringent medium. Photons can lose tiny amounts of energy in inelastic scattering processes such as Raman scattering, but these typically require absorption and reemission of that photon via a real or virtual intermediate state.

ok, thanks Cthugha

Photons can lose tiny amounts of energy

can these tiny amounts of energy be less than a quantum?...even if we cannot directly measure that

pardon my limited understanding of "the quantum"

 

[Cthugha]

can these tiny amounts of energy be less than a quantum?

These are different photons, so speaking of "a quantum" is a bit pointless. Each emission and absorption process still results in emission or absorption of a single quantum of light, but absorption and emission happen in different modes of the electromagnetic field, so one sees a difference in energy. The difference in energy then corresponds to the difference of two energy levels present in the scatterer.

 

[Evo]

If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

Dr Chinese didn't say that *he* didn't have access to journals, he was referring to the members that do not.

You really should be thankful that someone of Dr Chinese's stature is even trying to help you.

 

[DrChinese]

You really should be thankful that someone of Dr Chinese's stature is even trying to help you.

Why thank you Evo! I don't know how much stature I have, but I do have a bit more girth than I need.

 

[Spinalcold]

If you don't have access to physics journals, I don't see how you expect to be able to contribute to this thread.

From what I hear, physicists are proud that most of their papers are open access (the opinions are from my teachers, CERN, PHD comics). Physics is far ahead of the other fields in providing free access to their research, that is something we should be proud of. arXiv may not be the perfect solution to open access, but a lot of the content is posted, peer reviewed, then reposted.

Physics is daunting to most people, part of the reason I decided to take it on and go back to school is that I had access to the papers that interested me. I really hope the trend to open access continues.

 

[Spinalcold]

And I just got paywalled for a research paper: www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2012.261.html

I'm hoping I can get my school to pay, but for a student this completely unfair...

sorry to turn the thread into an ideological debate. But this is something I want to save others from. Students should have every means of access of learning avaliable open to them.

 

[Iforgot]

@DrChinese. Apologies for being a nudnik. But in my defense, arguing with you is like arguing with my mom. She's a hell of a smart lady (she's a computer programmer, so no surprise there), but her idea of an argument is to invalidate ones experiences and repeat her previous sentence. Very often as a rhetorical question.

Restricted access to publications annoys me too, and I'm doing my part to reduce it.

@Cthulga
Thanks for the article. This is more relevant to my original question. But I hope you do not take my gratitude as a sign of submission. Since when is the number of ones posts proportional to the validity of their argument? And I'm not so sure if reading up on the meaning of linear polarization for a single photon would help. How bout you try it out first and tell me how it goes. Before you escalate, remember that I have single handedly annoyed the largest contributor to entangled threads all by my self.

My original thought was why there have been no bell correlation experiments using electrons instead of photons? Is it technical or fundamental physics? There have been some really clever experiments on spin properties of electrons and holes in semiconductors so the technical stuff is in place.

One of the the main difference I can see between electron spin and photon linear polarization that could prevent experimentation in the former, is that electron spin is an angular momentum eigenstate while photon linear polarization is a super position of spin angular momentum eigenstates (which are circular polarization). This is the source of my original question.

 

[DrChinese]

1. Since when is the number of ones posts proportional to the validity of their argument?

2. My original thought was why there have been no bell correlation experiments using electrons instead of photons? Is it technical or fundamental physics? There have been some really clever experiments on spin properties of electrons and holes in semiconductors so the technical stuff is in place.

One of the the main difference I can see between electron spin and photon linear polarization that could prevent experimentation in the former, is that electron spin is an angular momentum eigenstate while photon linear polarization is a super position of spin angular momentum eigenstates (which are circular polarization). This is the source of my original question.

1. No one is saying this. But a prudent person would at least tread more softly while you gain a foothold. I hope you have at least learned that there is no fundamental difference with photon entanglement as to linear vs circular.

2. There have been a number of entanglement experiments on particles other than photons. Here is one you might appreciate (this should be the full article by the way):

http://www.nature.com/nature/journal/v409/n6822/full/409791a0.html

"Local realism is the idea that objects have definite properties whether or not they are measured, and that measurements of these properties are not affected by events taking place sufficiently far away1. Einstein, Podolsky and Rosen2 used these reasonable assumptions to conclude that quantum mechanics is incomplete. Starting in 1965, Bell and others constructed mathematical inequalities whereby experimental tests could distinguish between quantum mechanics and local realistic theories1, 3, 4, 5. Many experiments1, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 have since been done that are consistent with quantum mechanics and inconsistent with local realism. But these conclusions remain the subject of considerable interest and debate, and experiments are still being refined to overcome ‘loopholes’ that might allow a local realistic interpretation. Here we have measured correlations in the classical properties of massive entangled particles (9Be+ ions): these correlations violate a form of Bell's inequality. Our measured value of the appropriate Bell's ‘signal’ is 2.25 ± 0.03, whereas a value of 2 is the maximum allowed by local realistic theories of nature. In contrast to previous measurements with massive particles, this violation of Bell's inequality was obtained by use of a complete set of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called ‘detection’ loophole."

I would like to point out that this is a seminal experiment in the literature, as it closes the detection/fair sampling loophole. Also, the team was led (I believe) by David Wineland, who just received the Nobel prize in physics. Way to go Wineland and NIST!

 

[Iforgot]

1) I'm not adamant there is a fundamental difference between circular and linear, but I'm not 100% convinced there isn't. I'll need to really pour over Cthugha's article first.

2) Thanks for the article! Do you realize how long it takes me to really grasp these articles? If it took a week, it would be a productive week. Off the cusp, it's not clear they are dealing with spin eigenstates as opposed some linear combination. I.e. they are dealing with the equivalent of linearly polarized light, but for ions. But let me really get into the guts of these articles before I respond any further.

Thanks again

 

[DrChinese]

2) Thanks for the article! Do you realize how long it takes me to really grasp these articles? If it took a week, it would be a productive week. Off the cusp, it's not clear they are dealing with spin eigenstates as opposed some linear combination. I.e. they are dealing with the equivalent of linearly polarized light, but for ions. But let me really get into the guts of these articles before I respond any further.

Thanks again

Glad you like it. You may also benefit from the following article because it goes into a bit more detail on Bell tests. Don't let the title fool you, it fills in plenty of info you may not find in the 1998 Weihs paper (which is also good).

http://arxiv.org/abs/quant-ph/0205171
Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory

Also, these are probably outside your scope but in case I am wrong:

http://arxiv.org/abs/1010.4224
The Dynamical Nonlocality of Neutral Kaons and the Kaonic Quantum Eraser

http://arxiv.org/abs/1208.2592
Experimental realization of three-color entanglement at optical fiber communication and atomic storage wavelengths

The above 2 are examples that basically say: if you can find a quantum observable to measure, you might also be able to entangle something in that basis. That, and a Bell Inequality, gives you some quantum non-locality. Please keep in mind that in addition to non-locality, entanglement also defies normal bounds of time sequencing. I guess you could call it non-temporality. There are references on that too.

 

[dlgoff]

You really should be thankful that someone of Dr Chinese's stature is even trying to help you.

ditto.

Thanks DrChinese.

 

[Cthugha]

And I'm not so sure if reading up on the meaning of linear polarization for a single photon would help. How bout you try it out first and tell me how it goes. Before you escalate, remember that I have single handedly annoyed the largest contributor to entangled threads all by my self.

No problem. I just adopted the habit of answering in the tone the question was asked. ;)

Ok, the main point I tried to make about the meaning of linear polarization for single photons was just that it is a superposition of the two circular states, not a mixture. Therefore you can easily convert such a state into a superposition of the circular states using just quarter wave plates. Doing so just has an effect on the phase and therefore constitutes no measurement and also does not lead to collapse. So it is quite easy to get to the circular basis.

However, this is pretty much never done in experiments simply because the easiest way to measure polarization lies in using a simple Glan-Taylor or Glan-Thompson prism which is a polarizing beam splitter splitting linear polarizations. If you want to measure the circular degree of polarization (typically chosen as S3 on the Poincare sphere), it is quite common to just convert your state to the linear basis in a deterministic manner using a quarter wave plate and checking the transmission through the GT-prism. Measuring the degree of circular polarization directly in the circular basis is possible, but not quite easy and often not as exact as converting it to the linear basis. If you accept that this kind of measurement is ok, then I do not see why the choice of basis should pose any problem in experiments on entanglement.

My original thought was why there have been no bell correlation experiments using electrons instead of photons? Is it technical or fundamental physics? There have been some really clever experiments on spin properties of electrons and holes in semiconductors so the technical stuff is in place.

There have been experiments on "heavier" stuff like in the article DrChinese linked. However, preparing well defined entangled states in such systems is cumbersome and also all the heavier stuff tends to interact quite strongly with its surroundings which makes it quite complicated to perform the measurement before decoherence kicks in. This has been done even in unfriendly surrouondings. My boss did some work on entanglement in semiconductors way before he bacame my boss. See for example "Coupling and Entangling of Quantum States in Quantum Dot Molecules", Science 291, 451 (2001). http://www.sciencemag.org/content/291/5503/451.full. You should be able to find a free version of it if you just google for the name of the manuscript.

 

[Iforgot]

I posted to quickly. Will repost

 

[Iforgot]

@evo and dlgoff
While it's honorable of you to come to your friends defense, I feel that your friend was being dismissive of my question. I do not ask to be treated in any other fashion than I treat others As you can see from my discussion with San K in this post and his other post,
(https://www.physicsforums.com/showthread.php?p=4156001#post4156001)

I make an effort to understand their question and help them find the correct words. I believe being able to communicate clearly and effectively is not a talent all of us have, and that taking the time to understand what some one else is trying to communicate is important. If I do not sense a desire to understand from another party, I see no point in continuing a conversation with them and will try to extricate myself as quickly and bluntly as possible.

When I sense a desire to understand, I acknowledge it and try to show my appreciation.

@Cthugha

I don't mind (that much) if linear polarized photons are in bell states cause they don't impart any angular momentum onto the GT-prism. I want to say that in such an experiment, local realism in the surrounding environment is still preserved. Do you see what I'm trying to say?

 

[Cthugha]

I want to say that in such an experiment, local realism in the surrounding environment is still preserved. Do you see what I'm trying to say?

I do not really see your point. Please see my next comment for the reason why.

I don't mind (that much) if linear polarized photons are in bell states cause they don't impart any angular momentum onto the GT-prism.

The angular momentum transfer typically happens upon detection and therefore at the detector, not at the prism. However, linearly polarized single photons do not impart any angular momentum onto the detector only on average. Each single detection event will necessarily impart angular momentum in either direction onto the detector with 50% probability each for a linearly polarized single photon state. That is why I tried to emphasize the meaning of linear polarization for single photons before.

 

[Iforgot]

Ah! Here's the meat of our disagreement. Transfer of photon angular momentum with the GT polarizer!

My claim:
1) Angular momentum transfer happens at the GT-prism. Proof: Well defined incident circularly polarized light becomes linearly polarized after passing through. Loss of hbar (helicity reversal by a 1/2 wave plate would be 2hbar)

2) A single linearly polarized photon would transfer no angular momentum to a photon detector. I'm racking my brain to recall an experimental proof for this.

(I'm trying to take a rigid Bohm approach here. I.e. wavefunctions evolve in a deterministic fashion determined by dirac schrodiner. Measurements never completely collapse the wavefunction. E.g. Optical elements (polarizers, q-waveplates, etc...) force a known previous wave-function into a new wavefunction in a calculable way (the Dirac Schrodinger eq))

 

[DrChinese]

1. ...I feel that your friend was being dismissive of my question.

2. I do not ask to be treated in any other fashion than I treat others.

1. Dismissive? Question asked and answered.

2. Sadly, I believe you, but not for the reason you might think. On the other hand, around here we try to treat people nicely.

You seem to be pretty new to entanglement, this is a subject that well over a thousand papers a year are published on. You might want to learn a bit more about the subject before you pontificate further.

http://arxiv.org/find/quant-ph/1/AND+all:+2012+abs:+entanglement/0/1/0/2012/0/1?per_page=100

 

[Iforgot]

@ drchinese
1) When you say "You might want to learn a bit more about the subject before... ", I feel I'm being dismissed. I've heard that argument from every religious figure on the planet trying to take some sort of open minded/more educated high ground, and portray the people who disagree with them in a negative light.

I feel that statement is an attempt to discredit, humiliate, and dismiss by trying to demonstrate another's lack of education or expertise.

2) I have a hard time imagining anyone reading and understanding all those ArXiv papers in the course of a year. That would be 2+ papers a day. It's not intellectually honest for you to expect anyone to have read all of them. There's a finite amount of time in the day to gather information, and we need to reach conclusions with insufficient evidence.

3) Given the above, I don't feel you are treating me nicely. I feel you are using subtle approaches to discredit my ability to contribute to an intelligent discussion. If my posts offend you, you do not need to read them.

4) My rude posts earlier were just an attempt to regain credibility by demonstrating some knowledge on my part.@Cathugha
2) continued... Linear polarized photon on a semiconductor at the bandgap energy would excite 1 electron hole pair with a superposition of opposite spins. I.e. the spins would be precessing.

The linearly polarized photon would be annihilated without collapsing to a circular polarized state.

 

[jtbell]

To everybody to whom this might apply: please knock it off with the personal remarks and cattiness. If you have a personal dispute with someone, carry it on privately and not in public.

 

[San K]

Thanks Dr. Chinese and Cthugha for being patient and taking the time to answer questions and providing good knowledge & information.

 

[Cthugha]

Ah! Here's the meat of our disagreement. Transfer of photon angular momentum with the GT polarizer!

My claim:
1) Angular momentum transfer happens at the GT-prism. Proof: Well defined incident circularly polarized light becomes linearly polarized after passing through. Loss of hbar (helicity reversal by a 1/2 wave plate would be 2hbar)

This is not really exact. A GT-prism is more or less a polarizing beam splitter. You have s-polarized light at one exit port and p-polarized (or mostly p-polarized, depends on the prism design) at the other exit port. What you get if you shoot a single photon at the prism is a superposition state of having one linear polarization and the photon being transmitted and having the other polarization and the photon being reflected. You will of course know that a photon passing through will have some well defined polarization, but you will not know whether some photon went through the prism without performing a measurement. At that time you get angular momentum transfer (depending on what kind of light you used possibly also to the prism). However, as the prism is certainly not in an eigenstate of angular momentum, that change will most likely drown in uncertainty and you will not get the prism to another orthogonal eigenstate. Things would differ if you managed to create some very light polarizing beam splitter, where such a tiny change in angular momentum can change the state of the prism. That might actually be a measurement and momentum transfer might happen immediately.

(I'm trying to take a rigid Bohm approach here. I.e. wavefunctions evolve in a deterministic fashion determined by dirac schrodiner. Measurements never completely collapse the wavefunction. E.g. Optical elements (polarizers, q-waveplates, etc...) force a known previous wave-function into a new wavefunction in a calculable way (the Dirac Schrodinger eq))

Bohmian mechanics is deterministic, but the initial conditions are necessarily unknown to some degree, so you still have uncertainty and stuff. For example for a single photon state, phase is pretty much undefined. Bohmian mechanics is in some respects elegant as it distinguishes the particle and the guiding wave. Nevertheless, it also obeys the fundamental physical law for interpretations of physics: conservation of frustration. ;)
By the way wave plates and all the stuff acting solely on phase without probabilistic elements are also deterministic in other interpretations.

2) continued... Linear polarized photon on a semiconductor at the bandgap energy would excite 1 electron hole pair with a superposition of opposite spins. I.e. the spins would be precessing.

The linearly polarized photon would be annihilated without collapsing to a circular polarized state.

Annihilated? Yes. Detected? No!

You can create such a superposition of say +3/2 heavy hole /-1/2 electron and -3/2 heavy hole/ +1/2 electron spin for example. These might precess in a magnetic field. However, there still is no measurement. You cannot know whether such an absorption process took place without measuring the e-h pair or maybe exciton and collapsing it to some eigenstate.

 

[Iforgot]

1) "What you get if you shoot a single photon at the prism is a superposition state of having one linear polarization and the photon being transmitted and having the other polarization and the photon being reflected. "

To make sure we are on the same page you are saying the photon state after GT prism interaction is some like

ψ = (|1>+|-1>)e^k1*x +constant*(|1>-|-1>)e^k2*y

where the |1> and |-1> are spin eigenstates), k2 and k1 are the wave vectors along the x and y directions respectively. (I know, I know, the reflected and transmitted rays have a colinear component)

We could perform calculations to determine if that is true. If the CP photon doesn't exchange angular momentum with the GT prism, the super position state you describe would still have angular momentum. And for consistency, I would also show that calculations on a 45 degree polarized photon that is split into the state you described doesn't have any angular momentum.

Let me know if you agree with the above before I start going about doing the tedious equations.

I lose your train of thought starting at "the prism is certainly not in an eigenstate of angular momentum"... What's this very light polarizer you speak of?2) Detected. No! You cannot know whether such an absorption process took place without measuring the e-h pair or maybe exciton and collapsing it to some eigenstate.

I thought that's how PN junction photo-detectors work? Once the carriers are excited, the field across the PN junction (or the bias) sweeps the carriers to the electrodes for subsequent detection stages, e.g. a current amplifier/pre-amplifier.

Yes, precession requires a field. I meant to say a linear polarized photon (with wave vector along Z), would excite an electron with spin in a super position of z up and z down. We can show that an electron in such a super position is an eigenstate of the Pauli spin matrix along x or y.

 

[Cthugha]

To make sure we are on the same page you are saying the photon state after GT prism interaction is some like

ψ = (|1>+|-1>)e^k1*x +constant*(|1>-|-1>)e^k2*y

where the |1> and |-1> are spin eigenstates), k2 and k1 are the wave vectors along the x and y directions respectively. (I know, I know, the reflected and transmitted rays have a colinear component)

Hmm, I do not think we are on the same page here. Let me get one step back and start at something easier. Do you agree that any circular polarization state can also easily be expressed in a linear basis? This is possible in terms of a superposition of the two linear states with complex coefficients, which is simply equivalent to a well defined phase shift of +/- pi/2 between these two basis states. I just wanted to emphasize that it makes a huge difference whether you add the two linear polarization states coherently or incoherently, the former is more than just the sum of its parts.
As you seem to be interested in semiconductor physics: This can also be used in coherent control schemes. If you shine two short laser pulses (polarized horizontally and vertically, respectively) on e.g. a semiconductor quantum well, you will notice a huge increase in the degree of circular polarization of the excitons in the quantum well. This results from the coherent interaction of the second pulse with the coherent excitonic population created by the first pulse.
Anyway: relative phase matters.

I lose your train of thought starting at "the prism is certainly not in an eigenstate of angular momentum"... What's this very light polarizer you speak of?

I just wanted to emphasize that it does not make much sense to speak of momentum transfer before performing any measurement on the system. A VERY light polarizing beam splitter might actually end up in a different state when a single quantum of angular momentum is transferred to it, so that might constitute a measurement. For any real polarizing beam splitter, this will not be the case.

I thought that's how PN junction photo-detectors work? Once the carriers are excited, the field across the PN junction (or the bias) sweeps the carriers to the electrodes for subsequent detection stages, e.g. a current amplifier/pre-amplifier.

Yes, that is part of how they work. But you need to detect the current somehow to know that there was some light. This will in any case break the superposition state and put the electrons and holes into some eigenstates. While the superposition still exists you cannot even know whether a photon was detected (and also you will not know that the superposition exists).

Yes, precession requires a field. I meant to say a linear polarized photon (with wave vector along Z), would excite an electron with spin in a super position of z up and z down. We can show that an electron in such a super position is an eigenstate of the Pauli spin matrix along x or y.

Yes, I do not think we disagree here. Linearly polarized excitation can excite such a superposition.

 

[Iforgot]

1) Ahh, I forgot the "i"s in the equation. Other than that, I don't see what's wrong with the equation.

Yes I agree circularly polarized light can be expressed in a linear basis with a pi/2 phase shift.

"whether you add the two linear polarization states coherently or incoherently"
you mean putting the "i"s in the correct place/ properly accounting for the phases?

Could you write ψ for the photon explicitly for after the GT prism photon interaction. That would clear things up.

Regardless how we choose to write the photon ψ for the post GT prism interaction, do we agree that without angular momentum transfer, it must conserve its angular momentum? I.e. no change in angular momentum from before and after GT interaction. This can be verified evaluating the angular momentum operator on ψ.

2) I just wanted to emphasize that it does not make much sense to speak of momentum transfer before performing any measurement on the system. A VERY light polarizing beam splitter might actually end up in a different state when a single quantum of angular momentum is transferred to it, so that might constitute a measurement. For any real polarizing beam splitter, this will not be the case.

Ah! Another difference in our assumptions! I am of the opinion that we don't have to make a measurement or require the wavefunction to collapse into an eigenstate for there to be transfer of angular momentum.

3) Yes, that is part of how they work. But you need to detect the current somehow to know that there was some light. This will in any case break the superposition state and put the electrons and holes into some eigenstates. While the superposition still exists you cannot even know whether a photon was detected (and also you will not know that the superposition exists).

I don't agree that current detection would break the electron carrier spin super position. a) I thought we agreed the electron is in an spin eigenstate along the xy plane. b) And even if it was in a super position, detecting the charge (say using a transistor based amplifier) wouldn't break the spin superposition. (I'm thinking as to how I can support these assertions)

 

[Cthugha]

1) Ahh, I forgot the "i"s in the equation. Other than that, I don't see what's wrong with the equation.

Much, see below.

"whether you add the two linear polarization states coherently or incoherently"
you mean putting the "i"s in the correct place/ properly accounting for the phases?

Yes, but that was just a general remark.

Could you write ψ for the photon explicitly for after the GT prism photon interaction. That would clear things up.

No, I obviously cannot do that because there is no generally accepted formalism for wavefunctions of photons. There have been some efforts in that direction, but the results are far from giving a complete picture. In quantum optics the quantities of interest are therefore probability amplitudes for certain events.

Regardless how we choose to write the photon ψ for the post GT prism interaction, do we agree that without angular momentum transfer, it must conserve its angular momentum? I.e. no change in angular momentum from before and after GT interaction. This can be verified evaluating the angular momentum operator on ψ.

Well, if you can be sure of the angular momentum beforehand... The problem with the missing wavefunction of photons makes things complicated again.

Ah! Another difference in our assumptions! I am of the opinion that we don't have to make a measurement or require the wavefunction to collapse into an eigenstate for there to be transfer of angular momentum.

Hmm, so you insist on some kind of realistic interpretation of qm? Ok, but you are aware that every tenable realistic interpretation like the Bohmian one is non-local, right?

I don't agree that current detection would break the electron carrier spin super position. a) I thought we agreed the electron is in an spin eigenstate along the xy plane. b) And even if it was in a super position, detecting the charge (say using a transistor based amplifier) wouldn't break the spin superposition. (I'm thinking as to how I can support these assertions)

This puzzles me. For one you said beforehand that you want to follow a Bohmian approach. However, Bohmian mechanics does not even have superpositions. Anyway, please come up with some design for detecting the presence of photoexcited carriers (in terms of a strong measurement) which does not destroy the superposition. You might achieve something similar with weak measurements (and all their drawbacks), but it is basic first semester qm stuff that typical measurements spoil superpositions.

 

[Iforgot]

1)No, I obviously cannot do that because there is no generally accepted formalism for wavefunctions of photons. There have been some efforts in that direction, but the results are far from giving a complete picture. In quantum optics the quantities of interest are therefore probability amplitudes for certain events.

Ahh! I did not know this. I thought the dirac equation for a massless particle was generally accepted as providing the photon wavefunction.

2) Well, if you can be sure of the angular momentum beforehand... The problem with the missing wavefunction of photons makes things complicated again.

? Making a photon with a definite angular momentum can readily be done with a polarizer and a q-wave plate!

3) Hmm, so you insist on some kind of realistic interpretation of qm? Ok, but you are aware that every tenable realistic interpretation like the Bohmian one is non-local, right?

I was under the impression that the Bohm approach was a strict adherence to the (dirac-) schrodinger equations. That is, any modelling of a measurement had to be introduced in a rigorous fashion by introducing the potential contributed from the detector into the schrod-eq.

E.g. I can semi-quantitatively show how an electron in the double slit experiment is localized to one of the slits when one introduces the detector as a potential term in the Schrodinger equation. (Have you seen this mathematical derivation before?)

I have wanted to see the non-locality arise out of the photon wave-function by applying the appropriate operators. But in light of your comment in 1) this is not a trivial task.

4) Anyway: relative phase matters.

I couldn't agree more. My original post was an attempt to draw an analogy between a) super luminous phase velocity of light in medium with refractive index less than 1 and the fact that it doesn't transmit information faster than light and b) relative phase determining polarization states in bell experiments that also can't be used to transmit information faster than light.

Both of these strange properties (a & b) arise from some sort of phase term. This analogy led me to speculate that the underlying mathematical structure of both these phenomena might share other similarities.

(Thanks for listening and responding. I've really appreciated having some one to talk to about this.)

Oooh. My head is starting to spin. Conversation (meant to say conservation) of frustration is starting to kick-in :-)