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

San K

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

ok, thanks Cthugha

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

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

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

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

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: http://www.nature.com/nphoton/journa....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. 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.../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

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

ditto.

Thanks DrChinese.

Cthugha

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.

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,
(http://www.physicsforums.com/showthr...01#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 do not really see your point. Please see my next comment for the reason why.

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))