Undergrad Is Polarisation Entanglement Possible in Photon Detection?

[A. Neumaier]

For ensembles we have statistics. Probability is model for individual case based on statistics of ensemble.

No. Probability is the theoretical tool in terms of which statistics is formulated. For individual cases we just have observations, together with a sloppy (or subjective) tradition of misusing the notion of probability.

 

[stevendaryl]

No. Probability is the theoretical tool in terms of which statistics is formulated. For individual cases we just have observations, together with a sloppy (or subjective) tradition of misusing the notion of probability.

The subjective treatment of probability is anything but sloppy. It's much more careful than the usual frequentist approach.

 

[forcefield]

With physical collapse you mean that measurement of Alice's photon changes Bob's photon polarization? Meaning that if initially we model Bob's mixed state as statistical mixture of orthogonal pure states H/V then after Alice's measurement in H'/V' basis Bob's mixed state components change to H'/V' basis, right?

Let's say that Alice always "measures" first. Then when the photon pair interacts with her polarizer, it prepares the state for both Alice and Bob. I think Simon has said more or less the same thing.

Interestingly, I saw yesterday a Danish TV program from 2013, where the main message seemed to be that people should just accept the non-locality a la Bohr. I did not recognize other people talking there but they did have Zeilinger talking there. They also had "Bohr" and "Einstein" traveling back and forth with a train discussing Bohr's ideas and whether moon is there when nobody is looking. "Bohr" just said that "Einstein" can't prove it.

 

[A. Neumaier]

the preparation can be completely determining the state, which is described in the formalism by a pure state.

In most cases, when the model is sufficiently accurate, only by a mixed state. Whatever is prepares, the state is objectively given by the experimental setting. No subjective interpretation enters, except for the choice of a level of detail and accuracy with which the situation is modeled.

the protons in the LHC which have a pretty well-determined momentum

Even the state of protons will generally be mixed states, since their position/momentum uncertainty is larger than that required for a pure state.

you associate mixed states based on the (incomplete) information you have.

No. Otherwise the state would change if the experimenter gets a stoke and forgets the information, and the assistant who completes the experiment has not yet read the experimental logbook where this information was recorded.

One associates mixed states based on the knowledge (or hope) that these mixed states correctly describe the experimental situation. The predictions with a mixed state will be correct if and only if this mixed state actually describes the experiment, and this is completely independent of the knowledge various people have.

Introducing talk about knowledge introduces a nonscientific subjective aspect into the setting that is completely spurious. What counts is the knowledge that Nature has, not the one of one of the persons involved in an experiment. Whose knowledge should count in case of collision experiments at CERN where most experimental information is gathered completely automatically, and nobody ever looks at all the details?

 

[stevendaryl]

In most cases, when the model is sufficiently accurate, only by a mixed state. Whatever is prepares, the state is objectively given by the experimental setting. No subjective interpretation enters, except for the choice of a level of detail and accuracy with which the situation is modeled.

That's like saying no subjective interpretation enters, other than the parts that are subjective.

 

[jeremyfiennes]

A basic question from a beginner. Two polarization-entangled photons are generated, and set off in opposite directions across the universe. One of them bumps into a heavenly body and gets absorbed by one of its atoms, displacing an electron into a higher orbit. And then no longer exists as a photon. What happens to the other, still out in free space?

 

[secur]

A basic question from a beginner. Two polarization-entangled photons are generated, and set off in opposite directions across the universe. One of them bumps into a heavenly body and gets absorbed by one of its atoms, displacing an electron into a higher orbit. And then no longer exists as a photon. What happens to the other, still out in free space?

The simplest answer is: nothing at all happens to the other photon. That's according to some interpretations of QM. OTOH other interpretations might say its polarization wavefunction collapses. (Of course its energy or direction wouldn't be affected.)

To avoid that interpretation issue, change the question to "if we measure the other photon's polarization, can we say anything about the result?" That depends on whether the (first photon's) absorption is considered a measurement. Some interpretations would say it is, others not.

To avoid that interpretation issue, let's assume a scientist observes the "heavenly body" atom after the photon is absorbed. With an appropriate extremely sensitive detector, he can theoretically determine what its polarization was. All interpretations agree that constitutes a measurement.

Then the other photon would definitely be measured with the expected "entangled" polarization. Typically, opposite to the first photon.

AFAIK.

 

[jeremyfiennes]

Ok. Thanks. I got the measurement bit. My main doubt is that I have read that entangled photons are 'forever entangled". But what if one is absorbed, and hence ceases to exist as a photon, before 'forever' expires? A further more basic question arises from this. If a measurement is made on one photon, collapsing the common wave function and determining the polarization state of the other, after that are the photons still entangled?

 

[jeremyfiennes]

Excuse my ignorance: what is "AFAIK"?

 

[DrChinese]

Ok. Thanks. I got the measurement bit. My main doubt is that I have read that entangled photons are 'forever entangled". But what if one is absorbed, and hence ceases to exist as a photon, before 'forever' expires? A further more basic question arises from this. If a measurement is made on one photon, collapsing the common wave function and determining the polarization state of the other, after that are the photons still entangled?

Particles typically cease to be entangled when a measurement is performed on either of a pair. That is a general statement, and there are a number of caveats to consider. For one, no one knows the precise moment that entanglement ceases. Also, a particle can be measured on one basis and remain entangled on another.

 

[DrChinese]

Excuse my ignorance: what is "AFAIK"?

AFAIK = As far as I know...

 

[jeremyfiennes]

Both queries answered! Thanks.