Is Polarisation Entanglement Possible in Photon Detection?

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Discussion Overview

The discussion revolves around the nature of polarization entanglement in photon detection, specifically whether individual photons in an entangled pair can be considered to exist in superposition or mixed states prior to measurement. The scope includes theoretical aspects of quantum mechanics and the interpretation of entangled states.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that if the polarization state of a photon is unknown before detection, it may be reasonable to assume it is in a superposition of all possible states.
  • Others argue that the polarization can be described as a mixed state rather than a superposition, emphasizing the use of density matrices for representation.
  • A participant clarifies that in the case of entangled photons, the composite system can be in a pure state, while the individual photons are in mixed states.
  • There is a discussion about the distinction between superposition and mixture, with some asserting that superposition implies a lack of a well-defined polarization state, while others maintain that a mixture describes a probabilistic distribution of states.
  • One participant mentions that the entangled state can be represented as a maximum-entropy state, indicating that the individual photons are unpolarized.
  • Some participants express confusion over the terminology, debating whether it is appropriate to describe the state of individual photons in an entangled pair as superpositions or mixtures.
  • Another participant emphasizes that an entangled state cannot be decomposed into individual pure states, which contradicts the notion of individual photons being in superposition.

Areas of Agreement / Disagreement

Participants do not reach consensus on whether individual photons in an entangled state should be described as being in superposition or mixture. Multiple competing views remain, with ongoing debate regarding the definitions and implications of these terms in the context of entanglement.

Contextual Notes

Participants highlight the complexity of defining states in quantum mechanics, particularly regarding the terms "superposition" and "mixture," and how these relate to entangled systems. There is an acknowledgment of the need for clarity in terminology, as well as the implications of these definitions on understanding entanglement.

  • #241
zonde said:
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.
 
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  • #242
A. Neumaier said:
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.
 
  • #243
zonde said:
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.
 
  • #244
vanhees71 said:
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.
vanhees71 said:
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.
vanhees71 said:
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?
 
  • #245
A. Neumaier said:
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.
 
  • #246
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?
 
  • #247
jeremyfiennes said:
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.
 
  • #248
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?
 
  • #249
Excuse my ignorance: what is "AFAIK"?
 
  • #250
jeremyfiennes said:
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.
 
  • #251
jeremyfiennes said:
Excuse my ignorance: what is "AFAIK"?

AFAIK = As far as I know...
 
  • #252
Both queries answered! Thanks.
 

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