Is Polarisation Entanglement Possible in Photon Detection?

[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.