How entangled is laser light?

[rajama]

This is a spin-off from the FTL/entangement thread:

 

I've been reading papers describing a series of undergraduate experiments with entangled photons and they employ laser-fed down-converters to ensure conditions for coincidence counting to work, etc... and, well, doesn't the physical process within the laser cavity itself create entangled photons?

 

I guess I''m thinking the partial reflection of the light produced and the small size of modern lasers (as pointed out in another thread) might produce a variety of entanglements..?

[swansont]

Why do you think laser light would be entangled? If the lasers are polarized, all of the photons are in a known polarization state.

[rajama]

Why do you think laser light would be entangled? If the lasers are polarized, all of the photons are in a known polarization state.

 

Is laser light always polarised or is it a matter of degree or design? Ummm - I'll go check...

 

What about other kinds entanglement, maybe caused by the cavity?

[swansont]

Is laser light always polarised or is it a matter of degree or design? Ummm - I'll go check...

 

What about other kinds entanglement' date=' maybe caused by the cavity?[/quote']

 

It depends on the laser, but many have elements in them that suppress one polarization (e.g. Brewster plate), or one polarization simply isn't supported (e.g. waveguide effects in a laser diode).

[rajama]

Okay, but that doesn't mean there's no entanglement - the paper I mentioned (which is at about my level, though I've forgotten most of it already) is this pdf

 

See B. The Crystal, para 2 - the down converter produces parallel polarization in the output photons...

[swansont]

Okay' date=' but that doesn't mean there's no entanglement - the paper I mentioned (which is at about my level, though I've forgotten most of it already) is this pdf

 

See B. The Crystal, para 2 - the down converter produces parallel polarization in the output photons...

 

 

Yes, the down-converter produces entangled photons. Not the laser. The parametric down-conversion produces two photons with the same polarization - that's how they are entangled. If you measure one photon's polarization, you know the polarization of the other one.

[rajama]

Yes, the down-converter produces entangled photons. Not the laser. The parametric down-conversion produces two photons with the same polarization - that's how they are entangled. If you measure one photon's polarization, you know the polarization of the other one.

 

Yes.

 

I got that.

 

I was comparing the laser and the downconverter and what I perceived as the broad similarity of the processes occuring in each.

 

Wouldn't the difficulty in disinguishing between laser photons lead to any form of entanglement?

[swansont]

Yes.

 

I got that.

 

I was comparing the laser and the downconverter and what I perceived as the broad similarity of the processes occuring in each.

 

Wouldn't the difficulty in disinguishing between laser photons lead to any form of entanglement?

 

Not as far as I know. If there were, nobody would bother doing down-converting to do entanglement.

[rajama]

Not as far as I know. If there were, nobody would bother doing down-converting to do entanglement.

 

Okay.

 

It's just that in re-reading the papers I've looked at recently, the down converter seems to be there, in many respects, to provide the degree of control over the experiment to allow the coincidence detectors to be effective.

 

It all appears to be about painstakingly careful control - which may be something you wouldn't have a chance in achieving with the output of a laser..?

 

But wouldn't it be useful...

[rajama]

I really should have researched a little more before posting this thread (sorry) - I just found a handful of references to an 'entanglement laser' as a research topic at Grenoble:

 

http://www-lsp.ujf-grenoble.fr/eng/recherche/a1t4/a1t4a3/theory.htm

 

The related paper is available for purchase at:

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000091000005053601000001&idtype=cvips&gifs=yes

 

Okay, so its a down-converter in a laser cavity type thingy? I don't have access to the paper, so...

 

It may exploit the process described here (at a guess):

http://physicsweb.org/articles/news/5/8/20

 

This was all some time ago, so if no one has read anything elsewhere, I would guess it's still a 'work in progress'.