Why aren't photons from a laser considered entangled?

[LarryS]

TL;DR

Are photons from a laser entangled?

Photons produced by a laser are all in the same quantum state, right? But, I don't believe they are considered entangled. If not, then why not? Particles having the same quantum state, have the same eigenbases. That is about as entangled as you can get.

Thanks in advance.

 

[PeroK]

Particles having the same quantum state, have the same eigenbases. That is about as entangled as you can get.

Where did you get that idea from?

 

[PeterDonis]

TL;DR: Are photons from a laser entangled?

Photons produced by a laser are all in the same quantum state, right?

Wrong. What is emitted by a laser is not a bunch of photons that each have an individual quantum state. The quantum state emitted by a laser is one quantum state, which is not an eigenstate of photon number, so the concept of "photons produced by a laser" doesn't really make sense, nor does the concept of multiple photons in the state being entangled. These concepts simply don't apply to the state emitted by a laser.

 

[PeterDonis]

Particles having the same quantum state, have the same eigenbases. That is about as entangled as you can get.

Quite apart from the particular case of a laser, this is wrong. As a counterexample, the electron gun inside a cathode ray tube emits electrons (and in this case the state emitted is an eigenstate of electron number, so the concept of "electrons emitted by the tube" makes sense) that all (at least to a reasonably good approximation) have "the same quantum state", but they aren't entangled; the overall quantum state is a product state of all the individual electron states.

 

[Demystifier]

When the intensity of the laser beam is very strong, i.e., when the average number of photons is large, then the laser beam can very well be described as a classical electromagnetic wave. It's only for the low intensity laser beams that its quantum properties become pronounced. But those quantum properties do not involve entanglement, laser light is not entangled in any practical operational sense. (Of course, at the fundamental level any QFT state, even the vacuum, is entangled, but with current technology that kind of entanglement cannot be directly seen experimentally.) In quantum optics it takes an extra effort to produce photons which are entangled in a practical sense. The most popular way to do this is parametric down conversion.