How is measurement performed on a stream of polarized photons?

Question

Is it possible to use a simple laser to measure qubits in optical quantum computers, or is a single-photon emitter absolutely necessary? If you can do it with a simple laser, how would you go about measuring it since you wouldn't have to measure a single photon but rather a stream of them?

Answer 1

One idea is to do polarimetry. By using a polarizing beam splitter, the polarization qubit can have each of its polarization components directed to a different detector for photon counting (ideally a single-photon detector, here).

A polarizing beam splitter might send horizontally polarized photons in one direction and vertically polarized photons in another. If you want to measure in a different basis, you can first place a series of wave plates to rotate the qubit before the polarizing beam splitter, then interpret the measurement results in that new basis. For example, a quarter-wave plate allows one to switch between the linear and circular polarization bases.

I think your question may be also asking whether the polarization qubit itself needs to be a single photon, or if the polarization qubit may be the polarization state of an entire stream of photons coming from a laser. Is this correct? If so, there are more complications. Classically, the state of polarization coming from a laser seems identical to that of a single photon, but there are many many more quantum degrees of freedom when you consider the photons involved. A laser won't always behave like a single polarization qubit when subject to arbitrary transformations and so it isn't actually a qubit (more generally, this is the realm of continuous-variable quantum computation).