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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?

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  • $\begingroup$ what kind of qubits are you thinking about? Information can be encoded in multiple ways in photons/light. Are you thinking about single photons in polarisation specifically? Moreover, are you asking about how the measurement is performed, or how the states themselves are generated? The title and body of the post seem to contrast in this regard $\endgroup$
    – glS
    May 26 at 18:50
  • $\begingroup$ I was asking whether or not it is possible to somehow measure a stream of polarized photons rather than just a single one. $\endgroup$
    – Mimo Pakou
    May 28 at 5:46
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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).

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    $\begingroup$ Well yeah about the polarising beam splitter I am aware (thinking back to it I should have stated the question better), I was just looking for a way to get rid of a single photon emitter since those are expensive, your reply however was really helpful! $\endgroup$
    – Mimo Pakou
    May 26 at 4:50

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