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I've been studying quantum computers and while it's apparent to me how electrons are used as qubits, through their spin number, it's not as clear what makes a photon a viable candidate to be used as a qubit.

I understand how entanglement between qubits works, and how quantum gates work (though I haven't looked into optical gates yet) but I just can't seem to figure out how a photon could carry information!

Could anyone solve this conundrum? Thanks!

Corrado
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  • For a geometric point of view, the state space for photon (of fixed momentum) is represented as a Poincare sphere (see for example https://spie.org/publications/fg05_p10-11_poincare_sphere?SSO=1), which is related to the Bloch sphere picture for ordinary qubits. – QuantumEyedea May 29 '20 at 19:25

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A photon is a massless spin-1 particle. This means that a photon has exactly two spin states, just like an electron.

TimRias
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  • For anyone wondering why a "spin-1 particle" has as many states as the electron as a spin-1/2 particle and not more, see https://physics.stackexchange.com/q/46643/50583. – ACuriousMind May 30 '20 at 14:31
  • See e.g. "Gauge choice and general solution to equations of motion" in Sec. 4.4 here. – J.G. May 30 '20 at 14:34
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Just to add, any system that can be in two states can serve as qubit. So any elementary particle having two or more values of spin can be a qubit. Moreover, another parameter of a particle can be used for representing a qubit, for example location (you can have one electron and you are looking where is is placed).

Martin Vesely
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Photons have polarization state, so called right circularly polarized and left circularly polarized.

seyitliyev
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