I am studying Quantum Key Distribution which relies on photonic qubits, the photon.
Indeed, this one is viewed as a qubit, that is a two state system, just like the spin of an electron, an atom with ground/excited energy levels, ...
I wonder however what are the two states at stake for the photon.
I usually hear about the photon polarization. But polarization rather sounds for me as a classical property : that of the electromagnetic wave which can be either rectilinear, circular or elliptic polarized.
What is the property used for using photons as qubits ?
I know that a photon has a spin-1, which means its spin has three degrees of freedom, but one of them is forbidden (the degree of freedom parallel to its direction propagation).
Do the two remaining degree of freedom act like polarization ? Indeed, if you make a beam of polarized light go through two orthogonal classical filters, there will be extinction. But on the Bloch sphere, states $|0\rangle$ and $|1\rangle$ are not orthogonal in 3D space, as one is the $180^°$ rotation of the other.
How can I explain clearly what is the photon polarization ?