What degrees of freedom (other than polarization) of a photon can be used to store quantum information? Usually in quantum optics, one uses the polarization degree of freedom of the photon to create and manipulate photonic qubits. This, using various linear optics devices, can be coupled with the 'path degree of freedom' inside an interferometer for instance. So far, that makes two 'available' degrees of freedom to store quantum information as qubits.
But what else can we use in photons ? One can immediately discard the angular momentum degree of freedom, since this is just the left or right circular polarization state.
Maybe the wavelength ? But how could it be used, ie how could you create and manipulate quantum superpositions of wavelength states ?
I try to think of others, but I quite lack of imagination and experience in this field.
Thanks in advance,
 A: Just like an electron, light can have spin angular momentum and orbital angular momentum.
To make a beam of light with orbital angular momentum you can make a diffraction pattern where instead of a series of parallel lines you have one line that forks where the one line comes in and two (or more) lines come out. Then if you shin the right kind of light beam in you can get an outgoing beam with a dark part in the middle and have angular momentum around the dark part. This is in addition to regular polarization, and in particular you can have much more than two states.
Other forms seem to involve more than one photon, even wavelength to me is more than one photon so you are make qubits out of having a photon of this wavelength (versus not) and having a photon of that other wavelength (versus not).
But the semantics aren't important if it allows you do do what you want.  But obviously if you can store a quibit in a single photon you can store more if you have access to (potentially) more photons.
