Abstract: There is presented optical interferometer apparatus for generating at least a first and a second state for each of at least two different quantum cryptographic basis sets and associated methods for operating the apparatus. First and a second interferometer arm paths and at least two interferometer output paths are provided. Each of the first and second interferometer arm paths; and the first and second optical output paths comprises a controllable phase modulator for controllably changing the phase of light propagating along the respective paths. This maybe a silicon carrier depletion phase modulator. The output from the controllable phase modulators of the first and second optical output paths are used together for defining each of the first and second states for each respective basis set. In some examples at least one of the interferometer arm paths comprises a further controllable phase modulator such as a thermo-optic phase modulator.