An emitter for a quantum communication system, the emitter being configured to output a sequence of periodic light pulses with different polarisations, the emitter comprising: a first source configured to generate a first sequence of pulses of light. the first sequence comprising a series of pairs of pulses, each pair of pulses corresponding to a bit which is encoded in the phase difference between the pulses of each pair of pulses, the phase difference between the later pulse of one pair and the earlier pulse of the following pair in time being random; a beam splitter configured to divide the pulses of the first sequence of pulses, such that each pulse is split between a first path and a second path, the first sequence of pulses having a varying phase and a first polarisation; a polarisation rotator configured to rotate the polarisation state of pulses in one of the first path or the second path with respect to the polarisation state of pulses in the other path; a time delay component configured to provide a time delay such that the first sequence of pulses in the first path are delayed by one period with respect to the first sequence of pulses in the second path; and an optical combination component configured to combine the delayed first sequence of pulses from the first path with the first sequence of pulses from the second path to produce an output sequence of pulses, where each pulse in the output sequence is a combination of a pulse from the second path and a delayed pulse from the first path, and has a polarisation determined from the phase difference between combined pulses and the polarisations of the pulses of the first path and the second path just before combination at the optical combination component, the output sequence comprising the delayed earlier pulse of a pair of pulses from the second path to overlap with the non-delayed later pulse of the same pair of pulses which followed the first path and the delayed later pulse of a pair of pulses to overlap with the non-delayed first pulse of the following pair of pulses. a first source configured to generate a first sequence of pulses of light. the first sequence comprising a series of pairs of pulses, each pair of pulses corresponding to a bit which is encoded in the phase difference between the pulses of each pair of pulses, the phase difference between the later pulse of one pair and the earlier pulse of the following pair in time being random; a beam splitter configured to divide the pulses of the first sequence of pulses, such that each pulse is split between a first path and a second path, the first sequence of pulses having a varying phase and a first polarisation; a polarisation rotator configured to rotate the polarisation state of pulses in one of the first path or the second path with respect to the polarisation state of pulses in the other path; a time delay component configured to provide a time delay such that the first sequence of pulses in the first path are delayed by one period with respect to the first sequence of pulses in the second path; and an optical combination component configured to combine the delayed first sequence of pulses from the first path with the first sequence of pulses from the second path to produce an output sequence of pulses, where each pulse in the output sequence is a combination of a pulse from the second path and a delayed pulse from the first path, and has a polarisation determined from the phase difference between combined pulses and the polarisations of the pulses of the first path and the second path just before combination at the optical combination component, the output sequence comprising the delayed earlier pulse of a pair of pulses from the second path to overlap with the non-delayed later pulse of the same pair of pulses which followed the first path and the delayed later pulse of a pair of pulses to overlap with the non-delayed first pulse of the following pair of pulses.