Transmitter for a quantum communication system, a quantum communication system and a method of generating intensity modulated photon pulses

A method of generating intensity modulated photon pulses performed by a transmitter, the method comprising:

• generating light pulses at a photon source unit comprising a photon source;
• receiving an input light pulse from the photon source unit at a first intensity modulator;
• splitting the input light pulses into two components at a first element in the first intensity modulator;
• applying a phase shift between the two components at a phase modulator in the first intensity modulator;
• and
• interfering the two components at a second element in the first intensity modulator, wherein at least one of the first element and the second element has a splitting ratio less than 1, where the splitting ratio is the ratio of a signal intensity through a first port to a signal intensity through a second port, for a signal applied at a third port,
• wherein the transmitter is configured to implement decoy state quantum communication;
• wherein the photon source unit is configured to emit light pulses having a first intensity and a second intensity, the second intensity being lower than the first intensity, wherein the phase modulator comprises a first setting, corresponding to one of destructive interference or constructive interference and a second setting, corresponding to the other of destructive or constructive interference, and wherein the first intensity modulator outputs light pulses having three or more different intensities, dependent on selection between the first and second setting of the phase modulator and the first and second intensity of the photon source unit,
• wherein a first of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the first setting,
• wherein a second of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the first intensity and the phase modulator is configured to the second setting, and
• wherein a third of the three or more intensities is generated when the photon source unit is configured to emit light pulses having the second intensity.