Apparatus and method for decoy-state three-state quantum key distribution

A Quantum Key distribution system comprising a transmitter and a receiver for exchanging a Quantum key via a quantum channel through a decoy-state three-state protocol wherein the transmitter comprises

• a transmitter processing unit configured to use random numbers from a quantum random generator to select one quantum state out of nine possible states of intensity and basis,
• a Pulsed light source configured to generate an optical pulse,
• a time-bin interferometer through which the generated optical pulse passes and which transforms the generated optical pulse into two coherent pulses separated by a time bin duration,
• a single modulator configured to encode the two coherent pulses according to the choice made by the transmitter processing unit by changing the intensity of the two pulses individually, and
• a variable optical attenuator located in series and downstream of the single modulator and configured to reduce the overall signal intensity to the optimum photon number per pulse, wherein the single modulator is an intensity modulator configured to encode a first or a second quantum state by transmitting the pulse in an early or late time-bin, respectively.