Optical Coherent Scheme with m-PSK Modulation for Secure Key Distribution

In this work, the implementation of secure key distribution on the physical layer using an m-PSK modulation is discussed. We establish a theoretical model to estimate secret key-rates in a realistic scenario. We show that in order to obtain reasonable key-rates, the signal power is required to be very low, usually about 1 photon per symbol and less. On the other hand, a higher order of the m-PSK modulation potentially offers a higher secret key rate. Therefore, the challenge of such a key distribution system is the quantum-limited coherent demodulation of a high-order phase-modulated signal at SNR-values below 0 dB. In this article, we propose a simple transmission scheme using heterodyne detection to achieve this. System experiments are carried out to investigate the scheme regarding minimum required signal power levels using a 4-PSK modulation.