Quantum Key Distribution with Heterodyne Detection

In this paper, we focus on the exchange of quantum states with a prototype setup and present estimated secret bit rates. The continuous-variable QKD protocol employs binary encoding in optical coherent states and postselection. In our experiment, the protocol is adapted to a long range fibre-based setup at the telecommunication wavelength (1550nm). The system employs heterodyne detection, using free-space optics and exploiting the polarisation degree of freedom. This allows for measuring the Q- functions of the transmitted signal states and the estimation of the secret key rates for different fibre lengths. Our results show an excess noise of 0.025 shot noise units, originating from the electronic detector noise and stray light of the LO in the signal arm of the detection, for different channels up to 40 km. The values for attenuation and negligible channel noise allow for an estimated secret key of about 0.001 bit per pulse and a fibre channel of 20.5 km length.