Quantum Key Distribution with Heterodyne Detection

Quantum key distribution (QKD) is a two step protocol. In one step, quantum states are exchanged between two parties, usually called Alice and Bob. Afterwards, quantum correlations and classical communication are used to generate a shared secret key. 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 [1–3] 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 Qfunctions of the transmitted signal states and the estimation of the secret key rates for different fibre lengths.