Perspectives of microwave quantum key distribution*

One of the cornerstones of quantum communication is an unconditionally secure distribution of classical keys between remote parties. This can be achieved by exploiting quantum features of electromagnetic waves, such as entanglement or superposition. However, these quantum properties are known to be susceptible to noise and losses, which are essential for free-space communication scenarios. In this work, we theoretically investigate perspectives of continuous-variable free-space quantum key distribution (QKD) at microwave frequencies. Using a protocol based on displaced squeezed states, our model predicts that continuous-variable microwave quantum key distribution with propagating microwaves can be unconditionally secure at room temperatures up to distances of around 200 meters and even outperform conventional QKD protocols at telecom wavelengths under realistic weather conditions. Furthermore, we conduct experimental studies on microwave QKD with propagating microwave squeezed states. The latter are generated and manipulated by using superconducting Josephson parametric amplifiers. We demonstrate the experimental feasibility of unconditional security for microwave QKD in cryogenic enviroments and project these results to near-term open-air applications.