 "Quantum cryptography aims to exploit the laws of quantum mechanics to securely transmit data. Among different possible encryption methods, we focus on quantum key distribution (QKD). In this context, the no-cloning theorem limits the access of an eavesdropper to information communicated via a quantum channel between two parties. In this thesis, we implement a specific prepare-and-measure continuous-variable QKD protocol proposed by Cerf et al. [1], which encodes classical information in displacement amplitudes of squeezed coherent states of light. In our experimental implementation, we use propagating squeezed microwaves at the carrier frequency of f0 = 5.5231 GHz. The detection of these microwave signals relies on cryogenic amplification chains. Here, state-of-the-art cryogenic high-electron mobility transistor amplifiers add 10 − 20 noise photons, corresponding to a quantum efficiency of η < 5%. These phase-insensitive amplifiers are ill-suited for QKD, as they typically reduce the signal-to-noise ratio (SNR) below a threshold required for the secure communication and are also bound by the standard quantum limit (SQL), η = 50%. Therefore, we make use of superconducting phase-sensitive amplifiers which can even exceed the SQL to implement the aforementioned CV-QKD protocol with quantum microwaves in the single-shot regime. As our main experimental result, we achieve a positive secret key for our microwave CV-QKD protocol implementation and analyze its robustness against an eavesdropping attack. To this end, we use a Josephson parametric amplifier (JPA) in the phase-sensitive regime at the beginning of the cryogenic amplification chain. With this modification, we demonstrate a significant improvement in the experimental quantum efficiency, η = 38%. This step allows us to increase the SNR from 14% to 177% during the CV-QKD protocol sequence which results in the positive secret key. The current SNR is mainly limited by the dynamic range of our JPAs. In the future, the SNR can be further improved by exploiting traveling wave parametric amplifiers. Our results highlight the experimental feasibility of microwave CV-QKD protocols." [1] N. J. Cerf, M. Lévy, and G. V. Assche, Quantum distribution of Gaussian keys using squeezed states, Physical Review A 63, 052311 (2001). (Published at www.wmi.badw.de/fileadmin/WMI/Publications/Krueger_Philipp_Masterarbeit_2022.pdf) 
0 Comments
Leave a Reply. |
|||||||