Continuous Variable Quantum Cryptography: Post-Selection with Thermal Noise

Continuous variable quantum key distribution (CVQKD) utilizes bright beams to establish secure communication between two parties via an optical quantum channel such as an optical fiber. One of the main protocols that are practical and promising involves a selection process after the transmission of quantum states, called post-selection based CVQKD. The security of CVQKD using post-selection has been investigated both theoretically (Silberhorn et al., 2002) and demonstrated experimentally (Lance et al., 2002). However, it was subject to one fundamental assumption, namely that the quantum channel must have strictly zero excess noise (i.e. it is shot-noise-limited). In reality, nonlinearities in optical fibers and classical noise of laser beams commonly introduce Gaussian excess noise. The authors considered an eavesdropping attack based on entangling cloner setup where the channel was replaced with a lossless and shot-noise-limited line and remain undetectible. Two-way reconciliation was considered in the security analysis.The paper presents experiments based on the protocol developed by the authors.