Akash Nag Oruganti

We address and theoretically analyze continuous-variable quantum key distribution (CV QKD) with noisy squeezed states. The noise in such states unavoidably emerges due to optical losses in the state preparation and has to be taken into account in any practical scenario, with the outcomes depending on the trust assumption on such noise. We show that the untrusted noise should pessimistically be allocated to the anti-squeezed (AS) quadrature and can break the security of the protocols already in the asymptotic regime. In the finite-size regime we analyze the impact of the AS noise on the parameter estimation, showing that it limits the performance of the protocols even if assumed trusted and requires the protocol modifications in terms of modulation and detection schemes. We also consider the noisy squeezed-state CV QKD in the channels with transmittance fluctuations (typical for the atmospheric channels) and show that even trusted AS noise can break the security of the protocols in this regime due to fluctuations-related channel excess noise, which has to be assumed untrusted. Our results demonstrate the importance of the squeezing purity in practical realizations of squeezed-state CV QKD.