Akash Nag Oruganti

In recent years, continuous-variable (CV) quantum communication has become a nascent but promising alternative to secure multiple end-users sharing the same telecommunication backbone. Continuous-variable quantum key distribution (CV-QKD) with reverse reconciliation enables natural scalability from point-to-point communication to quantum access networks with passive quantum broadcasting channels. Here, we report two experimental demonstrations on a $1:4$ quantum communication network with protocols advancing from two fronts, either moving beyond the asymptotic limit or the Gaussian assumption of the modulation. We demonstrate for the first time a network of four active users simultaneously achieving high secret key rates with finite-size security against collective attacks. A comprehensive security analysis was performed to cover different scenarios on how each end user prefers to trust others. In addition, with the same optical configuration, we implement an initial CV quantum network accounting for four users (two active) with discrete modulation with the conservative untrusted broadcast protocol. The results establish the viability of CV quantum network for practical security with imperfect devices when broadly deployed to connect end users to existing infrastructures.

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.