Abstract:
Non local correlations, which was a long standing foundational topic in quantum information, have recently found application as a resource for cryptographic tasks where not all devices are trusted. For example, the asymmetric phenomena of Einstein-Podolsky-Rosen steering plays a key role in one-sided device-independent quantum key distribution (1sDI-QKD) protocols. In the context of continuous-variable (CV) QKD, we identify all Gaussian protocols that can be 1sDI and calculate their maximum loss tolerance. Surprisingly, this includes a protocol that uses only coherent states. We also establish a direct link between the relevant EPR steering inequality and the secret key rate, further strengthening the relationship between these asymmetric notions of non locality and device independence. We experimentally implement both entanglement-based and coherent-state protocols, and measure the correlations necessary for 1sDI key distribution up to an applied loss equivalent to 7.5 km and 3.5 km of optical fibre transmission respectively.
Entanglement distillation is an essential feature in the extended quantum communication networks. Distillation protocols are necessarily non-deterministic and necessitate advanced experimental techniques such as noiseless amplification. Recently, it was demonstrated that the benefits of noiseless amplification could be extracted by performing a post-selective filtering of the measurement record to improve the performance of quantum key distribution. We apply this protocol to entanglement degraded by transmission loss of up to the equivalent of 100 km of optical fibre. We measure an effective entangled resource stronger than that achievable by even a maximally entangled resource passively transmitted through the same channel. The measurement-based noiseless linear amplifier offers two advantages over its physical counterpart: easy implementation and near-optimal probability of success. It can provide an effective and versatile tool for a broad class of entanglement-based quantum communication protocols.