Nicolò Lo Piparo

Bell nonlocality is of broad interest not only because of its foundational significance in quantum theory, but also due to its applications in quantum technologies such as device-independent quantum key distribution (DI-QKD) and quantum randomness expansion. However, it is still challenging to obtain large Bell violation for long distance in experiment. In this work, we propose a loophole-free Bell test protocol using continuous-variable (CV) codes and cavity-QED system. We evaluate the performance of this protocol by calculating the CHSH parameter S and the secret key rate (SKR) in DI-QKD scenario. Various experimental imperfections are considered in the numerical calculation to provide insight into the feasibility and reliability of this protocol. Our results show that Bell violations are achievable up to approximately 18 km with a decent entanglement distribution rate and the SKR for DI-QKD can reach near 100 bits/s at 10 km, significantly outperforming the existing protocols. All the elements in this protocol are reachable with current technologies, so we believe this approach using CV codes provides a feasible and promising route towards practical realization of long-distance Bell nonlocality and secure DI-QKD systems.

Measurement-independent quantum key distribution (MDI-QKD) over probabilistic quantum repeaters (QRs) is addressed. We calculate, under practical assumptions, the secret key generation rate, as the main figure of merit, to estimate the performance of such protocols. First, we consider an MDI-QKD phase encoding scheme having a coherent state as the source on the one side and an imperfect single-photon source, with a nonzero double photon probability, on the other side. We compare this system with the case when both parties have an imperfect single-photon source. For this system we calculate the key rate versus the distance. Then, we combine MDI-QKD and QRs protocols, by introducing quantum memories (QMs) in the original MDI-QKD scheme. We, first, generate entangled states between the QMs using the protocol proposed by Sangouard et. al. in [Phys. Rev. A 76, 050301 (2007)]. We calculate the key rate of such a protocol versus the distance up to two nesting levels. We consider various sources of imperfection in both protocols, such as dark counts in detectors, and inefficiencies in the channel, photodetectors and memories.

Three network architectures, compatible with passive optical networks, for future hybrid quantum-classical networks are proposed and compared. These setups rely on three different schemes for quantum key distribution (QKD): BB84, entanglement-based QKD, and measurement-device-independent QKD (MDI-QKD). It turns out that, while for small-to-moderate-size networks BB84 supports the highest secret key generation rate, it may fail to support large numbers of users. Its cost implications are also expected to be higher than other setups. For large networks, MDI-QKD offers the highest key rate if fast single-photon detectors are employed. Entanglement-based networks offer the longest security distance among the three setups. MDI-QKD is, however, the only architecture resilient to detection loopholes and possibly the most favorable with its less demanding end-user technology.Entanglement-based and MDI-QKD setups can both be combined with quantum repeater systems to allow for long-distance QKD with no trust constraints on the service provider.