Maximum tolerable excess noise in CV-QKD and improved lower bound on two-way capacities

Francesco Mele
Scuola Normale Superiore and CNR Nano
Fabio Taddei, Stefan Heun

The two-way capacities of quantum channels determine the ultimate entanglement and secret-key distribution rates achievable by two distant parties that are connected by a noisy transmission line, in absence of quantum repeaters. Since repeaters will likely be expensive to build and maintain, a central open problem of quantum communication is to understand what performances are achievable without them. In our work we find a new lower bound on the energy-constrained and unconstrained two-way quantum and secret-key capacities of all phase-insensitive bosonic Gaussian channels, which are realistic models for the noise affecting optical fibres or free-space links. Ours is the first lower bound on the two-way quantum capacity that shows explicitly that entanglement distribution is always possible when the channel is not entanglement breaking. This completely solves a crucial open problem of the field, namely, establishing the maximum excess noise which is tolerable in continuous-variable quantum key distribution. In addition, our construction is fully explicit, i.e. we devise and optimise a concrete entanglement distribution and distillation protocol that works by combining recurrence and hashing protocols. The first part of the talk will provide a general introduction to quantum communication and entanglement distillation, while the second part will focus on our recent results, which are detailed in our paper ( co-authored with Prof. Ludovico Lami and Prof. Vittorio Giovannetti.


Seminar realized in the framework of the funded projects:

- PNRR MUR project PE0000023-NQSTI
- PRIN 2017 Taming complexity via Quantum Strategies: a Hybrid Integrated Photonic approach (QUSHIP) Id. 2017SRN-BRK
- PRO3 Quantum Pathfinder