Fundamental Limits of Repeaterless Quantum Communications

TL;DR

This paper establishes the ultimate limits of repeaterless quantum communication capacities for various channels, providing fundamental benchmarks for secure quantum key distribution and quantum networks.

Contribution

It introduces a novel dimension-independent technique called teleportation stretching to precisely determine capacities of key quantum channels without repeaters.

Findings

Derived upper bounds on quantum communication capacities.

Established fundamental rate-loss trade-offs for quantum key distribution.

Provided the most general benchmarks for quantum repeaters.

Abstract

Quantum communications promises reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure keys. For all these tasks, we need to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, without restrictions on their local operations and classical communication, which can be unlimited and two-way. These two-way assisted capacities represent the ultimate rates that are reachable without quantum repeaters. By constructing an upperbound based on the relative entropy of entanglement and devising a dimension-independent technique dubbed "teleportation stretching", we establish these capacities for many fundamental channels, namely bosonic lossy channels, quantum-limited amplifiers, dephasing and erasure channels in arbitrary dimension. In particular, we determine the fundamental rate-loss trade-off affecting any protocol of quantum key distribution. Our findings set the ultimate limits of point-to-point quantum communications and provide the most precise and general benchmarks for quantum repeaters.