Key Assistance, Key Agreement, and Layered Secrecy for Bosonic Broadcast Channels

TL;DR

This paper investigates quantum broadcast channels for secret sharing, determining capacity regions with and without keys, and introduces layered secrecy for multi-user bosonic networks, advancing quantum communication security understanding.

Contribution

It provides the first capacity region characterizations for bosonic broadcast channels with secret sharing and layered secrecy, including key-assisted scenarios and finite-dimensional formulas.

Findings

Confidential capacity region determined for pure-loss bosonic channels.

Achievable region established for lossy bosonic broadcast channels.

Layered secrecy region derived for multi-user bosonic networks.

Abstract

Secret-sharing building blocks based on quantum broadcast communication are studied. The confidential capacity region of the pure-loss bosonic broadcast channel is determined, both with and without key assistance, and an achievable region is established for the lossy bosonic broadcast channel. If the main receiver has a transmissivity of \eta<1/2, then confidentiality solely relies on the key-assisted encryption of the one-time pad. We also address conference key agreement for the distillation of two keys, a public key and a secret key. A regularized formula is derived for the key-agreement capacity region in finite dimensions. In the bosonic case, the key-agreement region is included within the capacity region of the corresponding broadcast channel with confidential messages. We then consider a network with layered secrecy, where three users with different security ranks communicate over the same broadcast network. We derive an achievable layered-secrecy region for a pure-loss bosonic channel that is formed by the concatenation of two beam splitters.