Robust Covert Quantum Communication under Bounded Channel Uncertainty

TL;DR

This paper develops a robust framework for covert quantum communication over uncertain optical channels, addressing environmental variability and deriving worst-case guarantees for secure, reliable transmission.

Contribution

It introduces a novel approach to certify covertness and reliability under channel uncertainty, revealing fundamental trade-offs and providing explicit bounds on secure payloads.

Findings

Derived a closed-form lower bound on worst-case covert qubits

Identified a sharp feasibility boundary for secure transmission

Validated covertness with QuTiP simulations of a bosonic model

Abstract

Covert quantum communication is usually analyzed under idealized assumptions that channel parameters, such as transmissivity and background noise, are perfectly known and constant. In realistic optical links, including satellite, fiber, and free-space systems, these parameters vary because of environmental fluctuations, calibration noise, and estimation errors. We study covert quantum communication over compound quantum optical channels with bounded uncertainty in both transmissivity and thermal noise, and derive guarantees that hold for all admissible channel realizations. We develop a robust framework for certifying both covertness and reliability under uncertainty. A central finding is that robustness cannot be obtained by simply inserting worst-case parameter values into known-channel bounds: the channel realizations that are most adverse for covertness and reliability generally occur at different corners of the uncertainty set. This creates a fundamental trade-off in secure system design. We derive a closed-form lower bound on the worst-case guaranteed number of covert qubits that can be transmitted reliably, identify a sharp feasibility boundary beyond which the guaranteed payload drops to zero, and quantify the security penalty caused by uncertainty. We validate the covertness term with QuTiP simulations of a four-mode bosonic model and combine it with an analytical reliability bound to evaluate the robust payload. Our results move covert quantum communication from nominal perfect-knowledge analysis to certified worst-case operation under uncertainty.