Quantum-Processing-Assisted Classical Communications

We describe a general quantum receiver protocol that maps laser-light-modulated classical communications signals into quantum processors for decoding with quantum logic. The quantum logic enables joint quantum measurements over a codeword to achieve the quantum limit of communications capacity. Our receiver design requires only logarithmically increasing qubit resources with the size of the codeword and accommodates practically relevant coherent-state modulation containing multiple photons per pulse. Focusing on classical-quantum polar codes, we outline the necessary quality of quantum operations and codeword lengths to demonstrate a quantum processing-enhanced communications rate surpassing that of any known classical optical receiver-decoder pair. Specifically, we show that a small quantum receiver of 4 qubits with operational errors of $\sim 0.2\%$ can already provide a $5$ percent gain in the communications rate in the weak signal limit. Additionally, we outline a possible hardware implementation of the receiver where efficient spin-photon interfaces such as cavity-coupled diamond color centers or atomic qubits are used to input the received photonic signal to a small scale quantum processor for decoding. Our results outline a new, promising route for potential quantum advantage in classical communication with near-term, small-scale quantum computers.