Quantum capacities of transducers

TL;DR

This paper introduces a unified quantum capacity metric for quantum transducers, analyzing optimal designs and effects of thermal noise, to improve quantum information transfer efficiency.

Contribution

It proposes using quantum capacity as a single figure of merit for transducers and identifies optimal transducer designs with flat frequency response under physical constraints.

Findings

Maximum quantum capacity is approximately 5 times the maximal coupling rate.

Maximally flat frequency response transducers achieve optimal capacity.

Thermal noise negatively impacts transducer performance.

Abstract

High-performance quantum transducers, which faithfully convert quantum information between disparate physical carriers, are essential in quantum science and technology. Different figures of merit, including efficiency, bandwidth, and added noise, are typically used to characterize the transducers' ability to transfer quantum information. Here we utilize quantum capacity, the highest achievable qubit communication rate through a channel, to define a single metric that unifies various criteria of a desirable transducer. Using the continous-time quantum capacities of bosonic pure-loss channels as benchmarks, we investigate the optimal designs of generic quantum transduction schemes implemented by transmitting external signals through a coupled bosonic chain. With physical constraints on the maximal coupling rate $g_{max}$, the highest continuous-time quantum capacity $Q^{max} \approx 5 g_{max}$ is achieved by transducers with a maximally flat conversion frequency response, analogous to Butterworth electric filters. We further investigate the effect of thermal noise on the performance of transducers.