High-throughput bidirectional electro-optic transduction assessed with a practical quantum capacity

A microwave-optical transducer of sufficiently low noise and high signal transfer rate would allow entanglement to be distributed between superconducting quantum processors reliably within the lifetimes of their quantum memories. To clarify the multifaceted performance required for such a task, we derive a broadband quantum channel capacity that bounds the maximum rate at which quantum information can be sent through realistic finite-bandwidth thermal-loss channels. This capacity serves as a comprehensive measure of transducer performance and provides insight into the relative importance of disparate metrics. We find that the broadband capacity depends on the throughput -- defined as the product of efficiency, bandwidth, and duty cycle -- and on the added noise. We present measurements of a membrane-based opto-electromechanical transducer with high throughput of 7 kHz and at an input-referred added noise of 3 photons in both upconversion and downconversion, demonstrating that bidirectional transducer capacities comparable to superconducting qubit decay rates are within reach. In downconversion, throughput of this magnitude at the few-photon noise level is unprecedented, marking an improvement of nearly four orders of magnitude over previous work.