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arXiv:2507.09873

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

Maxwell D. Urmey, S. Dickson, Kazemi Adachi, Sarang Mittal, Luca G. Talamo, Kyle, A., Nicholas Frattini, Syue-Liang Lin, Konrad W. Lehnert, Cindy A. Regal

Published Jul 14, 2025• 0 citations
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Abstract

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. We report the continuous operation of an efficient transducer at high enough bandwidth to realize kHz signal transfer rates, in downconversion as well as upconversion, with noise levels approaching that required for quantum information transfer. To define the significance of our advance and clarify the multifaceted performance required for quantum networking tasks, 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 capability and provides insight into the relative importance of disparate performance 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.

DOI
10.48550/arxiv.2507.09873

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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. We report the continuous operation of an efficient transducer at high enough bandwidth to realize kHz signal transfer rates, in downconversion as well as upconversion, with noise levels approaching that required for quantum information transfer. To define the significance of our advance and clarify the multifaceted performance required for quantum networking tasks, 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 capability and provides insight into the relative importance of disparate performance 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.
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 continuous-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 [Formula: see text], the highest continuous-time quantum capacity [Formula: see text] 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.
The demonstration of a quantum link between microwave and optical frequencies would be an important step toward the realization of a quantum network of superconducting processors.A major impediment to quantum electro-optic transduction in all platforms explored to date is noise added by thermal occupation of modes involved in the transduction process, and it has proved difficult to realize low thermal occupancy concurrently with other desirable features like high duty cycle and high efficiency.In this work, we present an efficient and continuously operating electro-optomechanical transducer whose mechanical mode has been optically sideband cooled to its quantum ground state.The transducer achieves a maximum efficiency of 47% and minimum input-referred added noise of 3.2 photons in upconversion.Moreover, the thermal occupancy of the transducer's microwave mode is minimally affected by continuous laser illumination with power more than 2 orders of magnitude greater than that required for optomechanical ground-state cooling.
The development of scalable quantum networks requires coherent interfaces capable of converting microwave photons used in superconducting quantum processors into optical photons suitable for long-distance fiber transmission. This review surveys recent progress in microwave-to-optical quantum transduction across optomechanical, electro-optic, and magneto-optic platforms, with emphasis on conversion efficiency, bandwidth, added noise, and operating temperature. In addition to standard metrics, we propose the internal efficiency eta_in and the magnon decay rate kappa_m/2pi as normalized parameters that enable fairer comparison across heterogeneous implementations. Optomechanical systems achieve internal phonon-to-photon efficiencies of 93% with sub-quantum added noise of 0.25 quanta at millikelvin temperatures. Electro-optic devices based on LiNbO3 and AlN have advanced from room-temperature efficiencies below 1% to millikelvin systems with internal efficiencies approaching 99.5%, added noise as low as 0.16 quanta at 60 mK, and bandwidths extending to several tens of megahertz. Magneto-optic (optomagnonic) platforms exhibit the lowest efficiencies (typically 10−1010^{-10}10−10 to 10−8)10^{-8})10−8), but offer intrinsic non-reciprocity and broadband magnonic operation, with emerging approaches based on topological heterostructures and magnon squeezing predicting enhancements up to 10−410^{-4}10−4. Optomechanical systems appear promising for high-fidelity quantum state transfer, electro-optic transducers for high-bandwidth coherent links, and magneto-optic devices for non-reciprocal network components. We discuss the fundamental trade-off between efficiency and added noise across all three platforms, and argue that heterogeneous microwave-optical transduction is emerging as a key enabling technology for distributed quantum computing and large-scale quantum networks.
A quantum transducer converts an input signal to an output probe at a distant frequency band while maintaining the quantum information with high fidelity, which is crucial for quantum networking and distributed quantum sensing and computing. In terms of microwave–optical quantum transduction, the state-of-the-art quantum transducers suffer low transduction efficiency from weak nonlinear coupling, wherein increasing pump power to enhance efficiency inevitably leads to thermal noise from heating. Moreover, we reveal that the efficiency-bandwidth product of a cavity electro-optical or electro-optomechanical transducer is fundamentally limited by pump power and nonlinear coupling coefficient, irrespective of cavity engineering efforts. To overcome this fundamental limit, we propose to noiselessly boost the transduction efficiency by consuming intraband entanglement (e.g., microwave–microwave or optical–optical entanglement in the case of microwave–optical transduction). Via a squeezer–coupler–antisqueezer sandwich structure, the protocol enhances the transduction efficiency to unity in the ideal lossless case, given an arbitrarily weak pump and nonlinear coupling. In practical cavity systems, our entanglement-assisted protocol surpasses the non-assisted fundamental limit of the efficiency-bandwidth product and reduces the threshold cooperativity for positive quantum capacity by a factor proportional to two-mode squeezing gain. Given a fixed cooperativity, our approach increases the broadband quantum capacity by orders of magnitude. The entanglement-assisted advantage is robust to ancilla loss and cavity detuning.
State transfer between light and microwaves is a key challenge in quantum networks. Promising transducers use a mechanical intermediary that couples to both fields via radiation pressure. Such electro-optomechanical devices have achieved high efficiencies, yet require resolved-sideband cavities, and generally compromise in scalability and noise performance. Here, we relax this constraint by extending the protocol of Navarathna et al. that transfers optical quantum information onto a mechanical resonator using a broadband, sideband-unresolved cavity and feedback. Combining this with parametric mechanical-to-microwave conversion, we show that continuous optical-to-microwave quantum state transfer is possible using measurement-based feedback, while all-optical coherent feedback enables bidirectional transfer. To assess the transfer, we introduce the quantum transfer witness WT\mathcal{W}_TWT​, which -- though similar to the input-referred added noise -- also identifies whether a channel is capable of both preserving Gaussian entanglement and outperforming classical transduction schemes. Finally, we show that quantum-compatible noise performance is within reach of current experimental capabilities. Our results unlock a new design space for electro-optomechanical transducers and strengthens their candidacy as scalable quantum links between distant nodes.
Transduction of quantum signals between the microwave and the optical ranges will unlock powerful hybrid quantum systems enabling information processing with superconducting qubits and low-noise quantum networking through optical photons. Most microwave-to-optical quantum transducers suffer from thermal noise due to pump absorption. We analyze the coupled thermal and wave dynamics in electro-optic transducers that use a two-step scheme based on an intermediate frequency state in the THz range. Our analysis, supported by numerical simulations, shows that the two-step scheme operating with a continuous pump offers near-unity external efficiency with a multiorder noise suppression compared to single-step transduction. As a result, two-step electro-optic transducers may enable quantum-noise-limited interfacing of superconducting quantum processors with optical channels at MHz-scale bit rates.