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arXiv:2503.10842 · Quantum Science and Technology

Distilled remote entanglement between superconducting qubits across optical channels

Nicolas Dirnegger, Moein Malekakhlagh, Vikesh Siddhu, Ashutosh Rao, Chi Xiong, Muir Kumph, Jason S. Orcutt, Abram L. Falk

Published Dec 5, 2025• 0 citations
arXivHTMLPDF

Abstract

Abstract A promising quantum computing architecture comprises modules of superconducting quantum processors linked via optical channels using quantum transducers. As quantum transducer hardware improves, a need has arisen to understand the quantitative relationship between transducer-device characteristics and the strength of the resulting remote entanglement. Using Monte Carlo simulations that incorporate 2-to-1 and 3-to-1 entanglement distillation methods, our model maps transducer device performance up to system-level channel performance, thereby allowing the performance of remote entanglement approaches to be compared and optimized. We find the extreme photon loss (EPL) distillation protocol to be particularly high performing. Moreover, even without distillation, present-day transducers with added noise of N add = 0.5 photons are at the threshold of enabling remote Bell pairs with fidelities exceeding 50%. If the next generation of transducers can improve by 3 orders of magnitude in added noise, efficiency, and repetition rates, then they would allow for remote two-qubit gates achieving 99.7% fidelities at MHz rates. These results set practical targets for transducers to be ready for deployment into modular quantum computing systems.

DOI
10.1088/2058-9565/ae2884

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Abstract A promising quantum computing architecture comprises modules of superconducting quantum processors linked via optical channels using quantum transducers. As quantum transducer hardware improves, a need has arisen to understand the quantitative relationship between transducer-device characteristics and the strength of the resulting remote entanglement. Using Monte Carlo simulations that incorporate 2-to-1 and 3-to-1 entanglement distillation methods, our model maps transducer device performance up to system-level channel performance, thereby allowing the performance of remote entanglement approaches to be compared and optimized. We find the extreme photon loss (EPL) distillation protocol to be particularly high performing. Moreover, even without distillation, present-day transducers with added noise of N add = 0.5 photons are at the threshold of enabling remote Bell pairs with fidelities exceeding 50%. If the next generation of transducers can improve by 3 orders of magnitude in added noise, efficiency, and repetition rates, then they would allow for remote two-qubit gates achieving 99.7% fidelities at MHz rates. These results set practical targets for transducers to be ready for deployment into modular quantum computing systems.
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A promising quantum computing architecture comprises modules of superconducting quantum processors linked via optical channels using quantum transducers. As quantum transducer hardware improves, a need has arisen to understand the quantitative relationship between transducerdevice characteristics and the strength of the resulting remote entanglement. Using Monte Carlo simulations that incorporate 2-to-1 and 3-to-1 entanglement distillation methods, our model maps transducer device performance up to system-level channel performance, thereby allowing the performance of remote entanglement approaches to be compared and optimized. We find the extreme photon loss (EPL) distillation protocol to be particularly high performing. Moreover, even without distillation, present-day transducers with added noise of Nadd = 0.5 photons are at the threshold of enabling remote Bell pairs with fidelities exceeding 50%. If the next generation of transducers can improve by 3 orders of magnitude in added noise, efficiency, and repetition rates, then they would allow for remote two-qubit gates achieving 99.7% fidelities at MHz rates. These results set practical targets for transducers to be ready for deployment into modular quantum computing systems.
Distributed quantum computing can potentially address the scalability challenge by networking processors through photon-mediated remote entanglement. Prior approaches assumed that remote Bell pairs require distillation before use, incurring substantial overhead, to achieve sufficiently high fidelity. However, recent results show that lattice-surgery operations at logical qubit boundaries tolerate significantly higher error rates than previously assumed. We quantify the resource trade-offs between distillation overhead and surface-code distance requirements under realistic constraints including probabilistic entanglement generation and memory decoherence. We identify the fidelity crossover point separating the two regimes. Below this threshold, the distillation strategy dominates, reducing resource overhead by up to two orders of magnitude. Above it, no-distillation becomes the more efficient choice, reducing resource overhead by more than half. We briefly describe the application of these methods to ion-trap and neutral-atom platforms. These results provide joint design guidelines for optimizing photonic interconnects and fault-tolerant architectures in distributed quantum computing.
Quantum networks linking quantum computers are predicted to be extremely useful for a wide range of applications. At the hardware level, superconducting quantum computers have emerged as one of the most promising scalable platforms for general quantum computation, and optical networks are clearly the best choice for long distance communication due to their minimal loss. To leverage the strengths of these two dissimilar platforms, quantum transducers, which are reachingquantum-enabled operation, are needed. This thesis presents progress towards designing a long distance optical network which will entangle two superconducting qubits and the implementation of the optical portion of this network. We have found simple, application independent, necessary and sufficient conditions for transducers to work in quantum networks, and we conclude that some previously demonstrated devices are ready for integration into a quantum network. Optimizing the integration of two transducers into a network is explored and it is found that the most robust topology to transmission loss consists of generating optical Gaussian entanglement and then using the two transducers to down-convert this entanglement to the microwave domain. To implement this topology we have constructed a tunable source of optical two-mode squeezing and two ultra-low noise photodetectors. Finally a new method for transferring and distilling Gaussian entanglement to qubits using the dispersive Jaynes-Cummings interaction is presented. This proof-of-principle two-node network should be able to entangle superconducting qubits at a rate of 5 ebits/second over 4 km of optical fiber using currently available technology.
Abstract Connecting superconducting quantum processors to telecommunications-wavelength quantum networks is critically necessary to enable distributed quantum computing, secure communications, and other applications. Optically-mediated entanglement heralding protocols offer a near-term solution that can succeed with imperfect components, including sub-unity efficiency microwave-optical quantum transducers. The viability and performance of these protocols relies heavily on the properties of the transducers used: the conversion efficiency, resonator lifetimes, and added noise in the transducer directly influence the achievable entanglement generation rate and fidelity of an entanglement generation protocol. Here, we use an extended Butterworth–van Dyke (BVD) model to optimize the conversion efficiency and added noise of a Thin film bulk acoustic resonator (FBAR) piezo-optomechanical transducer. We use the outputs from this model to calculate the fidelity of one-photon and two-photon entanglement heralding protocols in a variety of operating regimes. For transducers with matching circuits designed to either minimize the added noise or maximize conversion efficiency, we theoretically estimate that entanglement generation rates of greater than 160 kHz can be achieved at moderate pump powers with fidelities of > 90 % . This is the first time a BVD equivalent circuit model is used to both optimize the performance of an FBAR transducer and to directly inform the design and implementation of an entanglement generation protocol. These results can be applied in the near term to realize quantum networks of superconducting qubits with realistic experimental parameters.
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.