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arXiv:2602.00928

Electro-optic conversion of itinerant Fock states

Thomas Werner, Erfan Riyazi, Samarth Hawaldar, Rishabh Sahu, Georg Arnold, Paul Falthansl-Scheinecker, Jennifer A. Sánchez Naranjo, Dante Loi, Lucky N. Kapoor, Martin Zemlicka, Liu Qiu, Andrei Militaru, Johannes M. Fink

quant-phSubmitted Jan 31, 2026• Announced Feb 3, 2026• 0 citations
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Abstract

Superconducting qubits are a leading candidate for utility-scale quantum computing due to their fast gate speeds and steadily decreasing error rates. The requirement for millikelvin operating temperatures, however, creates a significant scaling bottleneck. Modular architectures using optical fiber links could bridge separate cryogenic nodes, but superconducting circuits do not have coherent optical transitions and microwave-to-optical conversion has not been shown for any non-classical photon state. In this work, we demonstrate the on-demand generation and tomographic reconstruction of itinerant single microwave photons at 8.9 GHz from a superconducting qubit. We upconvert this non-Gaussian state with a transducer added noise below 0.012 quanta and count the converted telecom photons at 193.4 THz with a signal-to-noise ratio of up to 5.1$\pm$1.1. We characterize the trade-offs between throughput and noise, and establish a viable path toward heralded entanglement distribution and gate teleportation. Looking ahead, these results empower existing superconducting devices to take a key role in distributed quantum technologies and heterogeneous quantum systems.

AI Summary

The paper addresses microwave-to-telecom conversion of itinerant non-Gaussian states for interconnecting superconducting quantum processors. A transmon–cavity source emits single microwave photons at $8.9006\,\mathrm{GHz}$, which travel through a coaxial link to a separate lithium-niobate electro-optic transducer. Synchronized $200\,\mathrm{ns}$ optical pump pulses convert the photons to $193.4\,\mathrm{THz}$; cascaded optical filters and a superconducting nanowire detector resolve the converted emission. Microwave heterodyne tomography establishes Wigner negativity of the emitted single-photon and vacuum–single-photon superposition states, with reported fidelities of $97.6\%$ and $99.7\%$. Optical photon-counting measurements recover the emission envelope and oscillations versus the source qubit’s preparation angle. Interleaved signal/vacuum measurements give a maximum conversion signal-to-noise ratio of $5.1\pm1.1$. External transducer efficiency is $2.2\times10^{-4}$, while the measured source-to-detection probability is $(1.32\pm0.03)\times10^{-5}$ per pulse. Independent microwave noise spectroscopy and optical counting identify input-referred transducer added noise of approximately $0.012$ quanta at $1\,\mathrm{kHz}$, rising to $0.12$ at $20\,\mathrm{kHz}$ as optical absorption heats the microwave mode. The optical output is not tomographically reconstructed: antibunching, phase-coherence preservation, and qubit–optical-photon entanglement remain future measurements. Low conversion efficiency, microwave loss, temporal-mode mismatch, fiber-generated optical noise, and heating limit performance. The quoted $87.7\%$ Bell fidelity is a conditional projection assuming perfect input entanglement and coherent conversion; the approximately $2.9\,\mathrm{s}^{-1}$ capacity figure is an upper bound, not a demonstrated entanglement-distribution rate.

Little sign of AI-generated textOverclaimsPossible error

Error found: The paper presents a calculated quantum-channel-capacity upper bound of approximately 2.91 Hz as a current capacity and uses it to support positivity, although an upper bound alone establishes neither an achievable rate nor positive capacity. The electro-optic vacuum coupling g_0/(2π) is reported inconsistently as 4.2 Hz in the implementation section and 2.9 Hz in the device-parameter table.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

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16 pages, 11 figures

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The paper addresses microwave-to-telecom conversion of itinerant non-Gaussian states for interconnecting superconducting quantum processors. A transmon–cavity source emits single microwave photons at 8.9006 GHz8.9006\,\mathrm{GHz}8.9006GHz, which travel through a coaxial link to a separate lithium-niobate electro-optic transducer. Synchronized 200 ns200\,\mathrm{ns}200ns optical pump pulses convert the photons to 193.4 THz193.4\,\mathrm{THz}193.4THz; cascaded optical filters and a superconducting nanowire detector resolve the converted emission.
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The optical output is not tomographically reconstructed: antibunching, phase-coherence preservation, and qubit–optical-photon entanglement remain future measurements. Low conversion efficiency, microwave loss, temporal-mode mismatch, fiber-generated optical noise, and heating limit performance. The quoted 87.7%87.7\%87.7% Bell fidelity is a conditional projection assuming perfect input entanglement and coherent conversion; the approximately 2.9 s−12.9\,\mathrm{s}^{-1}2.9s−1 capacity figure is an upper bound, not a demonstrated entanglement-distribution rate.
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.
The rapid development of superconducting quantum hardware is expected to run into substantial restrictions on scalability because error correction in a cryogenic environment has stringent input-output requirements. Classical data centres rely on fibre-optic interconnects to remove similar networking bottlenecks. In the same spirit, ultracold electro-optic links have been proposed and used to generate qubit control signals, or to replace cryogenic readout electronics. So far, these approaches have suffered from either low efficiency, low bandwidth or additional noise. Here we realize radio-over-fibre qubit readout at millikelvin temperatures. We use one device to simultaneously perform upconversion and downconversion between microwave and optical frequencies and so do not require any active or passive cryogenic microwave equipment. We demonstrate all-optical single-shot readout in a circulator-free readout scheme. Importantly, we do not observe any direct radiation impact on the qubit state, despite the absence of shielding elements. This compatibility between superconducting circuits and telecom-wavelength light is not only a prerequisite to establish modular quantum networks, but it is also relevant for multiplexed readout of superconducting photon detectors and classical superconducting logic.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.
Superconducting quantum processors operate at microwave frequencies in millikelvin environments, making it challenging to interconnect distant nodes using conventional microwave wiring. Coherent microwave-to-optical (M2O) transduction enables superconducting quantum networks by interfacing itinerant microwave photons with low-loss optical fiber. However, many state-of-the-art transducers provide efficient conversion only over a narrow frequency span, complicating deployment with heterogeneous superconducting devices that are detuned by gigahertz-scale offsets. Here we demonstrate a frequency-agile microwave-optical interface that overcomes this bandwidth mismatch by cascading an electro-optic M2O transducer with a multimode microwave-to-microwave (M2M) frequency converter, with in situ tunability of the microwave resonances in both stages. Using this architecture, we realize continuous frequency coverage from 5.0 to 8.5 GHz within a single system. As an application relevant to superconducting-qubit networking, we use the cascaded M2M-M2O interface to optically read out a superconducting qubit whose readout resonator is detuned by 1.7 GHz from the native M2O microwave resonance, demonstrating a scalable route toward fiber-linked superconducting quantum nodes.
Microwave photonics lends the advantages of fiber optics to electronic sensing and communication systems.In contrast to nonlinear optics, electro-optic devices so far require classical modulation fields whose variance is dominated by electronic or thermal noise rather than quantum fluctuations.Here we demonstrate bidirectional single-sideband conversion of X band microwave to C band telecom light with a microwave mode occupancy as low as 0.025 ± 0.005 and an added output noise of less than or equal to 0.074 photons.This is facilitated by radiative cooling and a triply resonant ultra-low-loss transducer operating at millikelvin temperatures.The high bandwidth of 10.7 MHz and total (internal) photon conversion efficiency of 0.03% (0.67%) combined with the extremely slow heating rate of 1.1 added output noise photons per second for the highest available pump power of 1.48 mW puts near-unity efficiency pulsed quantum transduction within reach.Together with the non-Gaussian resources of superconducting qubits this might provide the practical foundation to extend the range and scope of current quantum networks in analogy to electrical repeaters in classical fiber optic communication.
Quantum frequency conversion between superconducting (SC) microwave qubits and telecom optical photons is critical for long distance communication of networked SC quantum processors. While SC qubits operate at cryogenic temperatures to sustain their quantum coherence, converting them to the optical domain enables transferring the quantum states to room temperature and over long distances. For such a quantum state transduction process, several schemes have been investigated, including optomechanics, magnons, piezomechanics, and Pockels electro-optics (EO). The EO conversion approach is particularly attractive since it is broadband, low noise, mechanically and thermally stable (i.e., does not rely on freestanding structures), scalable (largescale integration of EO devices with superconducting circuits is possible), and tunable (e.g., using bias voltages). In this presentation we review our progress in an electro-optic based quantum transduction and discuss the promises and challenges.