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arXiv:2605.01239 · Nature Communications

Memory-assisted multimode microwave-to-optical transduction

Ujjwal Gautam, Nasser Gohari Kamel, Sourabh Kumar, Daniel Oblak

quant-phSubmitted May 2, 2026• Announced May 5, 2026• 0 citations
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Abstract

Microwave-to-optical quantum transducers will enable coherent interconnection between distant superconducting quantum devices. Ongoing explorations with several platforms have shown promising results at single-photon levels. However, in all these demonstrations, elimination of noise due to the concurrence of the weak transduced signal with intense pump pulses remains a challenge, requiring high suppression filtering setups. A memory-assisted transducer, on the other hand, offers a versatile approach that not only mitigates the noise but also enables the on-demand retrieval of the transduced signal. Here, we integrate a quantum memory protocol with transduction in a three-level atomic system to demonstrate on-demand retrieval of transduced signals. Due to the zero-first-order Zeeman transitions at zero magnetic fields, providing long optical and spin coherence times, and GHz range hyperfine splitting, we use a low-doping concentration $^{171}{\rm Yb}^{3+}$:${\rm Y}_2{\rm SiO}_5$ crystal at 30\,mK temperature. We achieve on-demand transduction assisted by memory with $0.4\ (\text{and }0.3)$ noise photons in the detection window at a storage duration of $460\ (\text{and }620) \, μ\textrm{s}$. To demonstrate the coherent nature of the protocol, we show interference patterns resulting from transduced signals due to varying phase or frequency of the input microwave pulses. Further, multimode transduction capacity is demonstrated, utilizing the spin and optical inhomogeneous broadening. The on-demand capability of the protocol allows synchronizing qubits in a quantum repeater protocol, while multimode capacity increases the entanglement generation rate. To the best of our knowledge, this is the first demonstration of an on-demand microwave-to-optical transducer assisted by memory.

Comments
22 pages, 19 figures

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Quantum technologies harness superposition and entanglement for computation, communication, and sensing. Superconducting quantum circuits (SQCs) are promising for scalable quantum information processing, but networking distant SQC nodes is challenging: microwave photons suffer severe thermal noise and loss at room temperature, while optical photons travel through fiber with negligible loss. A quantum transducer bridges this gap by converting microwave photons to optical photons with minimal added noise. This thesis presents a memory-assisted microwave-to-optical (M2O) transducer using an ensemble of Yb-ions doped in Y2SiO5 (YSO). Yb:YSO offers zero-first-order Zeeman (ZEFOZ) transitions at zero field, giving long coherence times, and GHz-range hyperfine splittings that interface directly with SQCs without an external magnetic field. The platform integrates a custom loop-gap resonator (LGR) with a free-space optical setup at 30 mK in a dilution refrigerator. The LGR, designed via CAD and COMSOL simulation and machined from oxygen-free copper, concentrates the microwave field for strong coupling to the spin ensemble. A Python-based automation framework synchronizes instrumentation for high-throughput acquisition of pulse sequences. Coherence is characterized via optical excited-state lifetime, spin population time, and optical coherence time. Optically detected magnetic resonance reveals coherent microwave-spin dynamics, including Rabi oscillations with collapse-and-revival features from spin inhomogeneity and cavity photon-number statistics. Continuous-wave M2O transduction at 2623 MHz achieves an internal efficiency of 2e-8 over 200 kHz bandwidth. Avoided-crossing measurements confirm strong spin-microwave coupling, with cooperativity exceeding unity. A theoretical framework links efficiency to microwave, optical, and atomic cooperativities, projecting ~1e-4 efficiency through improved doping, beam diameter, detuning, and cavity integration. A memory-assisted M2O protocol integrates quantum storage via rapid adiabatic passage phase imprinting, enabling on-demand retrieval separated from the pump and suppressing pump noise without lossy filtering. At 460 μs storage, this achieves 4e-8 efficiency with 0.4 noise photons, among the lowest noise figures reported for M2O transducers. Interference measurements confirm coherent information transfer, and multiplexing of up to ten transduced modes is demonstrated, exploiting Yb:YSO's broad inhomogeneous linewidths. This work, the first memory-assisted M2O interface, enables photon synchronization for path-erasure entanglement between remote SQCs, advancing scalable quantum repeaters and distributed quantum computing.
Microwave-to-optical quantum transducers are essential for entangling remote superconducting qubits. Among the available transduction platforms, ensembles of Er3+^{3+}3+ and Yb3+^{3+}3+ ions doped into solids have emerged as leading candidates. While external magnetic fields are needed to split the Zeeman levels of erbium ions and enable a microwave--qubit interface, superconducting qubits suffer decoherence in such fields. In contrast, ytterbium ions exhibit zero-first-order Zeeman transitions and large hyperfine splittings at zero magnetic field (when doped into inorganic crystals). Owing to its long optical and spin coherence times, Yb:YSO has been widely used as a quantum memory, yet its potential for quantum transduction remains largely unexplored. Investigating this material could enable the integration of quantum memory and transduction in a single platform. Here, we demonstrate microwave-to-optical transduction in the continuous-wave regime using a 5\,ppm doped Yb:YSO crystal. The internal transduction efficiency is 2×10−82\times10^{-8}2×10−8 with a bandwidth of 200\,kHz, achieved using a 3D loop-gap microwave resonator (LGR) and a single-pass optical configuration. We explore all the ground states that form a V-type three-level system with the first and second optical excited states and assert the use of the ground state, which provides the highest efficiency and isolated optical transition. We further establish strong spin-microwave coupling from avoided crossing measurements. With a strong microwave drive to saturate the spin transition, we estimate the spin population pumped into the excited state, close to the simulated value. Finally, we calculate target parameter values for maximum efficiency with our system and suggest using 50\,ppm doped Yb:YSO crystal. With the calculated target parameters, the internal transduction efficiency is predicted to reach up to 10−410^{-4}10−4 in the current LGR.
Microwave-optical transducers and quantum memories are essential for quantum repeaters enabling a quantum internet. Despite advances in both technologies, integrating these functionalities remains challenging. Here, we theoretically propose and experimentally demonstrate an on-demand microwave-optical quantum transducer based on a Rydberg ensemble. Using cascaded electromagnetically induced transparency, we store microwave photons in a highly excited collective state and convert them into optical photons during retrieval. Leveraging an optical depth of millions for microwave photons and minimal single-photon-level dephasing, our transducer achieves around 90% area-normalized storage efficiency, 2.3 MHz bandwidth, and noise-equivalent temperature of 26 K under cavity-free conditions. Furthermore, our system is cryogenically compatible and extendable for high single-photon conversion efficiency without requiring optical cavity coupling. These findings advance practical on-demand quantum interfaces with broad applications across atomic and solid-state platforms. A quantum internet would link computers over optical fibers, which would require converting their signals into frequencies that can travel by fiber and storing those signals to keep them in sync. The authors built a proof-of-concept from a Rydberg ensemble of cold rubidium atoms, that does both jobs.
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.
Abstract Optical networks that distribute entanglement among various quantum systems will form a powerful framework for quantum science but are yet to interface with leading quantum hardware such as superconducting qubits. Consequently, these systems remain isolated because microwave links at room temperature are noisy and lossy. Building long distance connectivity requires interfaces that map quantum information between microwave and optical fields. While preliminary microwave-to-optical transducers have been realized, developing efficient, low-noise devices that match superconducting qubit frequencies (gigahertz) and bandwidths (10 kilohertz – 1 megahertz) remains a challenge. Here we demonstrate a proof-of-concept on-chip transducer using trivalent ytterbium-171 ions in yttrium orthovanadate coupled to a nanophotonic waveguide and a microwave transmission line. The device′s miniaturization, material, and zero-magnetic-field operation are important advances for rare-earth ion magneto-optical devices. Further integration with high quality factor microwave and optical resonators will enable efficient transduction and create opportunities toward multi-platform quantum networks.
Hybrid quantum systems combining circuit QED with spin-doped solids are an attractive platform for distributed quantum information processing. There, the magnetic ions serve as coherent memory elements and reversible conversion elements of microwave to optical qubits. Among many possible spin-doped solids, erbium ions offer the unique opportunity for a coherent conversion of microwave photons into the telecom C band at $1.54\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ employed for long distance communication. In our work, we perform a time-resolved electron spin resonance study of an Er3+:Y2SiO5{\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ spin ensemble at millidegrees Kelvin temperatures and demonstrate multimode storage and retrieval of up to 16 coherent microwave pulses. The memory efficiency is measured to be ${10}^{\ensuremath{-}4}$ at a coherence time of ${T}_{2}=5.6\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$. We observe a saturation of the spin coherence time below 50 mK due to full polarization of the surrounding electronic spin bath.