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Light-matter Interactions for Memory-assisted Microwave-to-Optical Transducers

Ujjwal Gautam

Published Sep 2, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.11575/prism/51941

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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 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 171Yb3+^{171}{\rm Yb}^{3+}171Yb3+:Y2SiO5{\rm Y}_2{\rm SiO}_5Y2​SiO5​ crystal at 30\,mK temperature. We achieve on-demand transduction assisted by memory with 0.4 (and 0.3)0.4\ (\text{and }0.3)0.4 (and 0.3) noise photons in the detection window at a storage duration of 460 (and 620) μs460\ (\text{and }620) \, μ\textrm{s}460 (and 620)μs. 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.
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.
Long distance transmission of quantum information is a central ingredient of distributed quantum information processors for both computing and secure communication. Transmission between superconducting/solid-state quantum processors necessitates transduction of individual microwave photons to optical photons. Current approaches to transduction employ solid state links between electrical and optical domains, facing challenges from the thermal noise added by the strong classical pumps required for high conversion efficiency and bandwidth. Neutral atoms are an attractive alternative transducer: they couple strongly to optical photons in their ground states, and to microwave/millimeter-wave photons in their Rydberg states. Nonetheless, strong coupling of atoms to both types of photons, in a cryogenic environment to minimize thermal noise, has yet to be achieved. Here we demonstrate quantum-limited transduction of millimeter-wave (mmwave) photons into optical photons using cold 85^{85}85Rb atoms as the transducer. We achieve this by coupling an ensemble of atoms simultaneously to a first-of-its-kind, optically accessible three-dimensional superconducting resonator, and a vibration suppressed optical cavity, in a cryogenic (555 K) environment. We measure an internal conversion efficiency of 58(11)%58(11)\%58(11)%, a conversion bandwidth of 360(20)360(20)360(20) kHz and added thermal noise of 0.60.60.6 photons, in agreement with a parameter-free theory. Extensions to this technique will allow near-unity efficiency transduction in both the mmwave and microwave regimes. More broadly, this state-of-the-art platform opens a new field of hybrid mmwave/optical quantum science, with prospects for operation deep in the strong coupling regime for efficient generation of metrologically or computationally useful entangled states and quantum simulation/computation with strong nonlocal interactions.
Scaling superconducting quantum processors beyond single dilution refrigerators requires efficient optical interconnects, yet integrating microwave-to-optical (M2O) transducers poses challenges due to frequency mismatches and qubit decoherence. We propose a modular architecture using SNAIL-based parametric coupling to interface Brillouin M2O transducers with long-lived 3D cavities, while maintaining plug-and-play compatibility. Through numerical simulations incorporating realistic noises, including laser heating, propagation losses, and detection inefficiency, we demonstrate raw entangled bit fidelities of F~0.8 at kHz-level rates over 30 km using the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. Implementing asymmetric entanglement pumping tailored to amplitude damping errors, we achieve purified fidelities F~0.94 at 0.2 kHz rates. Our cavity-based approach outperforms transmon schemes, providing a practical pathway for distributed superconducting quantum computing.
Abstract Optical quantum networks can connect distant quantum processors to enable secure quantum communication and distributed quantum computing. Superconducting qubits are a leading technology for quantum information processing but cannot couple to long-distance optical networks without an efficient, coherent, and low noise interface between microwave and optical photons. Here, we demonstrate a microwave-to-optical transducer using an ensemble of erbium ions that is simultaneously coupled to a superconducting microwave resonator and a nanophotonic optical resonator. The coherent atomic transitions of the ions mediate the frequency conversion from microwave photons to optical photons and using photon counting we observed device conversion efficiency approaching 10 −7 . With pulsed operation at a low duty cycle, the device maintained a spin temperature below 100 mK and microwave resonator heating of less than 0.15 quanta.