A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of electromagnetically induced transparency, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for coherently transferring a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of nitrogen-vacancy centers and approaches 99% for cold atomic ensemble, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime in order to efficiently combine superconducting and optical components in quantum networks.
A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of electromagnetically induced transparency, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for coherently transferring a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of nitrogen-vacancy centers and approaches 99% for cold atomic ensemble, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime in order to efficiently combine superconducting and optical components in quantum networks.
Quantum conversion between microwave and optical photons is an important area of quantum information science and technology. Here we propose a one-step method to realize a coherent conversion between a microwave cavity and an optical cavity utilizing a nitrogen-vacancy center ensemble (NV ensemble). An NV ensemble considered here acts as a quantum transducer, dispersively coupled to the microwave and optical cavities, enabling transfer of the quantum state and generating entangled states between microwave and optical cavities. With state-of-the-art technology, the numerical simulation is performed to demonstrate that cat states and Fock states can be high-fidelity transferred and a maximally entangled state can be efficiently synthesized between microwave and optical cavities.
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.
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.
Abstract We propose an efficient scheme for microwave-to-optical quantum frequency conversion mediated by a single atom simultaneously coupled to a superconducting microwave resonator and an optical cavity. The atom mediates efficient interactions between the two electromagnetic modes via non-linear frequency mixing owing to the presence of huge microwave transition dipole moments between nearby Rydberg states as well as suitable optical transitions between low-lying states. In the regime of four-wave mixing with off-resonant fields, the excited states can be adiabatically eliminated, leading to an effective Hamiltonian that directly describes the two-mode interaction responsible for the frequency conversion. However, in this regime, the converter is slow, hence sensitive to losses due to cavity decays. To enhance the performance of the conversion process, we employ pulse engineering to tailor the atom-field couplings. The resulting protocol exhibits high robustness against decoherence and parameter fluctuations, making it well suited for realistic experimental implementations. Our scheme provides a promising route toward high-fidelity and scalable hybrid quantum interfaces connecting superconducting and optical quantum networks.
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.
A complete physical approach to quantum information requires a robust interface among flying qubits, long-lifetime memory, and computational qubits. Here we present a unified interface for microwave and optical photons, potentially connecting engineerable quantum devices such as superconducting qubits at long distances through optical photons. Our approach uses an ultracold ensemble of atoms for two purposes: quantum memory and to transduce excitations between the two frequency domains. Using coherent control techniques, we examine an approach for converting and storing quantum information between microwave photons in superconducting resonators, ensembles of ultracold atoms, and optical photons, as well as a method for transferring information between two resonators.