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Optics Express

Microwave-to-optical conversion via four-wave mixing in a cesium thermal vapor

Ning Ji, Yanzhao Liang, Haitao Lin, Thibault Vogt

Published Jul 27, 2026• 0 citations
Publisher

Abstract

Microwave-to-optical frequency conversion is a key enabling technology for quantum sensing of microwave fields and for hybrid quantum networks connecting superconducting processors with optical channels. Here, we demonstrate coherent microwave-to-optical conversion using four-wave mixing in a room-temperature cesium vapor. The system achieves an internal photon conversion efficiency close to 0.003 , a dynamic range exceeding 17 dB, and a wide bandwidth of 25 MHz despite using reduced powers of the auxiliary fields. Experimental results are qualitatively reproduced by a Maxwell–Bloch model incorporating Doppler averaging. Numerical analysis indicates that the conversion efficiency can potentially reach several percent by increasing the powers of the auxiliary lasers. These findings establish an experimental and theoretical framework for developing efficient, coherent, and broadband frequency conversion with cesium vapors.

DOI
10.1364/oe.607832

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Microwave-to-optical frequency conversion is a key enabling technology for quantum sensing of microwave fields and for hybrid quantum networks connecting superconducting processors with optical channels. Here, we demonstrate coherent microwave-to-optical conversion using four-wave mixing in a room-temperature cesium vapor. The system achieves an internal photon conversion efficiency close to 0.003 , a dynamic range exceeding 17 dB, and a wide bandwidth of 25 MHz despite using reduced powers of the auxiliary fields. Experimental results are qualitatively reproduced by a Maxwell–Bloch model incorporating Doppler averaging. Numerical analysis indicates that the conversion efficiency can potentially reach several percent by increasing the powers of the auxiliary lasers. These findings establish an experimental and theoretical framework for developing efficient, coherent, and broadband frequency conversion with cesium vapors.
Microwave-to-optics conversion is a core technology for hybrid quantum networks, enabling the integration of microwave and optical frequency domains essential for quantum communication and quantum information processing. However, the Doppler broadening effect in thermal atomic ensembles often severely limits the conversion efficiency. This study aims to propose a novel mechanism for microwave-to-optics conversion using four-wave mixing (FWM) in room-temperature Rydberg atoms, addressing the challenges posed by Doppler broadening and providing a theoretical framework for practical applications. We develop a theoretical model based on the coupled Maxwell-Bloch equations to describe the FWM process in a symmetric double-ladder four-level system. The density matrix method and perturbation method combined with Maxwell’s equations are used to derive an analytical expression for the coherence coefficient between the microwave field and the optical field. This coherence coefficient characterizes the energy transfer between the microwave and optical fields and is used to obtain an analytical expression for the FWM conversion efficiency. We use cesium vapor as a medium to analyze the propagation characteristics of the FWM efficiency and explore the effects of laser field intensity and the Doppler effect on the conversion efficiency. Our analysis reveals that the detuning effect caused by the thermal motion of atoms significantly reduces the resonance coupling efficiency. Specifically, when the Doppler frequency is lower than the natural linewidth, the conversion efficiency can be notably improved. In a Doppler-free environment, the conversion efficiency approaches unity at an optimal propagation distance. In contrast, in room-temperature cesium vapor (300 K), the conversion efficiency is significantly reduced due to Doppler broadening. However, by cooling the atoms to microkelvin temperatures, the Doppler broadening can be minimized, leading to a substantial increase in conversion efficiency. This study provides new theoretical guidance and experimental schemes for microwave-to-optics conversion at room temperature. The proposed mechanism based on Rydberg atoms provides a promising approach to overcoming the limitations imposed by Doppler broadening. Our findings are of great significance for advancing quantum information technology, especially in the context of developing efficient quantum networks.
Abstract Coherent microwave-to-optical conversion is crucial for transferring quantum information generated in the microwave domain to optical frequencies, where propagation losses can be minimized. Coherent, atom-based transducers have shown rapid progress in recent years. This paper reports an experimental demonstration of coherent microwave-to-optical conversion that maps a microwave signal to a large, tunable 550(30) MHz range of optical frequencies using room-temperature 87Rb atoms. The inhomogeneous Doppler broadening of the atomic vapor advantageously supports the tunability of an input microwave channel to any optical frequency channel within the Doppler width, along with the simultaneous conversion of a multi-channel input microwave field to corresponding optical channels. In addition, we demonstrate phase-correlated amplitude control of select channels, providing an analog to a frequency domain beam splitter across five orders of magnitude in frequency. With these capabilities, neutral atomic systems may also be effective quantum processors for quantum information encoded in frequency-bin qubits.
Abstract The coupling of microwave and optical systems presents an immense challenge due to the natural incompatibility of energies, but potential applications range from optical interconnects for quantum computers to next-generation quantum microwave sensors, detectors and coherent imagers. Several of the engineered platforms that have emerged are constrained by specific conditions, such as cryogenic environments, impulse protocols or narrowband fields. Here we employ Rydberg atoms that allow the wideband coupling of optical and microwave photons at room temperature with the use of a modest set-up. We present continuous-wave conversion of a 13.9 GHz field to a near-infrared optical signal using an ensemble of Rydberg atoms via a free-space six-wave mixing process designed to minimize noise interference from any nearby frequencies. The Rydberg photonic converter exhibits a conversion dynamic range of 57 dB and a wide conversion bandwidth of 16 MHz. Using photon counting, we demonstrate the readout of photons of free-space 300 K thermal background radiation at 1.59 nV cm −1 rad −1/2 s −1/2 (3.98 nV cm −1 Hz −1/2 ) with a sensitivity down to 3.8 K of noise-equivalent temperature, allowing us to observe Hanbury Brown and Twiss interference of microwave photons.
Diode-pumped alkali lasers, possessing high efficiency and narrow linewidth, can provide feasible solutions for wavelength ranges difficult to reach by commercial lasers. In this study, we investigate a generation of coherent blue light (CBL) via four-wave mixing (FWM)-based up-conversion processes in cesium (Cs) vapor. A bichromatic pumping scheme with 852- and 917-nm lasers drives the Cs atoms to the 6 D 5/2 excited level, followed by cascaded decay of 6 D 5/2 → 7 P 3/2 → 6 S 1/2 , producing 456-nm CBL under phase matching conditions. The fluorescence in multiple bands from blue to near- and far-infrared in the FWM process is demonstrated under different experimental conditions. To optimize the experimental parameters, we investigate the dependence of 456-nm CBL on the vapor temperature, frequency, and intensity of the two pump lasers. A maximum power of 2.94 mW is achieved with pump powers of 430 mW (for 852 nm) and 470 mW (for 917 nm). The corresponding conversion efficiency is 1.5%/W, three-fold higher than those in previous studies. Our results can contribute to fundamental research on atom—photon interactions and quantum metrology.
Abstract Frequency conversion between microwave and optical photons is a key enabling technology to create links between superconducting quantum processors and to realize distributed quantum networks. We propose a microwave-optical transduction platform based on long-coherence time superconducting radio-frequency (SRF) cavities coupled to electro-optic optical cavities to mitigate the loss mechanisms that limit the attainment of high conversion efficiency. We optimize the microwave-optical field overlap and optical coupling losses in the design while achieving long microwave and optical photon lifetime at milli-Kelvin temperatures. This represents a significant enhancement of the transduction efficiency up to 50% under incoming pump power of 140 μW, which allows the conversion of few-photon quantum signals. Furthermore, this scheme exhibits high resolution for optically reading out the dispersive shift induced by a superconducting transmon qubit coupled to the SRF cavity. We also show that low microwave losses enhance the fidelity of heralded entanglement generation between two remote quantum systems. Finally, high precision in quantum sensing can be reached below the standard quantum limit.
We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold ^{87}Rb atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon-conversion efficiency of ∼0.3% at low microwave intensities and a broad conversion bandwidth of more than 4 MHz. Theoretical simulations agree well with the experimental data, and they indicate that near-unit efficiency is possible in future experiments.