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Microwave-to-optical Conversion via Rydberg Excitons in Cu2O

Horatio Q. X. Wong

Published Aug 17, 2026• 0 citations
OpenAlex

Abstract

The primary objective of this thesis is to investigate the feasibility of cuprous oxide (cuprite) as a competitive platform for microwave-to-optical conversion utilising Rydberg excitons. While the ultimate goal is the simultaneous enhancement of both microwave and optical domains to reach the strong-coupling regime, this work focuses on characterising the two domains independently to establish the necessary foundational parameters. The approach follows a two-pronged strategy: increasing the optical field interaction by embedding cuprite within a Fabry-Pérot microcavity and, in parallel, enhancing the microwave field through integration of cuprite with superconducting microwave resonators. Within the microcavity, high-resolution spectroscopy enabled the resolution of Rydberg resonances as high as 17P. A coupled-oscillator model fitting confirmed the achievement of the strong-coupling regime for states $n \leq 4$. In the second-harmonic generation regime, the microcavity facilitated the identification of doubly resonant locations satisfying the quasi-phase matching conditions required to simultaneously enhance the pump field and the SHG output. This architecture permitted the observation of Rydberg states up to 10D, the highest reported to date in a microcavity sample and revealed significant cavity-induced splittings of the 3S and 3D excitonic resonances. Microwave field confinement was addressed through the development of superconducting lumped-element resonators. A nested split-ring resonator design achieved peak electric field intensities exceeding $1.2 \times 10^6$ V/m and a sub-wavelength mode volume of $V_m \approx 2156$ $\upmu\mathrm{m}^3$, satisfying the high-confinement requirements for efficient microwave-to-optical coupling. Leveraging the multi-mode response of this platform, we provide the first reported measurement of cuprite's relative permittivity at microwave frequencies and sub-Kelvin temperatures of $\varepsilon_r = 6.66 \pm 0.08$. Furthermore, the loss tangent of cuprite was constrained to the $10^{-5}$-$10^{-4}$ range using a lumped-element interdigitated capacitor resonator architecture. The convergence of these optical and microwave architectures provides a definitive roadmap for a hybrid quantum system where Rydberg excitons couple concurrently to both domains. By accessing high-n states within a co-localized field geometry, this work establishes the engineering strategies necessary to realise scalable, cuprite-based quantum transduction devices for next-generation communication systems.

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The primary objective of this thesis is to investigate the feasibility of cuprous oxide (cuprite) as a competitive platform for microwave-to-optical conversion utilising Rydberg excitons. While the ultimate goal is the simultaneous enhancement of both microwave and optical domains to reach the strong-coupling regime, this work focuses on characterising the two domains independently to establish the necessary foundational parameters. The approach follows a two-pronged strategy: increasing the optical field interaction by embedding cuprite within a Fabry-Pérot microcavity and, in parallel, enhancing the microwave field through integration of cuprite with superconducting microwave resonators. Within the microcavity, high-resolution spectroscopy enabled the resolution of Rydberg resonances as high as 17P. A coupled-oscillator model fitting confirmed the achievement of the strong-coupling regime for states n≤4n \leq 4n≤4. In the second-harmonic generation regime, the microcavity facilitated the identification of doubly resonant locations satisfying the quasi-phase matching conditions required to simultaneously enhance the pump field and the SHG output. This architecture permitted the observation of Rydberg states up to 10D, the highest reported to date in a microcavity sample and revealed significant cavity-induced splittings of the 3S and 3D excitonic resonances. Microwave field confinement was addressed through the development of superconducting lumped-element resonators. A nested split-ring resonator design achieved peak electric field intensities exceeding 1.2×1061.2 \times 10^61.2×106 V/m and a sub-wavelength mode volume of Vm≈2156V_m \approx 2156Vm​≈2156 \upmum3\upmu\mathrm{m}^3\upmum3, satisfying the high-confinement requirements for efficient microwave-to-optical coupling. Leveraging the multi-mode response of this platform, we provide the first reported measurement of cuprite's relative permittivity at microwave frequencies and sub-Kelvin temperatures of εr=6.66±0.08\varepsilon_r = 6.66 \pm 0.08εr​=6.66±0.08. Furthermore, the loss tangent of cuprite was constrained to the 10−510^{-5}10−5-10−410^{-4}10−4 range using a lumped-element interdigitated capacitor resonator architecture. The convergence of these optical and microwave architectures provides a definitive roadmap for a hybrid quantum system where Rydberg excitons couple concurrently to both domains. By accessing high-n states within a co-localized field geometry, this work establishes the engineering strategies necessary to realise scalable, cuprite-based quantum transduction devices for next-generation communication systems.
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.
Microwave–optical conversion is key to future networks of quantum devices, such as those based on superconducting technology. Conversion at the single quantum level requires strong nonlinearity, high bandwidth, and compatibility with a millikelvin environment. A large nonlinearity is observed in Rydberg atoms, but combining atomic gases with dilution refrigerators is technically challenging. Here, we demonstrate a strong microwave–optical nonlinearity in a cryogenic, solid-state system by exploiting Rydberg states of excitons in Cu2O. We measure a microwave–optical cross-Kerr coefficient of B0 = 0.022 ± 0.008 m V−2 at 4 K, which is several orders of magnitude larger than other solid-state systems. The results are in quantitative agreement with a nonlinear susceptibility model based on the giant microwave dipole moment between nearby excitonic states. Our results highlight the potential of Rydberg excitons for nonlinear optics and form the basis for a microwave–optical frequency converter based on Cu2O.
This thesis investigates hybrid circuit quantum electrodynamics (cQED) as a versatile platform that unites superconducting resonators with semiconductor quantum dots. The central motivation is to harness strong light-matter coupling at the single-photon level, which enables coherent photon-qubit interactions and paves the way for quantum technologies in computation, communication, and sensing. The first part of this work combines hybrid cQED with holes in planar Ge/SiGe, a material system that has only recently become accessible for quantum devices. Planar Ge offers unique advantages for this purpose and has attracted considerable attention in the context of quantum computing. In contrast, its potential for hybrid cQED has remained largely unexplored, making the present work one of the first demonstrations of strong light-matter interaction in this platform. Within this framework, we demonstrate strong charge-photon coupling in planar Ge by observing vacuum Rabi splitting between a hole charge qubit based on a double quantum dot (DQD) and a microwave resonator. We employ a frequency-tunable high-impedance resonator to perform qubit spectroscopy, enabling systematic exploration of coupling regimes and confirming device controllability. In addition, we use this setup to carry out excited-state spectroscopy of strongly correlated states in the DQD, revealing the formation of Wigner molecules and illustrating how the hybrid architecture can be harnessed to access many-body physics. While these experiments in Ge demonstrate how strong coupling can be harnessed to explore qubit physics and correlated states, they also exemplify the wider potential of hybrid cQED. A particularly pressing application is in quantum optics, where efficient single-photon detectors are still lacking in the microwave regime. Such detectors are essential for enabling experiments in quantum communication and microwave quantum optics. In the second part of this thesis, we address this challenge by realizing a photodetector based on a GaAs DQD coupled to a superconducting resonator. The detector operates via photon-to-charge conversion and achieves efficiencies approaching unity. Strong coupling plays a central role by ensuring that incoming photons are absorbed before being lost to the environment, which is key to the high performance of the detector. We further exploit the frequency tunability of the resonator to investigate device performance and identify the parameters that optimize detection efficiency. The broader impact of this work lies in advancing quantum technologies that rely on coherent microwave photons. Applications include spin qubit readout, long-distance photon-mediated coupling, quantum communication, and microwave quantum optics. Looking forward, planar Ge is identified as a highly competitive platform for hybrid cQED. Future directions include the development of single-photon sources, quantum- limited detectors, and analog quantum simulators based on engineered resonator-qubit arrays.
A hybrid circuit quantum electro-dynamics (circuit QED) setup consisting of helium atoms in high-n Rydberg states and superconducting co-planar waveguide (CPW) microwave resonators has been developed with the goal of performing hybrid quantum optics experiments with applications in quantum information processing. In this thesis an overview of the field of cavity QED is introduced, and numerical methods to calculate the atomic energy level structure and transition dipole moments in electric and magnetic fields are described. Using this background information, a new method for efficiently preparing high-n circular Rydberg states is presented. This was required to ac- cess circular-state–to–circular-state transition frequencies in experiments which are compatible with superconducting CPW microwave resonators. Experimental and numerical results demonstrating the implementation of this method for the preparation of the |n = 70, l = 69, ml = +69⟩ circular state in helium are reported. The design and fabrication of λ/4 superconducting CPW microwave resonators, compatible with these high-n circular Rydberg states of helium is then described. The effects of microwave driving power, temperature and magnetic field on the characteristics of these resonators are presented. Finally, experiments in which helium Rydberg atoms have been coherently coupled to the microwave field of a superconducting co-planar waveguide resonator are reported.
Hybrid quantum systems play an essential role in efforts towards the advancements of quantum information technology and quantum sensing by bridging distinct technological platforms. In particular, hybrid systems comprising superconducting microwave circuits are being pursued intensely in light of existing achievements in superconducting qubits and precise quantum control in microwave circuits. Their realization relies on the design of superconducting resonators capable of coupling to an oscillatory mode of a second quantum system, facilitating the probing and manipulation of one via the other. In order to accommodate the coupled system, its particular constraints and requirements must be respected, which may be quite extreme from the perspective of established superconducting circuit design and operation. Overcoming this challenge has proved to be a substantial obstacle for the attainment of strongly coupled hybrid systems, despite spectacular advances in superconducting microwave circuits individually. In this work, we examine two approaches tackling this issue for different hybrid quantum systems, focusing on the optimization and characterization of the superconducting circuits. The first device aims at a superconductor-atom hybrid system through the coupling to Rydberg-Rydberg transitions in ultra-cold Rb-87 atoms trapped near the superconducting chip. Using simulations in conjunction with theoretical considerations, we demonstrate a comprehensive geometry parameter optimization procedure for maximum coupling rate. Presenting an experimental implementation of the resulting circuit, we validate its suitability for use under the intended operation conditions, including the option of tuning the electronic transition frequencies using a dc voltage. Secondly, we investigate a device designed for applications in large magnetic in-plane fields used to mediate coupling to integrated micromechanical oscillators; more specifically it is a flux-tunable SQUID resonator comprising Josephson nano-constrictions as nonlinear inductive components. Performing a thorough characterization of its frequency and Kerr anharmonicity tuning behavior at fields up to several hundred mT, we discover a pronounced asymmetry in its flux response at large fields. We show this asymmetry to be caused by the nano-constrictions turning into Josephson diodes, develop an intuitive macroscopic model for its field-induced emergence and reconstruct the diode current-phase relation from the flux tuning data. Several figures of merit informing the suitability of the SQUID resonators for optomechanical systems as well as other applications are improved by the diode effect, highlighting the potential of Josephson diodes in superconducting microwave circuits. Our results provide a promising stepping stone for the advancement of hybrid systems involving superconducting microwave circuits, putting new and exciting operation and coupling regimes within reach.
Achieving efficient microwave–optical transduction is a critical challenge for quantum networks where information is shared optically across disparate microwave systems and for coherent optical detection and manipulation of microwave states. Resonant electro-optic systems are being pursued as a means of efficiently bridging these frequencies. Here, we present a platform that employs a bulk lithium niobate crystal whose large dielectric constant creates wavelength-scale confinement of microwave photons. By incorporating this crystal within a high-finesse Fabry–Pérot optical cavity, microwave photons couple to optical photons through the electro-optic effect. We demonstrate the ability to tune our system into triply resonant operation, where microwave photons, optical pump photons, and upconverted optical photons are simultaneously resonant with high quality factor electromagnetic modes of the system. The device achieves photon-number conversion efficiency at the percent level, comparable to that of state-of-the-art devices at room temperature—sufficient to resolve the thermal occupation of the microwave mode—by taking advantage of the high-power handling, low loss, and low noise of all-dielectric devices. These results establish our all-dielectric devices as a promising platform for high-precision sensing of optically detected microwave fields and as a viable route toward single-photon-level microwave–optical quantum transduction.