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Electro-optic Links for Cryogenic Radio Frequency Systems

Hana Kei Warner

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

Radio frequency (RF) radiation is a powerful tool in computing, communication, and sensing due to the ability to engineer strong electrical interactions. Within cryogenic systems, in particular, microwave (MW) frequency superconducting qubits have demonstrated promise as a platform for quantum computing, and MW and millimeter-wave (mmW) systems have been valuable tools in sensing. However, owing to their low energy, the weak, single-photon level signals employed by these systems suffer high sensitivity to thermal noise when they exit the thermal bath, as well as short attenuation lengths due to absorption and scattering in cables. Telecommunication frequency optical photons, meanwhile, have long propagation lengths and can be supported by low-loss optical fibers. The high photon energy makes signals insensitive to thermal noise, and the high frequency can allow many RF signals to be multiplexed onto a single optical carrier. Interfacing RF and optical radiation to leverage both strong interactions with macroscopic systems characteristic of electrical interactions and low-loss propagation characteristic of optical photons has been critical to realizing our global communication system. Leveraging the infrastructure developed to support this industry, such as single photon detectors, is attractive for scaling cryogenic RF systems. Coherent conversion processes, especially, can benefit a wide range of applications, especially in computing and sensing, by preserving quantum states, and retaining phase information necessary for quantum gates, and enable the use of measurement techniques that can increase detection sensitivity. This dissertation focuses on using the electro-optic effect in thin-film lithium niobate to form an efficient, coherent interface between radio- and telecommunication-frequency photons. Lithium niobate is an attractive platform due to its high electro-optic coefficient (approx. 30 pm/V), wide transparency window (DC to 400 THz), and ability to support low-loss optical waveguides (.2 dB/m), all of which deviate little from room temperature down to near-zero Kelvin environments. First, I explore the development of low-loss lithium niobate optical cavities and operation of lithium niobate waveguides at low temperatures. While lithium niobate is an electro-optic material with a DC response, using the electro-optic response at room temperature for stable biasing and low-frequency operation has remained a challenge. A temperature- and frequency-dependent study is conducted to identify lithium charge migration as a potential screening mechanism suppressed at low temperatures. Next, I explore the use of RF-cavity enhanced, triply resonant photonic molecules (created from two coupled low-loss optical cavities) for forming efficient electro-optic interfaces with microwave (3-10 GHz) and millimeter wave (100 GHz) radiation. In the microwave regime, I explore microwave-optical transduction for forming optical interfaces with microwave qubits, specifically focusing on superconducting qubits. The characterization of one such transducer is demonstrated, and the device is used to optically synthesize microwave signals at low temperatures, bypassing the need for input microwave control lines. Outlook for other interfaces, including optical photon mediated quantum information transfer, is discussed. At mmW frequencies, the triply resonant photonic molecule is explored for low-power ( mW) optical-frequency shifting.

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Cavity electro-optics in thin-film lithium niobate

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Quantum networks of superconducting qubits linked by optical channels could leverage both the quantum information processing capabilities of superconducting circuits and the long communication distances provided by optical photons. Such networks would require high efficiency, low noise, and wide bandwidth transducers between microwave and optical frequencies. Transducers based on the Pockels electro-optic (EO) effect are particularly promising in this application for their direct conversion mechanism and potential for strong performance. EO transducers could also be used for sensitive optical modulators and low-noise detection of microwave and millimeter-wave signals. This dissertation presents recent work to create cavity EO transducers in thin-film lithium niobate, an integrated photonics platform that provides low optical loss and strong EO coupling. I first describe the theory of cavity electro-optics and how it can be used to generate high-efficiency transduction between microwave and optical fields. An initial device is presented and characterized, demonstrating per-photon on-chip transduction efficiencies of up to (2.7 ± 0.3) × 10^−5. A key benefit of the device described here, which is based on photonic molecule modes, is the ability to use a static electro-optic bias to trim the transducer into resonance. However, we find the migration of free carriers in thin-film lithium niobate reduces the EO response of the device to static fields, making such trimming ineffective. This carrier migration is a key challenge for enabling the promise of low-power electro-optics provided by thin-film lithium niobate devices. I characterize the low-frequency electro-optic response, which suggests that conduction occurs on the etched surface of lithium niobate. I show how this conduction can be reduced – and low-frequency EO performance improved – by changing the electrode design and annealing the devices. Following this, I describe the design and initial characterization of an improved transducer device. Finally, I describe a scheme by which even relatively low-efficiency transducers could be used to generate remote entanglement using an optically heralding scheme. Demonstration of such a system appears possible with current devices, suggesting that small opticallymediated quantum networks of superconducting qubits may be feasible soon.

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Radio frequency (RF) radiation is a powerful tool in computing, communication, and sensing due to the ability to engineer strong electrical interactions. Within cryogenic systems, in particular, microwave (MW) frequency superconducting qubits have demonstrated promise as a platform for quantum computing, and MW and millimeter-wave (mmW) systems have been valuable tools in sensing. However, owing to their low energy, the weak, single-photon level signals employed by these systems suffer high sensitivity to thermal noise when they exit the thermal bath, as well as short attenuation lengths due to absorption and scattering in cables. Telecommunication frequency optical photons, meanwhile, have long propagation lengths and can be supported by low-loss optical fibers. The high photon energy makes signals insensitive to thermal noise, and the high frequency can allow many RF signals to be multiplexed onto a single optical carrier. Interfacing RF and optical radiation to leverage both strong interactions with macroscopic systems characteristic of electrical interactions and low-loss propagation characteristic of optical photons has been critical to realizing our global communication system. Leveraging the infrastructure developed to support this industry, such as single photon detectors, is attractive for scaling cryogenic RF systems. Coherent conversion processes, especially, can benefit a wide range of applications, especially in computing and sensing, by preserving quantum states, and retaining phase information necessary for quantum gates, and enable the use of measurement techniques that can increase detection sensitivity. This dissertation focuses on using the electro-optic effect in thin-film lithium niobate to form an efficient, coherent interface between radio- and telecommunication-frequency photons. Lithium niobate is an attractive platform due to its high electro-optic coefficient (approx. 30 pm/V), wide transparency window (DC to 400 THz), and ability to support low-loss optical waveguides (.2 dB/m), all of which deviate little from room temperature down to near-zero Kelvin environments. First, I explore the development of low-loss lithium niobate optical cavities and operation of lithium niobate waveguides at low temperatures. While lithium niobate is an electro-optic material with a DC response, using the electro-optic response at room temperature for stable biasing and low-frequency operation has remained a challenge. A temperature- and frequency-dependent study is conducted to identify lithium charge migration as a potential screening mechanism suppressed at low temperatures. Next, I explore the use of RF-cavity enhanced, triply resonant photonic molecules (created from two coupled low-loss optical cavities) for forming efficient electro-optic interfaces with microwave (3-10 GHz) and millimeter wave (100 GHz) radiation. In the microwave regime, I explore microwave-optical transduction for forming optical interfaces with microwave qubits, specifically focusing on superconducting qubits. The characterization of one such transducer is demonstrated, and the device is used to optically synthesize microwave signals at low temperatures, bypassing the need for input microwave control lines. Outlook for other interfaces, including optical photon mediated quantum information transfer, is discussed. At mmW frequencies, the triply resonant photonic molecule is explored for low-power ( mW) optical-frequency shifting.
Quantum networks of superconducting qubits linked by optical channels could leverage both the quantum information processing capabilities of superconducting circuits and the long communication distances provided by optical photons. Such networks would require high efficiency, low noise, and wide bandwidth transducers between microwave and optical frequencies. Transducers based on the Pockels electro-optic (EO) effect are particularly promising in this application for their direct conversion mechanism and potential for strong performance. EO transducers could also be used for sensitive optical modulators and low-noise detection of microwave and millimeter-wave signals. This dissertation presents recent work to create cavity EO transducers in thin-film lithium niobate, an integrated photonics platform that provides low optical loss and strong EO coupling. I first describe the theory of cavity electro-optics and how it can be used to generate high-efficiency transduction between microwave and optical fields. An initial device is presented and characterized, demonstrating per-photon on-chip transduction efficiencies of up to (2.7 ± 0.3) × 10^−5. A key benefit of the device described here, which is based on photonic molecule modes, is the ability to use a static electro-optic bias to trim the transducer into resonance. However, we find the migration of free carriers in thin-film lithium niobate reduces the EO response of the device to static fields, making such trimming ineffective. This carrier migration is a key challenge for enabling the promise of low-power electro-optics provided by thin-film lithium niobate devices. I characterize the low-frequency electro-optic response, which suggests that conduction occurs on the etched surface of lithium niobate. I show how this conduction can be reduced – and low-frequency EO performance improved – by changing the electrode design and annealing the devices. Following this, I describe the design and initial characterization of an improved transducer device. Finally, I describe a scheme by which even relatively low-efficiency transducers could be used to generate remote entanglement using an optically heralding scheme. Demonstration of such a system appears possible with current devices, suggesting that small opticallymediated quantum networks of superconducting qubits may be feasible soon.
Photonic integrated circuits operating at cryogenic temperatures are necessary for many quantum technologies such as quantum transduction, integrated single-photon emitters and detectors, as well as deep-space communication and sensing devices. Thin-film lithium niobate (TFLN) is an emerging platform that is a strong candidate for fully integrated quantum photonics, offering low loss, fast electro-optic reconfigurability, nonlinear quantum light sources, and the ability to host quantum emitters and single-photon detectors. To interface TFLN with technologies that require cryogenic operation, like superconducting single-photon detectors, microwave-to-optical transducers, and solid-state quantum emitters, it is important to study its optical and electrical properties from room temperature down to cryogenic temperatures. Here, we investigate linear and nonlinear photonic devices, including racetrack resonators, Mach-Zehnder modulators and periodically poled waveguides in TFLN using a cryogenic fiber probe station with full temperature control down to 5 K. We quantify a shift in resonances, a 22% increase in electro-optic modulator half-wave voltage, a blue shift of 18 nm for Type-0 phase-matching as well as a red shift of 64 nm for Type-II phase-matching as the sample temperature decreases. Our study of nonlinear processes in a cryogenic environment will contribute towards developing novel devices for inter-platform quantum information processing, secure communication, and enhanced sensing.
Abstract Encoding information onto optical fields using electro-optical modulation is the backbone of modern telecommunication networks, offering vast bandwidth and low-loss transport via optical fibers. For these reasons, optical fibers are also replacing electrical cables for short range communications within data centers. Compared to electrical coaxial cables, optical fibers also introduce two orders of magnitude smaller heat load from room to milli-Kelvin temperatures, making optical interconnects based on electro-optical modulation an attractive candidate for interfacing superconducting quantum circuits and hybrid superconducting devices. Yet, little is known about optical modulation at cryogenic temperatures. Here we demonstrate a proof-of-principle cryogenic electro-optical interconnect, showing that currently employed Ti-doped lithium niobate phase modulators are compatible with operation down to 800mK ---below the typical operation temperature of conventional microwave amplifiers based on high electron mobility transistors (HEMTs)---and maintain their room temperature Pockels coefficient. We utilize cryogenic electro-optical modulation to perform spectroscopy of a superconducting circuit optomechanical system, measuring optomechanically induced transparency (OMIT). In addition, we encode thermomechanical sidebands from the microwave domain onto an optical signal processed at room temperature. Although the currently achieved noise figure is significantly higher than that of a typical HEMT, substantial noise reduction should be attainable by harnessing recent advances in integrated modulators, by increasing the modulator length, or by using materials with a higher electro-optic coefficient, leading to noise levels on par with HEMTs. Our work highlights the potential of electro-optical modulators for massively parallel readout for emerging quantum computing or cryogenic classical computing platforms.
Superconducting circuits are important technological tools that have found various applications, notably hypothetical future superconducting quantum computers. Scaling such systems further is, however, proving challenging because of thermal heat load and space constraints in cryostats. One way of improving the scalability of such systems would be to resort to optical fibers for controlling the circuits in cryostats, as well as mediating long-distance interaction between them while preserving quantum coherence. Such a strategy relies on building a coherent interface between GHz frequencies and optical frequencies. This interface, referred to as "microwave-optical transducer", would need to exhibit high photon conversion efficiencies and very low added noise, ideally less than a quantum, in a cryogenic environment. Since the prospects of weaving networks of optically connected superconducting quantum processors have now attracted the attention of the scientific community for more than a decade, very efficient transducers have already been proposed and realized. However, despite their impressive performances against selected metrics, often equivalent to optimizing the on-chip conversion efficiency alone, none of the approaches pursued so far really solved the qubit interconnection problems in ways that could be applied to large-scale systems in the near future. The aim of this thesis was to develop transduction approaches that would ultimately enable optical driving of superconducting circuits and quantum state transfer between microwave and optical frequencies, while at the same time ensuring good scalability, both in terms of wafer manufacturability and integration in cryogenic environments. Integrated photonic circuits based on silicon nitride appeared as a promising platform, as silicon nitride is a CMOS-certified material, has a transparency window compatible with optical fibers and lacks two photon absorption. Realizing the coupling between microwave and optical frequencies in a way similar to what is done in optomechanics is desirable, because the field of optomechanics has seen the establishement of techniques that enable control of the low frequency degree of freedom at the quantum mechanical level, including ground state cooling, control of the susceptibility by dynamical backaction, quantum state transfer and entanglement. The heterogeneous integration of nonlinear crystalline thin films, such as lithium niobate and lithium tantalate, onto silicon nitride photonic circuitry endows it with an electro-optic activity that emulates the optomechanical interaction, while offering an inherent access to the microwave degree of freedom. However, material nonlinearities, including photorefraction, charge screening, and hysteresis, introduce complications that motivate exploring alternative approaches. Here, alongside an electro-optic implementation, a transducer that leverages the interaction of bulk acoustic resonances excited by micro-electromechanical systems with whispering-gallery-like modes of microresonators is presented. Coherent transduction between microwave and optical fields is demonstrated, and optomechanical dynamical backaction is observed. The transducers fast bandwidth, compatibility with multiplexing and ability to down-convert microwaves suggest interesting prospects, particularly upon further improvements of the optomechanical coupling.
Abstract The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. However, in such a device the optical pump can be a source of thermal noise and thus degrade the fidelity; the similarity of input microwave state to the output optical state. In order to investigate the magnitude of this effect we model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion.
We present a resonant electro-optic transducer for efficient conversion between microwave and telecom wavelength photons. Our platform 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 - Perot 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 state-of-the-art devices at room temperature -- sufficient to resolve the thermal occupation of the microwave mode -- while avoiding the noise and loss associated with metal electrodes. 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.