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Superfluid-Cooled Transmon Qubits under Optical Excitation

Chunzhen Li, Yufeng Wu, Manuel C. C. Pace, Matthew LaHaye, Michael Senatore, Hong X. Tang

Published Jun 9, 2025• 1 citation
Publisher

Abstract

Microwave-to-optical quantum state transfer enables the interconnection of remote superconducting quantum processors via optical-fiber links. Achieving high conversion efficiency necessitates an intense optical pump, yet direct optical exposure of superconducting circuits often degrades their performance. Efficient and fast thermalization of superconducting qubits is therefore essential. To achieve this, we immerse a laser-illuminated transmon qubit in superfluid helium-4 and investigate its behavior. We observe that the qubit recovers significantly faster in superfluid helium than in vacuum, with an improvement in power handling by over 10 dB. This enhanced cooling capacity supports higher optical pump powers, shortening recovery times after high-power optical pulses. These findings open a promising pathway for future efficient implementation of microwave-to-optical quantum transduction devices.

DOI
10.1103/q99s-lrnv

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Microwave-to-optical quantum state transfer enables the interconnection of remote superconducting quantum processors via optical-fiber links. Achieving high conversion efficiency necessitates an intense optical pump, yet direct optical exposure of superconducting circuits often degrades their performance. Efficient and fast thermalization of superconducting qubits is therefore essential. To achieve this, we immerse a laser-illuminated transmon qubit in superfluid helium-4 and investigate its behavior. We observe that the qubit recovers significantly faster in superfluid helium than in vacuum, with an improvement in power handling by over 10 dB. This enhanced cooling capacity supports higher optical pump powers, shortening recovery times after high-power optical pulses. These findings open a promising pathway for future efficient implementation of microwave-to-optical quantum transduction devices.
Interfacing superconducting microwave resonators with optical systems enables sensitive photon detectors, quantum transducers, and related quantum technologies. Achieving high optical pulse repetition is crucial for maximizing the device throughput. However, light-induced deterioration, such as quasiparticle poisoning, pair-breaking-phonon generation, and elevated temperature, hinders the rapid recovery of superconducting circuits, limiting their ability to sustain high optical pulse repetition rates. Understanding these loss mechanisms and enabling fast circuit recovery are therefore critical. In this work, we investigate the impact of optical illumination on niobium nitride and niobium microwave resonators by immersing them in superfluid helium-4 and demonstrate a three-order-of-magnitude faster resonance recovery compared to vacuum. By analyzing transient resonance responses, we provide insights into light-induced dynamics in these superconductors, highlighting the advantages of niobium-based superconductors and superfluid helium for rapid circuit recovery in superconducting quantum systems integrated with optical fields. High-speed optical pulses can enhance system performance and data throughput in photonic-microwave hybrid circuits, but repeated illumination can degrade superconducting resonators’ performance. Here, by conducting experiments in vacuum and superfluid helium, the authors reveal the mechanisms of light-induced loss and demonstrate a three order-of-magnitude faster resonance recovery of niobium-based superconducting resonators in the superfluid helium environment.
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.
Superfluid helium's low-loss dielectric properties, excellent thermal conductivity, and unique collective excitations make it an attractive candidate to incorporate into superconducting qubit systems. We controllably immerse a three-dimensional superconducting transmon qubit in superfluid 4He^{4}\mathrm{He}4He and measure the spectroscopic and coherence properties of the system. We find that the cavity, the qubit, and their coupling are all modified by the superfluid, which we analyze within the framework of circuit quantum electrodynamics. At temperatures relevant to quantum computing experiments, the energy relaxation time of the qubit is not significantly changed by the presence of the superfluid, while the pure dephasing time modestly increases, which we attribute to improved thermalization of the microwave environment via the superfluid.
Superfluid helium at milli-Kelvin temperatures is a dielectric liquid with an extremely low loss tangent at microwave frequencies. As such, it is a promising candidate for incorporation into hybrid quantum systems containing superconducting qubits. We demonstrate the viability of this hybrid systems approach by controllably immersing a three-dimensional superconducting transmon qubit in superfluid 4^44He. By measuring spectroscopic and coherence properties we find that the cavity, the qubit, and their coupling are all modified by the presence of the dielectric superfluid, which we analyze within the framework of circuit quantum electrodynamics (cQED). At temperatures relevant to quantum computing experiments, the energy relaxation time of the qubit is not significantly changed by the presence of the superfluid, while the pure dephasing time modestly increases, which we attribute to improved thermalization via the superfluid.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.
A quantum interface between microwave and optical photons is essential for entangling remote superconducting quantum processors. To preserve fragile quantum states, a transducer must operate efficiently while generating less than one photon of noise referred to its input. Here, we present a platform that meets these criteria, utilizing a combination of electrostatic and optomechanical interactions in devices made entirely from crystalline silicon. This platform's small mechanical dissipation and low optical absorption enable ground-state radiative cooling, resulting in quantum-enabled operation with a continuous laser drive. Under the optimal settings for high efficiency (low noise), we measure an external efficiency of 2.2%2.2\%2.2% (0.47%0.47\%0.47%) and an input-referred added noise of 0.940.940.94 (0.580.580.58) in microwave-to-optics conversion. We quantify the transducer throughput using the efficiency-bandwidth product, finding it exceeds previous demonstrations with similar noise performance by approximately two orders of magnitude, thereby paving a practical path to interconnecting remote superconducting qubits.