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arXiv:1910.01203 · Physical Review Letters

Radiative Cooling of a Superconducting Resonator

Mingrui Xu, Xu Han, Chang‐Ling Zou, Wei Fu, Yuntao Xu, Changchun Zhong, Liang Jiang, Hong X. Tang

Published Jan 23, 2020• 46 citations
arXivHTMLPDF

Abstract

Cooling microwave resonators to near the quantum ground state, crucial for their operation in the quantum regime, is typically achieved by direct device refrigeration to a few tens of millikelvin. However, in quantum experiments that require high operation power such as microwave-to-optics quantum transduction, it is desirable to operate at higher temperatures with non-negligible environmental thermal excitations, where larger cooling power is available. In this Letter, we present a radiative cooling protocol to prepare a superconducting microwave mode near its quantum ground state in spite of warm environment temperatures for the resonator. In this proof-of-concept experiment, the mode occupancy of a 10 GHz superconducting resonator thermally anchored at 1.02 K is reduced to 0.44±0.05 from 1.56 by radiatively coupling to a 70 mK cold load. This radiative cooling scheme allows high-operation-power microwave experiments to work in the quantum regime, and opens possibilities for routing microwave quantum states to elevated temperatures.

DOI
10.1103/physrevlett.124.033602

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Cooling microwave resonators to near the quantum ground state, crucial for their operation in the quantum regime, is typically achieved by direct device refrigeration to a few tens of millikelvin. However, in quantum experiments that require high operation power such as microwave-to-optics quantum transduction, it is desirable to operate at higher temperatures with non-negligible environmental thermal excitations, where larger cooling power is available. In this Letter, we present a radiative cooling protocol to prepare a superconducting microwave mode near its quantum ground state in spite of warm environment temperatures for the resonator. In this proof-of-concept experiment, the mode occupancy of a 10 GHz superconducting resonator thermally anchored at 1.02 K is reduced to 0.44±0.05 from 1.56 by radiatively coupling to a 70 mK cold load. This radiative cooling scheme allows high-operation-power microwave experiments to work in the quantum regime, and opens possibilities for routing microwave quantum states to elevated temperatures.
Current state-of-the-art superconducting microwave qubits are cooled to extremely low temperatures to avoid sources of decoherence. Higher qubit operating temperatures would significantly increase the cooling power available, which is desirable for scaling up the number of qubits in quantum computing architectures and integrating qubits in experiments requiring increased heat dissipation. To operate superconducting qubits at higher temperatures, it is necessary to address both quasiparticle decoherence (which becomes significant for aluminum junctions above 160 mK) and dephasing from thermal microwave photons (which are problematic above 50 mK). Using low-loss niobium-trilayer junctions, which have reduced sensitivity to quasiparticles due to the higher superconducting transition temperature of niobium, we fabricate transmons with higher frequencies than previously studied, up to 24 GHz. We measure decoherence and dephasing times of about 1 μ s , corresponding to average qubit quality factors of approximately 10 5 , and find that decoherence is unaffected by quasiparticles up to 1 K . Without relaxation from quasiparticles, we are able to explore dephasing from purely thermal sources, finding that our qubits can operate up to approximately 250 mK while maintaining similar performance. The thermal resilience of these qubits creates new options for scaling up quantum processors, enables hybrid quantum experiments with high heat-dissipation budgets, and introduces a material platform for even-higher-frequency qubits. Published by the American Physical Society 2024
Scalable quantum computing is limited by the dense network of electrical interconnects linking cryogenic quantum processors to room-temperature control electronics. To overcome this bottleneck, considerable effort has focused on cryogenic CMOS electronics and microwave-to-optical transduction, aiming to reduce wiring complexity and thermal loading. Wireless interconnects have recently emerged as a promising complementary approach, yet their compatibility with superconducting quantum hardware remains largely unexplored. Here, we demonstrate the wireless excitation of a superconducting microwave resonator of the type routinely employed for qubit readout, operating at millikelvin temperatures inside a dilution refrigerator. By directly comparing wired and wireless operation within the same cryogenic environment, we show that wireless coupling preserves the intrinsic resonator response while revealing parasitic electromagnetic pathways arising from stray radiation within the cryostat enclosure. These results establish a framework for the co-design of wireless interconnects, cryogenic packaging and superconducting quantum hardware.
Manipulating the electromagnetic spectrum at the single-photon level is fundamental for quantum experiments. In the visible and infrared ranges, this can be accomplished with atomic quantum emitters, and with superconducting qubits such control is extended to the microwave range (below 10 GHz). Meanwhile, the region between these two energy ranges presents an unexplored opportunity for innovation. We bridge this gap by scaling up a superconducting qubit to the millimeter-wave range (near 100 GHz). Working in this energy range greatly reduces sensitivity to thermal noise compared to microwave devices, enabling operation at significantly higher temperatures, up to 1 K. This has many advantages by removing the dependence on rare 3 He for refrigeration, simplifying cryogenic systems, and providing orders-of-magnitude higher cooling power, lending the flexibility needed for novel quantum sensing and hybrid experiments. Using low-loss niobium trilayer junctions, we realize a qubit at 72 GHz cooled to 0.87 K using only 4 He . We perform Rabi oscillations to establish control over the qubit state, and measure relaxation and dephasing times of 15.8 and 17.4 ns, respectively. This demonstration of a millimeter-wave quantum emitter offers exciting prospects for enhanced sensitivity thresholds in high-frequency photon detection, provides new options for quantum transduction and for scaling up and speeding up quantum computing, enables integration of quantum systems where 3 He refrigeration units are impractical, and, importantly, paves the way for quantum experiments exploring a novel energy range.
We have fabricated a superconducting resonator capable of generating a strong microwave magnetic field in a small (100 μm) volume for low temperature magnetic resonance experiments. While the resonator was specifically developed for use at millikelvin temperatures in a dilution refrigerator, where the total cooling power is limited to a few hundred microwatts, it is also useful at temperatures up to 5 K. The resonator consists of a 220 μm diameter, 2-1/2 turn niobium coil resonating with a short section of niobium microstripline. At a resonance frequency of 3 GHz, the loaded Q of the resonator was 780. The field strength was characterized by performing electron spin nutations. Operating at 100 mK with 320 μW of dissipated power, the resonator generated a field of 4 G at a distance of 100 μm from the coil.
High-frequency mechanical resonators subjected to low thermal phonon occupancy are easier to be prepared to the ground state by direct cryogenic cooling. Their extreme stiffness, however, poses a significant challenge for external interrogations. Here we demonstrate a superconducting cavity piezoelectromechanical system in which multiple modes of a bulk acoustic resonator oscillating at 10 GHz are coupled to a planar microwave superconducting resonator with a cooperativity exceeding 2×10^{3}, deep in the strong coupling regime. By implementation of the noncontact coupling scheme to reduce mechanical dissipation, the system exhibits excellent coherence characterized by a frequency-quality-factor product of 7.5×10^{15} Hz. Interesting dynamics of classical temporal oscillations of the microwave energy is observed, implying the coherent conversion between phonons and photons. The demonstrated high-frequency cavity piezoelectromechanics is compatible with superconducting qubits, representing an important step towards hybrid quantum systems.
We demonstrated microwave-induced cooling in a superconducting flux qubit. The thermal population in the first-excited state of the qubit is driven to a higher-excited state by way of a sideband transition. Subsequent relaxation into the ground state results in cooling. Effective temperatures as low as approximately 3 millikelvin are achieved for bath temperatures of 30 to 400 millikelvin, a cooling factor between 10 and 100. This demonstration provides an analog to optical cooling of trapped ions and atoms and is generalizable to other solid-state quantum systems. Active cooling of qubits, applied to quantum information science, provides a means for qubit-state preparation with improved fidelity and for suppressing decoherence in multi-qubit systems.