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Physics Today

Quantum-circuit refrigerator

Published Jun 1, 2017• 2 citations
Publisher

Abstract

Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling.However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits.Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator.This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator.Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures.In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature.Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices.In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.

DOI
10.1063/pt.3.3607

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•Jan 29, 2019•0 citations
Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling.However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits.Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator.This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator.Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures.In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature.Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices.In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.
Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.
Superconducting microwave circuits are promising candidates for a variety of different applications, ranging from extremely sensitive photon detectors to large-scale quantum computer. Thus, they may revolutionize our everyday lives in the future through substantial technological development. Superconducting quantum circuits typically operate at low power levels and consist of linear and nonlinear resonators. In order to enhance their operation fidelity, they need to be cooled efficiently, and the dissipation of the circuit needs to be well engineered. In this chapter, we describe a refrigerator for electric quantum circuits based on photon-assisted electron tunneling in hybrid superconductor–insulator–normal-metal junctions. Furthermore, such junctions can also serve as incoherent microwave sources. Importantly, the photon absorption and emission rates can be tuned over several orders of magnitude. In addition, these junctions are used for refrigeration and heating of the normal-metal electrons as well as for probing the electron temperature. Here, we present their operation principle and provide insight to their potential applications as well as briefly describe fabrication methods.
Abstract The recent progress in direct active cooling of the quantum‐electric degrees of freedom in engineered circuits, or quantum‐circuit refrigeration is reviewed. In 2017, the discovery of a quantum‐circuit refrigerator (QCR) based on photon‐assisted tunneling of quasiparticles through normal‐metal–insulator–superconductor junctions inspired a series of experimental studies demonstrating the following main properties: i) the direct‐current (dc) bias voltage of the junction can change the QCR‐induced damping rate of a superconducting microwave resonator by orders of magnitude and give rise to nontrivial Lamb shifts, ii) the damping rate can be controlled in nanosecond time scales, and ii) the dc bias can be replaced by a microwave excitation, the amplitude of which controls the induced damping rate. Theoretically, it is predicted that state‐of‐the‐art superconducting resonators and qubits can be reset with an infidelity lower than 10−4 in tens of nanoseconds using experimentally feasible parameters. A QCR‐equipped resonator has also been demonstrated as an incoherent photon source with an output temperature above 1 K yet operating at millikelvin. This source has been used to calibrate cryogenic amplification chains. In the future, the QCR may be experimentally used to quickly reset superconducting qubits, and hence assist in the great challenge of building a practical quantum computer.
We focus on a recently experimentally realized scenario of normal-metal--insulator--superconductor tunnel junctions coupled to a superconducting resonator. We develop a first-principles theory to describe the effect of photon-assisted electron tunneling on the quantum state of the resonator. Our results are in very good quantitative agreement with the previous experiments on refrigeration and heating of the resonator using the photon-assisted tunneling, thus providing a stringent verification of the developed theory. Importantly, our results provide simple analytical estimates of the voltage-tunable coupling strength and temperature of the thermal reservoir formed by the photon-assisted tunneling. Consequently, they are used to introduce optimization principles for initialization of quantum devices using such a quantum-circuit refrigerator. Thanks to the first-principles nature of our approach, extension of the theory to the full spectrum of quantum electric devices seems plausible.
The emerging quantum technological applications call for fast and accurate initialization of the corresponding devices to low-entropy quantum states. To this end, we theoretically study a recently demonstrated quantum-circuit refrigerator in the case of nonlinear quantum electric circuits such as superconducting qubits. The maximum refrigeration rate of transmon and flux qubits is observed to be roughly an order of magnitude higher than that of usual linear resonators, increasing flexibility in the design. We find that for typical experimental parameters, the refrigerator is suitable for resetting different qubit types to fidelities above 99.99% in a few or a few tens of nanoseconds depending on the scenario. Thus the refrigerator appears to be a promising tool for quantum technology and for detailed studies of open quantum systems.
A quantum-circuit refrigerator draws energy from the quantum electric degrees of freedom of a coupled circuit, thus decreasing the entropy of the circuit. This allows rapid and on-demand initialization of the circuit, such as a qubit, imperative for quantum computing. The initialization efficiency of the refrigerator depends on the detailed properties of the coupled device and on the coupling scheme. In this thesis, we study the efficiency and controllability of initializing a resonator supported by an additional resonator coupled to the refrigerator. The role of the supporting resonator is to provide additional paths for the lower-frequency primary resonator to relax, enhancing the operation of the quantum-circuit refrigerator. This elementary system provides information also for the initialization of other components, such as qubits. The refrigeration is studied by deriving the dynamics of the device from the first-principles, yielding transition rates between different resonator states from which the resonator quality factor and the effective temperature are derived. The validity of the formulated model is verified by experimentally measuring the quality factor of a superconducting resonator, which is found to be tunable over three orders of magnitude. Furthermore, instead of applying dc bias, the refrigeration effect can be achieved by driving the supporting resonator. Although the goal of this thesis is to study the quality factor, we also observe an oscillatory Lamb shift at high drive powers on the supporting mode.