Recent Developments in Quantum‐Circuit Refrigeration

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.

Exceptional-point-assisted entanglement, squeezing, and reset in a chain of three superconducting resonators

“…The EP-2 used for stabilization is thus achieved with κ 2 /(2π) ≈ 5.66 MHz and κ 3 /(2π) = 1.0 kHz, while the EP-3 with κ 2 /(2π) ≈ 2.83 MHz and κ 3 /(2π) ≈ 5.66 MHz. Even though the almost four-orders-of-magnitude tunability required to interchange between this particular EP-2 and the EP-3 may be technically challenging, the maximum achievable decay rates with the QCR are beyond the ones considered here and their demonstrated on/off ratios are close to these requirements [23].…”

“…The decay rates of resonators R2 and R3 can be controlled by quantum-circuit refrigerators (QCRs) placed at the resonator input ports. Each QCR is comprised of a normalmetal-insulator-superconducting (NIS) junction and can remove photons incoherently from the system mediated by electron tunneling at specific bias-voltage pulses [22,23].…”

“…4b. Using the lower envelopes of the fitting functions (23) in Eq. ( 24), one can estimate the quasi-stabilization time as…”

“…In circuit quantum electrodynamics (cQED), for example, efforts have been made to integrate devices with in-situ-tunable dissipation to prepare specific quantum states [5][6][7][8][9][10][11][12], produce fast reset [13][14][15][16][17][18][19][20][21][22][23][24], and to exploit the potential benefits of open-system degeneracies, referred to as exceptional points (EPs) [17,21,[25][26][27][28][29]. In contrast to Hermitian degeneracies, EPs induce the coalescence of eigenvalues and eigenvectors of the dynamical matrix governing the open-system evolution leading to critical dynamics manifested by polynomial solutions in time [30,31].…”

Non-Hermitian topological quantum states in a reservoir-engineered transmon chain

“…In reservoir engineering, the idea is to turn the usually detrimental effects of dissipation into a resource. In this article, we demonstrate that the flexibility to engineer dissipation in a controllable manner in transmon circuits [41][42][43][44][45] can be utilized for realizing NH topological quantum phases. In our proposal, the NH topological phase is created by introducing a spatial modulation of dissipation [7,46] in the one-dimensional Bose-Hubbard transmon chain [47,48], where the dissipation strength in each transmon is controlled by the tunable coupling of the transmon to a quantum circuit refrigerator (QCR) [41][42][43][44] (see Fig.…”