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

The interplay between coherent and dissipative dynamics required in various control protocols of quantum technology has motivated studies of open-system degeneracies, referred to as exceptional points (EPs). Here, we introduce a scheme for fast quantum-state synthesis using exceptional-point engineering in a lossy chain of three superconducting resonators. We theoretically find that the rich physics of EPs can be used to identify regions in the parameter space that favor a fast and quasi-stable transfer of squeezing and entanglement, or a fast reset of the system. For weakly interacting resonators with the coupling strength $g$, the obtained quasi-stabilization time scales are identified as $1/(2\sqrt{2}g)$, and reset infidelities below $10^{-5}$ are obtained with a waiting time of roughly $6/g$ in the case of weakly squeezed resonators. Our results shed light on the role of EPs in multimode Gaussian systems and pave the way for optimized distribution of squeezing and entanglement between different nodes of a photonic network using dissipation as a resource.Comment: 14 pages, 6 figure

Fully in situ Nb/InAs-nanowire Josephson junctions by selective-area growth and shadow evaporation

Josephson junctions based on InAs semiconducting nanowires and Nb superconducting electrodes are fabricated in situ by a special shadow evaporation scheme for the superconductor electrode. Compared to other metallic superconductors such as Al, Nb has the advantage of a larger superconducting gap which allows operation at higher temperatures and magnetic fields. Our junctions are fabricated by shadow evaporation of Nb on pairs of InAs nanowires grown selectively on two adjacent tilted Si (111) facets and crossing each other at a small distance. The upper wire relative to the deposition source acts as a shadow mask determining the gap of the superconducting electrodes on the lower nanowire. Electron microscopy measurements show that the fully in situ fabrication method gives a clean InAs/Nb interface. A clear Josephson supercurrent is observed in the current–voltage characteristics, which can be controlled by a bottom gate. The large excess current indicates a high junction transparency. Under microwave radiation, pronounced integer Shapiro steps are observed suggesting a sinusoidal current–phase relation. Owing to the large critical field of Nb, the Josephson supercurrent can be maintained to magnetic fields exceeding 1 T. Our results show that in situ prepared Nb/InAs nanowire contacts are very interesting candidates for superconducting quantum circuits requiring large magnetic fields

Single-junction quantum-circuit refrigerator

We propose a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single normal-metal–insulator–superconductor (NIS) junction. In contrast to previous studies with multiple junctions and an additional charge island for the QCR, we directly connect the NIS junction to an inductively shunted electrode of a superconducting microwave resonator making the device immune to low-frequency charge noise. At low characteristic impedance of the resonator and parameters relevant to a recent experiment, we observe that a semiclassical impedance model of the NIS junction reproduces the bias voltage dependence of the QCR-induced damping rate and frequency shift. For high characteristic impedances, we derive a Born–Markov master equation and use it to observe significant non-linearities in the QCR-induced dissipation and frequency shift. We further demonstrate that, in this regime, the QCR can be used to initialize the linear resonator into a non-thermal state even in the absence of any microwave drive

Recent Developments in Quantum‐Circuit Refrigeration

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

Compact inductor-capacitor resonators at sub-gigahertz frequencies

| openaire: EC/HE/101053801/EU//ConceptQ | openaire: EC/HE/101113946/EU//OpenSuperQPlus100Compact inductor-capacitor (LC) resonators, in contrast to coplanar waveguide (CPW) resonators, have a simple lumped-element circuit representation but usually call for sophisticated finite-element method (FEM) simulations for an accurate modeling. Here we present a simple analytical model for a family of coplanar LC resonators where the electrical properties are directly obtained from the circuit geometry with a satisfying accuracy. Our experimental results on ten high-internal-quality-factor resonators (Qi≳2×105), with frequencies ranging from 300MHz to 1GHz, show an excellent consistency with both the derived analytical model and detailed FEM simulations. These results showcase the ability to design sub-gigahertz resonators with less than 2% deviation in the resonance frequency, which has immediate applications, for example, in the implementation of ultrasensitive cryogenic detectors. The achieved compact resonator size of the order of a square millimeter indicates a feasible way to integrate hundreds of microwave resonators on a single chip for realizing photonic lattices.Peer reviewe

Hard-Gap Spectroscopy in a Self-Defined Mesoscopic InAs/Al Nanowire Josephson Junction

Superconductor-semiconductor-nanowire hybrid structures can serve as versatile building blocks to realize Majorana circuits or superconducting qubits based on quantized levels such as Andreev qubits. For all these applications, it is essential that the superconductor-semiconductor interface is as clean as possible. Furthermore, the shape and dimensions of the superconducting electrodes need to be precisely controlled. We fabricated self-defined InAs/Al core-shell nanowire junctions by a fully in-situ approach, which meet all these criteria. Transmission electron microscopy measurements confirm the sharp and clean interface between the nanowire and the in-situ deposited Al electrodes that are formed by means of shadow evaporation. Furthermore, we report on tunnel spectroscopy, gate, and magnetic field-dependent transport measurements. The achievable short junction lengths, the observed hard gap, and the magnetic field robustness make this hybrid structure very attractive for applications that rely on a precise control of the number of subgap states, like Andreev qubits or topological systems

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