arXiv subjects

A. Kringhøj

Publications and source records attributed to A. Kringhøj.

10 recordsLinked to original sources

arXiv2022

Gate-Tunable Transmon Using Selective-Area-Grown Superconductor-Semiconductor Hybrid Structures on Silicon

We present a gate-voltage tunable transmon qubit (gatemon) based on planar InAs nanowires that are selectively grown on a high resistivity silicon substrate using III-V buffer layers. We show that low loss superconducting resonators with an internal quality of $2\times 10^5$ can readily be realized using these substrates after the removal of buffer layers. We demonstrate coherent control and readout of a gatemon device with a relaxation time, $T_{1}\approx 700\,\mathrm{ns}$, and dephasing times, $T_2^{\ast}\approx 20\,\mathrm{ns}$ and $T_{\mathrm{2,echo}} \approx 1.3\,\mathrm{μs}$. Further, we infer a high junction transparency of $0.4 - 0.9$ from an analysis of the qubit anharmonicity.

cond-mat.mes-hall↗

arXiv2022

Parity switching in a full-shell superconductor-semiconductor nanowire qubit

The rate of charge-parity switching in a full-shell superconductor-semiconductor nanowire qubit is measured by directly monitoring the dispersive shift of a readout resonator. At zero magnetic field, the measured switching time scale $T_P$ is on the order of 100 ms. Two-tone spectroscopy data post-selected on charge-parity is demonstrated. With increasing temperature or magnetic field, TP is at first constant, then exponentially suppressed, consistent with a model that includes both non-equilibrium and thermally activated quasiparticles. As TP is suppressed, qubit lifetime T1 also decreases. The long $T_P\sim 0.1$ s at zero field is promising for future development of qubits based on hybrid nanowires.

cond-mat.mes-hall↗

arXiv2022

Signatures of a topological phase transition in a planar Josephson junction

A growing body of work suggests that planar Josephson junctions fabricated using superconducting hybrid materials provide a highly controllable route toward one-dimensional topological superconductivity. Among the experimental controls are in-plane magnetic field, phase difference across the junction, and carrier density set by electrostatic gate voltages. Here, we investigate planar Josephson junctions with an improved design based on an epitaxial InAs/Al heterostructure, embedded in a superconducting loop, probed with integrated quantum point contacts (QPCs) at both ends of the junction. For particular ranges of in-plane field and gate voltages, a closing and reopening of the superconducting gap is observed, along with a zero-bias conductance peak (ZBCP) that appears upon reopening of the gap. Consistency with a simple theoretical model supports the interpretation of a topological phase transition. While gap closings and reopenings generally occurred together at the two ends of the junction, the height, shape, and even presence of ZBCPs typically differed between the ends, presumably due to disorder and variation of couplings to local probes.

cond-mat.mes-hall↗

arXiv2021

Magnetic-Field-Compatible Superconducting Transmon Qubit

We present a hybrid semiconductor-based superconducting qubit device which remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations with magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.

cond-mat.mes-hall↗

arXiv2020

Andreev Modes from Phase Winding in a Full-shell Nanowire-based Transmon

We investigate transmon qubits made from semiconductor nanowires with a fully surrounding superconducting shell. In the regime of reentrant superconductivity associated with the destructive Little-Parks effect, numerous coherent transitions are observed in the first reentrant lobe, where the shell carries 2π winding of superconducting phase, and are absent in the zeroth lobe. As junction density was increased by gate voltage, qubit coherence was suppressed then lost in the first lobe. These observations and numerical simulations highlight the role of winding-induced Andreev states in the junction.

cond-mat.mes-hall↗

arXiv2020

A Parity-Protected Superconductor-Semiconductor Qubit

Coherence of superconducting qubits can be improved by implementing designs that protect the parity of Cooper pairs on superconducting islands. Here, we introduce a parity-protected qubit based on voltage-controlled semiconductor nanowire Josephson junctions, taking advantage of the higher harmonic content in the energy-phase relation of few-channel junctions. A symmetric interferometer formed by two such junctions, gate-tuned into balance and frustrated by a half-quantum of applied flux, yields a cos(2ϕ) Josephson element, reflecting coherent transport of pairs of Cooper pairs. We demonstrate that relaxation of the qubit can be suppressed tenfold by tuning into the protected regime.

cond-mat.mes-hall↗

arXiv2019

Suppressed Charge Dispersion via Resonant Tunneling in a Single-Channel Transmon

We demonstrate strong suppression of charge dispersion in a semiconductor-based transmon qubit across Josephson resonances associated with a quantum dot in the junction. On resonance, dispersion is drastically reduced compared to conventional transmons with corresponding Josephson and charging energies. We develop a model of qubit dispersion for a single-channel resonance, which is in quantitative agreement with experimental data.

cond-mat.mes-hall↗

arXiv2019

Controlled DC Monitoring of a Superconducting Qubit

Creating a transmon qubit using semiconductor-superconductor hybrid materials not only provides electrostatic control of the qubit frequency, it also allows parts of the circuit to be electrically connected and disconnected in situ by operating a semiconductor region of the device as a field-effect transistor (FET). Here, we exploit this feature to compare in the same device characteristics of the qubit, such as frequency and relaxation time, with related transport properties such as critical supercurrent and normal-state resistance. Gradually opening the FET to the monitoring circuit allows the influence of weak-to-strong DC monitoring of a live qubit to be measured. A model of this influence yields excellent agreement with experiment, demonstrating a relaxation rate mediated by a gate-controlled environmental coupling.

cond-mat.mes-hall↗

arXiv2018

Voltage-Controlled Superconducting Quantum Bus

We demonstrate the ability of an epitaxial semiconductor-superconductor nanowire to serve as a field-effect switch to tune a superconducting cavity. Two superconducting gatemon qubits are coupled to the cavity, which acts as a quantum bus. Using a gate voltage to control the superconducting switch yields up to a factor of 8 change in qubit-qubit coupling between the on and off states without detrimental effect on qubit coherence. High-bandwidth operation of the coupling switch on nanosecond timescales degrades qubit coherence.

cond-mat.mes-hall↗

arXiv2017

Anharmonicity of a Gatemon Qubit with a Few-Mode Josephson Junction

Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of two compared to conventional metallic junctions, and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.

cond-mat.mes-hall↗