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arXiv:1904.00208 · Physical Review Research

Transmon qubit in a magnetic field: Evolution of coherence and transition frequency

Andre Schneider, Tim Wolz, Marco Pfirrmann, Martin Spiecker, Hannes Rotzinger, Alexey V. Ustinov, Martin Weides

Published Sep 4, 2019• 29 citations
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Abstract

Quantum bits require isolation from outer fields to maintain their coherence, and the superconducting state is destroyed by magnetic fields. In this work, however, the authors measure a superconducting transmon qubit in relatively large magnetic fields. By demonstrating quantum coherence up to field values of 40mT, new avenues in hybrid systems and sensing applications based on superconducting quantum circuits are opened up.

DOI
10.1103/physrevresearch.1.023003

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Quantum bits require isolation from outer fields to maintain their coherence, and the superconducting state is destroyed by magnetic fields. In this work, however, the authors measure a superconducting transmon qubit in relatively large magnetic fields. By demonstrating quantum coherence up to field values of 40mT, new avenues in hybrid systems and sensing applications based on superconducting quantum circuits are opened up.
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Hybrid semiconductor-superconductor systems are an appealing platform for both fundamental studies in condensed matter physics and quantum technologies. The authors demonstrate coherent operation of a hybrid superconducting qubit in magnetic fields as strong as 1 T, with the rich qubit spectrum revealing the underlying Josephson physics of these systems. The results open several avenues for research on e.g.e.g.e.g. robust Majorana transmon qubits, the probing of Andreev physics, and the coupling of superconducting qubits to spin qubits or spin ensembles.
The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 μs even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.
We propose to exploit currently available tunnel ferromagnetic Josephson junctions to realize a hybrid superconducting qubit. We show that the characteristic hysteretic behavior of the ferromagnetic barrier provides an alternative and intrinsically digital tuning of the qubit frequency by means of magnetic field pulses. To illustrate functionalities and limitation of the device, we discuss the coupling to a readout resonator and the effect of magnetic fluctuations. The possibility to use the qubit as a noise detector and its relevance to investigate the subtle interplay of magnetism and superconductivity is envisaged.
We present an experimental study of flux- and gate-tunable nanowire transmons with state-of-the-art relaxation time allowing quantitative extraction of flux and charge noise coupling to the Josephson energy. We evidence coherence sweet spots for charge, tuned by voltage on a proximal side gate, where first order sensitivity to switching two-level systems and background 1/f noise is minimized. Next, we investigate the evolution of a nanowire transmon in a parallel magnetic field up to 70 mT, the upper bound set by the closing of the induced gap. Several features observed in the field dependence of qubit energy relaxation and dephasing times are not fully understood. Using nanowires with a thinner, partially covering Al shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation and other applications.