Evolution of Nanowire Transmons and Their Quantum Coherence in Magnetic Field

Extending the range of application of circuit QED to magnetic fields of order 0.5 T promises interesting applications, including the control and readout of Majorana systems and the coupling to electron-spin systems. In this talk, we present an experimental study of nanowire transmon qubits with state-of-the-art relaxation and dephasing times exceeding 10 μs at zero magnetic field. We investigate the evolution of transmon transition frequencies, anharmonicity, and coherence up 70 mT, limited by the closing of the superconducting gap induced in the InAs nanowire by the thick, epitaxially contacting Al shell. We investigate various sources contributing to the decoherence. In particular, we find that, unlike in conventional transmons, on-chip charge noise coupling to the Josephson energy plays a dominant role in qubit dephasing. This noise takes the form of strongly-coupled two-level systems switching on 100 ms timescale and a more weakly coupled background producing 1/f noise. We conclude with an update on ongoing efforts to extend operation to 0.5 T using nanowires contacted with thinner, partially covering Al shells.