G. Marchegiani

Nonequilibrium regimes for quasiparticles in superconducting qubits

  1. G. Marchegiani,
  2. and G. Catelani
Qubits with gap asymmetry larger than their transition energy are less susceptible to quasiparticle decoherence as the quasiparticles are mostly trapped in the low-gap side of the junction.
Because of this trapping, the gap asymmetry can contribute to maintaining the quasiparticles out of equilibrium. Here we address the temperature evolution of the quasiparticle densities in the two sides of the junction. We show that four qualitatively different regimes are possible with increasing temperature: i) nonequilibrium, ii) local quasiequilibrium, iii) global quasiequilibrium, and iv) full equilibrium. We identify shortcomings in assuming global quasiequilibrium when interpreting experimental data, highlighting how measurements in the presence of magnetic field can aid the accurate determination of the junction parameters, and hence the identification of the nonequilibrium regimes.
arxiv pdf

Quasiparticle effects in magnetic-field-resilient 3D transmons

  1. J. Krause,
  2. G. Marchegiani,
  3. L. M. Janssen,
  4. G. Catelani,
  5. Yoichi Ando,
  6. and C. Dickel
Recent research shows that quasiparticle-induced decoherence of superconducting qubits depends on the superconducting-gap asymmetry originating from the different thicknesses of the
top and bottom films in Al/AlOx/Al junctions. Magnetic field is a key tuning knob to investigate this dependence as it can change the superconducting gaps in situ. We present measurements of the parity-switching time of a field-resilient 3D transmon with in-plane field up to 0.41T. At low fields, small parity splitting requires qutrit pulse sequences for parity measurements. We measure a non-monotonic evolution of the parity lifetime with in-plane magnetic field, increasing up to 0.2T, followed by a decrease at higher fields. We demonstrate that the superconducting-gap asymmetry plays a crucial role in the observed behavior. At zero field, the qubit frequency is nearly resonant with the superconducting-gap difference, favoring the energy exchange with the quasiparticles and so enhancing the parity-switching rate. With a higher magnetic field, the qubit frequency decreases and gets detuned from the gap difference, causing the initial increase of the parity lifetime, while photon-assisted qubit transitions increase, producing the subsequent decrease at higher fields. Besides giving a deeper insight into the parity-switching mechanism in conventional transmon qubits, we establish that Al-AlOx-Al JJs could be used in architectures for the parity-readout and manipulation of topological qubits based on Majorana zero modes.
arxiv pdf