Denis M. Basko

Theory of quasiparticle-induced errors in driven-dissipative Schrödinger cat qubits

  1. Kirill Dubovitskii,
  2. Denis M. Basko,
  3. Julia S. Meyer,
  4. and Manuel Houzet
Understanding the mechanisms of qubit decoherence is a crucial prerequisite for improving the qubit performance. In this work we discuss the effects of residual Bogolyubov quasiparticles
in Schrödinger cat qubits, either of the dissipative or Kerr type. The major difference from previous studies of quasiparticles in superconducting qubits is that the Schrödinger cat qubits are operated under non-equilibrium conditions. Indeed, an external microwave drive is needed to stabilize „cat states“, which are superpositions of coherent degenerate eigenstates of an effective stationary Lindbladian in the rotating frame. We present a microscopic derivation of the master equation for cat qubits and express the effect of the quasiparticles as dissipators acting on the density matrix of the cat qubit. This enables us to determine the conditions under which the quasiparticles give a substantial contribution to the qubit errors.
arxiv pdf

A photonic crystal Josephson traveling wave parametric amplifier

  1. Luca Planat,
  2. Arpit Ranadive,
  3. Remy Dassonneville,
  4. Javier Puertas Martinez,
  5. Sebastien Leger,
  6. Cecile Naud,
  7. Olivier Buisson,
  8. Wiebke Hasch-Guichard,
  9. Denis M. Basko,
  10. and Nicolas Roch
An amplifier combining noise performances as close as possible to the quantum limit with large bandwidth and high saturation power is highly desirable for many solid state quantum technologies
such as high fidelity qubit readout or high sensitivity electron spin resonance for example. Here we introduce a new Traveling Wave Parametric Amplifier based on Superconducting QUantum Interference Devices. It displays a 3 GHz bandwidth, a -102 dBm 1-dB compression point and added noise near the quantum limit. Compared to previous state-of-the-art, it is an order of magnitude more compact, its characteristic impedance is in-situ tunable and its fabrication process requires only two lithography steps. The key is the engineering of a gap in the dispersion relation of the transmission line. This is obtained using a periodic modulation of the SQUID size, similarly to what is done with photonic crystals. Moreover, we provide a new theoretical treatment to describe the non-trivial interplay between non-linearity and such periodicity. Our approach provides a path to co-integration with other quantum devices such as qubits given the low footprint and easy fabrication of our amplifier.
arxiv pdf