Shantanu R. Jha

Theory of Quasiparticle Generation by Microwave Drives in Superconducting Qubits

  1. Shoumik Chowdhury,
  2. Max Hays,
  3. Shantanu R. Jha,
  4. Kyle Serniak,
  5. Terry P. Orlando,
  6. Jeffrey A. Grover,
  7. and William D. Oliver
Microwave drives are commonly employed to control superconducting quantum circuits, enabling qubit gates, readout, and parametric interactions. As the drive frequencies are typically
an order of magnitude smaller than (twice) the superconducting gap, it is generally assumed that such drives do not disturb the BCS ground state. However, sufficiently strong drives can activate multi-photon pair-breaking processes that generate quasiparticles and result in qubit errors. In this work, we present a theoretical framework for calculating the rates of multi-photon-assisted pair-breaking transitions induced by both charge- and flux-coupled microwave drives. Through illustrative examples, we show that drive-induced QP generation may impact novel high-frequency dispersive readout architectures, as well as Floquet-engineered superconducting circuits operating under strong driving conditions.
arxiv pdf

Fast Universal Control of an Oscillator with Weak Dispersive Coupling to a Qubit

  1. Alec Eickbusch,
  2. Volodymyr Sivak,
  3. Andy Z. Ding,
  4. Salvatore S. Elder,
  5. Shantanu R. Jha,
  6. Jayameenakshi Venkatraman,
  7. Baptiste Royer,
  8. S. M. Girvin,
  9. Robert J. Schoelkopf,
  10. and Michel H. Devoret
Efficient quantum control of an oscillator is necessary for many bosonic applications including error-corrected computation, quantum-enhanced sensing, robust quantum communication,
and quantum simulation. For these applications, oscillator control is often realized through off-resonant hybridization to a qubit with dispersive shift χ where typical operation times of 2π/χ are routinely assumed. Here, we challenge this assumption by introducing and demonstrating a novel control method with typical operation times over an order of magnitude faster than 2π/χ. Using large auxiliary displacements of the oscillator to enhance gate speed, we introduce a universal gate set with built-in dynamical decoupling consisting of fast conditional displacements and qubit rotations. We demonstrate the method using a superconducting cavity weakly coupled to a transmon qubit in a regime where previously known methods would fail. Our demonstrations include preparation of a single-photon state 30 times faster than 2π/χ with 98±1(%) fidelity and preparation of squeezed vacuum with a squeezing level of 11.1 dB, the largest intracavity squeezing reported in the microwave regime. Finally, we demonstrate fast measurement-free preparation of logical states for the binomial and Gottesman-Kitaev-Preskill (GKP) code, and we identify possible fidelity limiting mechanisms including oscillator dephasing.
arxiv pdf