Shoumik Chowdhury

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

Dynamical sweet spot engineering via two-tone flux modulation of superconducting qubits

  1. Joseph A. Valery,
  2. Shoumik Chowdhury,
  3. Glenn Jones,
  4. and Nicolas Didier
Current superconducting quantum processors require strategies for coping with material defects and imperfect parameter targeting in order to scale up while maintaining high performance.
To that end, in-situ control of qubit frequencies with magnetic flux can be used to avoid spurious resonances. However, increased dephasing due to 1/f flux noise limits performance at all of these operating points except for noise-protected sweet spots, which are sparse under DC flux bias and monochromatic flux modulation. Here we experimentally demonstrate that two-tone flux modulation can be used to create a continuum of dynamical sweet spots, greatly expanding the range of qubit frequencies achievable while first-order insensitive to slow flux noise. To illustrate some advantages of this flexibility, we use bichromatic flux control to reduce the error rates and gate times of parametric entangling operations between transmons. Independent of gate scheme, the ability to use flux control to freely select qubit frequencies while maintaining qubit coherence represents an important step forward in the robustness and scalability of near-term superconducting qubit devices.
arxiv pdf