Seung-Bo Shim

Evidence for unexpectedly low quasiparticle generation rates across Josephson junctions of driven superconducting qubits

  1. Byoung-moo Ann,
  2. Sang-Jun Choi,
  3. Hee Chul Park,
  4. Sercan Deve,
  5. Robin Dekker,
  6. Gary A. Steele,
  7. Jaseung Ku,
  8. Seung-Bo Shim,
  9. and Junho Suh
Microwave drives applied to superconducting qubits (SCQs) are central to high-fidelity control and fast readout. However, recent studies find that even drives far below the superconducting
gap frequency may cause drive-induced quasiparticle generation (QPG) across Josephson junctions (JJs), posing a serious concern for fault-tolerant superconducting quantum computing. Here, we find experimental evidence that the actual QPG rates in strongly driven SCQs are remarkably lower than expected. We apply intense drive fields through readout resonators, reaching effective qubit drive amplitudes up to 300 GHz. The nonlinear response of the resonators enables quantification of the energy loss from SCQs into their environments, including the contribution from QPG. Even when conservatively attributing all measured dissipation to QPG, the observed energy dissipation rates are far lower than expected from the ideal QPG model. Meanwhile, calculations incorporating high-frequency cutoffs (HFCs) near 17-20 GHz in the QPG conductance can explain the experiments. These HFCs yield QPG rates a few orders of magnitude smaller than those without HFCs, providing evidence that the QPG rates are lower than predicted by the ideal model. Our findings prevent overestimation of drive-induced QPG and provide crucial guidance for operating superconducting quantum processors. Identifying the microscopic origin of the discrepancy opens new material and device opportunities to further mitigate QPG.
arxiv pdf

Superconducting Circuitry for Quantum Electromechanical Systems

  1. Matthew D. LaHaye,
  2. Francisco Rouxinol,
  3. Yu Hao,
  4. Seung-Bo Shim,
  5. and Elinor K. Irish
Superconducting systems have a long history of use in experiments that push the frontiers of mechanical sensing. This includes both applied and fundamental research, which at present
day ranges from quantum computing research and efforts to explore Planck-scale physics to fundamental studies on the nature of motion and the quantum limits on our ability to measure it. In this paper, we first provide a short history of the role of superconducting circuitry and devices in mechanical sensing, focusing primarily on efforts in the last decade to push the study of quantum mechanics to include motion on the scale of human-made structures. This background sets the stage for the remainder of the paper, which focuses on the development of quantum electromechanical systems (QEMS) that incorporate superconducting quantum bits (qubits), superconducting transmission line resonators and flexural nanomechanical elements. In addition to providing the motivation and relevant background on the physical behavior of these systems, we discuss our recent efforts to develop a particular type of QEMS that is based upon the Cooper-pair box (CPB) and superconducting coplanar waveguide (CPW) cavities, a system which has the potential to serve as a testbed for studying the quantum properties of motion in engineered systems.
arxiv pdf