Digital coherent control of a superconducting qubit

  1. Edward Leonard Jr.,
  2. Matthew A. Beck,
  3. JJ Nelson,
  4. Brad G. Christensen,
  5. Ted Thorbeck,
  6. Caleb Howington,
  7. Alexander Opremcak,
  8. Ivan V. Pechenezhskiy,
  9. Kenneth Dodge,
  10. Nicholas P. Dupuis,
  11. Jaseung Ku,
  12. Francisco Schlenker,
  13. Joseph Suttle,
  14. Christopher Wilen,
  15. Shaojiang Zhu,
  16. Maxim G. Vavilov,
  17. Britton L. T. Plourde,
  18. and Robert McDermott
High-fidelity gate operations are essential to the realization of a fault-tolerant quantum computer. In addition, the physical resources required to implement gates must scale efficiently
with system size. A longstanding goal of the superconducting qubit community is the tight integration of a superconducting quantum circuit with a proximal classical cryogenic control system. Here we implement coherent control of a superconducting transmon qubit using a Single Flux Quantum (SFQ) pulse driver cofabricated on the qubit chip. The pulse driver delivers trains of quantized flux pulses to the qubit through a weak capacitive coupling; coherent rotations of the qubit state are realized when the pulse-to-pulse timing is matched to a multiple of the qubit oscillation period. We measure the fidelity of SFQ-based gates to be ~95% using interleaved randomized benchmarking. Gate fidelities are limited by quasiparticle generation in the dissipative SFQ driver. We characterize the dissipative and dispersive contributions of the quasiparticle admittance and discuss mitigation strategies to suppress quasiparticle poisoning. These results open the door to integration of large-scale superconducting qubit arrays with SFQ control elements for low-latency feedback and stabilization.

A tunable quantum dissipator for active resonator reset in circuit QED

  1. Clement H. Wong,
  2. Chris Wilen,
  3. Robert McDermott,
  4. and Maxim G. Vavilov
We propose a method for fast, deterministic resonator reset based on tunable dissipative modes. The dissipator is based on a Josephson junction with relatively low quality factor. When
the dissipator is tuned into resonance with a high quality microwave resonator, resonator photons are absorbed by the dissipator at a rate orders of magnitude faster than the resonator relaxation rate. We determine the optimal parameters for realization of the tunable dissipator, and examine application of the dissipator to removing spurious photon population in the qubit readout resonator in circuit quantum electrodynamics. We show that even in the nonlinear large photon occupation regime, this enhanced resonator decay rate can be attained by appropriate modulation of the dissipator frequency.