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.

High fidelity single-shot readout of a transmon qubit using a SLUG μwave amplifier

  1. Yanbing Liu,
  2. Srikanth Srinivasan,
  3. D. Hover,
  4. Shaojiang Zhu,
  5. R. McDermott,
  6. and A. A. Houck
We report high-fidelity, quantum nondemolition, single-shot readout of a superconducting transmon qubit using a DC-biased superconducting low-inductance undulatory galvanometer(SLUG)
amplifier. The SLUG improves the system signal-to-noise ratio by 7 dB in a 20 MHz window compared with a bare HEMT amplifier. An optimal cavity drive pulse is chosen using a genetic search algorithm, leading to a maximum combined readout and preparation fidelity of 91.9% with a measurement time of Tmeas = 200ns. Using post-selection to remove preparation errors caused by heating, we realize a combined preparation and readout fidelity of 94.3%.