Stabilizing and improving qubit coherence by engineering noise spectrum of two-level systems

  1. Xinyuan You,
  2. Ziwen Huang,
  3. Ugur Alyanak,
  4. Alexander Romanenko,
  5. Anna Grassellino,
  6. and Shaojiang Zhu
The coherence times of many widely used superconducting qubits are limited by material defects that can be modeled as an ensemble of two-level systems (TLSs). Among them, charge fluctuators
inside amorphous oxide layers are believed to contribute to both low-frequency 1/f charge noise and high-frequency dielectric loss, causing fast qubit dephasing and relaxation. Here, we propose to mitigate those noise channels by engineering the relevant TLS noise spectral densities. Specifically, our protocols smooth the high-frequency noise spectrum and suppress the low-frequency noise amplitude via relaxing and dephasing the TLSs, respectively. As a result, we predict a drastic stabilization in qubit lifetime and an increase in qubit pure dephasing time. Our detailed analysis of feasible experimental implementations shows that the improvement is not compromised by spurious coupling from the applied noise to the qubit.

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%.