Readout-induced suppression and enhancement of superconducting qubit lifetimes

  1. Ted Thorbeck,
  2. Zhihao Xiao,
  3. Archana Kamal,
  4. and Luke C.G. Govia
It has long been known that the lifetimes of superconducting qubits suffer during readout, increasing readout errors. We show that this degradation is due to the anti-Zeno effect, as
readout-induced dephasing broadens the qubit so that it overlaps ‚hot spots‘ of strong dissipation, likely due to two-level systems in the qubit’s bath. Using a flux-tunable qubit to probe the qubit’s frequency dependent loss, we accurately predict the change in lifetime during readout with a new self-consistent master equation that incorporates the modification to qubit relaxation due to measurement-induced dephasing. Moreover, we controllably demonstrate both the Zeno and anti-Zeno effects, which explain suppression and the rarer enhancement of qubit lifetimes during readout.

TLS Dynamics in a Superconducting Qubit Due to Background Ionizing Radiation

  1. Ted Thorbeck,
  2. Andrew Eddins,
  3. Isaac Lauer,
  4. Douglas T. McClure,
  5. and Malcolm Carroll
Superconducting qubit lifetimes must be both long and stable to provide an adequate foundation for quantum computing. This stability is imperiled by two-level systems (TLSs), currently
a dominant loss mechanism, which exhibit slow spectral dynamics that destabilize qubit lifetimes on hour timescales. Stability is also threatened at millisecond timescales, where ionizing radiation has recently been found to cause bursts of correlated multi-qubit decays, complicating quantum error correction. Here we study both ionizing radiation and TLS dynamics on a 27-qubit processor, repurposing the standard transmon qubits as sensors of both radiation impacts and TLS dynamics. Unlike prior literature, we observe resilience of the qubit lifetimes to the transient quasiparticles generated by the impact of radiation. However, we also observe a new interaction between these two processes, „TLS scrambling,“ in which a radiation impact causes multiple TLSs to jump in frequency, which we suggest is due to the same charge rearrangement sensed by qubits near a radiation impact. As TLS scrambling brings TLSs out of or in to resonance with the qubit, the lifetime of the qubit increases or decreases. Our findings thus identify radiation as a new contribution to fluctuations in qubit lifetimes, with implications for efforts to characterize and improve device stability

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.