Full characterization of measurement-induced transitions of a superconducting qubit

  1. Thomas Connolly,
  2. Pavel D. Kurilovich,
  3. Vladislav D. Kurilovich,
  4. Charlotte G. L. Bøttcher,
  5. Sumeru Hazra,
  6. Wei Dai,
  7. Andy Z. Ding,
  8. Vidul R. Joshi,
  9. Heekun Nho,
  10. Spencer Diamond,
  11. Daniel K. Weiss,
  12. Valla Fatemi,
  13. Luigi Frunzio,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
Repeated quantum non-demolition measurement is a cornerstone of quantum error correction protocols. In superconducting qubits, the speed of dispersive state readout can be enhanced
by increasing the power of the readout tone. However, such an increase has been found to result in additional qubit state transitions that violate the desired quantum non-demolition character of the measurement. Recently, the readout of a transmon superconducting qubit was improved by using a tone with frequency much larger than the qubit frequency. Here, we experimentally identify the mechanisms of readout-induced transitions in this regime. In the dominant mechanism, the energy of an incoming readout photon is partially absorbed by the transmon and partially returned to the transmission line as a photon with lower frequency. Other mechanisms involve the excitation of unwanted package modes, decay via material defects, and, at higher qubit frequencies, the activation of undesired resonances in the transmon spectrum. Our work provides a comprehensive characterization of superconducting qubit state transitions caused by a strong drive.

Recovery dynamics of a gap-engineered transmon after a quasiparticle burst

  1. Heekun Nho,
  2. Thomas Connolly,
  3. Pavel D. Kurilovich,
  4. Spencer Diamond,
  5. Charlotte G. L. Bøttcher,
  6. Leonid I. Glazman,
  7. and Michel H. Devoret
Ionizing radiation impacts create bursts of quasiparticle density in superconducting qubits. These bursts severely degrade qubit coherence for a prolonged period of time and can be
detrimental for quantum error correction. Here, we experimentally resolve quasiparticle bursts in 3D gap-engineered transmon qubits by continuously monitoring qubit transitions. Gap engineering allowed us to reduce the burst detection rate by a factor of a few. This modest reduction falls several orders of magnitude short of the reduction expected if the quasiparticles quickly thermalize to the cryostat temperature. We associate the limited effect of gap engineering with the slow thermalization of the phonons in our chips after the burst.

High-frequency readout free from transmon multi-excitation resonances

  1. Pavel D. Kurilovich,
  2. Thomas Connolly,
  3. Charlotte G. L. Bøttcher,
  4. Daniel K. Weiss,
  5. Sumeru Hazra,
  6. Vidul R. Joshi,
  7. Andy Z. Ding,
  8. Heekun Nho,
  9. Spencer Diamond,
  10. Vladislav D. Kurilovich,
  11. Wei Dai,
  12. Valla Fatemi,
  13. Luigi Frunzio,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
Quantum computation will rely on quantum error correction to counteract decoherence. Successfully implementing an error correction protocol requires the fidelity of qubit operations
to be well-above error correction thresholds. In superconducting quantum computers, measurement of the qubit state remains the lowest-fidelity operation. For the transmon, a prototypical superconducting qubit, measurement is carried out by scattering a microwave tone off the qubit. Conventionally, the frequency of this tone is of the same order as the transmon frequency. The measurement fidelity in this approach is limited by multi-excitation resonances in the transmon spectrum which are activated at high readout power. These resonances excite the qubit outside of the computational basis, violating the desired quantum non-demolition character of the measurement. Here, we find that strongly detuning the readout frequency from that of the transmon exponentially suppresses the strength of spurious multi-excitation resonances. By increasing the readout frequency up to twelve times the transmon frequency, we achieve a quantum non-demolition measurement fidelity of 99.93% with a residual probability of leakage to non-computational states of only 0.02%.

Coexistence of nonequilibrium density and equilibrium energy distribution of quasiparticles in a superconducting qubit

  1. Thomas Connolly,
  2. Pavel D. Kurilovich,
  3. Spencer Diamond,
  4. Heekun Nho,
  5. Charlotte G. L. Bøttcher,
  6. Leonid I. Glazman,
  7. Valla Fatemi,
  8. and Michel H. Devoret
The density of quasiparticles typically observed in superconducting qubits exceeds the value expected in equilibrium by many orders of magnitude. Can this out-of-equilibrium quasiparticle
density still possess an energy distribution in equilibrium with the phonon bath? Here, we answer this question affirmatively by measuring the thermal activation of charge-parity switching in a transmon qubit with a difference in superconducting gap on the two sides of the Josephson junction. We then demonstrate how the gap asymmetry of the device can be exploited to manipulate its parity.