Ugur Alyanak

Disentangling the Impact of Quasiparticles and Two-Level Systems on the Statistics of Superconducting Qubit Lifetime

  1. Shaojiang Zhu,
  2. Xinyuan You,
  3. Ugur Alyanak,
  4. Mustafa Bal,
  5. Francesco Crisa,
  6. Sabrina Garattoni,
  7. Andrei Lunin,
  8. Roman Pilipenko,
  9. Akshay Murthy,
  10. Alexander Romanenko,
  11. and Anna Grassellino
Temporal fluctuations in the superconducting qubit lifetime, T1, bring up additional challenges in building a fault-tolerant quantum computer. While the exact mechanisms remain unclear,
T1 fluctuations are generally attributed to the strong coupling between the qubit and a few near-resonant two-level systems (TLSs) that can exchange energy with an assemble of thermally fluctuating two-level fluctuators (TLFs) at low frequencies. Here, we report T1 measurements on the qubits with different geometrical footprints and surface dielectrics as a function of the temperature. By analyzing the noise spectrum of the qubit depolarization rate, Γ1=1/T1, we can disentangle the impact of TLSs, non-equilibrium quasiparticles (QPs), and equilibrium (thermally excited) QPs on the variance in Γ1. We find that Γ1 variances in the qubit with a small footprint are more susceptible to the QP and TLS fluctuations than those in the large-footprint qubits. Furthermore, the QP-induced variances in all qubits are consistent with the theoretical framework of QP diffusion and fluctuation. We suggest these findings can offer valuable insights for future qubit design and engineering optimization.
arxiv pdf

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.
arxiv pdf