Fang Zhao

Niobium coaxial cavities with internal quality factors exceeding 1.5 billion for circuit quantum electrodynamics

  1. Andrew E. Oriani,
  2. Fang Zhao,
  3. Tanay Roy,
  4. Alexander Anferov,
  5. Kevin He,
  6. Ankur Agrawal,
  7. Riju Banerjee,
  8. Srivatsan Chakram,
  9. and David I. Schuster
Group-V materials such as niobium and tantalum have become popular choices for extending the performance of circuit quantum electrodynamics (cQED) platforms allowing for quantum processors
and memories with reduced error rates and more modes. The complex surface chemistry of niobium however makes identifying the main modes of decoherence difficult at millikelvin temperatures and single-photon powers. We use niobium coaxial quarter-wave cavities to study the impact of etch chemistry, prolonged atmospheric exposure, and the significance of cavity conditions prior to and during cooldown, in particular niobium hydride evolution, on single-photon coherence. We demonstrate cavities with quality factors of Qint≳1.4×109 in the single-photon regime, a 15 fold improvement over aluminum cavities of the same geometry. We rigorously quantify the sensitivity of our fabrication process to various loss mechanisms and demonstrate a 2−4× reduction in the two-level system (TLS) loss tangent and a 3−5× improvement in the residual resistivity over traditional BCP etching techniques. Finally, we demonstrate transmon integration and coherent cavity control while maintaining a cavity coherence of \SI{11.3}{ms}. The accessibility of our method, which can easily be replicated in academic-lab settings, and the demonstration of its performance mark an advancement in 3D cQED.
arxiv pdf

Improved Coherence in Optically-Defined Niobium Trilayer Junction Qubits

  1. Alexander Anferov,
  2. Kan-Heng Lee,
  3. Fang Zhao,
  4. Jonathan Simon,
  5. and David I. Schuster
Niobium offers the benefit of increased operating temperatures and frequencies for Josephson junctions, which are the core component of superconducting devices. However existing niobium
processes are limited by more complicated fabrication methods and higher losses than now-standard aluminum junctions. Combining recent trilayer fabrication advancements, methods to remove lossy dielectrics and modern superconducting qubit design, we revisit niobium trilayer junctions and fabricate all-niobium transmons using only optical lithography. We characterize devices in the microwave domain, measuring coherence times up to 62 μs and an average qubit quality factor above 105: much closer to state-of-the-art aluminum-junction devices. We find the higher superconducting gap energy also results in reduced quasiparticle sensitivity above 0.16 K, where aluminum junction performance deteriorates. Our low-loss junction process is readily applied to standard optical-based foundry processes, opening new avenues for direct integration and scalability, and paves the way for higher-temperature and higher-frequency quantum devices.
arxiv pdf