Reducing TLS loss in tantalum CPW resonators using titanium sacrificial layers

  1. Zachary Degnan,
  2. Chun-Ching Chiu,
  3. Yi-Hsun Chen,
  4. David Sommers,
  5. Leonid Abdurakhimov,
  6. Lihuang Zhu,
  7. Arkady Fedorov,
  8. and Peter Jacobson
We demonstrate a substantial reduction in two-level system loss in tantalum coplanar waveguide resonators fabricated on high-resistivity silicon substrates through the use of an ultrathin
titanium sacrificial layer. A 0.2nm titanium film, deposited atop pre-sputtered {\alpha}-tantalum, acts as a solid-state oxygen getter that chemically modifies the native Ta oxide at the metal-air interface. After device fabrication, the titanium layer is removed using buffered oxide etchant, leaving behind a chemically reduced Ta oxide surface. Subsequent high-vacuum annealing further suppresses two-level system loss. Resonators treated with this process exhibit internal quality factors Qi exceeding an average of 1.5 million in the single-photon regime across ten devices, over three times higher than otherwise identical devices lacking the titanium layer. These results highlight the critical role of interfacial oxide chemistry in superconducting loss and reinforce atomic-scale surface engineering as an effective approach to improving coherence in tantalum-based quantum circuits. The method is compatible with existing fabrication workflows applicable to tantalum films, offering a practical route to further extending T1 lifetimes in superconducting qubits.

Near-field localization of the boson peak on tantalum films for superconducting quantum devices

  1. Xiao Guo,
  2. Zachary Degnan,
  3. Julian Steele,
  4. Eduardo Solano,
  5. Bogdan C. Donose,
  6. Karl Bertling,
  7. Arkady Fedorov,
  8. Aleksandar D. Rakić,
  9. and Peter Jacobson
Superconducting circuits are among the most advanced quantum computing technologies, however their performance is currently limited by losses found in surface oxides and disordered
materials. Here, we identify and spatially localize a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy (s-SNOM). Making use of terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle x-ray scattering (GIWAXS) shows that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. By localizing defect centers at the nanoscale, our characterization techniques and results will inform the optimization of fabrication procedures for new low-loss superconducting circuits.