Yanhao Wang

Rhenium as a material platform for long-lived transmon qubits

  1. Yanhao Wang,
  2. Suhas Ganjam,
  3. Ishan Narra,
  4. Luigi Frunzio,
  5. and Robert J. Schoelkopf
Dielectric loss at the interfaces of superconducting films has long been recognized as limiting the performance of state-of-the-art superconducting circuits. Notably, the presence of
a native oxide layer on the film is hypothesized to contribute to dielectric loss at the metal-air interface. Here, we explore rhenium as a candidate for the film, motivated by its remarkable property to suppress native oxide formation. We demonstrate rhenium on sapphire as a promising material platform for superconducting circuits through the realization of transmons with mean relaxation times T1 up to 407 microseconds at 5 GHz. Our transmons are supplemented with a loss characterization study, in which we separate the dominant loss mechanisms and construct a loss budget that agrees with our T1 measurements. Further characterization may establish rhenium as a leading candidate for maximizing decoherence time.
arxiv pdf

Low loss lumped-element inductors made from granular aluminum

  1. Vishakha Gupta,
  2. Patrick Winkel,
  3. Neel Thakur,
  4. Peter van Vlaanderen,
  5. Yanhao Wang,
  6. Suhas Ganjam,
  7. Luigi Frunzio,
  8. and Robert J. Schoelkopf
Lumped-element inductors are an integral component in the circuit QED toolbox. However, it is challenging to build inductors that are simultaneously compact, linear and low-loss with
standard approaches that either rely on the geometric inductance of superconducting thin films or on the kinetic inductance of Josephson junctions arrays. In this work, we overcome this challenge by utilizing the high kinetic inductance offered by superconducting granular aluminum (grAl). We demonstrate lumped-element inductors with a few nH of inductance that are up to 100 times more compact than inductors built from pure aluminum (Al). To characterize the properties of these linear inductors, we first report on the performance of lumped-element resonators built entirely out of grAl with sheet inductances varying from 30−320pH/sq and self-Kerr non-linearities of 0.2−20Hz/photon. Further, we demonstrate ex-situ integration of these grAl inductors into hybrid resonators with Al or tantalum (Ta) capacitor electrodes without increasing total internal losses. Interestingly, the measured internal quality factors systematically decrease with increasing room-temperature resistivity of the grAl film for all devices, indicating a trade-off between compactness and internal loss. For our lowest resistivity grAl films, we measure quality factors reaching 3.5×106 for the all-grAl devices and 4.5×106 for the hybrid grAl/Ta devices, similar to state-of-the-art quantum circuits. Our loss analysis suggests that the surface loss factor of grAl is similar to that of pure Al for our lowest resistivity films, while the increasing losses with resistivity could be explained by increasing conductor loss in the grAl film.
arxiv pdf