Elizabeth Hedrick

2D transmons with lifetimes and coherence times exceeding 1 millisecond

  1. Matthew P. Bland,
  2. Faranak Bahrami,
  3. Jeronimo G.C. Martinez,
  4. Paal H. Prestegaard,
  5. Basil M. Smitham,
  6. Atharv Joshi,
  7. Elizabeth Hedrick,
  8. Alex Pakpour-Tabrizi,
  9. Shashwat Kumar,
  10. Apoorv Jindal,
  11. Ray D. Chang,
  12. Ambrose Yang,
  13. Guangming Cheng,
  14. Nan Yao,
  15. Robert J. Cava,
  16. Nathalie P. de Leon,
  17. and Andrew A. Houck
Materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because such improvements can be readily translated to large scale processors.
Recent work improved transmon coherence by utilizing tantalum (Ta) as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.
arxiv pdf

Machine-guided Design of Oxidation Resistant Superconductors for Quantum Information Applications

  1. Carson Koppel,
  2. Brandon Wilfong,
  3. Allana Iwanicki,
  4. Elizabeth Hedrick,
  5. Tanya Berry,
  6. and Tyrel M.McQueen
Decoherence in superconducting qubits has long been attributed to two level systems arising from the surfaces and interfaces present in real devices. A recent significant step in reducing
decoherence was the replacement of superconducting niobium by superconducting tantalum, resulting in a tripling of transmon qubit lifetimes (T1). One of these surface variables, the identity, thickness, and quality of the native surface oxide, is thought to play a major role as tantalum only has one oxide whereas niobium has several. Here we report the development of a thermodynamic metric to rank materials based on their potential to form a well-defined, thin, surface oxide. We first compute this metric for known binary and ternary metal alloys using data available from Materials Project, and experimentally validate the strengths and limits of this metric through preparation and controlled oxidation of 8 known metal alloys. Then we train a convolutional neural network to predict the value of this metric from atomic composition and atomic properties. This allows us to compute the metric for materials that are not present in materials project, including a large selection of known superconductors, and, when combined with Tc, allow us to identify new candidate superconductors for quantum information science (QISE) applications. We test the oxidation resistance of a pair of these predictions experimentally. Our results are expected to lay the foundation for tailored and rapid selection of improved superconductors for QISE.
arxiv pdf