Paal H. Prestegaard

Beta Tantalum Transmon Qubits with Quality Factors Approaching 10 Million

  1. Atharv Joshi,
  2. Apoorv Jindal,
  3. Paal H. Prestegaard,
  4. Faranak Bahrami,
  5. Elizabeth Hedrick,
  6. Matthew P. Bland,
  7. Tunmay Gerg,
  8. Guangming Cheng,
  9. Nan Yao,
  10. Robert J. Cava,
  11. Andrew A. Houck,
  12. and Nathalie P. de Leon
Tantalum-based transmon qubits are a promising platform for building large-scale quantum processors. So far, these qubits have been made from tantalum films grown exclusively in the
alpha phase ({\alpha}-Ta). The beta phase of tantalum (\{beta}-Ta) readily nucleates at room temperature, making it attractive for scalable qubit fabrication. However, \{beta}-Ta is widely believed to be detrimental to qubit performance because it has a lower superconducting critical temperature than {\alpha}-Ta. We challenge this prevailing belief by fabricating low-loss transmon qubits from \{beta}-Ta films on sapphire. Across 11 qubits, the mean time-averaged quality factor is (5.6 +/- 2.3) x 10^6, with the best qubit recording a time-averaged quality factor of (10.1 +/- 1.3) x 10^6. Resonator studies demonstrate that the dominant microwave loss channel is surface two-level systems, with the surface loss contribution for \{beta}-Ta being about twice that of {\alpha}-Ta. \{beta}-Ta films exhibit significant kinetic inductance, consistent with an estimated magnetic penetration depth of (1.78 +/- 0.02) {\mu}m. This work establishes \{beta}-Ta on sapphire as a material platform for realizing low-loss transmon qubits and other superconducting devices such as compact resonators, kinetic inductance detectors, and quasiparticle traps.
arxiv pdf

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