Ferdinand Kuemmeth

Efficient Qubit Calibration by Binary-Search Hamiltonian Tracking

  1. Fabrizio Berritta,
  2. Jacob Benestad,
  3. Lukas Pahl,
  4. Melvin Mathews,
  5. Jan A. Krzywda,
  6. Réouven Assouly,
  7. Youngkyu Sung,
  8. David K. Kim,
  9. Bethany M. Niedzielski,
  10. Kyle Serniak,
  11. Mollie E. Schwartz,
  12. Jonilyn L. Yoder,
  13. Anasua Chatterjee,
  14. Jeffrey A. Grover,
  15. Jeroen Danon,
  16. William D. Oliver,
  17. and Ferdinand Kuemmeth
We present a real-time method for calibrating the frequency of a resonantly driven qubit. The real-time processing capabilities of a controller dynamically compute adaptive probing
sequences for qubit-frequency estimation. Each probing time and drive frequency are calculated to divide the prior probability distribution into two branches, following a locally optimal strategy that mimics a conventional binary search. We show the algorithm’s efficacy by stabilizing a flux-tunable transmon qubit, leading to improved coherence and gate fidelity. By feeding forward the updated qubit frequency, the FPGA-powered control electronics also mitigates non-Markovian noise in the system, which is detrimental to quantum error correction. Our protocol highlights the importance of feedback in improving the calibration and stability of qubits subject to drift and can be readily applied to other qubit platforms.
arxiv pdf

Superconducting Gatemon Qubit based on a Proximitized Two-Dimensional Electron Gas

  1. Lucas Casparis,
  2. Malcolm R. Connolly,
  3. Morten Kjaergaard,
  4. Natalie J. Pearson,
  5. Anders Kringhøj,
  6. Thorvald W. Larsen,
  7. Ferdinand Kuemmeth,
  8. Tiantian Wang,
  9. Candice Thomas,
  10. Sergei Gronin,
  11. Geoffrey C. Gardner,
  12. Michael J. Manfra,
  13. Charles M. Marcus,
  14. and Karl D. Petersson
The coherent tunnelling of Cooper pairs across Josephson junctions (JJs) generates a nonlinear inductance that is used extensively in quantum information processors based on superconducting
circuits, from setting qubit transition frequencies and interqubit coupling strengths, to the gain of parametric amplifiers for quantum-limited readout. The inductance is either set by tailoring the metal-oxide dimensions of single JJs, or magnetically tuned by parallelizing multiple JJs in superconducting quantum interference devices (SQUIDs) with local current-biased flux lines. JJs based on superconductor-semiconductor hybrids represent a tantalizing all-electric alternative. The gatemon is a recently developed transmon variant which employs locally gated nanowire (NW) superconductor-semiconductor JJs for qubit control. Here, we go beyond proof-of-concept and demonstrate that semiconducting channels etched from a wafer-scale two-dimensional electron gas (2DEG) are a suitable platform for building a scalable gatemon-based quantum computer. We show 2DEG gatemons meet the requirements by performing voltage-controlled single qubit rotations and two-qubit swap operations. We measure qubit coherence times up to ~2 us, limited by dielectric loss in the 2DEG host substrate.
arxiv pdf