Brian D. Isakov

Junction-Intrinsic Dissipation in Hybrid Superconductor-Semiconductor Gatemon Qubits

  1. Zhenhai Sun,
  2. David Feldstein-Bofill,
  3. Ksenia Shagalov,
  4. Amalie T. J. Paulsen,
  5. Casper Wied,
  6. Shikhar Singh,
  7. Brian D. Isakov,
  8. Jacob Hastrup,
  9. Christopher W. Warren,
  10. Svend Krøjer,
  11. Anders Kringhøj,
  12. András Gyenis,
  13. and Morten Kjaergaard
Superconducting transmon qubits based on hybrid superconductor-semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those
of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.
arxiv pdf

Gatemon Qubit Revisited for Improved Reliability and Stability

  1. David Feldstein-Bofill,
  2. Zhenhai Sun,
  3. Casper Wied,
  4. Shikhar Singh,
  5. Brian D. Isakov,
  6. Svend Krøjer,
  7. Jacob Hastrup,
  8. András Gyenis,
  9. and Morten Kjaergaard
The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building novel superconducting
devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit. However, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times. Here we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1 MHz precision over a range of several GHz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design.
arxiv pdf