Svend Krøjer

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

Fast universal control of a flux qubit via exponentially tunable wave-function overlap

  1. Svend Krøjer,
  2. Anders Enevold Dahl,
  3. Kasper Sangild Christensen,
  4. Morten Kjaergaard,
  5. and Karsten Flensberg
Fast, high fidelity control and readout of protected superconducting qubits are fundamentally challenging due to their inherent insensitivity. We propose a flux qubit variation which
enjoys a tunable level of protection against relaxation to resolve this outstanding issue. Our qubit design, the double-shunted flux qubit (DSFQ), realizes a generic double-well potential through its three junction ring geometry. One of the junctions is tunable, making it possible to control the barrier height and thus the level of protection. We analyze single- and two-qubit gate operations that rely on lowering the barrier. We show that this is a viable method that results in high fidelity gates as the non-computational states are not occupied during operations. Further, we show how the effective coupling to a readout resonator can be controlled by adjusting the externally applied flux while the DSFQ is protected from decaying into the readout resonator. Finally, we also study a double-loop gradiometric version of the DSFQ which is exponentially insensitive to variations in the global magnetic field, even when the loop areas are non-identical.
arxiv pdf