Fast gates for bit-flip protected superconducting qubits

  1. C. A. Siegele,
  2. A.A. Sokolova,
  3. L. N. Kapoor,
  4. F. Hassani,
  5. and J. M. Fink
Superconducting qubits offer an unprecedentedly high degree of flexibility in terms of circuit encoding and parameter choices. However, in designing the qubit parameters one typically
faces the conflicting goals of long coherence times and simple control capabilities. Both are determined by the wavefunction overlap of the qubit basis states and the corresponding matrix elements. Here, we address this problem by introducing a qubit architecture with real-time tunable bit-flip protection. In the first, the `heavy‘ regime, the energy relaxation time can be on the order of hours for fluxons located in two near-degenerate ground states, as recently demonstrated in Ref. [Hassani et al., Nat.~Commun.~14 (2023)]. The second, `light‘ regime, on the other hand facilitates high-fidelity control on nanosecond timescales without the need for microwave signals. We propose two different tuning mechanisms of the qubit potential and show that base-band flux-pulses of around 10 ns are sufficient to realize a universal set of high-fidelity single- and two-qubit gates. We expect that the concept of real-time wavefunction control can also be applied to other hardware-protected qubit designs.

Single-atom maser with engineered circuit for population inversion

  1. A.A. Sokolova,
  2. G.P. Fedorov,
  3. E.V. Il'ichev,
  4. and O. V. Astafiev
We present a blueprint for a maser with a single three-level artificial atom. The artificial atom is a superconducting quantum system of a transmon layout coupled to two resonators.
The system is pumped via a two-photon process. To achieve a population inversion, we engineer the quantum system and optimize its parameters, particularly the relaxation via an auxiliary low-Q cavity coupled to a transition between two excited states. We show numerically that such a maser can operate both in the intermediate coupling regime with super-Poissonian photon statistics and in the strong coupling regime, where the statistics is sub-Poissonian. For the former, the maser exhibits thresholdless behavior and for the latter, there is a well-defined pumping threshold. An interesting side-effect of the auxiliary resonator is that it allows overcoming the photon blockage effect for the pump, which would otherwise prohibit reaching high photon population. Finally, we observe the bistability of the steady-state Wigner function and the self-quenching effect for some parameters.