Fast flux-control of T₁-protected fluxonium qubits via tunable coherent quantum phase tunnelling

Fluxon states localized in ultra-heavy fluxonium circuits (IST qubits) have remarkably reached lifetimes exceeding three hours close to the half-flux point [1]. This exponential protection with the Josephson Junction energy holds the promise of a strongly noise-biased qubit. Yet, coherent control is challenging due to the suppressed matrix elements. In this talk, we present universal control of such T₁-protected qubits operated at both the half-flux and integer-flux points.

The first part focuses on our theoretical proposal [2] and experimental implementation of flux gates in fluxonium qubits biased at half-flux, in which the Josephson energy E_J is tuned in-situ through the flux in a dc-SQUID. This enables tunable control of the coherent quantum phase tunnelling rate to realize continuous X-rotations within a few nanoseconds. Continuous Z-rotations are implemented by tuning the flux in the rf-SQUID loop, to temporarily break the symmetry between the two fluxon states. We report single-qubit gate fidelities for our fabricated devices, in which the T₁ time in the idle regime reaches several seconds.

In the second part, we present fluxonium qubits biased at the integer-flux point. In this configuration, the logical qubit is encoded in a doublet that is gapped from the ground state. The dominant decay processes are towards the ground state and not within the logical subspace, offering a rich erasure qubit structure. The erasure error detection can be performed via the readout resonator, resulting in a hardware-efficient approach compared to other erasure qubits. Given their exponential noise-bias, the introduced qubits are promising candidates as ancilla qubits and as building blocks for scalable, low-overhead error-correcting codes.

 

[1] Hassani, Farid, et al.  Nature Communications 14.1 (2023): 3968.

[2] Siegele, Christian, et al.  arxiv (2025): 2504.04807.