Simultaneous sweet-spot locking of gradiometric fluxonium qubits

/2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux-locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux bias within Φ

_{eff}

=−3 ×10

^{−4}

⁢Φ

_0

from the target, corresponding to only 15% degradation in 𝑇

_{2,𝐸}

when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing crosstalk and potentially eliminating the need for local flux bias.

DOI: 10.5445/ir/1000187229

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_0

_{eff}

=−3 ×10

^{−4}

⁢Φ

_0

from the target, corresponding to only 15% degradation in 𝑇

_{2,𝐸}

T_1

and pure dephasing

T_φ

, thereby quantifying the tradeoff between them. For fluxonium qubits with realistic noise spectra, our method enhances

T_φ

by a factor of 3-5 compared with existing DSS strategies while maintaining

T_1

in the hundred-microsecond range. We further prove that, although DSSs eliminate first-order sensitivity to low-frequency noise, relaxation rate cannot be reduced arbitrarily close to zero, establishing an upper bound on achievable

T_1

. At the optimized working points, we identify double-DSS regions that are insensitive to both DC and AC flux, providing robust operating bands for experiments. As applications, we design single- and two-qubit control protocols at these operating points and numerically demonstrate high-fidelity gate operations. These results establish a general and useful framework for Pareto-front engineering of DSSs that substantially improves coherence and gate performance in superconducting qubits.