Simultaneous sweet-spot locking of gradiometric fluxonium qubits

TL;DR

This paper presents a technique for simultaneously locking multiple gradiometric fluxonium qubits at their sweet spots, reducing crosstalk and simplifying integration in superconducting quantum processors.

Contribution

The authors demonstrate a flux locking method using symmetric gradiometric loops with trapped fluxons, achieving stable biasing with minimal external field influence.

Findings

Effective flux bias within 15% of target achieved

Locking reduces crosstalk and simplifies qubit integration

Minimal degradation of coherence time ($T_{2,E}$)

Abstract

Efforts to scale up superconducting processors that employ flux-qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary to bias the qubits at the zero-field and $\Phi_0/2$ 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 $\Phi_{eff} = -3 \times 10^{-4} \Phi_0$ 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 cross-talk and potentially eliminating the need for local flux bias.