Heavy Fluxonium Qubit

Description

Heavy fluxonium is a fluxonium regime engineered for maximal coherence by increasing effective mass in phase space (large capacitance, moderate $E_J/E_C$), producing wavefunctions with disjoint support and suppressed relaxation matrix elements.

Compared with conventional fluxonium, heavy fluxonium prioritizes coherence and error bias over speed, and has demonstrated millisecond-scale $T_1$ in sweet-spot operation.

Figure

Hamiltonian

Same base fluxonium Hamiltonian:

$H=4E_C{n}^2-E_J\cos \phi +\frac{1}{2}{E}_L(\phi -2\pi {\Phi }_{ext}/{\Phi }_0{)}^2$

Heavy-fluxonium regime typically uses lower $E_C$ and operating points where wavefunctions are localized in different wells, suppressing dipole matrix elements.

Motivation

Heavy fluxonium targets a “hardware-protected” operating regime where coherence is improved by circuit design rather than solely by control optimization. By engineering large effective mass in phase space and operating at sweet spots, matrix elements for dominant noise channels are strongly suppressed. This architecture is a practical bridge between conventional superconducting qubits and more strongly protected designs such as 0-π.

Key Findings

• Millisecond-scale $T_1$ has been reported in heavy-fluxonium-like parameter regimes.
• Large anharmonicity enables selective control with low leakage.
• Sweet-spot operation reduces flux-noise sensitivity to first order.
• Heavy fluxonium is a strong candidate for low-error bosonic/encoded hybrid stacks.

Key Metrics

Metric	Value	Notes	Fidelity reference
$T_1$	0.5–1.5 ms	Sweet-spot operation	Nguyen et al. 2019
1Q fidelity	99.9%+	Microwave control	Somoroff et al. 2023
Anharmonicity	GHz-scale	Much larger than transmon	—
Operating temperature	10–20 mK	Dilution refrigerator	—

Linked Papers

• lin-2018-heavy-fluxonium

Related Entries

• fluxonium
• 0-pi-qubit
• transmon