Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction

TL;DR

This paper introduces a new type of superconducting qubit based on a geometric superinductor, enabling precise control of phase delocalization and energy scales, with potential for quantum sensing and protected qubits.

Contribution

It demonstrates the implementation of a fluxonium qubit using a linear superinductor, offering high precision and new qubit variants with enhanced properties.

Findings

Realized fluxonium qubits with a single aluminum wire superinductor

Achieved high precision in inductive and capacitive energies

Demonstrated qubits with large magnetic dipole moments

Abstract

There are two elementary superconducting qubit types that derive directly from the quantum harmonic oscillator. In one the inductor is replaced by a nonlinear Josephson junction to realize the widely used charge qubits with a compact phase variable and a discrete charge wavefunction. In the other the junction is added in parallel, which gives rise to an extended phase variable, continuous wavefunctions and a rich energy level structure due to the loop topology. While the corresponding rf-SQUID Hamiltonian was introduced as a quadratic, quasi-1D potential approximation to describe the fluxonium qubit implemented with long Josephson junction arrays, in this work we implement it directly using a linear superinductor formed by a single uninterrupted aluminum wire. We present a large variety of qubits all stemming from the same circuit but with drastically different characteristic energy scales. This includes flux and fluxonium qubits but also the recently introduced quasi-charge qubit with strongly enhanced zero point phase fluctuations and a heavily suppressed flux dispersion. The use of a geometric inductor results in high precision of the inductive and capacitive energy as guaranteed by top-down lithography - a key ingredient for intrinsically protected superconducting qubits. The geometric fluxonium also exhibits a large magnetic dipole, which renders it an interesting new candidate for quantum sensing applications.