Granular aluminum: A superconducting material for high impedance quantum circuits

TL;DR

This paper introduces a fluxonium qubit using granular aluminum as a superinductor, achieving high impedance and coherence times, offering a promising material for advanced quantum circuits.

Contribution

The study demonstrates the use of granular aluminum as a self-assembled superinductor in fluxonium qubits, enabling high impedance and coherence with compatible fabrication processes.

Findings

Achieved qubit coherence time up to 30 microseconds.

Granular aluminum provides high kinetic inductance suitable for superinductors.

Compatible with standard aluminum circuit fabrication.

Abstract

Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, of about 100 Ohm, and small anharmonicity, which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum $R_Q = 6.4$ k$\Omega$. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here we present a fluxonium qubit design using a granular aluminum (grAl) superinductor strip. Granular aluminum is a particularly attractive material, as it self-assembles into an effective junction array with a remarkably high kinetic inductance, and its fabrication can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time $T_2^R$ up to 30 $\mu$s illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors.