Implementation of a transmon qubit using superconducting granular aluminum

TL;DR

This paper demonstrates that granular aluminum can be used to create a transmon qubit with high anharmonicity and long coherence times, resilient to magnetic fields, offering a promising platform for quantum computing.

Contribution

The authors show that a small volume of granular aluminum can form a transmon qubit with significant anharmonicity and coherence, expanding the material options for superconducting qubits.

Findings

Achieved anharmonicity of 4.48 MHz in a granular aluminum transmon.

Measured qubit lifetime of 16 microseconds.

Qubit linewidth remains below 150 kHz under magnetic fields up to 70 mT.

Abstract

The high kinetic inductance offered by granular aluminum (grAl) has recently been employed for linear inductors in superconducting high-impedance qubits and kinetic inductance detectors. Due to its large critical current density compared to typical Josephson junctions, its resilience to external magnetic fields, and its low dissipation, grAl may also provide a robust source of non-linearity for strongly driven quantum circuits, topological superconductivity, and hybrid systems. Having said that, can the grAl non-linearity be sufficient to build a qubit? Here we show that a small grAl volume ($10 \times 200 \times 500 \,\mathrm{nm^3}$) shunted by a thin film aluminum capacitor results in a microwave oscillator with anharmonicity $\alpha$ two orders of magnitude larger than its spectral linewidth $\Gamma_{01}$, effectively forming a transmon qubit. With increasing drive power, we observe several multi-photon transitions starting from the ground state, from which we extract $\alpha = 2 \pi \times 4.48\,\mathrm{MHz}$. Resonance fluorescence measurements of the $|0> \rightarrow |1>$ transition yield an intrinsic qubit linewidth $\gamma = 2 \pi \times 10\,\mathrm{kHz}$, corresponding to a lifetime of $16\,\mathrm{\mu s}$. This linewidth remains below $2 \pi \times 150\,\mathrm{kHz}$ for in-plane magnetic fields up to $\sim70\,\mathrm{mT}$.