Gatemon qubit in a narrow Ge/Si core/shell nanowire*

Gatemon qubits are conceptually similar to the more standard superconducting transmon qubits, with the metallic Josephson junction (JJ) replaced by a gate-tunable superconducting weak link [1,2], which allows one to tune the qubit transition frequency by a simple gate voltage.  Currently, gatemons are mostly realized in InAs platforms, with a unique epitaxially grown aluminum layer providing a high-quality interface. Here, we demonstrate the full functionality of a gatemon qubit based on Ge/Si core/shell nanowires (NWs), exploiting the high-quality JJs we obtain in a very simple, ex-situ annealing step in which superconducting Al penetrates into the Ge core of a NW from the reservoirs on both sides. We report full coherent control of the gatemon qubit, with energy relaxation times up to ~1.27 μs. In addition, we characterize the anharmonicity of the excitation spectrum in the device and show that the supercurrent through the nanowire junction is dominated by at most two conducting channels. Our results establish Ge/Si core/shell nanowires as a unique platform for novel quantum technologies, potentially also useful for other types of qubits like Andreev spin qubits [3], or for circuitQED experiments [4] with hole-carriers sporting a very large spin-orbit interaction.

[1] Physical review letters 115, 127001 (2015).

[2] Physical review letters 115, 127002 (2015).

[3] Science 373, 430 (2021).

[4] Physical Review Applied 15, 064050 (2021).