# Broadband microwave spectroscopy of semiconductor nanowire-based Cooper-pair transistors

## TL;DR

This study demonstrates control over the energy structure of a semiconductor nanowire-based Cooper-pair transistor, revealing insights into its quantum dynamics and parity occupation, with implications for superconducting and topological qubits.

## Contribution

It provides the first direct mapping of the energy level structure of an indium arsenide nanowire-based CPT with detailed quantum modeling and temperature-dependent parity measurements.

## Key findings

- Controlled the Josephson energy via gate-tunable semiconductor channels.
- Mapped the energy level structure of the nanowire CPT.
- Measured the parity occupation ratio as a function of temperature.

## Abstract

The Cooper-pair transistor (CPT), a small superconducting island enclosed between two Josephson weak links, is the atomic building block of various superconducting quantum circuits. Utilizing gate-tunable semiconductor channels as weak links, the energy scale associated with the Josephson tunneling can be changed with respect to the charging energy of the island, tuning the extent of its charge fluctuations. Here, we directly demonstrate this control by mapping the energy level structure of a CPT made of an indium arsenide nanowire (NW) with a superconducting aluminum shell. We extract the device parameters based on the exhaustive modeling of the quantum dynamics of the phase-biased nanowire CPT and directly measure the even-odd parity occupation ratio as a function of the device temperature, relevant for superconducting and prospective topological qubits.