# Microwave spectroscopy of a carbon nanotube charge qubit

## TL;DR

This paper demonstrates microwave spectroscopy of a carbon nanotube charge qubit, combining radio-frequency reflectometry with microwave control to improve readout and quantify charge coherence and relaxation times.

## Contribution

It introduces a microwave spectroscopy method for a carbon nanotube charge qubit, enabling efficient readout and coherence characterization in these quantum devices.

## Key findings

- Charge coherence time of 5 ns
- Charge relaxation time of 48 ns
- Operation at a noise-insensitive point

## Abstract

Carbon nanotube quantum dots allow accurate control of electron charge, spin and valley degrees of freedom in a material which is atomically perfect and can be grown isotopically pure. These properties underlie the unique potential of carbon nanotubes for quantum information processing, but developing nanotube charge, spin, or spin-valley qubits requires efficient readout techniques as well as understanding and extending quantum coherence in these devices. Here, we report on microwave spectroscopy of a carbon nanotube charge qubit in which quantum information is encoded in the spatial position of an electron. We combine radio-frequency reflectometry measurements of the quantum capacitance of the device with microwave manipulation to drive transitions between the qubit states. This approach simplifies charge-state readout and allows us to operate the device at an optimal point where the qubit is first-order insensitive to charge noise. From these measurements, we are able to quantify the degree of charge noise experienced by the qubit and obtain an inhomogeneous charge coherence of 5 ns. We use a chopped microwave signal whose duty-cycle period is varied to measure the decay of the qubit states, yielding a charge relaxation time of 48 ns.