# Dispersive readout of a weakly coupled qubit via the parity-time-symmetric phase transition

## TL;DR

This paper introduces a novel dispersive readout method for weakly coupled qubits using parity-time-symmetric phase transitions, enabling improved measurement sensitivity and reduced backaction compared to traditional techniques.

## Contribution

The authors propose leveraging parity-time symmetry and exceptional points in auxiliary cavities to enhance the dispersive readout of weakly coupled qubits, a significant advancement over conventional methods.

## Key findings

- The method amplifies qubit perturbations at the exceptional point.
- It narrows transmission spectrum linewidths for better resolution.
- Backaction on the qubit is reduced compared to strong-coupling dispersive readout.

## Abstract

For some cavity-quantum-electrodynamics systems, such as a single electron spin coupled to a passive cavity, it is challenging to reach the strong-coupling regime. In such a weak-coupling regime, the conventional dispersive readout technique cannot be used to resolve the quantum states of the spin. Here we propose an improved dispersive readout method to measure the quantum states of a weakly coupled qubit by harnessing either one or two auxiliary cavities linearly coupled to the passive cavity containing the qubit. With appropriate parameters in both cases, the system excluding the qubit can exhibit a parity-time-symmetric phase transition at the exceptional point (EP). Because the EP can amplify the perturbation induced by the qubit and the parity-time symmetry can narrow the linewidths of the peaks in the transmission spectrum of the passive cavity, we can measure the quantum states of the weakly coupled qubit via this transmission spectrum. Owing to the weak coupling between the qubit and the passive cavity, the backaction due to the measurement of the qubit can also be reduced in comparison with the conventional dispersive readout technique in the strong-coupling regime.