# Quantum-limited measurement of spin qubits via curvature coupling to a cavity

## TL;DR

This paper proposes a novel quantum non-demolition readout method for spin qubits using curvature coupling to a microwave resonator, enabling high-fidelity measurement while maintaining qubit stability at the charge noise sweet-spot.

## Contribution

It introduces a curvature coupling approach for spin qubits that allows dispersive-like readout without Purcell effect, and enables entanglement via measurement with gate voltage modulation.

## Key findings

- Achieves measurement strength of tens to hundreds of MHz.
- Maintains qubit at full charge noise sweet-spot with zero dipole moment.
- Enables selective longitudinal readout and multi-qubit entanglement.

## Abstract

We investigate coupling an encoded spin qubit to a microwave resonator via qubit energy level curvature versus gate voltage. This approach enables quantum non-demolition readout with strength of tens to hundred MHz all while the qubit stays at its full sweet-spot to charge noise, with zero dipole moment. A "dispersive-like" spin readout approach similar to circuit-QED but avoiding the Purcell effect is proposed. With the addition of gate voltage modulation, selective longitudinal readout and n-qubit entanglement-by-measurement are possible.