# Fast high fidelity quantum non-demolition qubit readout via a non-perturbative cross-Kerr coupling

## TL;DR

This paper introduces a non-perturbative cross-Kerr coupling technique for superconducting qubits that enhances readout speed, fidelity, and QND properties while reducing decay effects, demonstrated with high fidelity and QND-ness.

## Contribution

It presents the first experimental realization of a non-perturbative cross-Kerr coupling for improved qubit readout in superconducting circuits.

## Key findings

- Achieved 97.4% single-shot fidelity with 50 ns pulses.
- Quantified 99% QND-ness for repeated measurements.
- Reduced decay effects like the Purcell effect.

## Abstract

Qubit readout is an indispensable element of any quantum information processor. In this work, we experimentally demonstrate a non-perturbative cross-Kerr coupling between a transmon and a polariton mode which enables an improved quantum non-demolition (QND) readout for superconducting qubits. The new mechanism uses the same experimental techniques as the standard QND qubit readout in the dispersive approximation, but due to its non-perturbative nature, it maximizes the speed, the single-shot fidelity and the QND properties of the readout. In addition, it minimizes the effect of unwanted decay channels such as the Purcell effect. We observed a single-shot readout fidelity of 97.4% for short 50 ns pulses, and we quantified a QND-ness of 99% for long measurement pulses with repeated single-shot readouts.