# Quantum rifling: protecting a qubit from measurement back-action

## TL;DR

Quantum rifling is a regime where fast-driven qubits during measurement yield averaged expectation values, suppress back-action, and enable selective qubit readout, aiding scalable quantum computing.

## Contribution

This work introduces the concept of quantum rifling, demonstrating how rapid qubit dynamics during measurement can protect states and enable selective readout in superconducting qubits.

## Key findings

- Fast-driven qubits yield time-averaged measurement outcomes.
- Quantum rifling suppresses measurement back-action.
- Selective readout of multiple qubits is achievable.

## Abstract

Quantum mechanics postulates that measuring the qubit's wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate that the qubit collapsed into. We show that this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement back-action. We call this regime `quantum rifling', as the fast spinning of the Bloch vector protects it from deflection into either of its eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the qubits on demand while protecting the state of the other from measurement back-action. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to the efficient scaling up of quantum processors for future quantum technologies.