Transition from weak to strong measurements by nonlinear quantum feedback control

TL;DR

This paper demonstrates how nonlinear quantum feedback control can induce nonlinear dynamics in a damped harmonic oscillator, enabling enhanced quantum state measurement and signal amplification in circuit QED systems.

Contribution

It introduces a feedback control method that induces pseudo nonlinear dynamics, facilitating quantum state measurement and signal amplification in harmonic oscillators.

Findings

Feedback control induces classical-like nonlinear dynamics.

Nonlinear bifurcation can be used for quantum measurement.

Method applied to superconducting charge qubits.

Abstract

We find that feedback control may induce "pseudo" nonlinear dynamics in a damped harmonic oscillator, whose centroid trajectory in the phase space behaves like a classical nonlinear system. Thus, similar to nonlinear amplifiers (e.g., rf-driven Josephson junctions), feedback control on the harmonic oscillator can induce nonlinear bifurcation, which can be used to amplify small signals and further to measure quantum states of qubits. Using the circuit QED systems as an example, we show how to apply our method to measure superconducting charge qubits.