Evading quantum mechanics

TL;DR

This paper introduces quantum-mechanics-free subsystems (QMFS) that evade measurement back action, enabling classical-like dynamics in quantum systems for improved sensing and information processing.

Contribution

It proposes a novel method to engineer coupled quantum systems that create QMFS, allowing classical dynamics without quantum noise limitations, challenging previous beliefs about QND observables.

Findings

Demonstrated generation of broad-band squeezed light for gravitational-wave detection

Implemented quantum Toffoli gate using dynamical QMFS

Experimental entanglement in atomic spin ensembles

Abstract

Quantum mechanics is potentially advantageous for certain information-processing tasks, but its probabilistic nature and requirement of measurement back action often limit the precision of conventional classical information-processing devices, such as sensors and atomic clocks. Here we show that by engineering the dynamics of coupled quantum systems, it is possible to construct a subsystem that evades the measurement back action of quantum mechanics, at all times of interest, and obeys any classical dynamics, linear or nonlinear, that we choose. We call such a system a quantum-mechanics-free subsystem (QMFS). All of the observables of a QMFS are quantum-nondemolition (QND) observables; moreover, they are dynamical QND observables, thus demolishing the widely held belief that QND observables are constants of motion. QMFSs point to a new strategy for designing classical information-processing devices in regimes where quantum noise is detrimental, unifying previous approaches that employ QND observables, back-action evasion, and quantum noise cancellation. Potential applications include gravitational-wave detection, optomechanical force sensing, atomic magnetometry, and classical computing. Demonstrations of dynamical QMFSs include the generation of broad-band squeezed light for use in interferometric gravitational-wave detection, experiments using entangled atomic spin ensembles, and implementations of the quantum Toffoli gate.