  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.
