Abstract
Quantum mechanics is potentially advantageous for certain information-processing tasks, but its probabilistic nature and requirement of measurement backaction 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 backaction 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, backaction 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 broadband squeezed light for use in interferometric gravitational-wave detection, experiments using entangled atomic-spin ensembles, and implementations of the quantum Toffoli gate.
- Received 26 March 2012
DOI:https://doi.org/10.1103/PhysRevX.2.031016
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Published by the American Physical Society
Popular Summary
In the 1920s, Einstein objected to the peculiar laws of quantum mechanics with the famous line “God does not play dice.” What upset him most was Heisenberg’s uncertainty principle, which prohibits one from simultaneously knowing both the exact position and the momentum of a particle. In spite of the apparent absurdity in relation to everyday experience, every experiment performed to measure such quantities has confirmed that quantum mechanics is correct. Do we therefore have to accept a world full of quantum randomness? In our paper, we show theoretically that in principle it is possible to design a quantum device such that a part of it is immune from all laws of quantum mechanics. Although the entire system must respect quantum mechanics, the immune subsystem can have any dynamics we choose, and all of its observables can be measured to arbitrary precision.
We propose a way to create a “quantum-mechanics-free system” by concentrating on so-called quantum nondemolition observables. These quantities have the property that the quantum backaction produced by a measurement affects only their conjugate observables, not the observables themselves. If a subsystem is characterized by only quantum-nondemolition observables, then it can evade the effects of backaction, which are absorbed only by the conjugate variables not part of the subsystem. Our work shows that, not only can such subsystems exist, but the class of quantum nondemolition observables is also much larger than previously thought.
Our discovery has technological as well as conceptual implications because quantum randomness has increasingly affected classical information-processing applications. For example, gravitational-wave detectors that measure the miniscule vibrations of mirrors suffer from the quantum nature of the mirrors. Using our method, the detector could be rendered quantum-mechanics-free by coupling the mirrors to laser light in a special way. Although technically challenging, experiments on atomic vapor, ion traps, or superconducting microwave circuits might serve as first demonstrations of our proposal.