PerspectiveQuantum Mechanics

The quantum nondemolition derby

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Science  13 Jun 2014:
Vol. 344, Issue 6189, pp. 1224-1226
DOI: 10.1126/science.1255313

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Heisenberg's uncertainty principle demands that a very precise measurement of a particle's position x not only will increase the uncertainty in a measurement of its momentum p, but also affects any additional measurement of x because the high momentum uncertainty affects the spreading of the particle's wave function. However, it is possible to avoid this problem by cleverly choosing what to measure. A measurement variable can be constructed so that the inevitable increases in uncertainties in other quantities will have no influence on the outcome of its repeated measurement. Such “quantum nondemolition” (QND) measurements can be used to detect the turning on of an arbitrarily small force. A specific QND method, referred to as back-action evading (BAE), was conceived in the context of detecting gravitational waves by their effect on large test masses or on the freely suspended mirrors of interferometers (13). This technically demanding method was originally based on optomechanical systems, and on page 1262 of this issue, Suh et al. (4) now report an implementation of BAE using microwaves in a superconducting resonator circuit with a movable capacitor plate.