A group of physicists in Spain has shown how to make a quantum measurement that overcomes a limit related to Werner Heisenbergs uncertainty principle. The researchers confirmed a theoretical prediction of how to beat the Heisenberg limit by using interacting photons to measure atomic spin, and they say that their approach could lead to more sensitive searches for the ripples in space–time known as gravitational waves and perhaps also to improved brain imaging.The standard limit on the precision with which a quantum measurement can be carried out is due to the statistical error associated with counting discrete particles rather than continuous quantities. So, for example, when measuring the phase difference between the waves sent down two arms of an interferometer, the error in this quantity will scale with the square root of the total number of photons measured, N. Since the signal scales with N, the signal-to-noise ratio also scales in the same way. Or, put another way, the sensitivity of the measurement, which is the minimum signal that can be measured with a given level of noise, will scale with 1/N1/2.It is possible to improve on this scaling, however, by entangling the photons, because this correlates what would otherwise be independent sources of noise from the individual particles. Such entanglement allows measurements to approach the so-called Heisenberg limit, which means that sensitivity scales with 1/N. Until recently it was thought that this scaling represented an absolute limit on the sensitivity of quantum measurements.

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