Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Measurement of ultra-small displacement of a light beam beyond the quantum limit

It has been known for a long time, and experimentally demonstrated, that the sensitivity of optical measurements performed on the global intensity, or on the global phase, of a light beam can be improved by using single mode non-classical states of light, such as sub-Poissonian or squeezed states. This is no longer true for measurements performed in optical images, in which one only monitors a change in the transverse distribution of the light.

Just before the beginning of the project, ENS had shown theoretically that the simplest of these measurements, that of the position of the centre of a beam, could be also improved beyond the standard quantum limit by using a multi-mode non-classical light, namely the mixing of a Gaussian coherent beam with a squeezed vacuum state in a transverse mode of specific shape. This mixing actually creates a perfect quantum correlation between the intensities measured on the different pixels of the detector. During the QUANTIM project this effect was experimentally demonstrated, in a common work between ENS and the ANU (Canberra, Australia), using the very efficient squeezed light generators developed at ANU.

In a first experiment, ENS and ANU were able to improve the measurement of the displacement in a given direction of the transverse plane of a 0.5 mm diameter beam well below the standard quantum noise limit, at the angstrom level, using a split detector and a two-mode non-classical light. In a second experiment, using a quadrant detector and a three-mode non-classical light, they could measure simultaneously the two transverse coordinates of the beam centre below the standard quantum noise limit.

Meanwhile the theoretical understanding of such measurements in images advanced, and the transverse mode responsible for the noise in a measurement of any quantity derived from the combination of the intensities measured on different pixels was identified. This opens the possibility of improving many image processing and analysis functions, such as pattern recognition, image segmentation, or wavefront analysis.

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ENS Laboratoire Kastler Brossel
UPMC Case 74
75252 Paris
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