Surpassing the standard quantum limit for high sensitivity measurements in optical images using multimode non classical light

The existence of quantum fluctuations of light inevitably degrades the quality of information extraction from any optical image. We define the corresponding limit when the image is carried by coherent light ("standard quantum limit"). We then show that single-transverse-mode non-classical light, such as TEM00 sub-Poissonian light, cannot be used to go beyond this limit. We describe two experimental techniques for generating multimode non-classical light: the first one uses an Optical Parametric Oscillator inserted in a confocal cavity; the second one synthesizes the multi-transverse mode beam from its single mode components. We finally show that this latter technique has allowed us to beat the standard quantum limit in the ultra-precise positioning of the center of a light beam in the transverse plane.