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Quantum imaging and the detection of weak objects

E. Brambilla, A. Gatti, Lucia Caspani, L. A. Lugiato, Ottavia Jedrkiewicz, P. Di Trapani, M. Genovesi

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

A recent experiment has demonstrated the quantum nature of spatial fluctuations in parametric down-conversion. We show theoretically and numerically that, by exploiting the sub-shot-noise character of spatial correlations, it is possible to detect weak amplitude objects with a sensitivity beyond the standard quantum limit.

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Quantum properties of the optical field represent a resource of the utmost relevance for the development of quantum technologies, allowing unprecedented results in disciplines ranging from quantum information and metrology to quantum imaging. Spatial quantum correlations generated by of parametric down conversion (PDC) represents a tool for quantum imaging because they are intrinsically multimode, a requirement for obtaining large degree of correlation over small portions of the beams, allowing to register the spatial structure of an object. In particular a very interesting example is provided by the detection of weak objects, a result that could have important practical applications. The principle of this technique is to take advantage of the correlation in the noise of two conjugated branches of PDC emission: in fact, subtracting the noise measured on one branch from the image of a weak object obtained in the other branch, the image of the object, eventually previously hidden in the noise, could be restored. Here, after a general summary of quantum imaging techniques, firstly we will show how we have reached a sub shot noise regime and then improved this result up to reach a regime where it was possible to achieve the first experimental realisation of sub shot noise imaging of a weak absorbing object.
Sub-Shot Noise quantum imaging is a novel quantum method allowing in principle to recover the image of a weak absorbing object, even if illuminated by a low photon flux, with a signal to noise ratio larger than classical imaging techniques. Here we present the first realization of a genuine quantum imaging schemes exploiting correlation of twin beams produced by Parametric Down conversion. We compare its performances with respect to the corresponding classical scheme, showing a clear improvement of the sensitivity in the reconstruction of the absorbtion profile of a weak object.
Abstract. The field of Quantum Imaging exploits the quantum nature of light and the intrinsic parallelism of optical signals to devise novel techniques for optical imaging and for parallel information processing at the quantum level (see [1] and references quoted therein). In this presentation, we will shortly discuss two topics in this area: the first is the so-called ghost imaging which, however, is not necessarily quantum; the second is the detection of faint amplitude objects with a sensitivity beyond the standard quantum limit, and in this case we are fully in the quantum domain. Both topics are related to the phenomenon of optical parametric down-conversion (PDC), in which a fraction of the pump photons of a laser beam, injected into a crystal with a quadratic non-linearity, are down-converted to a pair of signal and idler photons, with conservation of total energy and total momentum. A feature of paramount importance is that the signal and idler beams are spatially correlated both in the near field (position correlation) and in the far field (momentum correlation).The simultaneous presence of position and momentum correlation implies quantum entanglement, as it has been also observed experimentally [2]. 1. Ghost Imaging
Properties of quantum states have disclosed new and revolutionary technologies, ranging from quantum information to quantum imaging. This last field is intended to overcome the limits of classical imaging by exploiting specific properties of quantum states of light. One of the most interesting proposed schemes exploits spatial quantum correlations between twin beams for realizing sub-shot-noise imaging of weakly absorbing objects, leading ideally to a noise-free imaging. Here we discuss in detail the experimental realization of this scheme, showing its capability to reach a larger signal-to-noise ratio with respect to classical imaging methods and therefore its potential for future practical applications.