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arXiv:1911.07039 · International Journal of Quantum Information

Quantum enhanced imaging of nonuniform refractive profiles

Giuseppe Ortolano, Ivano Ruo Berchera, E. Predazzi

Published Dec 1, 2019• 11 citations
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Abstract

In this work, quantum metrology techniques are applied to the imaging of objects with a nonuniform refractive spatial profile. A sensible improvement on the classical accuracy is shown to be found when the “Twin Beam (TWB) State” is used. In particular, exploiting the multimode spatial correlation, naturally produced in the Parametric Down Conversion (PDC) process, allows a 2D reconstruction of complex spatial profiles, thus enabling an enhanced imaging. The idea is to use one of the spatially multimode beams to probe the sample and the other as a reference to reduce the noise. A similar model can also be used to describe wave front distortion measurements. The model is meant to be followed by a first experimental demonstration of such enhanced measurement scheme.

DOI
10.1142/s0219749919410107

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In this work, quantum metrology techniques are applied to the imaging of objects with a nonuniform refractive spatial profile. A sensible improvement on the classical accuracy is shown to be found when the “Twin Beam (TWB) State” is used. In particular, exploiting the multimode spatial correlation, naturally produced in the Parametric Down Conversion (PDC) process, allows a 2D reconstruction of complex spatial profiles, thus enabling an enhanced imaging. The idea is to use one of the spatially multimode beams to probe the sample and the other as a reference to reduce the noise. A similar model can also be used to describe wave front distortion measurements. The model is meant to be followed by a first experimental demonstration of such enhanced measurement scheme.
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.
Twin entangled beams produced by single-pass parametric down-conversion (PDC) offer the opportunity to detect weak amount of absorption with an improved sensitivity with respect to standard techniques which make use of classical light sources. We propose a differential measurement scheme which exploits the spatial quantum correlation of type-II PDC to image a weak amplitude object with a sensitivity beyond the standard quantum limit imposed by shot noise. By applying a quantitative numerical model to a specific PDC system (a type-II beta-barium borate crystal), we are able to identify conditions in parameter space suitable for a substantial enhancement of the imaging sensitivity, and we investigate the robustness of this enhancement against experimental imperfections.
Istituto Nazionale di Ricerca Metrologica (INRIM), Strada delle Cacce 91, 10135 Torino, Italy. E-mail:i.ruoberchera@inrim.it Quantum correlations of twin beams represent a resource of great importance for the developments of quantum technologies allowing unprecedented results in quantum information, metrology and quantum imaging. Focusing on the measurement of the photon numbers, spatial multimode correlations beyond the shot noise limit have been experimentally demonstrated by using parametric down conversion (PDC) and CCD camera (both at the single photon level and at higher photon flux) [1], and also with other interesting techniques [2]. One of their natural application is the quantum imaging of weak absorbing objects, showed in Fig.1, with sensitivity beyond the shot-noise-level [3]. The idea consists in measuring the intensity pattern on one branch of PDC, where the object has been inserted, and then subtracting the correlated noise pattern measured in the other branch that does not interact with the object. In our presentation we overview the sub-shot noise imaging protocol mentioned above and other applications where the spatial quantum correlation can give some advantages over the classical ones. As an example, we studied the signalto-noise ratio (SNR) of ghost imaging protocols with quantum and classical light [4]. We realized a ghost imaging experiment in the mesoscopic regime (one thousand of photon per pixel per shot), obtaining substantial quantum enhanced performances (see Fig.1). We also demonstrated that spatial quantum correlations provide the possibility for the absolute calibration of analog and/or photon number resolving detectors, providing the first absolute calibration (without any comparison with reference standards, neither optical source nor detector) of a scientific CCD camera [5]. In this first realization we already reached interesting uncertainty level. All these findings indicate that the control and detection of quantum spatial correlation is a rather mature field and other application to different sensing scenarios can be envisaged.
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.
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.
Quantum metrology takes advantage of quantum correlations to enhance the sensitivity of sensors and measurement techniques beyond their fundamental classical limit, given by the shot-noise limit. The use of both temporal and spatial correlations present in quantum states of light can extend quantum-enhanced sensing to a parallel configuration that can simultaneously probe an array of sensors or independently measure multiple parameters. To this end, we use multispatial-mode bright twin beams of light, which are characterized by independent quantum-correlated spatial subregions in addition to quantum temporal correlations, to probe a four-sensor quadrant plasmonic array. We show that it is possible to independently and simultaneously measure local changes in refractive index for all four sensors with a quantum enhancement in sensitivity in the range of 22% to 24% over the corresponding classical configuration. These results provide a first step toward highly parallel spatially resolved quantum-enhanced sensing techniques and pave the way toward more complex quantum sensing and quantum imaging platforms.