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Optics Express

Sub-shot-noise shadow sensing with quantum correlations

Ermes Toninelli, M. Edgar, Paul‐Antoine Moreau, Graham M. Gibson, Giles D. Hammond, Miles J. Padgett

Published Aug 29, 2017• 20 citations
Publisher

Abstract

The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector's quantum efficiency.

DOI
10.1364/oe.25.021826

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The quantised nature of the electromagnetic field sets the classical limit to the sensitivity of position measurements. However, techniques based on the properties of quantum states can be exploited to accurately measure the relative displacement of a physical object beyond this classical limit. In this work, we use a simple scheme based on the split-detection of quantum correlations to measure the position of a shadow at the single-photon light level, with a precision that exceeds the shot-noise limit. This result is obtained by analysing the correlated signals of bi-photon pairs, created in parametric downconversion and detected by an electron multiplying CCD (EMCCD) camera employed as a split-detector. By comparing the measured statistics of spatially anticorrelated and uncorrelated photons we were able to observe a significant noise reduction corresponding to an improvement in position sensitivity of up to 17% (0.8dB). Our straightforward approach to sub-shot-noise position measurement is compatible with conventional shadow-sensing techniques based on the split-detection of light-fields, and yields an improvement that scales favourably with the detector's quantum efficiency.
In this thesis I discuss the experimental demonstration of quantum-enhanced imaging and sensing schemes able to surpass the performance of their classical counterparts. This is achieved by exploiting the spatial properties of quantum correlated biphotons. Over the next chapters I first discuss the production and detection of quantum correlated photons using a type-I nonlinear crystal and a single-photon sensitive electron-multiplying CCD camera. I then provide a simple yet powerful description of the spatially resolved detection of biphotons, allowing to accurately model and assess the performance of the quantum-enhanced schemes featured in this thesis. These consist of a shadow-sensing and an imaging scheme able to respectively beat the shot-noise-limit in the optical measurement of the position of a shadow and the diffraction limit in the full-field imaging of real-world objects. A combination of simulated and experimental results are used to investigate both the achieved and theoretically available quantum advantage. Optical losses and detector noise are found to limit the better-than-classical performance of the schemes, which rely on the ability to jointly detect an as high as possible number of spatially correlated biphotons.
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.
Quantum-intensity-correlated twin beams of light can be used to measure absorption with precision beyond the classical shot-noise limit. The degree to which this can be achieved with a given estimator is defined by the quality of the twin-beam intensity correlations, which is quantified by the noise reduction factor. We derive an analytical model of twin-beam experiments, incorporating experimental parameters such as the relative detection efficiency of the beams, uncorrelated optical noise, and uncorrelated detector noise. We show that for twin beams without excessive noise, measured correlations can be improved by increasing the detection efficiency of each beam; notwithstanding, this may unbalance detection efficiency. However, for beams with excess intensity or other experimental noise, one should balance detection efficiency, even at the cost of reducing detection efficiency—we specifically define these noise conditions and verify our results with statistical simulation. This has application in design and optimization of absorption spectroscopy and imaging experiments.
Abstract Imaging, detection and ranging of objects in the presence of significant background noise is a fundamental challenge in optical sensing. Overcoming the limitations imposed in conventional methods, quantum light sources show higher resistance against noise in a time-correlation-based quantum illumination. Here, we introduce the advantage of using not only time correlations but also polarization correlations in photon pairs in the detection of an object that is embedded in a noisy background. In this direction, a time- and polarization-correlated photon pair source using the spontaneous parametric down-conversion process is exploited. We found that the joint measurement of correlated pairs allows distinguishing the signal from the noise photons and that leads to an improved signal-to-noise ratio. Our comparative study revealed that using polarization correlations in addition to time correlations provides improved noise rejection. Furthermore, we show that polarization correlation allows undoing the detector limitation where high background often leads to detector saturation.