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Journal of Modern Optics

Quantum noise analysis in correlated two-photon imaging

Yanfeng Bai, Shensheng Han

Published Apr 6, 2009• 2 citations
Publisher

Abstract

The theoretical analysis of quantum noise is presented in a correlated two-photon imaging system with entangled photons. The noise is strongly influenced by the size and the entanglement degree of the source. An increase of the source size results in a remarkable decrease of the amplitude of the noise when the size is much smaller than the aperture of the lens, whereas the change will not be obvious if the size is close to the aperture. The difference of the effects from the source between correlated imaging formed with entangled photons and with partially coherent light is also discussed.

DOI
10.1080/09500340802082368

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The theoretical analysis of quantum noise is presented in a correlated two-photon imaging system with entangled photons. The noise is strongly influenced by the size and the entanglement degree of the source. An increase of the source size results in a remarkable decrease of the amplitude of the noise when the size is much smaller than the aperture of the lens, whereas the change will not be obvious if the size is close to the aperture. The difference of the effects from the source between correlated imaging formed with entangled photons and with partially coherent light is also discussed.
We use entangled photons as the light source in a normal microscope. Using this quantum illumination improves both the resolution and noise rejection in the image. The resolution of any classical imaging system is limited by the diffraction created by the finite size of the imaging lens. By using a light source that emits pairs of entangled photons and carefully controlling a camera that is capable of detecting single photons it is possible to observe these photons arriving two-by-two. Applying camera analysis software to log only these two-by-two events one can form an image from the mid-point (bisector) of each of these pair events. The image formed from the sum of all these bisectors has an improved resolution that surpasses the classical limit. Another advantage of using a quantum illumination is that it allows the two-by-two events from entangled photon pairs to be distinguished from background (classical) light and sensor noise. When processed in this way the resulting image is free of camera noise and any background light may be eliminated. The use of a quantum illumination protocol such as that used here results in the elimination of background light and therefore allows the development of the optical equivalent of quantum radar – a quantum enhanced LIDAR scheme. Using quantum illumination, combined with commercially available cameras that are sensitive to single photons creates a new approach to super-resolution and low noise microscopy. Imaging through noise with quantum illumination, T Gregory, P-A Moreau, E Toninelli, and M J Padgett, Science Advances 6, eaay2652 (2020) Resolution-enhanced quantum imaging by centroid estimation of biphotons, E Toninelli, P-A Moreau, T Gregory, A Mihalyi, M Edgar, N Radwell, and M J Padgett, Optica 6, 347-353 (2019)
Quantum fluctuations in image irradiance for spatially partially coherent optical imaging are studied as a function of the spatial coherence of the source. The advantage of using incoherent or partially coherent illumination for the quantum noise performance is discussed. The results are compared for illumination by coherent, spatially single-mode squeezed, and spatially multimode squeezed light. It is shown that a signal-to-noise ratio can be improved beyond the shot-noise limit employing the two latter sources.
Quantum imaging utilizes the correlation of photons to enhance the security, covertness, and noise resilience of optical imaging. Despite recent advances, developing a quantum imaging system with long-distance and strong noise resilience remains challenging. Here, we present an experimental demonstration of high-performance quantum imaging at telecom band with enhanced noise resilience. We use a fiber-integrated quantum light source via cascaded second-order nonlinear processes to generate high-quality correlated/entangled photon pairs at telecom band and a fiber-coupled confocal optical system to effectively suppress the background noise and improve the collection efficiency of echo signal photons by optimizing its bandwidth and aperture. In our demonstration, the quantum imaging with correlated/entangled photon pairs or heralded single photons is achieved at a distance of 11 m under indoor illumination. When the detected environment noise is 33 dB stronger than the signal photons, corresponding to a coincidence-to-accidental coincidence ratio of 1.7 ± 0.1, an imaging contrast of 0.23 ± 0.09 is achieved, which is 46 times higher than single-photon imaging without correlation. The results will open venues for the development of practical quantum imaging techniques.
Classical light sources emit a randomly-timed stream of individual photons, the spatial distribution of which can be detected with a camera to form an image. Quantum light sources, based on parametric down conversion, emit photons as correlated photon-pairs. The spatial correlations between the photons enables imaging systems where the preferential selection of photon-pairs allows for enhancements in the noise performance over what is possible using classical light sources. However, until now the technical challenge of measuring, and correlating both photons has led to system complexity. Here we show that a camera capable of resolving the number of individual photons in each pixel of the detector array can be used to record an image formed from these photon-pair events and hence achieve a greater contrast than possible using a classical light source. We achieve an enhancement in the ratio of two-photon events compared to one-photon events using spatially correlated SPDC light compared to uncorrelated illumination by a LED. These results indicate the potential advantages of using photon counting cameras in quantum imaging schemes and these advantages will further increase as the technology is developed. Operating in photon sparse regimes such systems have potential applications in low-light microscopy and covert imaging.
Optical measurements in quantum communication and quantum radiometry are quite often based on two-photon correlated channels and the coincidence between them. To quantify the noise level in these measurements, the authors focus on the statistics of photon coincidence in a basic experimental setup, accounting for all experimental effects contributing to the noise increment. By the proposed theory the measurement performance is evaluated in terms of noise fluctuations. Particularly, an ultimate limit in noise reduction is established by the optimal estimation model herein presented.
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