Quantum Ghost Imaging in reverse START-STOP with a 2D SPAD array

Ghost Imaging (GI) is a fascinating imaging technique that exploits second-order correlations of light to investigate samples without directly observing them with a camera [1]. Quantum GI (QGI) version exploits correlations stemming from photon pairs generated by spontaneous parametric down-conversion (SPDC) [2]. It retrieves images from two-path correlation measurements, where single-path detection is not sufficient to obtain an image of the target. Since early demonstrations, this technique has suffered in terms of scalability due to the difficulty of efficiently performing spatially resolved single-photon detection. Early demonstrations suffered from detection efficiency limitations, due to raster scanning spatial detection. More recent camera-based experiments relying on intensified CCD cameras obtain only a limited frame rate and need complex image-preserving delay-lines [3]. More recently, SPAD arrays, two-dimensional arrangement of single-photon avalanche detectors, have been developed and employed in a number of quantum imaging and spectroscopy schemes, dramatically improving detection performances [4]. Here we report on a QGI implementation exploiting a 2D SPAD array detector for the spatially resolving path, solving limitations in terms of frame-rate and the need for optical delay lines. Our experiment enables the acquisition of an image in under one minute, and with much higher frame rate compared to the state-of-the-art. We further take advantage of non-degenerate SPDC to interrogate a Si-Au target. While light interaction with the sample happens at infrared wavelengths, no infrared cameras are needed to obtain an image, and spatial detection is performed in the visible region. This two-color scheme allows to make use of the advanced and cost-efficient silicon-technology while still investigating a sample in spectral regions where cameras are too expensive or not yet available [5]. Our findings advance the state-of-the-art of QGI schemes towards practical applications.