Progress on quantum imaging with cameras was achieved using intensified CMOS and CCD cameras 16, 17, 18, 19, 20, 21. Furthermore, to achieve single-photon level sensitivity the EMCCD camera operated at a low temperature of − 85 ☌, which is typical for this type of cameras. Although the EMCCD quantum efficiency was up to 90%, a long exposure time of about 1 ms was necessary to operate this device at minimum photon rates to avoid multiple photons in the same frame. These experiments did not perform entanglement characterization, as it would require to analyze the pair coincidences.Įarly studies of entanglement with modern imagers used an EMCCD camera with an active area of 201 × 201 pixels and frame readout-rate of 5 Hz 8. The main motivation in the above studies was a demonstration of sub-shot noise that can be achieved by exploiting quantum correlations. However, the exposure time was 33 ms, so albeit the individual single photons were indeed spatially resolved, multiple photon pairs were registered in a single shot. EMCCDs were also used in the single-photon mode to study spatial correlations using an SPDC source 15. In those cases, the temporal resolution was determined by the pulsed nature of the source, albeit the cameras were not operated in the single-photon regime, as the results required integrating millions of photons per laser pulse. More recent experiments used low noise CCD 12, 13 and electron-multiplying CCD (EMCCD) 14 cameras to study spatial quantum correlations between twin beams of pulsed parametric down-conversion photon sources. However, most of these measurements used resource-intensive methods, such as sequential scanning or multiple stand-alone detectors. Recent developments have shown that spatial characterization of entangled states with single-photon sensitive cameras provides access to a myriad of new possibilities, such as imaging high-dimensional entanglement 8, generalized Bell inequalities 9 and the study of Einstein-Podolsky-Rosen non-localities 10, 11. Easy-to-use, scalable, and compact characterization devices, providing all the information regarding entanglement in near-real-time are fundamental for further large-scale network developments.
The creation of quantum networks of many such quantum devices in which entanglement is shared among multiple network nodes is the next technological frontier for the successful development of these applications. Ever since the original experiments with entangled photons 1, photonic entanglement has become a remarkable resource in the development of quantum technologies, including entanglement over long-distance for quantum communication 2, entanglement swapping 3, teleportation between a photon and an atomic ensemble 4, violation of the CHSH (Clauser-Horne-Shimony-Holt) inequality measured over long distances 5, 6 and entanglement of spin waves among four quantum memories 7.
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