Scientists at the University of Glasgow have photographed quantum entanglement, capturing for the first time visible evidence of the phenomenon described by Einstein as “'spooky action at a distance.” The image pictures a form of entanglement called Bell entanglement. Although Bell entanglement is being used today to develop quantum computing and cryptography applications, it has never been captured in a single photo. Physicists took a photograph of quantum entanglement. Courtesy of the University of Glasgow/CC by 4.0. To prepare the image, the researchers used a crystal pumped by an ultraviolet laser as a source of entangled photon pairs. The two photons were separated on a beamsplitter. The researchers set up a camera triggered by a single photon avalanche diode (SPAD) to acquire ghost images of a phase object placed on the path of the first photon and nonlocally filtered by four different spatial filters. The camera, which was sensitive enough to take images of single photons, was able to acquire coincidence images of the phase object probed by photons from an entangled pair source. Full-frame images recording the violation of a Bell inequality in four images. (A) The four coincidence counting images are presented, which correspond to images of the phase circle acquired with the four phase filters with different orientations necessary to perform the Bell test. (B to E) The coincidence counts graphs as a function of the orientation angle of the phase step along the object are presented. As shown, these results are obtained by unfolding the regions of interest (ROIs) represented as red rings and are extracted from the images presented in (A). The blue dots in the graphs are the coincidence counts per angular region within the ROIs, and the red curves correspond to the best fits of the experimental data by a cosine-squared function. Courtesy of Science Advances (2019). DOI: 10.1126/sciadv.aaw2563. The scientists took a photo that captured when a photon arrived with its entangled partner and found that the entangled photons were correlated in a manner that violated Bell’s inequalities. The image exhibited a violation of a Bell inequality with S = 2.44 ± 0.04. “The image we’ve managed to capture is an elegant demonstration of a fundamental property of nature, seen for the very first time in the form of an image,” researcher Paul-Antoine Moreau said. “It’s an exciting result which could be used to advance the emerging field of quantum computing and lead to new types of imaging.” The research was published in Science Advances (https://doi.org/10.1126/sciadv.aaw2563). Imaging setup to perform a Bell inequality test in images. A crystal pumped by an ultraviolet laser is used as a source of entangled photon pairs. The two photons are separated on a beamsplitter. An intensified camera triggered by a SPAD is used to acquire ghost images of a phase object placed on the path of the first photon and nonlocally filtered by four different spatial filters that can be displayed on a spatial light modulator (SLM 2 in the diagram) placed in the other arm. By being triggered by the SPAD, the camera acquires coincidence images that can be used to perform a Bell test. Courtesy of Science Advances (2019). DOI: 10.1126/sciadv.aaw2563.