As we aim for ever-higher bit rates in optical communications, the ability to detect the spatial state of light is becoming increasingly important. Often this is done using linear optics, for example phase or intensity modulation. Sephton et al. take a different approach, using a nonlinear crystal to compare the spatial modes of two light beams. Their experiment is based on the phenomenon of upconversion, where two photons combine in a nonlinear crystal and create a single photon of higher frequency. The reverse process, where one high-energy photon produces two lower-energy ones, is often used at very low intensities in quantum optics experiments; the spatial modes of the two photons created this way must match, due to momentum and energy conservation. In this experiment, the same conservation rules apply; that means that, if we know the spatial mode of one of the input photons, the presence of a signal at the detector indicates that the second photon is in a conjugate spatial mode. This paper describes the performance of this system in detail for orbital angular momentum modes of light, but the process works even for more complicated images, as the authors show later in the paper. This could allow the measurement of infrared light, using the much better detectors that are available for visible light, making it easier to work in the infrared bands that are favored for telecommunications fibers and free space optical communication.

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