Remote Sensing with a Twist

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Science  15 Feb 2013:
Vol. 339, Issue 6121, pp. 738
DOI: 10.1126/science.339.6121.738-b

Our eyes sense light that has bounced off objects, with our brain then making sense of the input flowing through the optic nerve. In this case, the photons that hit our eyes must interact with the object. However, it is possible to get information about an object using photons that have not actually hit the object. This counterintuitive process of "ghost imaging" follows from the quantum-mechanical properties of photons and the ability to find correlations between specially generated pairs of photons. Uribe-Patarroyo et al. use such correlations between beams of light that have been imprinted with specific values of optical angular momentum: The light beam is effectively twisted, with the photons mapping out a spiral as they propagate. Using correlated photons produced by parametric downconversion, whereby a high-energy photon is converted into a pair of correlated photons of lower energy, they send one of the photons to the object and hold on to the other. When the photon hits the object, the process of that interaction changes its degree of twist and in turn affects the correlation between the pair. The object can then be determined by looking at changes in the correlations between the two photons. With a modification of the experimental setup, it should be possible to remotely sense reflective targets at large distances, with the target unaware that it is being watched.

Phys. Rev. Lett. 110, 043601 (2013).

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