Entangled photons conspire to create interference patterns that would normally be associated with a wavelength much smaller than that of the individual photons―beating the diffraction limit. If a double slit is illuminated with a laser beam of wavelength λ, the familiar rippling pattern of interference fringes arises, even if the attenuation of the laser is so strong that only single photons pass through the double slit at any given instant. To emphasize this quantum feature, Paul Dirac wrote that "Each photon interferes only with itself. Interference between two different photons never occurs." Dirac would probably have been more cautious had he read this issue of Nature. Starting on page 158, Mitchell et al. and Walther et al. report their demonstrations of interference patterns produced by specific entangled states of three and four photons.
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