Morgan W. Mitchell and his colleagues at the University of Toronto started with three photons of different polarizations, which they coaxed into an entangled state by converging them into a single beam, essentially creating a superphoton with three times the energy of one photon alone. (The image above depicts part of the apparatus used in their experiment.) Physicists at the University of Vienna led by Philip Walther took a different tack: they combined the energy from two separate pairs of entangled photons, producing a four-photon entanglement. Both studies, published today in the journal Nature, show that entangled photons acting together have a higher energy¿and thus a shorter wavelength--than that which they can attain as free agents. By altering these two properties, the entanglement pushes the diffraction limit down to a third or a quarter, respectively, of the light¿s wavelength. Dirk Bouwmeester, a physicist at the University of California Santa Barbara and author of an accompanying commentary, notes that although this principle has already been demonstrated for two photons, "it is quite a technical challenge to increase to 3 or 4." Both methods, he says, offer "quite a beautiful demonstration of what entanglement can actually do."
Although these techniques must be refined before they can be put to commercial use, in the future they may provide new approaches for developing more powerful optical data storage systems such as CDs and even quantum computers. They could also endow devices used to measure astronomical bodies with significantly greater accuracy. "Entanglement comes in different varieties, like different wines or different colors," Bouwmeester observes. "In high-resolution measurement, a particular kind of entanglement is needed¿-a kind now demonstrated by these experiments." --Alla Katsnelson