How many photons does it take to light up an atom? Researchers have shown theoretically that it’s possible to excite two atoms—or even more—simultaneously by absorbing just a single photon between them. The process should be reversible, so that the atoms can return to a lower-energy state by collectively emitting one photon. This nonintuitive phenomenon might find applications in quantum information processing, the researchers say.
Excitation of a single atom by the absorption of two photons is a well-known process. Together the photons convey enough energy to boost the atom from a lower-energy state to a higher-energy state, even without an intermediate rung of the energy ladder. This two-photon absorption is now commonly used in spectroscopy and microscopy, while double emission of photons by an excited atom is a standard way to make so-called entangled photons, which have interdependent quantum states.
But Salvatore Savasta of the University of Messina in Italy and his co-workers wondered about the complementary processes—double-atom absorption and emission of a single photon. They considered a simple system in which two identical atoms are placed inside an optical cavity, a space that can support standing waves of light. The cavity's dimensions determine the specific wavelength and frequency of photons that it can accommodate.
According to the calculations of Savasta and colleagues, if this resonant frequency of the cavity is twice the frequency of a photon that could boost either of these atoms from its ground state to an excited state, then the phenomenon can occur. A photon in the cavity can be simultaneously absorbed by both atoms at once, each atom taking half of the photon’s energy and both ending up in the excited state. The same holds, the researchers say, for three atoms with transition frequencies a third of that of the cavity resonance.
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