In a development that could have far-reaching ramifications for single-atom science, physicists have created an "atom-cavity microscope" that uses single photons from a laser beam to track the motion of individual atoms. Researchers at the California Institute of Technology and the University of Auckland in New Zealand have constructed an optical cavity in which single photons manipulate an atom by quantum mechanical means. They placed a pair of highly reflective concave mirrors 10 µm apart to create a cavity and enclosed the setup in a vacuum chamber. Next, they shone an 852-nm continuous-wave single-mode Ti:sapphire laser into the cavity. The beam's weak illumination -- the laser was attenuated to a few picowatts -- created a standing-wave pattern between the mirrors. As shown by an artist's rendition, a single-photon field traps an atom, causing it to orbit several times before escaping. Courtesy of Caltech Quantum Optics. Single ultracold atoms drop one at a time through single segments of this field. The quantum mechanical coupling between the atom and the single-photon field is so strong that it traps the atom, causing it to orbit the segment several times before it escapes. Each photon eventually leaks out through the mirrors and is replaced by another. The researchers used variations in this output stream of photons to track the trajectory of the atom. They exploited the idea that a single atom would interact very strongly with light if confined to a small space, such as between mirrors, said Christina J. Hood, a member of the research team. "We can use this interaction to monitor the position of the atom or particle by watching the effect the atom's motion has on the light." In the experiment -- detailed in the Feb. 25 issue of Science -- the atom moved around and changed the resonance properties of the optical cavity, affecting the intensity of the photon's light. The physicists could use the intensity changes to reconstruct the trajectories of single atoms, Hood said. 'Photon in a box' The idea of using this kind of "photon in a box" to monitor and manipulate atoms has been around since the 1960s, but only very recently has technology enabled researchers to watch these single-particle interactions one at a time. Laser cooling and trapping atoms -- slowing them down enough for researchers to measure them easily -- are crucial components of the setup. Demonstrating photon-atom interactions through future experiments is critical to developing quantum computing and communications devices. The researchers will continue studies of single-atom/single-photon interactions with the goal of monitoring atomic position to implement real-time quantum feedback and to control atomic motion within the cavity. They are also considering such applications for their setup as the study of specific biochemical processes.