Physicists at the Max Planck Institute of Quantum Optics (MPQ) are claiming the ability to control the optical properties of a single atom using laser light. The group, led by professor Gerhard Rempe, director of MPQ and head of the Quantum Dynamics Div., based its work on the phenomenon of electromagnetically induced transparency, in which the transmission of laser light through an optically dense medium is controlled by a second laser beam. The researchers scaled down the effect to a single atom by taking advantage of the strong interaction between light and matter provided by an optical cavity. Figure 1. Optical transistor using a quantum of matter. (a) The presence of the atom results in light reflection (“off” action of the transistor); (b) Using single atom EIT, the atom becomes transparent, and full transmission is recovered (“on” action of the transistor). A key element to this process is electromagnetically induced transparency (EIT) — when the interaction of an atomic medium with a weak laser field can be controlled and manipulated coherently with a second, strong laser field. Practically, this is achieved by irradiating the medium with two laser beams: The action of a strong control laser causes the medium to become transparent for a weak probe laser. The properties derived from EIT allow the storing and retrieval of information between an atomic sample and light pulses, thus providing a powerful interface between photonic information and stationary atoms. In all experiments performed so far, the medium comprised a very large number of atoms. In contrast, in the experiment described here, only a single rubidium atom is addressed. The atom is trapped inside a high-finesse optical cavity to amplify the atom-light interaction such that atom and cavity form a strongly coupled system. Then the transmission of laser light — the probe laser — incident on the cavity axis is measured. When there is no atom inside the cavity, the laser light is transmitted. On the other hand, the presence of the atom causes the light to be reflected, and the transmission drops (see Fig. 1a). With an additional control laser of very high intensity applied transverse to the cavity axis, the single-atom EIT condition is achieved, and maximum transmission is recovered (see Fig. 1b). The single atom effectively acts as a quantum optical transistor, coherently controlling the transmission of light through the cavity. In addition, Rempe's team succeeded in performing EIT experiments when more atoms were added inside the cavity, one by one in a very controlled way. “Using EIT with a controlled number of atoms provides the possibility to manipulate many quantum properties of light fields transmitted by the cavity,” said Martin Mücke, who works on this experiment as a doctoral student. “Usually photons don’t interact with each other. With this scheme, we may be able to achieve a long-sought goal: strong interaction between photons, mediated by a single atom. Such a setup is a potential building block for a working quantum computer.” For more information, visit: www.mpq.mpg.de