Strong coupling without touching

Abstract

If an excited atom is put in between highreflectance mirrors, the light emitted by the atom can be bounced back and be re-absorbed and be re-emitted, again and again until the light leaks out of the mirrors or is re-emitted into a wrong direction. This coherent oscillation between atom and light, called vacuum Rabi oscillation, results from interference between two superposition states of the atom and the photon, which have different energies or frequencies, or vacuum Rabi splitting. The vacuum Rabi oscillation and splitting demonstrate the quantum nature of light. They also provide single-photon non-linearity for photonic applications. In recent years, these effects have been considered particularly useful for quantum information processing [1] since the oscillation presents a natural mechanism for information exchange between a stationary quantum bit carried by the atom and a flying quantum bit carried by the photon, making scalable and distributed quantum computing possible. Realization of the vacuum Rabi oscillation and splitting, however, is highly non-trivial. It requires the so-called strong coupling condition, that is, the exchange between the atom and the photon must be faster than the photon leaks out of the space enclosed by the mirrors, i.e. the cavity, and faster than the atom emits the light into a wrong direction. The technique to realize the strong coupling condition is to make the cavity small so that the photon is bounced back more frequently to the atom and to make the mirrors more reflective so that the photon can be bounced more times. That means to decrease the cavity volume V and to increase the quality factor Q. Remarkable progresses have been made to make optical microcavities with small V and high Q, facilitating observation of vacuum Rabi oscillation and splitting in highly dissipative solidstate systems [2,3]. However, there is no systematic method to reduce V/Q—the key factor for realizing strong coupling (see Fig. 1a). The reason appears to be fundamental. When the size of a cavity is made as small as comparable to the light wavelength, small deformation of the cavity would result in large evanescent wave and hence photon leakage [4]. A recent proposal by [5] provides a general solution, using a divide-andconquer strategy. A small cavitywith rela-