Optical switching with cold atoms

At room temperature and atmospheric pressure, atoms travel hundreds of meters per second and collide frequently with one another and their surroundings. Although this unceasing thermal motion keeps us warm, it causes serious problems for anyone trying to probe the internal state of an atom. To reduce the deleterious effects of atomic motion, experimentalists often rely on optical fields to trap and cool atoms [1]. In a paper appearing in Physical Review Letters[2], Michal Bajscy and colleagues at the Harvard-MIT Center for Ultracold Atoms demonstrate a system that is capable of trapping and cooling atoms and shuffling them into the tiny core of a hollow optical fiber (Fig. 1) The confined atoms are then in prime position to interact with beams of light guided by the fiber. This simultaneous confinement of light and atoms leads to increased optical nonlinearities with which the Harvard group can create an all-optical switch: a few hundred photons are capable of controlling the transmission of a weak (∼ 1 pW) probe beam. In a prototypical quantum information system, single photons transmit information. The control of one photon by another is typically mediated by a nonlinear interaction with atoms [3, 4] and for this light-matter coupling to be strong requires a long interaction length and a high optical intensity. An optical beam in free space can only remain focused for a distance on the order of the Rayleigh range, given by zR = πw2 0/λ, where λ is the wavelength of light and w0 is the narrowest radial size of the beam. This relationship shows that a large interaction length (large zR) and a high intensity (small w0) are not simultaneously possible with focused beams in free space. Waveguides, such as optical fibers, offer a straightforward way to increase both the interaction length and the optical intensity. The challenge for designing a quantum information system is therefore to combine a microscopic waveguide with a medium that exhibits large optical nonlinearities. Photonic crystal fibers are a type of optical fiber where FIG. 1: Rubidium atoms are loaded into and trapped within a hollow-core photonic crystal fiber. A cross section of the hollow fiber is shown in the upper right. A dipole trapping beam (large red arrow) serves to localize the atoms (red, not drawn to scale) within the hollow core of the fiber. Additional probe, coupling, and switch fields (red, green, and blue wave packets, respectively) can then control the atomic state and implement all-optical switching and electromagnetically induced transparency. The relevant atomic levels are indicated as states |1〉, |2〉, |3〉, and |4〉 in the upper left. Note: the actual experimental orientation of the fiber is vertical to enable loading from a magneto-optical trap (not shown). (Illustration: Alan Stonebraker)