Single-atom detection
Figure 1. (a) Part of the optics and the vacuum system used to trap a single atom in a high-finesse optical cavity at the Max Planck Institute for Quantum Optics in Garching, Germany. From left to right: Thomas Fischer, the author, Pepijn Pinkse, Thomas Puppe and Peter Maunz. (b) In the MPQ experiment, rubidium atoms (green) collected in a magneto-optical trap and cooled are ejected upwards towards a cavity that is 110 µm long and has a finesse of 430 000. Light from a weak laser with a wavelength of 780 nm sets up a standing wave in the cavity. The atoms are detected by measuring the light transmitted through the cavity. (c) If the wavelength of the light is resonant with the atom, then the presence of the atom is signalled by a dip in the transmission. (d) The presence of an atom can still be detected even if the light is not resonant with the empty cavity or the atom by tuning the external laser so that a cavity containing exactly one atom is resonant with the input light. In other words, the atom tunes the cavity into resonance with the light. Each of the three peaks displayed is characteristic of a single atom.