Quantum Non-Demolition
How to detect a single photon repeatedly without destroying it. Photons are stored in the cavity (red) in the center of the figure. The ring (yellow) between the two mirrors is cut open to reveal the inside of the cavity, which contains a pair of highly reflective, superconducting mirrors which allow a photon to bounce back and forth on the order of 100 million times before they hit a mirror defect and are lost.
Using lasers, rubidium atoms traveling along the main arrow are prepared in an appropriate state for interacting with the electric and magnetic fields associated with the photon inside the cavity. These atoms (in high-lying "Rydberg" states) have a very long lifetime, well suited for the experiment. The velocity selection area allows the position of each atom to be known. A device known as the Ramsey interferometer (green) interacts with the photon box through holes in the ring (not shown). Combined with the detector (grey) it measures the phase of the atomic wavefunction. The value of this phase tells you whether the cavity contains zero or one photon--without destroying it. [Reprinted by permission from Nature, 400, 215 (15 July 1999). Special thanks also to Jean-Michel Raimond, Laboratoire Kastler Brossel, École Normale Supérieure, France.]
This research is being reported by G. Nogues, A. Rauschenbeutel, S. Osnaghi, M. Brune, J.M. Raimond & S. Haroche in Nature, 400, 239 (15 July 1999).
