Optical photons detected without being destroyed

Nature : A Nobel Prize was awarded last year, in part, for research that demonstrated that the presence of microwave photons in a cavity can be detected without destroying them. Now, Stephan Ritter of the Max Planck Institute for Quantum Optics in Germany and his colleagues have performed a similar feat using optical photons. An arrangement of two mirrors separated by just 0.5 mm created a cavity that could confine photons of a specific resonant energy. In the cavity, they trapped a single atom in a superposition of states, one of which had the same resonant energy as the cavity. Photons fired at the cavity either bounced away—if they had the same energy—or reflected off the mirrors and passed through. Although the photons that passed into and out of the cavity were unchanged, the state of the atom was. The method provides a way to detect individual photons without altering their state. By using visible-wavelength photons, the technique may prove useful for quantum communication.