It's possible to detect something as fragile as a single photon without destroying it, new research suggests. Single-photon sources have important applications in quantum networking, cryptography and information processing. While there are several methods capable of detecting single light particles, none are very efficient, and most destroy (use all the photon's energy) the particle in the detection process. But Andreas Reiserer and colleagues at the Max Planck Institute of Quantum Optics have developed a device that leaves the photon untouched upon detection. Conceptual illustration of a feat achieved by Andreas Reiserer and colleagues at the Max Planck Institute of Quantum Optics, who developed a method for detecting a photon without absorbing its energy. With a single atom trapped in an optical resonator, the presence of a reflected photon can be detected nondestructively. Courtesy of Andreas Reiserer, MPQ. In their experiment, Reiserer, Dr. Stephan Ritter and professor Gerhard Rempe developed a cavity consisting of two highly reflecting mirrors closely facing each other. When a photon is put inside the cavity, it travels back and forth thousands of times before it is transmitted or lost, leading to strong interaction between the light particle and a rubidium atom trapped in the cavity. By reflecting the photon away from the device, the team was able to detect the photon by changing its phase rather than its energy. If a photon is not destroyed upon detection, it can be detected more than once, which can boost the efficiency of future detectors. Additionally, in quantum networks photons are carriers of quantum information. These results suggest that it’s possible to detect the presence of a photon without “touching” (and destroying) the information that it is encoded in it. "The trick is that we prepare the atom in a superposition of the two ground states. The very moment the photon is reflected from the cavity, the resonant state experiences a phase shift relative to the off-resonant one. This phase shift can then be read out from the atom," said Reiserer, doctoral candidate. "In this way, the photon has survived its detection with its properties — for example, its pulse shape or polarization — untouched." The phase shift of the atomic state is detected using a well-known technique. "Loosely speaking, the atom lights up when probed after reflection of a photon," said Ritter. To prove that the nondestructive detection works, the reflected photons are also registered by conventional photodetectors. "In this way, we detect the photon twice, which is impossible with destructive detectors alone," he said. "In our proof-of-principle experiment, we have achieved a single-photon detection efficiency of 74 percent, which is already more than the 60 percent of typical destructive detectors," Ritter said. The value they achieved might be improved upon in the future, he said, if they could eliminate some imperfections in the current system. The research, “Nondestructive Detection of an Optical Photon,” appears in the Nov. 14 issue of Science Express. doi: 10.1126/science.1246164 For more information, visit: www.mpq.mpg.de