Researchers have shown that an atom interacting with light inside a cavity can alter the wavelike properties of the light, reducing its amplitude or phase fluctuations below the level allowed for classical electromagnetic radiation. The team, led by Gerhard Rempe, director of the Max Planck Institute of Quantum Optics, reported this first observation of so-called “squeezed” light produced by a single atom in the June 30 issue of Nature. The “graininess” of photons causes tiny fluctuations of the light wave’s amplitude and phase. For classical beams, the minimal amount of fluctuations is equal. However, by creating interactions between the photons, one can “squeeze” the fluctuations of the amplitude below this “shot noise” level at the expense of increasing the fluctuations of the phase, and vice-versa. A single rubidium atom in a cavity squeezes the quantum fluctuations of a weak laser beam, decreasing the fluctuations of the amplitude at the expense of the phase. The effect is exaggerated for clarity. The photonic interactions inside standard optical media, however, are very weak, and require bright light beams to be observed. Single atoms are promising candidates to enable such interactions at a few-photon level. Their ability to generate squeezed light was predicted 30 years ago, but the amount of light they emit is very tiny, and all attempts to set this idea into realization had failed until now. Rempe’s group trapped a single rubidium atom inside a cavity made of two very reflective mirrors about 0.1 mm apart. After they injected a weak laser beam into this cavity, the atom interacted with one photon many times, and formed a kind of artificial molecule with the photons of the light field. As a consequence, two photons can enter the system at the same time and become correlated. “According to the model of Bohr, a single atom emits exactly one single energy quantum; i.e., one photon. That means that the number of photons is exactly known, but the phase of the light is not defined,” Rempe said. “But the two photons that are emitted by this strongly coupled atom are indistinguishable and oscillate together. Therefore, this time, the wavelike properties of the propagating light field are modified.” The ability of a single atom to induce strong coherent interactions between propagating photons opens up new possibilities for photonic quantum logic using single emitters. For more information, visit: www.mpq.mpg.de