A research group at the Max-Planck Institute for Quantum Optics in Garching has generated a bound state between an individual atom and single light quanta (Nature, March 23, 2000). The scientists have observed the motion of the trapped atom in real time. Fundamental questions concerning the mechanical interaction of light with matter can now be investigated. The results find applications in the interdisciplinary research field of quantum information processing.

Atoms and molecules absorb light of a characteristic colour. Hence, a light beam with this colour is significantly attenuated by the atoms. In this way even small amounts of trace gases can easily be detected. However, the method fails if only a single gas atom is present. In this case, the attenuation of the light beam is much too small to be measurable. To detect single atoms, the following trick can be used: the atom is placed between two highly reflecting mirrors. These mirrors form an optical resonator, in which the light is reflected to and fro many times. Although the attenuation of the light beam in presence of the atom is small between two consecutive reflections, a large effect results after many reflections.

This method to observe single atoms works best at low light intensity. This is because for increasing intensity one has a growing probability that the atom absorbs a light quantum thereby making a transition to an excited energy level. In this level the atom cannot absorb another light quantum and further attenuation of the light beam is impossible. To avoid the undesired excitation of the atom, the light intensity is reduced until at most a single light quantum is inside the resonator at any given time. In this case, the atom can still absorb a light quantum and populate an excited energy level - but the light is then turned off. As this effect is easy to detect, it can be used to observe single atoms.

If the spacing between the two mirrors is very small the energy absorbed by the atom is radiated back into the resonator. This leads to a periodic exchange of energy between the atom and the light field. In principle, such an oscillatory energy exchange between two coupled systems is not new. For example, it is responsible for the chemical binding between two neutral atoms. The reason for this binding is that electrons can jump from one atom to another thereby acting as a kind of glue between the atoms. In case of the atom and the light quantum, it is also possible to form a molecule with a bound state. This means, that the light quantum is able to hold the atom like optical tweezers between the mirrors. The bound state decays only when the light quantum is no longer reflected from one of the two mirrors. Therefore, mirror losses limit the lifetime of the molecule. To compensate this loss, new light quanta from an external laser must be brought into the resonator again and again.

For a long time it was not clear, whether this exotic molecule actually exists. This is because the binding energy is so small, that even the kinetic energy of an atom at room temperature is sufficient to destroy the molecule. Nevertheless, a research group consisting of Pepijn Pinkse, Thomas Fischer, Peter Maunz and Gerhard Rempe from the Max-Planck Institute for Quantum Optics in Garching has now successfully produced this novel molecule in the laboratory. For this purpose, the scientist have realised an atomic fountain where atoms are first cooled by light pressure to a very low temperature. Then, the atoms are thrown up vertically, with gravity decelerating them on their way upwards. The resonator is placed at the highest point of the atomic trajectory, exactly where the atoms turn around and, hence, have a very small velocity. The resonator consists of two mirrors with a reflectivity exceeding 99.999 percent, so that light is reflected to and fro several 100.000 times. The distance between the two mirrors amounts to about 100 micrometer. In the experiment, an extremely weak light beam passing through both mirrors one after the other is used to detect an approaching atom. Exactly when the atom is in the middle between the two mirrors, the intensity of the laser beam is increased until a single light quantum is inside the resonator. The sudden increase of the light intensity is necessary to capture the atom. This resembles the situation of a marble in a bowl. When the marble is thrown into the bowl, rolling downhill on one side, it will escape on the other side unless one increases the height of the walls exactly when the marble is at the lowest point in the centre of the bowl. Only then it is possible to trap the marble which, of course, will be moving around in the bowl. The atom trapped by single light quanta doesn't sit still either. In contrast, the periodic energy exchange between the atom and the light field induces a periodic motion. In the experiment, the scientist could continuously observe this motion in real time.

The system is an ideal tool to explore fundamental problems concerning the interaction between light and matter. Possible applications are in the field of information processing with individual quanta. In the new system, the light-matter interaction is so strong, that it should be possible to realise a photon pistol. Here, individual light quanta would be emitted on demand in a well-defined direction. Such a light source would be new and particularly useful for the transmission of single quantum bits in a future network of quantum computers.

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