Interfering Lasers Deliver Atoms

Richard Gaughan

The promise of nanotechnology is leading researchers to search for control over smaller and smaller amounts of matter. But the ideal, the manipulation of individual atoms, has been problematic, particularly when they must be delivered in a well-controlled manner with reasonably low kinetic energy. Now scientists at the University of Bonn in Germany have demonstrated smooth control of single atoms using a pair of interfering laser beams.

To do so, Stefan Kuhr and his colleagues in Dieter Meschede's laser physics laboratory split the output of an Nd:YAG laser and routed the two beams through independent acousto-optic modulators. The beams, propagating in opposite directions, were then focused to a 30-µm beam waist, forming a standing-wave interference pattern.

Series of optical traps

The researchers recognized that this standing-wave pattern effectively forms a series of optical traps. The electric field introduces an electric dipole moment in the irradiated atoms, and the interaction of the induced dipole moment and the electric field creates local potential minima. By overlapping the interference region with a fixed magneto-optical trap, they transferred a single ultracold cesium atom to the optical traps by turning the lasers on and off. The next step was to move the atom.

When the frequency of one acousto-optical modulator changes with respect to the other, the frequency difference also changes, which moves the standing-wave pattern. If an atom is caught within one of the traps in the pattern, it moves along with the pattern in a precise fashion.

Using 852-nm light to induce fluorescence in the cesium, the researchers monitored the position of the atoms in the optical traps. They translated single atoms as far as 10 mm from their initial positions and returned them with better than 80 percent efficiency.

"The transportation procedure works perfectly for our purposes," Kuhr said. "To be honest, it works much better than ever expected." He is confident that the incorporation of higher-power lasers into the setup will enable the researchers to precisely transport individual atoms over even greater distances.

The ability to control the number and the position of atoms will enable experiments that were previously impossible because of the statistical nature of the arrival process. Kuhr said that the research team is planning to deliver single atoms to a resonator. Another potential study could involve two or more atoms, entangled through the exchange of cavity photons, which, Kuhr noted, could lead to the creation of simple quantum gates for a quantum computer.