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Optical Lattices Shape Up

Cell dimensions in conventional optical lattices are constrained by the wavelength of the laser light.

Cell dimensions in conventional optical lattices are constrained by the wavelength of the laser light.

An optical lattice is formed by the intersection of multiple laser beams, producing a standing wave pattern. Within that pattern, as the beams interact with each other, there are regions with higher and lower energy intensity. As a result, an atom placed in the lattice will naturally tend to seek the minimal energy points, represented as deep wells in the figures to the right. Because lattice configurations resemble the geometrical arrangements of atoms in crystalline solids, they can be used to study atomic behaviors in a highly controlled environment.

The JQI/NIST team took a cluster of rubidium atoms, ultra-cooled it to the point at which it formed a Bose-Einstein condensate, and then nudged it into the lattice. If nothing else had been done, the energy landscape would have looked like the top figure above, and one atom would have settled into each well with only one degree of freedom -- its spin. But the researchers then gradually began applying an RF field and tuned the amplitude and frequency until the interaction of laser and RF effects created a fine, sub-wavelength structure in each well. Thus, in addition to spin, each atom in such a well would also have a positional degree of freedom in the reconfigured energy shape. That structure more closely resembles the complex symmetries of certain materials (such as high-temperature superconductors) that are of urgent interest to science. It may also make it easier to study some aspects of tunneling, the process whereby a quantum object passes through a barrier that it could not cross classically.

RF “dressing” has never been applied at these tiny dimensions before. “We’re trying to use optical lattices to gain insight into the tough problems of condensedmatter physics,” says Lundblad, “and anything we can do to take the tailored tradition of optical-lattice experiments and then increase the complexity level -- because real solids are quite complex -- is a goal to be sought after.”