In an experimental tour de force that could assist in the development of highly sensitive sensors for electric and magnetic fields with frequencies ranging from direct current (DC) to the high terahertz (THz) region, as well as fast quantum computers, a University of Michigan group led by Georg Raithel has succeeded in trapping an array of Rydberg atoms in an optical lattice. Such advances could dramatically alter the landscape of surveillance technology, airport scanners and computer calculation capabilities that enter the realm of science-fiction.

A Rydberg atom is simply a conventional atom which is excited to the point that one or more of its outer (or valence) electrons have a very large quantum number. The result is a Rydberg atom which is much larger in size than a conventional atom. In fact, atoms nearly 1 millimeter in diameter have been produced – almost 6 orders of magnitude larger than the conventional atom from which they were produced!

A Rydberg atom is very sensitive to external influences due to its small size and small binding energy. Despite this sensitivity, the lifetime of a Rydberg atom is relatively long because the wavefunction of the highly excited electron has almost no overlap with its tightly bound states. Together with nearly perfect and highly controllable optical lattices, this makes Rydberg atoms an ideal component in the implementation of fast quantum circuits.

Prof. Raithel’s team began with a gas of ground-state rubidium atoms (the alkali metals -- Li, Na, K, Rb, and Cs -- have particularly favorable properties for Rydberg studies.) Using laser-cooling techniques, the velocity of the rubidium atoms was slowed from 300 m/s to 0.1 m/s, corresponding to a reduction in temperature of 5 orders of magnitude, yielding sub-microKelvin temperatures. The very low velocities avoid Doppler shift from limiting the process of excitation into a Rydberg atom.

These remarkably slow Rb atoms were trapped in an optical lattice, and then excited to their 10th excited state, at which point their radius had expanded by a factor of 100, and their valence electron binding energy had been made smaller by a factor of 100, to about 0.0136 eV. These are relatively stable, but still are sufficiently large to qualify as Rydberg atoms.

To trap the Rydberg atoms, an optical lattice consisting of 1064 nm laser light is utilized. An optical lattice is a standing wave of light of sufficient intensity that it can trap atoms within the spatial and time-dependent variations in optical intensity. A free electron reacts to a laser-cooling techniques, the velocity of the rubidium atoms was slowed from 300 m/sec to 0.1 m/s electric field by being forced toward regions of small field gradient. In a stationary optical lattice, these points are at the peaks and valleys of the optical field intensity. In Rydberg atoms, the highly excited valence electron is still sufficiently strongly attached to the nucleus that in trapping the distant (and hence nearly free) valence electron, you trap the entire Rydberg atom.

Unfortunately, the position of the pre-excitation atoms in the optical lattice does not change once they are cooled. The problem is that they are no longer in equilibrium locations, near the peaks of the lattice potential. As a result, the cooled atoms are generally not trapped by the lattice, but rather fall down the potential hills, or simply migrate rather freely throughout the lattice.

Prof. Raither and his group found that if the optical lattice were simply inverted once the Rydberg atoms are excited (requiring a λ/4 phase shift in the optical lattice), nearly all of them become trapped in what were now strong local potential minima. Trapping efficiency is better than 90% with a non-optimized apparatus, and thought likely to approach 100% with optimization of the experiment. This remarkable technique will allow technologists to begin studying both the theoretical behavior and some practical applications of Rydberg atoms – a field of study that until recently was thought of as having no potential for application.