From the perspective of an atomic nucleus, all electrons are far-flung voyagers. Whereas a nucleus measures femtometers in diameter, a bound electron typically travels 100,000 nuclear diameters away from the core. But Rydberg atoms, the colossi of the atomic world, have outer electrons so pumped with energy that they can travel 100 billion nuclear diameters — tens or hundreds of micrometers — from their nucleus. The largest Rydberg atoms even approach the size of the full stop at the end of this sentence.
Named after nineteenth-century Swedish physicist Johannes Rydberg, these giant atoms have been studied extensively since the 1970s, with the introduction of lasers that could excite electrons out to such vast distances. Like any distant traveler, the outer electron in a Rydberg system can be lonely and vulnerable. The attraction to the distant core is faint and easily disturbed by stray electromagnetic fields or collisions, so the atoms must be created in high vacuum. If carefully isolated from outside forces, such inflated atoms can be maintained for anything from a few hundredths of a second up to multiple seconds.
For Barry Dunning, a physicist at Rice University in Houston, Texas, the joy of Rydberg atoms is that they give physicists exquisite control over the motion of an electron. That is not possible with normal atoms because the electrons move much too quickly for even the fastest lasers. But the motion of an inflated electron in a Rydberg atom is much slower: it can be controlled with carefully directed nanosecond electric-field pulses, which allow researchers to herd the electron cloud by knocking it back and forth.
In 2008, researchers led by Dunning reported that they had managed to squeeze the normally spread-out electron into a tight packet that briefly orbited the nucleus. Last year, they added radio waves that enabled that motion to be maintained indefinitely. “It only took a century, but we recreated Bohr's atom,” says Dunning proudly. His next idea is to try exciting and controlling two outer electrons at once, creating a system analogous to how Bohr might have pictured helium.
This kind of atom-stretching has some potential applications. Two gaseous atoms a few micrometers apart cannot normally affect each other. But inflate one (or both) to a Rydberg state, and the negatively charged electron clouds start to repel each other, distorting the energy levels of the atoms so that they are no longer isolated systems. Mark Saffman, a physicist at the University of Wisconsin-Madison, has used this property to make a quantum logic gate—a fundamental part of a quantum computer — with lasers switching on a Rydberg interaction between two atomic quantum bits, or qubits.
He and other researchers hope next to add more atoms. A cloud of cold gas atoms might, if suitably excited, create a kind of hovering crystalline array of Rydberg interactions, says Matthew Jones, a physicist at Durham University, UK.
That approach might prove a useful model for studying the physics of 'strongly correlated' solid-state systems. These are systems, such as high-temperature superconductors, in which unusual properties emerge because particles interact strongly with their neighbors. An array of Rydberg atoms would not be a perfect model for the messy interactions in real solid-state systems, but the simplicity of the approach is a strength, says Burgdörfer. “It's a wonderful testing ground for probing many of these ideas about how strongly correlated physics actually works,” he says.