Cold atoms + lasers = synthetic magnetic field

Hank Hogan, hank.hogan@photonics.com

Thanks to lasers, researchers have persuaded ultracold, neutral atoms to do something new: respond as if they were charged particles to a magnetic field that isn’t there. This synthetic magnetic field will help scientists create new states of matter and probe their fundamental properties. The findings also could lead someday to new types of computers.

Research team leader Ian B. Spielman, a physicist at the National Institute of Standards and Technology (NIST), said that the experiment required new scientific concepts and was technically challenging to pull off. However, it did not demand breakthroughs in instrumentation or equipment. “We use established laser techniques, but with a lot of finesse.”

In the Dec. 3, 2009, issue of Nature, the team described how it achieved synthetic magnetism. It began by cooling a cloud of rubidium atoms, using optical and other methods to trap them and then chill them to 100 nK. Hovering just above absolute zero, the atoms formed a Bose-Einstein condensate, with all of them residing in the lowest energy quantum mechanical state. Such a condensate has been described as a superatom because its constituents behave identically.

After creating the condensate, the researchers applied a small, real magnetic field across the ensemble that varied along a single direction. At the same time, they illuminated the atoms with two near-infrared (801.7 nm) laser beams at right angles to each other. The two beams differed in frequency by about 3 MHz, or about one part in a billion. The beams coupled to the internal spin-state of the atoms through the Raman effect.

The result of the laser beams and magnetic field was that the neutral particles moved as though they were charged particles traveling through a uniform real magnetic field. The synthetic magnetic field caused the atoms to spiral as they moved. The researchers created vortices in the condensate by varying parameters. Imaging the rubidium cloud captured these vortices.

Vortices, the dark spots in these images, are the result of synthetic magnetism acting on an ultracold condensate of rubidium atoms. The vortices are not present before two laser beams are switched on (–19 ms in upper left) but are there afterward (rest of images). Courtesy of Ian Spielman, NIST.

Spielman noted that synthetic magnetic fields could help reveal some of the fundamentals of matter and its interactions with magnetic fields. Researchers, for instance, will be able to investigate more completely the energy spectrum of particles, such as electrons, in a crystal lattice when a magnetic field is applied. One possible outcome of this research could be novel materials with unusual properties.

New states of matter also could help resolve a problem: Quantum computers can solve some problems impossible to tackle with current technology. However, actually building these new machines involves practical issues, one of which is the loss of quantum coherence.

Bosons, as with the condensate, might theoretically eliminate some of these problems, Spielman explained. “These quasiparticles are important for a proposed method of quantum computation, known as topological quantum computation, that is naturally robust against the decoherence that plagues current implementations.”