Published online 22 July 1999 | Nature | doi:10.1038/news990722-8


Crystals through the looking glass

Researchers from Austria have devised a looking-glass world in which the normal roles of light and matter are reversed. In the experiments of Anton Zeilinger's group at the University of Innsbruck, crystals are made of pure light, and the rays that bounce off them are made of atoms.

This strange reversal is made possible by quantum physics, which ascribes some very counterintuitive properties to atoms. Albert Einstein's studies of the so-called photoelectric effect, in which light knocks electrons out of metals, showed at the beginning of the century that light can sometimes act like a stream of particles, not a continuous wave. And in 1923 the French aristocrat and scientist Louis de Broglie argued that if light could be both wave and particle, then so should matter be capable of this duality. De Broglie's hypothesis built on the new framework of quantum mechanics, the science of very small particles, and it paved the way for Erwin Schrödinger's 'wave mechanics', which described the fundamental constitution of matter in terms of electron waves.

That electrons - the tiny subatomic particles that circulate round an atom's heavy nucleus - can act like waves has been known for a long time. But it is only in the past decade or so that scientists have been able to study the wave properties of whole atoms. This is because the atoms need to be very cold and slow-moving before their wave-like nature can show itself; and methods for extreme cooling of atoms are a fairly recent invention. The field of 'atom optics', in which beams of atoms are manipulated like light beams, is now producing exciting developments such as the atom laser, from which the 'laser' beam is a stream of atoms.

The experiments of Zeilinger and colleagues reproduce a looking-glass version of a well-known effect in physics called diffraction. In ordinary diffraction, a beam of electromagnetic radiation, such as light or X-rays, bounces off a regular array of stacked atoms, as in a crystal, and forms a series of bright spots through wave interference effects. Diffraction of light causes the iridescent qualities of opal, and diffraction of X-rays from crystals allows scientists to deduce the positions of the atoms they contain.

But the Austrian team used crystals of light: 'standing wave' patterns, like the vibrations of a guitar string. The light standing waves are made by reflecting a laser beam off a gold mirror, which sets up a light field of bright and dark bands, analogous to layers of atoms in a real crystal. The researchers then fire a beam of argon atoms at this light field and measure the 'brightness' of the scattered beam.

Zeilinger's group first conducted experiments of this sort in 1996, when they showed that diffraction-type effects could affect the transmission of atoms through the 'light crystals'. They now report in the journal Physical Review A observations of diffraction that are more directly analogous to the scattering of X-rays from crystals: so-called Bragg diffraction, which was pioneered by William Bragg and his son Lawrence in the 1910s.

One of the attractions of this unusual approach to diffraction, say the authors, is that because their crystals are 'handmade' from light, they can "tailor crystal structures which were not available previously" by overlapping various different light standing waves. In other words, they can invent new types of crystal, or alternatively, new types of interaction between the 'atoms' of the crystal and the incoming beam. In this upside-down world where light plays the role of matter and vice versa, entirely new kinds of 'matter' become possible.