A SIMPLE way of guiding matter waves鈥攁nalogous to the way optical
fibres guide light waves鈥攈as been demonstrated by Austrian physicists. The
trick is a vital step towards building devices based upon 鈥渕atter optics鈥� such
as atom beam splitters, tiny highly sensitive motion detectors and switches for
quantum computers.
The technique鈥檚 appeal is its simplicity: all it requires is a wire. When a
current flows through the wire, it generates a magnetic field. Electrically
neutral atoms are affected by magnetic fields because their nuclei have
spin.
If the field is strong enough, and the atoms are moving slowly enough, the
magnetic field around a wire can trap atoms into circling the wire, just as the
Sun can trap passing asteroids. Johannes Denschlag and his colleagues at the
University of Innsbruck have done just that with super-cold lithium atoms,
chilled to within 200 millionths of a degree of absolute zero (Physical
Review Letters, vol 82, p 2014).
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They also applied an external magnetic field at right angles to the wire,
producing a region parallel to the wire where the magnetic fields cancel. Atoms
prefer to reside in that region.
鈥淥nce you can control, coherently, the motion of atoms, you can build
billions of things that are useful, like interferometers on a small scale,鈥� says
Denschlag. Atom streams moving along a wire could be steered in different
directions by manipulating the wire, in much the same way as a fibre-optic cable
steers light beams.
Denschlag also predicts that a quantum computer could be made using this
technology: 鈥淛ust as electronic integrated circuits control the motion of
electrons, you can control the flow of atoms and do computations.鈥� Quantum
computers aim to use the quantum states of objects such as photons, electrons or
atoms to store more states than the mere 0s and 1s of today鈥檚 computers.
No one is yet sure whether the wire-guided atoms produced by Denschlag鈥檚
technique maintain their coherence鈥攖hat is, if the matter waves all stay
in the same phase. 鈥淚t has a lot of promise, but it鈥檚 too early to
tell鈥攁ll that has been done here is that they guided atoms, piping them
along wires,鈥� says William Phillips of the National Institute for Standards and
Technology, who won a Nobel prize in 1997 for his atom-cooling experiments. 鈥淭he
important thing that has yet to be done is guiding the atoms to make some kind
of interference experiment.鈥�
However, the simplicity of the arrangement gives Phillips hope that it will
be easier to build matter-wave switches with the wire-guided method than with a
rival hollow-tube method. 鈥淲ith a physically closed tube, it鈥檚 a demanding
task,鈥� says Phillips. 鈥淚f you could build a beam splitter with this, it would be
驳谤别补迟.鈥�
If the wire guides do produce coherent matter waves, they could represent a
breakthrough as significant as that of fibre optics. And since matter waves have
much smaller wavelengths than light waves, they can be used for much more
precise measurements, just as microscopes that use electrons instead of photons
get much more detailed pictures.
The fairly weak magnetic fields used in the Innsbruck experiments were able
to trap lithium atoms for tens of milliseconds. It should be easy to apply
stronger magnetic fields, and this increase the confinement times significantly.
鈥淭here鈥檚 not a real border that we cannot cross,鈥� says Denschlag.