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Quantum optics

Quantum optics

Single atoms on demand

14 Jun 2001

Quantum computers - in which data is stored as the quantum state of an atomic particle - could in principle outperform conventional computers. But it is difficult to control the single atoms that are necessary for the technique. Stefan Kuhr and colleagues at the University of Bonn in Germany may have overcome this barrier with the development of an 'atom trap' that can manipulate single atoms with sub-micron precision and can deliver them on demand (S Kuhr et al 2001 Science to appear).

In previous experiments, physicists have controlled the motion of ions and clouds of neutral atoms. But neutral atoms lack an electric charge, and combined with the intricacies of quantum statistics, it is much trickier to pin down a single one. Existing sources of neutral atoms include atom beams, but these produce an incoherent stream of neutral atoms, while ‘one-atom masers’ sometimes emit more than one atom.

Kuhr’s team created a trap for a single atom – or a specific number of atoms – using the interference pattern produced by a pair of infrared lasers. A magneto-optical device gathers single atoms from chilled caesium gas and deposits the desired number of them into the trap. The electric component of the laser light induces a dipole moment in each atom, causing the atom to interact with the electric field. This attraction between the atom and the electric field is dependent on the light intensity, and the atom is therefore drawn towards the brightest regions – that is, the maxima of the interference pattern. This means that the position of the atom is known to within half the wavelength of the laser light. By tuning the wavelengths of the lasers, Kuhr and colleagues were able to shift the interference pattern by about a centimetre – moving the atom with it. The position of the atom is tracked by ‘fluorescence detection’, in which the atom is excited by a probe laser, and its glow is measured by a photon counter.

After successfully moving an atom, Kuhr’s team investigated what would happen if the lasers were suddenly switched off while an atom was in motion. They found that the atom was catapulted into free flight with a velocity of a few metres per second. Although the velocity of the atom was uncertain in their set-up, Kuhr and colleagues have proved that single atoms can be ejected on demand. They are optimistic that – with some refinements – their system could evolve into a practical device.

“Our next goal is to place two atoms between two mirrors of ultrahigh reflectivity – that is, an optical resonator”, coauthor Dominik Schrader told PhysicsWeb. The team’s ‘atom ejection’ technique should make this possible for the first time. “This will allow us to study the quantum interactions between the two atoms, which is fundamental for the implementation of quantum information processing”, says Schrader.

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