ScienceWise - Jul/Aug 2006

A Double Photon Atomic Laser

Article Illustration
The researchers behind the atomic laser. Clockwise from top right are Nick Robins, Julien Dugue, Paul Summers, Matt Jeppesen, and Oliver Topic.
Article Illustration
A beam of identical atoms, shown in red, pour out of a Bose-Einstein Condensate, shown in blue, to form an atomic laser. Atomic lasers may make a whole new generation of sensors possible. This is a shadow image. To capture it the researchers fire a near resonant laser at the condensate and the atom laser beam. The atoms scatter photons out of the laser beam leaving a ‘shadow’ which is directly proportional to the local atomic density, just like sunlight shinning through a cloud. This shadow is then imaged onto a CCD detector and what you see is simply the raw data from the experiment - a density map of the atom laser.

Using BECs to generate matter waves

An optical laser beam consists of a stream of identical photons each with the exact same wavelength and phase. But the physics of lasers isn’t restricted to photons; you can also make a laser from a stream of identical atoms. So-called atomic lasers begin with a super-cold cloud of atoms known as a Bose-Einstein Condensate or BEC. The atoms in a BEC are so cold that they take on an entirely new state of matter, each adopting identical quantum properties to the others, just like the individual photons in an optical laser.

In principle, by slowly releasing atoms from a BEC in a steady stream, it’s possible to generate an atomic beam with all the coherence properties of an optical laser. As with so many things in science, the practice is a lot more difficult than the principle.

To achieve a BEC in the first place, scientists use optical lasers to slow or cool atoms to almost absolute zero. At such low temperatures the atoms become sensitive to the pull of magnetic fields and it becomes possible to trap them in a carefully designed magnetic cell. As the atoms cool further in the magnetic trap, they begin to condense into a BEC.

To release atoms from the BEC a radio frequency pulse is used to flip their Zeeman state, making them less sensitive to the magnetic trapping field and allowing them to tumble from the trap under gravity. However to make a really good bright atom laser, it’s better if they all travel in a single direction with more enthusiasm than a mere tumble.

For almost a decade now theorists have been proposing a clever way of achieving atom release from a BEC by using two photons of light to induce a double transition. With the angles, wavelengths and energy all just right, the effect is to flip an atom’s Zeeman state thus freeing it from the trap, and also giving it a little kick using the momentum of one of the photons.

This is very complex to achieve in practice, and until recently had only been done using very short pulses to release little packets of atoms. However, scientists in the Department of Physics recently became the first group in the world to be able to utilise this double photon process to create a continuous steady release of atoms from a BEC forming a bright and highly coherent atomic laser. The work is one of the flagship projects of the ARC Centre of Excellence for Quantum-Atom Optics.

Because of their inherent physical properties, bright atomic lasers offer the potential to revolutionise many sensing applications. For example the theoretical limit to the accuracy of a rotation sensor using an atomic laser is 100,000,000,000 times as high as the same limit for an optical laser based system. Atomic beams also offer major improvements in time keeping which is vital to global positioning systems and to super quiet sensing used in gravity wave detection.

More info: Nick.Robins@anu.edu.au

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