Physicists in Australia have demonstrated the first pumped atom laser. Their achievement marks another step on the road to a continuously operating atom laser, which should enable high-precision measurements of rotations, accelerations and magnetic fields (Nature Physics doi:10.1038/nphys1027).
An atom laser is made from a Bose–Einstein condensate (BEC), a collection of atoms that have been cooled until they fall into the same quantum state. Normally a BEC is contained completely within magnetic fields, but if some of a BEC's atoms are allowed to escape, they stream away in a coherent state – just like the photons in a conventional laser.
Eventually the atoms in an atom laser's BEC will run out, and the lasing will stop. The idea of the pumped atom laser, therefore, is to continuously replenish the lasing BEC with atoms from a similar BEC a short distance away. "The pumped atom laser has been a goal of our group for five years," said John Close
, the lead author of the research at the Australian National University.
It's not a case of doing it slowly, as you might if you were trying to fill a bucket of water without splashing
John Close, Australian National University
'Subtle' technique
Close and colleagues situate the lasing BEC some 8 µm beneath the second BEC, so that it can be "drip fed" with atoms from above. But as the atoms from the second BEC are falling they are slowed by an optical laser pointed upwards, which forces each atom to absorb a photon. Shortly after, the atoms emit their own photon downwards, thereby slowing their descent further.
The advantage of this mechanism, which is known as a Bose-stimulated irreversible transition, is that the total upwards momentum produced by the photon absorption and emission exactly cancels the downwards momentum. This means that the atoms from the second BEC can join those in the lasing BEC without disturbing them, and therefore without creating any noise.
"It's not a case of doing it slowly or carefully, as you might if you were trying to fill a bucket of water without splashing it," explained Close. "It is more subtle than that."
Wolfgang Ketterle, the Nobel Prize-winning physicist who, together with his team at the Massachusetts Institute of Technology, invented the atom laser in 1996, told physicsworld.com that Close and colleagues have "cleverly" implemented their mechanism. "It is nice to see how the analogy with an optical laser is getting closer," he added.
Towards a continuous atom laser
The next step for Close's team is to combine the new pumping mechanism with a previously realised "conveyor belt" mechanism that takes the BEC atoms into the lasing mode. Once they have achieved this, says Close, they will have a "truly" continuous atom laser that can perform measurements of moving systems and magnetic fields.
"The sensitivity of an atom-based measurement ultimately depends on the flux of atoms, or how many atoms per second go through the measurement system," he explained. "More atoms gives you higher precision."
