Atomic lasers that can continuously produce a beam of matter are one step closer. Light lasers, which we use in printers, measuring equipment and ophthalmology, among other things, emit narrow beams of coherent light, which consists of light waves moving completely synchronously. According to quantum mechanics, particles such as atoms can also be described as waves. This means that physicists can create an atomic laser by having the matter waves of atoms move synchronously, like one large matter wave.
Atomic lasers can be used in precise sensors with applications in navigation and physics research into gravitational effects. For these applications it is necessary to maintain the matter waves for a long time. A group of physicists from the University of Amsterdam (UvA) has developed a method for this. Their Results appeared last week in Nature†
The basis of an atomic laser are thousands to millions of atoms that are all in the same state at the same time, thus forming one coherent wave of matter. This is called a Bose-Einstein condensate. You can compare it to a group of soldiers marching in perfect step, making them look like a whole.
It is not easy to get atoms to form a Bose-Einstein condensate. For this you have to cool them in vacuum to almost absolute zero (-273°C), so that they hardly move. This is done with laser light that slows down the atoms, causing them to cool. When a cloud of atoms is cold and compact enough, they naturally form a Bose-Einstein condensate.
Bose-Einstein condensates were first made over 25 years ago. This soon produced the first atomic lasers. But these can only produce matter wave pulses of a fraction of a second. A Bose-Einstein condensate only exists for a short time and is vulnerable, because you lose the atoms in it when they heat up or form molecules. A little scattered laser light can destroy it. That is difficult, because laser light is needed for cooling.
“To maintain an atomic laser without pulse, you have to continuously add ultra-cold atoms to the Bose-Einstein condensate to compensate for the atoms you lose,” says UvA physicist Florian Schreck by telephone. He and colleagues developed a technique for this. “In 2012, we showed that it is possible to make a Bose-Einstein condensate surrounded by a cloud of laser-cooled atoms,” he says. In that laser-cooled cloud, atoms can collide like billiard balls. One gets all the kinetic energy so that the other is so slow that it can enter the Bose-Einstein condensate.
The latest development has to do with laser cooling of the atoms, which is done step by step. At each step, a different laser cools the atoms further. “Other experiments carry out these cooling steps one after the other, in the same place. In our setup, every step takes place somewhere else,” says Schreck. “This lowers the risk of light from one step interfering with the next.”
This results in an arrangement with a constant flow of atoms that are cooled step by step to eventually end up in the laser-cooled cloud. From there they feed the Bose-Einstein condensate that is maintained for such arbitrarily long.
This is almost an atomic laser, but not quite yet. Schreck: “The next step is to add some sort of output so that we can extract a continuous atomic laser beam from the Bose-Einstein condensate.” If successful, the laser is ready to use.