Lasers produce coherent light waves. This means that all the light inside a laser is vibrating at the same time. Meanwhile, quantum mechanics suggests that particles, like atoms, should be considered waves. As a result, the idea emerges that we can create "atomic lasers" containing waves of coherent matter.
But can we keep these matter waves alive long enough to be used in applications? A group of Amsterdam physicists shows that the answer to that question is yes, in a study published this week in Nature.
Synchronized Movements of Bosons
Bose-Einstein Condensation, or BEC for short, is the underlying concept of the atomic laser. Fermions and bosons are two types of elementary particles found in nature. Fermions are electron and quark-like particles that are the structural components of matter. Bosons differ from fermions in that they are not hard like fermions but are soft, which allows them to pass through each other without difficulty. The photon, the smallest imaginable amount of light, is the best-known example of a boson. However, matter particles can combine to form bosons, and all atoms can behave similarly to light particles.
What distinguishes bosons from other particles is that they can all be in the same state at the same time, or in other words, "condense" into a coherent wave. When matter particles condense in this way, physicists call the resulting matter Bose-Einstein Condensation.
We are not familiar with these condensations in daily life. This is because it is extremely difficult for atoms to function as one. The reason for the disappearance of synchrony is temperature.
As a substance heats up, the particles that make it up begin to spatter around, making it very impossible for them to work as a whole.
Coherent matter waves of a BEC can only occur at extremely low temperatures, roughly one millionth of a degree above absolute zero (about 273 degrees below zero on the Celsius scale).
The first Bose-Einstein Condensates were produced in physics laboratories a quarter of a century ago. This allowed the development of atomic lasers (devices that emit beams of matter), but these devices could only work for a short time. Lasers could produce pulses of matter waves, but each pulse required a new BEC to be created before the next was sent.
That still wasn't bad for a first step towards an atomic laser. In fact, conventional optical lasers were also made in a pulsed form before physicists built continuous lasers. However, with optical lasers advancing rapidly, the first continuous laser appeared six months after its pulsed counterpart, with the continuous version of atomic lasers elusive for more than 25 years.
The problem was clear: BECs are extremely sensitive and quickly disappear when exposed to light.
However, the presence of light is critical in the formation of condensate: to cool a substance by one millionth of a degree, laser light must be used to cool its atoms.
As a result, BECs were limited to short bursts and lacked the ability to sustain them sensibly.
A team of physicists from the University of Amsterdam has managed to solve the difficult problem of creating a continuous Bose-Einstein Condensation.
Team leader Florian Schreck explains what the trick is. “In previous experiments, the gradual cooling of atoms was done in one place.
In our setup, we decided to spread the cooling steps over space, not time. We can move atoms as they go through successive cooling steps.
Eventually, the ultracold atoms reach the heart of the experiment, where they can be used to create coherent waves of matter in a BEC.
But while these atoms are being used, new atoms are already on the way to replenish the BEC. That way we can keep the process going—indeed, essentially.”
While the basic concept was easy, putting it into action was not. Researcher Chun-Chia Chen recalls: “As early as 2012, the team at Innsbruck implemented technology that isolates a BEC from laser cooling light and allows laser cooling to the degenerate state necessary for consistency. While this was an important first step in resolving the longstanding problem of creating a continuous atom laser, it was clear that further work would be required.
In 2013 we started to run a project supported by personal donations. The experiment finally paid off after six years.
To fix one last technical issue, we added an extra laser beam and every image we took instantly revealed a BEC, the first continuous wave BEC.”
Addressing the longstanding open problem of creating a continuous Bose-Einstein Condensate, the researchers have now decided on the next goal.
This is to use the laser to create a stable material exit beam.
Nothing stands in the way of technical applications any more, when their lasers can not only work indefinitely, but also produce stable beams.
Matter lasers could begin to play an equally important role in technology as ordinary lasers currently do.