Physicists are building an atom laser that can stay on forever

The central part of the experiment in which coherent matter waves are created. New atoms (blue) fall and move towards the Bose-Einstein condensate in the center. In reality, atoms are not visible to the naked eye. Credit: Scixel.

Lasers use coherent light waves: all the light inside a laser vibrates completely in sync. Meanwhile, quantum mechanics tells us that particles like atoms should also be considered waves. As a result, we can construct “atom lasers” containing coherent waves of matter. But can we make these waves of matter last, so that they can be used in applications? In research published in Nature this week, a team of physicists from Amsterdam shows that the answer to this question is yes.

Running bosons in sync

The concept behind the atom laser is something called the Bose-Einstein Condensate, or BEC for short. Elementary particles in nature come in two types: fermions and bosons. Fermions are particles like electrons and quarks, the building blocks of the matter we are made of. Bosons are of a very different nature: they are not hard like fermions, but soft: for example, they can cross each other without any problem. The best-known example of a boson is the photon, the smallest amount of light possible. But particles of matter can also combine to form bosons – in fact, whole atoms can behave like particles of light. What makes bosons so special is that they can all be in the exact same state at the same time, or put in more technical terms, they can “condense” into a coherent wave. When this type of condensation occurs for particles of matter, physicists call the resulting substance a Bose-Einstein condensate.

In everyday life, we do not know these condensates at all. The reason: it is very difficult to make the atoms all behave as one. The culprit that destroys synchronicity is temperature – when a substance heats up, the constituent particles begin to twitch and it becomes nearly impossible to get them to behave as one. Only at extremely low temperatures, about one millionth of a degree above absolute zero (about 273 degrees below zero on the Celsius scale), is there a chance of the waves forming. consistent material of a BEC.

Fleeting Shards

A quarter of a century ago, the first Bose-Einstein condensates were created in physics laboratories. This opened up the possibility of building atom lasers – devices that literally produce beams of matter – but these devices could only work for a very short time. Lasers could produce pulses of matter waves, but after sending such a pulse, a new BEC had to be created before the next pulse could be sent. For a first step towards an atom laser, it was still not bad. In fact, ordinary optical lasers were also made in a pulsed variant before physicists could create continuous lasers. But while developments for optical lasers had moved very quickly, with the first continuous laser being produced within six months of its pulsed counterpart, for atom lasers the continuous version remained elusive for over 25 years.

The problem was clear: BECs are very fragile and are quickly destroyed when light falls on them. However, the presence of light is crucial in the formation of the condensate: to cool a substance to a millionth of a degree, its atoms must be cooled using laser light. As a result, BECs were limited to fleeting bursts, with no way to sustain them consistently.

A Christmas present

A team of physicists from the University of Amsterdam has now succeeded in solving the difficult problem of creating a continuous Bose-Einstein condensate. Florian Schreck, the team leader, explains what the trick was. “In previous experiments, the gradual cooling of the atoms was done in a single place. In our setup, we decided to spread the cooling steps not in time, but in space: we make the atoms move for as they progress through consecutive cooling stages. In Ultimately, ultracool atoms arrive at the heart of the experiment, where they can be used to form coherent matter waves in a BEC. But while these atoms are used, new atoms are already on their way to replenish the BEC. This way we can continue the process, practically forever.”

While the idea behind it was relatively simple, its execution certainly wasn’t. Chun-Chia Chen, first author of the publication in Naturerecalls: “Already in 2012, the team, then still in Innsbruck, developed a technique that made it possible to shield a BEC from laser cooling light, allowing for the first time laser cooling to the state degenerate needed for coherent waves.While this was a critical first step towards the long-standing challenge of building a continuous atom laser, it was also clear that a dedicated machine would be needed to go further away. faith, borrowed funds, an empty room, and a team funded entirely by personal grants. Six years later, in the early hours of Christmas morning 2019, the experiment was finally about to work. We had the idea of ​​adding an extra laser beam to solve one last technical difficulty, and instantly every image we took showed a BEC, the first continuous wave BEC.”

After tackling the long-open problem of creating a continuous Bose-Einstein condensate, the researchers have now turned their attention to the next goal: using the laser to create a stable output beam of matter. Once their lasers can not only operate indefinitely, but also produce stable beams, nothing will stand in the way of technical applications, and matter lasers could begin to play as important a role in technology as ordinary lasers.


Laser cooling for quantum gases


More information:
Chun-Chia Chen et al, Bose-Einstein Continuous Condensation, Nature (2022). DOI: 10.1038/s41586-022-04731-z

Provided by the University of Amsterdam

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