Lasers use coherent light waves: all the light in a laser vibrates completely synchronously. Meanwhile, quantum mechanics tells us that particles such as atoms should also be considered waves. As a result, we can build “atomic lasers” that contain 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 Amsterdam physicists shows that the answer to this question is affirmative.
Making bosons march synchronously
The concept underlying the atomic laser is the so-called Bose-Einstein Condensate, or BEC for short. Elemental particles occur in nature in two types: fermions and bosons. Fermions are particles like electrons and quarks – the building blocks of matter from which we are made. Bosons are very different in nature: they are not hard like fermions, but soft: they can move through each other without any problem, for example. The best-known example of a boson is the photon, which is the smallest possible amount of light. But matter particles can also combine to form bosons — in fact, whole atoms can behave just like particles of light. What makes bosons so special is that they can all be in the exact same state at the exact 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 are not at all familiar with these condensates. The reason: It’s very difficult to get atoms all to behave as one. The culprit destroying synchronicity is temperature – when a substance heats up, its constituent particles begin to wiggle around and make it virtually impossible for them to behave as one. Only at extremely low temperatures, about a millionth of a degree above absolute zero (about 273 degrees below zero on the Celsius scale), is there a chance for the coherent matter waves of a BEC to form.
A quarter of a century ago, the first Bose-Einstein condensates were created in physics labs. This opened up the possibility of building atomic lasers – devices that literally produce bundles of matter – but these devices were only able to function for a very short time. The lasers could produce pulses of matter waves, but after emitting such a pulse, a new BEC had to be created before the next pulse could be emitted. This was not bad for a first step towards an atomic laser. 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 atomic lasers the continuous version remained elusive for more than 25 years.
It was clear what the problem was: BECs are very fragile and are quickly destroyed when light falls on them. Still, the presence of light is crucial in forming the condensate: to cool a substance to one millionth of a degree, you have to cool its atoms with laser light. As a result, BECs were limited to fleeting eruptions, with no way to sustain them coherently.
A Christmas present
A team of physicists from the University of Amsterdam has now managed to solve 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 atoms was done all in one place. In our setup, we decided not to spread the cooling steps in time, but in space: we let the atoms move as they go through successive cooling steps. Ultracold atoms come into the heart of the experiment, where they can be used to form 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. keep the process going — essentially forever.”
The idea behind it was relatively simple, but the execution was certainly not. Chun-Chia Chen, lead author of the publication in Naturerecalls: “As early as 2012, the team – then still in Innsbruck – realized a technique that allowed a BEC to be protected from laser cooling light, allowing for the first time laser cooling to the degenerate state needed for coherent waves. Although this is a crucial first step was on the way to the long-held challenge of building a continuous atomic laser, it was also clear that a special machine would be needed to move forward.When we moved to Amsterdam in 2013, we started with a leap of faith, borrowed money, 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 to add an extra laser beam to solve one last technical difficulty, and immediately every photo we took showed a BEC, the first continuous-wave BEC.”
After tackling the long-standing open problem of creating a continuous Bose-Einstein condensate, the researchers set their sights on the next goal: to use the laser to create a stable output beam of matter. Once their lasers can not only work forever, but also produce stable beams, nothing will stand in the way of technical applications and matter lasers can play as important a role in technology as regular lasers do today.
Laser cooling for quantum gases
Chun-Chia Chen et al, Continuous Bose-Einstein Condensation, Nature (2022). DOI: 10.1038/s41586-022-04731-z
Provided by University of Amsterdam
Quote: Physicists build an atomic laser that can stay on forever (2022, June 14) retrieved June 14, 2022 from https://phys.org/news/2022-06-physicists-atom-laser.html
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