Physicists Build Atom Laser That Can Stay On Forever These days, imagining our everyday life without lasers is difficult. Lasers are used in printers, CD players, measuring devices, pointers, and so on.
What makes lasers so special is that they use coherent waves of light: all the light inside a laser vibrates completely in sync. Meanwhile, quantum mechanics tells us that particles like atoms should also be thought of as waves. As a result, we can build ‘atom lasers’ containing coherent waves of matter. But can we make these matter waves last, so that they may be used in applications? In research that was published in the journal Nature on June 8, a team of physicists from the University of Amsterdam shows that the answer to this question is affirmative The concept that underlies the atom laser is the so-called Bose-Einstein Condensate, or BEC for short. Elementary particles in nature occur in two types: fermions and bosons. Fermions are particles like electrons and quarks – the building blocks of the matter that we are made of. Bosons are very different in nature: they are not hard like fermions, but soft: for example, they can move through one another without a problem. The best-known example of a boson is the photon, the smallest possible quantity of light.
But matter particles can also combine to form bosons – in fact, entire 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 phrased in more technical terms: they can ‘condense’ into a coherent wave. When this type of condensation happens for matter particles, physicists call the resulting substance a Bose-Einstein Condensate. In everyday life, we are not at all familiar with these condensates. The reason: it is very difficult to get atoms to all behave as one. The culprit destroying the synchronicity is temperature: when a substance heats up, the constituent particles start to jiggle around, and it becomes virtually impossible to get 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 of forming the coherent matter waves of a BEC.
A quarter of a century ago, the first Bose-Einstein Condensates were created in physics labs. This opened up the possibility to build atom lasers – devices that literally output beams 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 sending out such a pulse, a new BEC had to be created before the next pulse could be sent out. What makes lasers so special is that they use coherent waves of light: all the light inside a laser vibrates completely in sync. Meanwhile, quantum mechanics tells us that particles like atoms should also be thought of as waves. As a result, we can build ‘atom lasers’ containing coherent waves of matter. But can we make these matter waves last, so that they may be used in applications? Elementary particles in nature occur in two types: fermions and bosons. Fermions are particles like electrons and quarks – the building blocks of the matter that we are made of. Bosons are very different in nature: they are not hard like fermions, but soft: for example, they can move through one another without a problem. The best-known example of a boson is the photon, the smallest possible quantity of light.
But matter particles can also combine to form bosons – in fact, entire 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 phrased in more technical terms: they can ‘condense’ into a coherent wave. When this type of condensation happens for matter particles, physicists call the resulting substance a Bose-Einstein Condensate. The central part of the experiment in which the coherent matter waves are created. Fresh atoms (blue) fall in and make their way to the Bose-Einstein Condensate in the center. In reality, the atoms are not visible to the naked eye. Image processing by Scixel. Credit: Uv In everyday life, we are not at all familiar with these condensates. The reason: it is very difficult to get atoms to all behave as one. The culprit destroying the synchronicity is temperature: when a substance heats up, the constituent particles start to jiggle around, and it becomes virtually impossible to get 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 of forming the coherent matter waves of a BEC.
A quarter of a century ago, the first Bose-Einstein Condensates were created in physics labs. This opened up the possibility to build atom lasers – devices that literally output beams of matter – but these devices were only able to function for a very short time. Physicists Build Atom Laser could produce pulses of matter waves, but after sending out such a pulse, a new BEC had to be created before the next pulse could be sent out. For a first step towards an atom laser, this was still not bad. In fact, ordinary, optical lasers were also made in a pulsed variant before physicists were able to create continuous lasers. But while the developments for optical lasers had gone very fast, the first continuous laser being produced within six months after its pulsed counterpart, for atom lasers the continuous version remained elusive for more than 25 years.
Florian Schreck, the team leader, explains what the trick was. “In previous experiments, the gradual cooling of atoms was all done in one place. In our setup, we decided to spread the cooling steps not over time, but in space: we make the atoms move while they progress through consecutive cooling steps. In the end, ultracold 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 being used, new atoms are already on their way to replenish the BEC. In this way, we can keep the process going – essentially forever.” Having tackled the long-standing open problem of creating a continuous Bose-Einstein Condensate, the researchers have now set their minds on the next goal: using the Physicists Build Atom Laser to create a stable output beam of matter. Once their lasers can not only operate forever but can also produce stable beams, nothing stands in the way of technical applications anymore, and matter lasers may start to play an equally important role in technology as ordinary lasers currently do.
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