“A really big step. Yes, a breakthrough.”
These are the words of Danish physicist and professor Klaus Mølmer from the Niels Bohr Institute after Dutch researchers announced that they have succeeded in creating a stable and continuous atom laser in a laboratory at the University of Amsterdam.
This is an atom laser that physicists have been trying to create for decades because it allows for some precision measurements that far surpass the tools and sensors that we have access to today. For example, just one atom laser can measure gravitational waves, dark matter, and quantum mechanical properties of gravity.
“It opens up completely new measurement properties for us physicists, so it will undoubtedly be a candidate for the most accurate time measurement ever achieved,” Klaus Mølmer says.
The atom laser from the Netherlands is quite complex, and it is actually based on Indian physicist Satyendra Nath Bose’s and Albert Einstein’s theories from 1920s.
The atoms in the laser are in a so-called Bose–Einstein condensate, which is a special quantum state that can occur when the temperature is lowered to close to absolute zero.
To understand the state, one has to look at atoms through a quantum mechanics perspective—i.e. look at them as waves. Because as the temperature decreases, the wavelength increases, and at some point, the wavelength becomes greater than the distance between the atoms, and then a state arises in which all atoms behave like a single wave—sort of like the light in a laser beam:
“For us physicists, it’s an incredibly magical moment. All particles are suddenly completely identical,” Klaus Mølmer says.
However, creating a Bose–Einstein condensate (BEC) in a laboratory is no longer something to write home about. It has been done in laboratories around the world ever since American researchers demonstrated it in an experiment in 1995 and 1996, for which they also received the Nobel Prize.
The difficult part, however, is maintaining the state for a long time AND using it to emit a beam of atoms.
Scientists have typically only succeeded in creating a BEC state by evaporating the hottest molecules in a collection of atoms until only the very coldest ones were left. But this means that the BEC state occurred only when there were almost no atoms left—and within a few milliseconds, the state ceased again.
A new way of cooling the atoms was needed, and already in 2012, the research team had begun to cool the atoms with laser light instead of evaporation.
But it would be another seven years before a continuous atom laser became a reality. And the crucial trick was to use strontium atoms, which are also used in high-accuracy watches. They have very precisely defined properties when it comes to the absorption and emission of light, and can be cooled with lasers to as close to absolute zero as it is necessary to achieve BEC.
The next step for the researchers was to create a setup in which new atoms are added to the atom laser as the beam sends atoms away. And here it was absolutely crucial that the newly arrived atoms have exactly the same temperature—because the BEC state would collapse otherwise.
One solution was to make two vacuum chambers, one of which acts as a kind of charging station for atoms that are cooled and that move towards the atom laser.
And in this process, strontium atoms are also a very smart choice. The type of strontium atoms used are so-called bosons that behave like photons. They naturally undergo a so-called stimulated process, in which they seek to be in the same state as the atoms that are already there—just as photons are formed by stimulated emission in a laser (the letters S and E in the word laser stand for “stimulated emission”):
“You could call it a kind of peer pressure, where all atoms seek to behave the same way—and in this case, it means they all want to go in and move with the beam. That way, the researchers can maintain the atom laser,” Klaus Mølmer says.
According to the Dutch researchers, the atom laser can in principle run for as long as it needs to, and the next step will be to refine the technology so that it emits stable rays of atoms.
And when that becomes possible, “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,” the researchers say.
The great advantage of atom lasers is, for example, that the wavelength of an atom beam is several hundred times shorter than the wavelength of light—and therefore can be used to take readings of much smaller structures.
The atom laser can also be used to measure gravitational waves, as the beam is a perfectly synchronous orbit of atoms that is only affected by gravity.
But Klaus Mølmer focuses on a completely different application, which he refers to as a kind of holy grail:
“In physics, we have slowly begun to investigate whether gravity in itself is subject to quantum properties, i.e. whether it has different values at the same time. If this is the case, then it is very possible that an atom laser can be used to get closer to an answer to that question,” Klaus Mølmer says.
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