A new breakthrough has made it possible for physicists to create a ray of atoms that behaves in the same way as a laser, and which could theoretically remain “forever”.
Finally, this may mean that the technology is on its way to practical application, although significant limitations still apply.
Nevertheless, this is a major advance for what is known as an “atomic laser” – a beam made of atoms that marches as a single wave that can one day be used to test basic physical constants, and technical precision technology.
Atomic lasers have been around for a minute. The first atomic laser was created by a team of MIT physicists back in 1996. The concept sounds simple: just as a traditional light-based laser consists of photons moving with their waves synchronized, a laser made of atoms will require its own wave – nature likes to adjust before it is shuffled out like a beam.
As with many things in science, however, it is easier to conceptualize than to realize. At the root of the atomic laser is a state of matter called a Bose-Einstein condensate, or BEC.
A BEC is created by cooling a cloud of bosons to only a fraction above absolute zero. At such low temperatures, the atoms sink to the lowest possible energy state without stopping completely.
When they reach these low energies, the quantum properties of the particles can no longer interfere with each other; they move close enough to each other to overlap, resulting in a cloud of high-density atoms that behave like a “superatom” or a wave of matter.
However, BECs are something of a paradox. They are very fragile; even light can destroy a BEC. Given that the atoms in a BEC are cooled by optical lasers, this usually means that the existence of a BEC is volatile.
Atomic lasers that scientists have managed to achieve to date have been of pulsating, rather than continuous variation; and involves firing only one pulse before a new BEC must be generated.
To create a continuous BEC, a team of researchers at the University of Amsterdam in the Netherlands realized that something had to change.
“In previous experiments, the gradual cooling of atoms was done in one place. In our setup, we decided to disperse the cooling steps not over time, but in space: we make the atoms move as they go through subsequent cooling steps.” explained physicist Florian Schreck.
“Finally, ultra-cold atoms come to 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 the way to fill up the BEC. In this way we can keep the process started – really forever. “
The “heart of the experiment” is a trap that keeps the BEC shielded from light, a reservoir that can be continuously replenished as long as the experiment lasts.
Protecting the BEC from the light produced by the cooling laser, but even though it was easy in theory, was again a little more difficult in practice. Not only were there technical barriers, but there were also bureaucratic and administrative barriers.
“When we moved to Amsterdam in 2013, we started with a leap of faith, borrowed funds, an empty room and a team entirely funded by personal scholarships,” said physicist Chun-Chia Chen, who led the research.
“Six years later, early on Christmas morning 2019, the experiment was finally on the verge of working. We had the idea of adding an extra laser beam to solve one last technical problem, and immediately each image we took showed a BEC, the first continuous the wave BEC. “
Now that the first part of the continuous atomic laser has been realized – the “continuous atom” part – the next step, the team said, is working to maintain a stable atomic beam. They could achieve this by transferring the atoms to an untrapped state, thereby extracting a propagating wave of matter.
Because they used strontium atoms, a popular choice for BECs, the prospect opens up exciting opportunities, they said. Atomic interferometry using strontium BECs, for example, can be used to perform relativity and quantum mechanics studies, or to detect gravitational waves.
“Our experiment is the matter-wave analog of a continuous optical laser with a fully reflective cavity mirror,” the researchers wrote in their paper.
“This proof-of-principle demonstration provides a new, hitherto missing piece of nuclear optics, which enables the construction of continuous coherent matter wave units.”
The research is published in Nature.