Physicists Say They've Built an Atom Laser That Can Run 'Forever'

A new innovation has enabled scientists to construct an atom beam that acts like a laser and can theoretically stay on "indefinitely."

This might finally signal that the technology is on its approach to being used in the real world, albeit there are still certain limits.

Nonetheless, this is a significant step forward for a "atom laser" — a beam made up of atoms marching in a single wave that might one day be utilized for verifying fundamental physical constants and manufacturing precise technologies.

Atom lasers have existed for quite some time. In 1996, a group of MIT scientists developed the first atom laser. A laser constructed of atoms would require their own wave-like nature to align before being moved out as a beam, much as a standard light-based laser consists of photons moving with their waves in sync.

It is, however, simpler to comprehend than to actualize, as is the case with many things in science. A condition of matter known as a Bose-Einstein condensate, or BEC, is at the heart of the atom laser.

A BEC is made by cooling a cloud of bosons to a fraction of the temperature of absolute zero. The atoms descend to their lowest conceivable energy state at such low temperatures without totally halting.

The quantum qualities of the particles can no longer interfere with one other at these low energies, so they move close enough to overlap, resulting in a high-density cloud of atoms that acts like one'super atom' or matter wave.

BECs, on the other hand, are a bit of a conundrum. They're extremely delicate, and even light may kill a BEC. Given that the atoms in a BEC are cooled using optical lasers, the existence of a BEC is generally brief.

To yet, scientists have only been able to create pulsed atom lasers, rather than continuous lasers, which require just one pulse to be fired before a new BEC is needed.

A group of researchers at the University of Amsterdam in the Netherlands discovered that something had to change in order to construct a continuous BEC.

"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," explained physicist Florian Schreck.

"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." 

The 'heart of the experiment' is a light-shielding trap, a reservoir that may be continually supplied for the duration of the experiment.

However, protecting the BEC from the cooling laser's light, while easy in principle, proved to be more problematic in practice. There were not only technological barriers to overcome, but also governmental and administrative ones.

"On moving to Amsterdam in 2013, we began with a leap of faith, borrowed funds, an empty room, and a team entirely funded by personal grants," said physicist Chun-Chia Chen, who headed the research.

"Six years later, in the early hours of Christmas morning 2019, the experiment was finally on the verge of working. We had the idea of adding an extra laser beam to solve a last technical difficulty, and instantly every image we took showed a BEC, the first continuous-wave BEC." 

The next stage, according to the scientists, is to focus on maintaining a steady atom beam now that the first component of the continuous atom laser has been accomplished — the "continuous atom" part. They could do it by releasing the confined atoms and extracting a propagating matter wave in the process.

The idea presents interesting prospects because scientists employed strontium atoms, a typical choice for BECs, they added. Atom interferometry based on strontium BECs, for example, might be used to study relativity and quantum physics, as well as detect gravity waves.

"Our experiment is the matter wave analogue of a continuous-wave optical laser with fully reflective cavity mirrors," the researchers noted in their publication.

"This proof-of-principle demonstration provides a new, hitherto missing piece of atom optics, enabling the construction of continuous coherent-matter-wave devices."