A recent breakthrough has allowed physicists to create an atomic beam that behaves like a laser and that can theoretically stay on “forever”.
This could mean that the technology is finally on the way to practical application, although significant limitations remain.
Nonetheless, this is a major advance for a so-called “atomic laser” – a beam of atoms marching as a single wave that could one day be used for testing fundamental physical constants and precision engineering technology.
Atomic lasers have been around for a minute. The first atomic laser was developed in 1996 by a team of MIT physicists. The concept sounds pretty simple: just as a traditional light-based laser is made up of photons moving in sync with their waves, a laser made up of atoms would need a wave-like nature of their own to align before they are pushed out as a beam.
However, as with many things in science, it is easier to conceive than to realize. At the root of the atomic laser is a state of matter called the Bose-Einstein condensate, or BEC.
A BEC is formed by cooling a boson cloud to a fraction above absolute zero. At such low temperatures, atoms sink to their 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 high-density atomic cloud that behaves like a “superatom,” or matter wave.
However, BECs are something of a paradox. They are very fragile; even light can destroy a BEC. Because the atoms in a BEC are cooled with optical lasers, this usually means that the existence of a BEC is fleeting.
Atomic lasers that scientists have developed to date have been of the pulsed rather than continuous type; and involve firing only one pulse before a new BEC has to be generated.
In order to create a continuous BEC, a team of researchers from the University of Amsterdam in the Netherlands realized that something had to change.
“In previous experiments, the gradual cooling of atoms in one place was performed. In our setup, we decided not to distribute the cooling steps in time but in space: we make the atoms move while they go through successive cooling steps,” explains 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 those atoms are being used, new atoms are already on their way to refill the BEC. That way we can keep the process going — basically forever.”
The “heart of the experiment” is a trap that protects the BEC from light, a reservoir that can be continuously refilled as long as the experiment is running.
Shielding the BEC from the light from the cooling laser was easy in theory, but a little more difficult in practice. There were not only technical, but also bureaucratic and administrative hurdles.
“When we moved to Amsterdam in 2013, we started with a leap of faith, borrowed funds, an empty space and a team funded entirely by personal grants,” said physicist Chun-Chia Chen, who led the research.
“Six years later, in the early hours of Christmas morning 2019, the experiment was finally about to go to work. We came up with the idea of adding an extra laser beam to solve a final technical difficulty and immediately every image captured showed a BEC, the first continuous wave BEC.”
With the first part of the continuous atom laser — the “continuous atom” part — realized, the next step, the team says, is to work on maintaining a stable atom beam. They could achieve this by converting the atoms into 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 possibilities, they said. Atomic interferometry with strontium BECs could be used, for example, to study relativity and quantum mechanics, or to detect gravitational waves.
“Our experiment is the matter-wave analogue of an optical continuous-wave laser with fully reflecting resonator mirrors,” the researchers write in their paper.
“This proof of principle provides a new, previously missing piece of atomic optics that will enable the construction of continuous coherent matter wave devices.”
The research was published in Nature.