Imagining our everyday life without lasers is difficult. We use lasers in
printers, CD players, pointers, measuring devices, 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 a team of Amsterdam physicists shows
that the answer to this question is affirmative.
Getting bosons to march in sync
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.
Fleeting bursts
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. 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.
It was clear what the problem was: BECs are very fragile, and are rapidly
destroyed when light falls on them. Yet the presence of light is crucial in
forming the condensate: to cool a substance down to a millionth of a degree,
one needs to cool down its atoms using laser light. As a result, BECs were
restricted to fleeting bursts, with no way to coherently sustain them.
A Christmas present
A team of physicists from the University of Amsterdam has now managed to
solve the difficult problem of creating a continuous Bose-Einstein
Condensate. 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."
While the underlying idea was relatively simple, carrying it out was
certainly not. Chun-Chia Chen, first author of the publication in Nature,
recalls: "Already in 2012, the team - then still in Innsbruck - realized a
technique that allowed a BEC to be protected from laser cooling light,
enabling for the first time laser cooling all the way down to the degenerate
state needed for coherent waves. While this was a critical first step
towards the long-held challenge of constructing a continuous atom laser, it
was also clear that a dedicated machine would be needed to take it further.
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. 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."
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 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.
Reference:
Chen, CC., González Escudero, R., Minář, J. et al. Continuous Bose–Einstein
condensation. Nature (2022).
DOI: 10.1038/s41586-022-04731-z
Tags:
Physics