A slice of
crystalline light sounds like something from passionate poetry or an imaginative
unreal novel. Scientists, though, consider it a possible result of an accomplishment
that sounds remarkably unlikely on its own; making photons repel each other.
Photons,
the units from which light is created, do not attract or repel each other. They
may convey momentum to atoms, for example, but do no such thing to others photons,
inhibiting the possibility of making lightsabers, for example. Nevertheless
that is the norm, but scientists learned long ago that in the world of quantum
behavior calling anything impossible is an uncertain step. Back in 2013 one
exception was found, with photons traveling through an ultracold gas generating
an attraction for each other, as if huddling together for warmness.
Team
members accountable for that discovery pondered whether, if photons could
attract, they could also repel. Now, their efforts have been appreciated, stated
in a new paper in Nature Physics. They've even described both repulsion and
attraction in photon triads.
Adhesive
or repulsive photons are not something you'll come across in your daily life. For
this, MIT's Professor Vladan Vuletić and co-authors had to cool rubidium gas to
50 millionths of a degree above absolute zero and use electromagnetic fields to
make the gas completely transparent. In this environment photons passing
through go into what is known as a Rydberg state, similar to an atom with
electrons excited enough to achieve ionization levels. The physicists then
caused two photons to each become fixed to a different atomic state of the same
atom.
After
shining photons onto the rubidium the team members was able to illustrate the possibility
of finding two photons leaving the gas at the same time was reduced suitably so
they must be repelling each other. In the same way, by tuning the atoms with a
different field, the physicists could demonstrate more than chance co-location
for the photons, showing attraction.
Circumstances
that pushed the photons apart also triggered repulsion from both to a third
photon when it was added.
The work reveals
the probability of fine photon control, so that they are attracted to each
other from a certain distance, but repelled as they get closer, maintaining a steady
relationship that can be built into something the structure of a crystal.
“This
opens possibilities to studying exotic phases of matter, including
self-organization in open quantum systems, along with photonic quantum
materials,” the paper states. Photonic crystals could have uses in quantum
communication, but we're good with the lightsaber idea too.
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