Scientists create never-before-seen form of matter
Harvard and MIT scientists are challenging the conventional wisdom about
light, and they didn't need to go to a galaxy far, far away to do it.
Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a
group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of
Physics Vladan Vuletic have managed to coax photons into binding together to form molecules – a state of matter that,
until recently, had been purely theoretical. The work is described in a
September 25 paper in Nature.
The discovery, Lukin said, runs contrary to decades of accepted wisdom about
the nature of light. Photons have long been described as massless particles
which don't interact with each other – shine two laser beams at each other, he said, and they simply pass through one
another.
"Photonic molecules," however, behave less like traditional lasers and more
like something you might find in science fiction – the
light saber.
"Most of the properties of light we know about originate from the fact that
photons are massless, and that they do not interact with each other," Lukin
said. "What we have done is create a special type of medium in which photons
interact with each other so strongly that they begin to act as though they have
mass, and they bind together to form molecules. This type of photonic bound
state has been discussed theoretically for quite a while, but until now it
hadn't been observed.
"It's not an in-apt analogy to compare this to light sabers," Lukin added.
"When these photons interact with each other, they're pushing against and
deflect each other. The physics of what's happening in these molecules is
similar to what we see in the movies."
To get the normally-massless photons to bind to each other, Lukin and
colleagues, including Harvard post-doctoral fellow Ofer Fisterberg, former
Harvard doctoral student Alexey Gorshkov and MIT graduate students Thibault
Peyronel and Qiu Liang couldn't rely on something like the Force – they instead
turned to a set of more extreme conditions.
Researchers began by pumped rubidium atoms into a vacuum chamber, then used
lasers to cool the cloud of atoms to just a few degrees above absolute zero.
Using extremely weak laser pulses, they then fired
single photons into the cloud of atoms.
As the photons enter the cloud of cold atoms, Lukin said, its energy excites
atoms along its path, causing the photon to slow dramatically. As the photon
moves through the cloud, that energy is handed off from atom to atom, and
eventually exits the cloud with the photon.
"When the photon exits the medium, its identity is preserved," Lukin said.
"It's the same effect we see with refraction of light in a water glass. The
light enters the water, it hands off part of its energy to the medium, and
inside it exists as light and matter coupled together, but when it exits, it's
still light. The process that takes place is the same it's just a bit more
extreme – the light is slowed considerably, and a lot more energy is given away
than during refraction."
When Lukin and colleagues fired two photons into the cloud, they were
surprised to see them exit together, as a single molecule.
The reason they form the never-before-seen molecules?
An effect called a Rydberg blockade, Lukin said, which states that when an
atom is excited, nearby atoms cannot be excited to the same degree. In practice,
the effect means that as two photons enter the atomic cloud, the first excites
an atom, but must move forward before the second photon can excite nearby
atoms.
The result, he said, is that the two photons push and pull each other through
the cloud as their energy is handed off from one atom to the next.
"It's a photonic interaction that's mediated by the atomic interaction,"
Lukin said. "That makes these two photons behave like a molecule, and when they
exit the medium they're much more likely to do so together than as single
photons."
While the effect is unusual, it does have some practical applications as
well.
"We do this for fun, and because we're pushing the frontiers of science,"
Lukin said. "But it feeds into the bigger picture of what we're doing because
photons remain the best possible means to carry quantum information. The
handicap, though, has been that photons don't interact with each other."
To build a quantum computer, he explained, researchers need to build a system
that can preserve quantum information, and process it using quantum logic
operations. The challenge, however, is that quantum logic requires interactions
between individual quanta so that quantum systems can be switched to perform
information processing.
"What we demonstrate with this process allows us to do that," Lukin said.
"Before we make a useful, practical quantum switch or photonic logic gate we
have to improve the performance, so it's still at the proof-of-concept level,
but this is an important step. The physical principles we've established here
are important."
The system could even be useful in classical computing, Lukin said,
considering the power-dissipation challenges chip-makers now face. A number of
companies – including IBM – have worked to develop systems that rely on optical
routers that convert light signals into electrical signals, but those systems
face their own hurdles.
Lukin also suggested that the system might one day even be used to create
complex three-dimensional structures – such as crystals – wholly out of light.
"What it will be useful for we don't know yet, but it's a new state of
matter, so we are hopeful that new applications may emerge as we continue to
investigate these photonic molecules' properties," he said.
Explore further: MIT researchers build an all-optical transistor
More information: Paper: dx.doi.org/10.1038/nature12512
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