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
"Photonic molecules," however, behave less like traditional lasers
and more like something you might find in science fiction -- the light
"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
"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.