Single-photon transmitter could enable new quantum devices
Long-sought goal for quantum devices — the ability to transmit single photons while
blocking multiple photons — is finally achieved.
An artist's conception shows how any number of incoming photons (top)
can be absorbed by a cloud of ultra-cold atoms (center),
tuned so that only one single photon can pass through at a time.
Being able to produce a controlled beam of single photons has
been a goal of research toward creating quantum devices.
Graphic: Christine Daniloff
July 25, 2012
David L. Chandler, MIT News Office
In theory, quantum computers should be able to perform certain kinds of complex calculations much faster than conventional
computers, and quantum-based communication could be invulnerable to eavesdropping. But producing quantum components for
real-world devices has proved to be fraught with daunting challenges.
Now, a team of researchers at MIT and Harvard University has achieved a crucial long-term goal of such efforts: the ability
to convert a laser beam into a stream of single photons, or particles of light, in a controlled way. The successful demonstration
of this achievement is detailed in a paper published this week in the journal Nature by MIT doctoral student Thibault Peyronel
Senior author Vladan Vuletić, the Lester Wolfe Professor of Physics at MIT, says the achievement
“could enable new quantum devices” such as quantum gates, where a single photon switches the direction of travel or polarization
of another photon. This goal has been very hard to attain, Vuletić explains, because photons ordinarily interact, at best,
only very weakly with one another.
Encouraging such interactions requires atoms that interact strongly with photons — as well as with other atoms that,
in turn, can affect other photons. For example, a single photon traveling through a cloud of such atoms might pass through easily,
but change the state of the atoms so that a second photon is blocked when it tries to pass through.
That means that if two photons try to pass through at once, only one will succeed, while the other is absorbed.
So, in the new system, no matter how many photons are sent into such a cloud of atoms, only one at a time emerges from
the other side. The cloud acts as a kind of turnstile for photons, forcing a jumbled mob into an orderly succession of individuals.
Atac Imamoglu, professor of physics at ETH Zurich, who was not involved in this research, says “I view this work as a true
breakthrough in quantum optics, as the authors realize a completely novel way of inducing strong interactions between
The system is based on a phenomenon called electromagnetically induced transparency (EIT), used previously as a way of
slowing a beam of light. (The well-known invariance of the speed of light, first formulated by Albert Einstein, only applies
to light in a vacuum. Light traveling through matter can move at different speeds.) Various research groups,
including members of this team of MIT and Harvard researchers, had published results a decade ago showing that light,
and even single photons, could be slowed to a walking pace — or even stopped altogether — and then allowed to resume a normal speed.
This slowing of light is achieved by passing a focused laser beam through a dense cloud of ultracold atoms
(in this case, rubidium atoms) chilled to about 40 microkelvins, or 40 millionths of a degree above absolute zero.
This cloud is normally opaque to light, but a separate laser beam produces the EIT state that lets photons pass through
at a slow speed while elevating atoms to an excited state. Atoms in this state (called a Rydberg state) interact very strongly
with each other, meaning that a second photon does not meet the EIT condition if the first photon is still in the medium.
So whenever a single photon enters, it passes through the temporarily transparent medium; when two or more enter, the gas
becomes opaque again, blocking all but the first photon.
“If you send in one photon, it just passes through, but if you send in two or three, forcing them to squeeze through the tight
focus of the laser beam, just one passes,” says Ofer Firstenberg, a Harvard postdoc who is one of the paper’s co-authors.
“It’s like a lot of sand going into an hourglass, but only one grain at a time can pass through,” he says.
As a result, a conventional laser beam — a bundle of photons — fired into one end of this new apparatus comes out the other
end as a sequential string of individual photons.
Stephen Harris, professor of electrical engineering and professor of applied physics emeritus at Stanford University,
who was not connected with the project, says the team’s experiment “worked significantly better than I might have guessed that
it would. This is likely due to the, in my mind, unexpectedly robust interactions of nearby Rydberg atoms.” As a result of this
work, he says, “For the first time, non-resonant single photon physics is a reality.”
The technique can be used to alter the state of the atoms according to the number of photons striking them, with a second
laser beam detecting those changed states. “One big goal has been to measure a photon without affecting it,” Vuletić says.
“We know how to detect individual optical photons, but only by destroying them. This technique should allow you to measure your
photon and keep it, too.”
Eugene S. Polzik, professor of physics at the Niels Bohr Institute at Copenhagen University and director of the Danish Center
for Quantum Optics, says, “Demonstration of the efficient nonlinear interaction at a single photon level is one of the most
important goals in quantum information processing. This work is an exciting new development in this direction.
It paves the way towards new implementations of photon quantum logic.”
The system could lead to the development of a single-photon switch, the team says. It could also be used to develop quantum logic
gates, an essential component of an all-optical quantum information-processing system. Such systems, in principle, could be immune
from eavesdropping when used for communication, and could also allow much more efficient processing of certain kinds of
Besides potential commercial applications, the system offers new insights into the basic interactions between light and matter,
Stanford’s Harris calls this a “wonderful and major scientific advance.”
The work was conducted in collaboration with Mikhail Lukin’s group at Harvard and was partly supported by the National Science
Foundation, the MIT-Harvard Center for Ultracold Atoms, and the Air Force Office of Scientific Research’s Quantum Memories
Multidisciplinary University Research Initiative.