An artist’s conception shows how any number of incoming photons (top) can be absorbed by a cloud of ultracold 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. Image: Christine Daniloff |
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 Massachusetts
Institute of Technology (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 Nature by
MIT doctoral student Thibault Peyronel and colleagues.
Senior author Vladan Vuleti?, 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, Vuleti? 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 single photons.”
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,” Vuleti? 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 computation tasks.
Besides potential commercial applications,
the system offers new insights into the basic interactions between light and
matter, Vuleti? says.
