PASADENA—In a promising development with applications to science at the single-atom level, physicists have constructed an "atom-cavity microscope" that tracks the motion of individual atoms.
California Institute of Technology physics professor H. Jeff Kimble, his Caltech colleagues, and collaborators from New Zealand report in the February 25 issue of Science that they have succeeded in monitoring the motion of individual cesium atoms bound in orbit by single photons inside a high-quality optical resonator. The atom is trapped in orbit by a weak light field, and the same light field can be used to observe the atom's motion within the cavity.
This advance is an important development toward the eventual realization of quantum technologies, which would enable quantum computation and communication.
The stage for this microscopic dance is the optical cavity, a pair of highly reflective mirrors that face each other only 10 microns (0.0004 inches) apart. The mirrors are so reflective that a photon, the fundamental quantum of light, enters the cavity and bounces back and forth between the mirrors hundreds of thousands of times before it escapes again through one of the mirrors.
In this way a single photon confined in the cavity builds up an electric field strong enough to influence the motion of an atom and even to bind the atom in orbit within the cavity.
Collaborating theorists A. Scott Parkins and Andrew Doherty in New Zealand first recognized the potential of this trapping technique, in which the atom and the cavity share a quantum of excitation.
"I like to think of it as an atom-cavity molecule," says Christina Hood, a Caltech graduate student and primary author of the paper. "In a molecule, two atoms give up their own electron orbits, their separate identities, to share electrons and form something qualitatively different. In the same sense, in our experiment the atom and the cavity field are bound together strongly by sharing a series of single photons."
How do the scientists actually "see" what is going on inside the tiny optical system? The Caltech group and others had already used similar cavities to sense single atoms whizzing through the cavity. To do this, they illuminate one mirror of the cavity and measure the light escaping from the opposite mirror. "The cavity is a resonator for light, like a half-filled soda bottle is for sound," says Theresa Lynn, a Caltech graduate student and coauthor of the paper. "What we do is similar to holding a tuning fork up to the bottle and listening to hear it resonate. You'll only hear a ring if the right amount of water is in the bottle."
In this case, amazingly, it's a single atom that plays the role of the water in the bottle, dramatically altering the resonance properties of the cavity by its presence or absence. By measuring the amount of light emerging from the cavity, the researchers can tell whether an atom is in the cavity or not.
The major step of the current work is that now they can determine precisely where the atom is located within the light field, creating "movies" of atomic motion in the space between the cavity mirrors. Examples of these movies can be viewed at the group's web site:
http://www.its.caltech.edu/~qoptics/atomorbits/
The movies show atoms trapped in the cavity as they orbit in a plane parallel to the cavity mirrors. The atoms have orbital periods of about 150 microseconds and are typically confined to within about 20 microns of the cavity's center axis.
The Kimble team was able to measure the atomic position to within about 2 microns in measurement times of about 10 microseconds. Continuous position measurements at this level of accuracy and speed allowed them to capture the orbital motions of the atoms.
"The interaction of the atom with the cavity field gives us advantages in two distinct ways," says Kimble. "On the one hand, it provides forces sufficient to trap the atom within the cavity at the level of single photons. On the other hand and more importantly, the strong interaction enables us to sense atomic motion in a fashion that has not been possible before," he says.
Both aspects are important to physicists who probe the limits of our ability to observe and to control the microscopic world, in which the rules and regulations are set by quantum mechanics. According to one basic rule of quantum mechanics, the Heisenberg uncertainty principle, any measurement performed on a system inherently disturbs the future evolution of that system. The principle presents a challenge to physicists who strive to control or "servo" individual quantum systems for use in quantum computation and other quantum technologies.
In collaboration with Caltech assistant professor Hideo Mabuchi, the Caltech team is pursuing extensions of the current research to implement real-time quantum feedback to control atomic motion within the cavity. The operating principles for such "quantum servos" are a topic of contemporary theoretical investigation at Caltech being pursued by Mabuchi, Doherty, and their colleagues.
The cavity as a powerful sensing tool by itself also presents possibilities outside the quantum realm. The same techniques that produce movies of orbiting atoms could be adapted to more general settings, such as to "watch" the dynamics of molecules engaged in chemical and biochemical reactions. Mabuchi is pursuing an independent effort along these lines to monitor single molecules engaged in important biological processes such as conformationally gated electron transfer.
