Feature

Tracking the Atom

New Zealand physicists have played a crucial role developing an atom-cavity microscope that tracks the motion of individual atoms.

Researchers at the University of Auckland and colleagues at the California Institute of Technology (Caltech) have succeeded in trapping individual caesium atoms in microscopic orbits inside a high-quality optical resonator, while simultaneously monitoring their motion with high resolution.

The ability to "watch" such tiny objects may in the future allow researchers to gather new kinds of information about chemically and biologically important processes by monitoring the individual molecules involved in chemical and biochemical reactions.

The experiment is an important step forward in controlling and observing a system in which the physics must be described using quantum mechanics, the theory of the microscopic world developed early last century by, amongst others, Einstein, Heisenberg, and Schroedinger.

This new capability will be important for the eventual creation of quantum technologies such as quantum computers and quantum communication.

Quantum computers make use of some of the strange and unique properties of quantum mechanics to facilitate a kind of massive parallelism, which can in principle be used to solve certain problems that are currently intractable using conventional computers. An important example is finding the prime factors of very large numbers (with, say, 250 or more digits) which form the basis of present encryption schemes for the secure transfer of confidential information.

"The atom is trapped in orbit with forces exerted by a tiny amount of light bouncing backwards and forwards between two mirrors," says University of Auckland theoretical physicist Dr Scott Parkins.

"So tiny in fact that there is on average just one photon," a photon being the elementary quantum of light. "Remarkably though, with the two-mirror set-up this tiny quantity of light is sufficient both to trap the atoms and to provide a means of tracking their movement."

This single-photon trapping and monitoring technique was first proposed by Parkins. Along with co-author Andrew Doherty, also at the University of Auckland, he developed models of the motion of the atoms in the experiment as part of a research programme led by the late Professor Dan Walls and funded by the Marsden Fund.

The two mirrors that form the optical resonator are very highly reflective and are spaced just 10 microns (1/100th of a millimetre) apart.

In order to detect the atoms the experimental group at Caltech shines weak laser light through one of the mirrors and measures the light coming out through the opposite mirror.

"Only certain colours (frequencies) of light can pass through the cavity but exactly which colours depends on whether there is an atom in the cavity and where that atom is," says New Zealander Christina Hood, a postgraduate student of Professor Jeff Kimble at Caltech and primary author of a paper published in the February issue of Science. "The amount of light coming out through the second mirror changes dramatically as the atom moves around, thus providing a very sensitive measure of the atom's position."

The group used this information to calculate the path taken by each atom and create "movies" of their motion. Examples of these movies can be obtained from the Caltech group's Web site:
http://www.its.caltech.edu/~qoptics/atomorbits/

The movies show atoms orbiting around the centre axis of the cavity in a plane parallel to the mirrors. Each orbit takes about 150 microseconds and the radius of the orbit is usually less than 20 microns.

The group was able to determine the atomic position to within about 2 microns in a time of about 10 microseconds. This combination of accuracy and speed made it possible to track the orbital motion of the atoms.

This demonstration represents an important step forward in measurement and control capability at the microscopic level, and experimental work is continuing at Caltech to refine the technique and apply it to problems of interest.

Theoretical research at Auckland University is also continuing along these lines, with an accent on possible applications of the configuration to quantum computing and quantum communication, including "teleporters" for atoms, which can faithfully transfer the complete quantum state and properties of an atom from one location to another without having to physically move the atom.

Images courtesy of Caltech Quantum Optics group.

Vicki Hyde is the editor of New Zealand Science Monthly.