World's Smallest Atom Storage Ring Is First To Guide Ultra-Cold Neutral Atoms; A Step Toward "Atom Fiber Optics"

Dubbed the "Nevatron," the storage ring -- a circular waveguide that uses magnetic fields from tiny electrical wires to direct low-energy atoms -- marks a step toward "atom fiber optics" that could ultimately do for ordinary uncharged atoms what optical fiber has done for light. Details of the project are reported in the December 31 issue of the journal Physical Review Letters.

"In contrast to high-energy particle storage rings in which the goal is to increase the energy of the confined particles up to and beyond the tera-electron (TeV) volt scale, we are interested in the opposite regime, using ultra-cold atoms with nano-electron (neV) volt energies," explained Michael Chapman, assistant professor of physics at the Georgia Institute of Technology. "In keeping with the theme of naming storage rings according to the energy scale, we call our device the Nevatron."

The two-centimeter storage ring could serve as the foundation for a miniaturized atom interferometer that would improve the accuracy of inertial guidance systems used in commercial aircraft. Such systems now use optical interferometers in which a beam of light is split into two separate beams that travel in opposite directions through coils of optical fiber. By observing how changes in aircraft speed and direction differentially affect the two beams by recombining them with an interferometer, the instrument measures changes in aircraft motion.

Much heavier atoms traveling in rings would be affected more dramatically by aircraft directional changes, Chapman said. An atom interferometer would measure phase shifts in the deBroglie wave, a quantum effect associated with atoms.

"The sensitivity of these gyroscopes is proportional to the area enclosed by the interferometer and the mass of the particle," he explained. "The mass of an atom is about ten orders of magnitude higher than the (relativistic) mass of an optical photon."

Atomic interferometers now exist, but they are too large for aircraft use. If Chapman's team can split an atom beam and make the beams travel in opposite directions around a circular ring, they could have the basis for an instrument small enough to fly.

"If our experiment were an interferometer, it would already have the potential to be a thousand times more sensitive than the best optical interferometer," said Chapman. "This is really going to be a major direction in the field of ultra-cold atoms. Making an atomic storage ring is the first step toward useful devices."

Developed with collaborators Jake Sauer and Murray Barrett, the Nevatron also provides new opportunities for creating continuous monochromatic atomic beams that could one day lead to the development of an atom laser with a continuous output. It also offers new opportunities for studying collisions between ultra-cold atoms.

Other researchers have produced straight-line waveguides for neutral atoms, but the Georgia Tech ring is the first to make neutral atoms move around a closed circle using magnetic confinement. Chapman believes the most significant accomplishment was a technique for loading atoms into the ring from a standard magneto-optical trap used to cool the atoms to micro-Kelvin temperatures.

The experiment takes place within a vacuum chamber. First, a standard magneto-optical trap (MOT) uses a combination of magnetic fields and intense laser beams to confine a few million atoms of rubidium while reducing their speed to a "crawl" (less than a 10 cm/sec).

"It's kind of like slowing a car with a million ping-pong balls," explained Sauer, a graduate student in Chapman's team. "You just keep throwing the balls at the car until it slows down. The atoms are like moving cars, and we slow them by firing lasers that consist of photons."

When the atoms in the trap reach the appropriate temperature -- about three micro-Kelvin, a fraction of a degree above absolute zero -- the magnetic fields and laser beams confining them are switched off. That allows the cold atoms to flow by gravity into a "funnel" made up of two current-carrying wires about a millimeter apart.

The funnel guides the atoms into the storage ring, where they are confined by magnetic fields created by parallel wires each carrying a few amps of electrical current.

"You can think of each atom as a tiny bar magnet," Chapman explains, "and the magnetic fields from the wires are arranged to keep the atoms guided between the wires." A CCD camera records the passage of atom clouds by observing light scattered by the atoms from laser beams passing through tiny holes in the ring.

In the paper, Chapman's team reported observing atom clouds making up to seven revolutions around the ring at velocities averaging one meter per second. The atoms ultimately stop moving due to 'bumps' in the ring and encounters with stray atoms left in the vacuum chamber. In subsequent experiments, they have measured up to ten revolutions, and Chapman believes an improved ring could increase the number of revolutions ten-fold.

The team has also developed techniques for loading multiple batches of atoms into the ring, a first step toward a continuous atom flow. "If you get the timing right, you can get multiple atom clouds moving around in the ring," Sauer said.

In May, Chapman's group announced development of the first all-optical technique for producing a Bose-Einstein condensate, a unique form of matter in which all atoms exist in the same state. The Bose-Einstein condensate requires even lower temperatures, in the nano-Kelvin range.

The research was partially supported by the National Security Agency (NSA) and the Advanced Research and Development Activity (ARDA) of the Army Research Office (ARO).