In a development that could lead to dramatic improvements in aircraft guidance systems and open new areas of study in basic physics, researchers at the Georgia Institute of Technology have demonstrated the first storage ring able to confine and guide the flow of ultra-cold neutral atoms in a circular path.
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 fibre optics’ that could do for ordinary uncharged atoms what optical fibre has done for light.
The two-centimetre storage ring could serve as the foundation for a miniaturised atom interferometer that would reportedly 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 fibre. 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 travelling in rings would be affected more dramatically by aircraft directional changes, said Michael Chapman, professor of physics at the Georgia Institute of Technology. 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,’ said Chapman. ‘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.
Developed with collaborators Jake Sauer and Murray Barrett, the Nevatron is also said to provide new opportunities for creating continuous monochromatic atomic beams that could 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 believed to be 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 less than 10 cm/sec.
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 millimetre 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.
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 is salso said to have 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.