Time to shrink the atomic clock 14 March 2009 by Anil Ananthaswamy Magazine issue 2699. Subscribe and get 4 free issues. ATOMIC clocks, currently the size of fridges, could shrink to the microscale thanks to a new way of measuring the second. The technique could also see aluminium displace caesium as the standard of time.
The world's most accurate atomic clocks are at the National Institute of Standards and Technology (NIST) at Boulder, Colorado. Known as fountain clocks, they send clouds of caesium atoms through a vacuum chamber in a magnetic field. Large atoms like caesium and aluminium have multiple energy levels that are so close together they appear indistinguishable. The magnetic field separates these levels into two "hyperfine" states. The chamber is also filled with microwaves, which excite the atoms. They then emit light as they drop to the lower hyperfine state. The microwave frequency that maximises this fluorescence is used to define the length of a second, currently the time it takes for 9,192,631,770 cycles of microwave radiation. All this takes place in a large vacuum chamber and so fountain clocks are big devices, about a cubic metre in size. That makes it hard to keep the magnetic field and the device's temperature uniform over the whole area, which can lead to errors of measurement. That's why Andrei Derevianko and Kyle Beloy of the University of Nevada in Reno and colleagues have come up with the idea of trapping the atoms in place using lasers. This means their energy states could be monitored in an area only a few micrometres across, potentially leading to more accurate measurements. This is difficult to get right, though, because the lasers distort an atom's energy levels in a complex way, making it impossible to define a jump that equates to a second. Derevianko's team overcome this problem by finding a laser frequency that alters both hyperfine states by exactly the same amount - a trick that works in aluminium and gallium but not as well in caesium (www.arxiv.org/abs/0808.2821). "Then, the energy difference between the levels is the same as if the atoms are in vacuum," says Derevianko. Using this method, the team has calculated the second to be 1506 million cycles of microwaves for aluminium-27 and 2678 million cycles for gallium-69. Although the atoms can be trapped in an area only a few micrometres across, the lasers, and cooling and computing equipment will add to the bulk. Nevertheless, the team say the clocks may be portable and could be used in space-based experiments that require extremely accurate timekeeping, such as those for detecting gravitational waves or for testing Einstein's theories. Tom Heavner, who works on fountain clocks at NIST, describes the proposal as forward-thinking and original. "It is a really clever way to meld together the old-style clocks with new laser technology," he says. http://www.newscientist.com/article/mg20126994.900-time-to-shrink-the- atomic-clock.html _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.