Much of fundamental science requires the precision measurement of time and frequency. Both broadband communication networks and global positioning systems (GPS), for example, use technologies that require extremely precise measurements of time. New research on optical lattice clocks from the University of Tokyo (Tokyo, Japan) demonstrates a method for determining time based on atomic fountain clocks that could exceed today's standards by several orders of magnitude.
Today the International System (SI) second is defined as 9,192,631,770 periods of the microwave transition between two hyper-fine levels of the ground state of the cesium 133 (Cs) atom. This measurement has a fractional uncertainty of 10-15. The ground-state Cs transition provides a good standard because it has the highest microwave transition frequency among alkali atoms. An optical frequency is up to five orders of magnitude higher than Cs transition frequency, however, and as the comb technique now allows precision measurement of optical frequency, researchers at the University of Tokyo now believe optical clocks can exceed Cs fountain clocks in stability and accuracy (see oemagazine, June/July 2005, p. 9).
Hidetoshi Katori and research team members with the apparatus in their laboratory at the University of Tokyo.
The Japanese research team may have found the answer by using atoms trapped in an optical lattice as quantum references for determining time. Hidetoshi Katori of the University of Tokyo says, "Our optical lattice clock demonstrates a linewidth an order of magnitude narrower than that of neutral-atom optical clocks. What's more, its stability will far exceed that of single-ion clocks in a few years." In collaboration with a group from the National Metrology Institute of Japan, Katori's team used an optical frequency comb referenced to the SI second and determined the Strontium (Sr) optical lattice clock's transition frequency to be 429,228,004,229,952(15) Hz.
Katori and his colleagues put laser-cooled Sr atoms into an optical lattice created by the spatial interference patterns of lasers and confined atoms in a submicrometer region. In this way, the lattice's periodicity produces billions of microtraps within a single cubic centimeter. Light field perturbation in the lattice would ordinarily modify the internal states of the trapped atoms considerably, but development of a light-shift cancellation technique allowed removal of such perturbations and opened up a new approach for atomic frequency measurements.
"We used the frequency comb and measured the clock laser frequency against a commercial Cs clock calibrated with international atomic time via GPS time," Katori says, "and we used this data to deduce the absolute frequency of our lattice clock." Katori's team carried out frequency measurement for τ ≈ 9.4 X 104 s over nine days and reduced frequency measurement uncertainty to 9 Hz.
Katori explains that optical lattice clocks can operate with other divalent atoms such as calcium, ytterbium, and mercury. "A frequency comparison of lattice clocks operated with different atomic species would allow us to study the time variation of the fine structure constant," he says. "The very high potential stability of this scheme would enable us to measure the fractional frequency difference at the 10-18 level in one second. This fractional precision corresponds to the gravitational red shift for a 1 cm height difference. It might open up a new way for metrology that employs Einstein's relativistic effects as a probe."
The team expects optical lattice clock features to come into play in metrology, fundamental physics, and such engineering applications as the remote sensing of natural resources.
Atsuo Morinaga of the Department of Physics at Tokyo University of Science (Chiba, Japan) says, "Katori has found a 'magic' wavelength where the light shifts of the reference transition are cancelled out, so we expect him to achieve an accuracy better than that of the current microwave Cs frequency standard."
Pierre Lemonde of the Paris Observatory (Paris, France) comments, "I think Katori's technique is very promising. He shows that a lot of neutral atoms can be trapped in a lattice with no perturbation of the clock transition by the trap. With this type of clock, the ultimate stability at one second drops down to about 10-18. If this range is indeed reached one day," he continues, "it would represent more than two orders of magnitude improvement as compared to what is achieved today. This could lead to important applications in terms of geophysics: One could map the Earth's gravitational field 'in an absolute way' with this effect, which, for example, could give new methods for oil prospecting."