October 4, 2010 — Ottawa, Ontario
NRC researchers have achieved an important milestone toward the development of a new global standard for measuring time.
In May 2010, scientists at the NRC Institute for National Measurement Standards (NRC-INMS) in Ottawa successfully captured and isolated a single ion of strontium and then detected its fluorescence, using an imaging camera system and photon-counting photomultiplier. This feat marks a major step toward the team's goal of building, operating and evaluating an ultra-accurate optical atomic clock - and could ultimately help to open up new frontiers in physics.
All clocks have two main components: a way of producing accurate time intervals and a way to count the intervals. In a mechanical clock, a pendulum swings back and forth in one second intervals, while gears record the number of times the pendulum has swung.
The NRC optical clock produces tiny time intervals — specifically, the regular periods of an oscillating light wave derived from an optical frequency standard, which is based on a laser whose frequency is controlled by monitoring the absorption of light by a strontium ion. To count the number of oscillations, NRC’s prototype optical clock uses a fibre-based optical frequency comb, based on a femtosecond laser system, which can now operate unattended for weeks at a time.
"An optical atomic clock is a revolution in timekeeping, as well as in measurements of atomic and molecular physics," says Dr. Alan Madej, the NRC-INMS project co-leader, along with his colleague Dr. Pierre Dubé. "It is a vital part of how we will be able to control light and do precision measurements in the future."
Like other clocks, an optical atomic clock is basically a device for measuring the time elapsed between events. However, it has the potential to measure time "more precisely and accurately than any timepiece that now exists," says Dr. Madej. "Right now, the worldwide level of accuracy for a microwave atomic clock is about 3-5 x 10-16 seconds, while the theoretical limit of accuracy for an optical atomic clock is about 10-18 seconds, or up to one hundred times greater." In other words, a future optical clock would neither gain nor lose more than a small fraction of a billionth of a second in a year.
So why does the world need this level of accuracy? Dr. Madej suggests that without steady improvements in our ability to measure fundamental units such the second, metre and kilogram, advances in physics will eventually grind to a halt. “A few decades ago, nobody ever thought we would need to measure better than 10-10 seconds for civilian timekeeping,” he says. “But along came microwave atomic clocks with even greater accuracy, which allowed the development of the global positioning system (GPS). Now that satellites have atomic clocks on board, you can instantly find out your position anywhere in the world.”
“As we further improve timekeeping accuracy, front-line applications will involve fundamental physics experiments,” predicts Dr. Madej. “We may test Einstein’s theory of general relativity at the highest level of accuracy.” Optical clocks may also allow scientists to study gravity's influence on time, or permit spacecraft to precisely navigate in the farthest reaches of the solar system. And while commercial applications are difficult to predict, optical clocks are sure to open new possibilities, just as the harnessing of light opened up the world of fibre optics and fibre telecommunications.
Scientists at national metrology institutions around the world expect that the cesium clock that now serves as the primary standard for defining the second in the SI (metric) system will be replaced with an optical clock — perhaps within a few years. However, it is not clear yet which type of optical clock will be used as the redefined standard. A clock based on either a single trapped atomic ion such as a strontium ion, or a lattice of trapped neutral atoms, appear to be good candidates, but which atom is the best choice is not yet obvious. However, even after optical clocks are adopted around the world, cesium clocks (and other non-optical atomic clocks) will continue to play an important role in technological applications since they are simpler and more compact than optical clocks.
The cesium clock now serves as the world’s primary standard for defining the second. Since 1967, the second has been defined as the period of time elapsed after exactly 9,192,631,770 oscillations of a radiation fixed to excite the reference energy levels in cesium.
