Light clock promises finer time

By Eric Smalley, Technology Research News

Making the most of time requires that time be well defined. And in order to be precise, a definition of time must involve something that happens very quickly.

The current definition of a second is the duration of 9,192,631,770 oscillations of cesium atoms excited by microwaves. Today's cesium atomic clocks are accurate to within one million billionth of a second or 1 second in 30 million years.

This is precise enough for a cluster of orbiting satellites to calculate the position of a stationary object to within a millimeter. For moving objects like cars and planes, however, the accuracy is a few meters, which is not enough to allow a global positioning system to, for instance, automatically land a plane.

Researchers from the National Physical Laboratory in England have made a prototype atomic clock that divides time into slices based on optical radiation, or lightwaves, rather than microwave radiation. Such clocks could eventually improve global positioning systems, make space exploration more accurate, and more accurately test the laws of physics, said Helen Margolis, a principal research scientist at the National Physical Laboratory.

Because lightwaves are smaller and faster than microwaves, optical clocks have operating frequencies as many as 100,000 times higher than today's cesium microwave clocks, said Margolis. "They divide time into much finer slices, and therefore have the potential to give much higher accuracy," she said.

Today's atomic clocks measure the vibration frequency of cesium atoms to calibrate quartz crystal electronic oscillators. A laser excites the atoms to the energy level where they resonate with a microwave field that is tuned by an electronic oscillator. The microwave field cycles through a range of frequencies close to the cesium atoms' resonant frequency, and as the microwaves resonate with the atoms the atoms give off energy in the form of photons. A photodetector measures the peak amount of light and locks the microwave field on that frequency.

Optical atomic clocks use lasers instead of microwaves to resonate the atoms, and atoms that have higher resonant frequencies than cesium. The researchers' prototype uses a single strontium ion that is held in an electromagnetic trap and laser cooled to near absolute zero, said Margolis. Another laser causes the ion to oscillate at its resonant frequency -- 444,779,044,095,484.6 cycles per second.

The difference between the strontium frequency and the cesium frequency is the difference between 1 second and 13 and a half hours. This higher frequency could lead to optical atomic clocks that are so accurate they would lose less than a second over the lifetime of the universe.

One challenge is that electronic photodetectors are too slow to measure such high frequencies. Researchers have recently developed another tool that is fast enough, the femtosecond laser frequency comb. Frequency combs are lasers that emit a pulse every million billionth of a second, or femtosecond. The light emitted by the lasers covers the wavelengths of the spectrum of visible light in discrete, narrowly-spaced intervals equal to the frequency of the pulses. "This acts like an optical frequency ruler... with the frequency of every tooth of the comb known precisely," said Margolis.

Researchers measure optical atomic clocks by matching their output to a frequency on a frequency comb.

The researchers' strontium ion prototype is accurate to 3.4 million billionths of a second, which is three times more accurate than a prototype optical atomic clock based on a single mercury ion demonstrated by U.S. National Institute for Standards and Technology scientists but about three times less accurate than the best cesium clocks, said Margolis.

The strontium ion clock is potentially precise enough that it would be limited by the current definition of the second, said Margolis. The frequency combs are calibrated by cesium clocks. Given a redefined second, optical clocks could be considerably more accurate, she said. "We believe that future generations of such optical clocks could be nearly a thousand times more accurate than the best clocks of today," she said.

Such a clock would not lose a second over the lifetime of the universe.

Optical clocks based on a more precise definition of the second would improve global positioning systems, and are crucial to deep space exploration, said Margolis. "To send a spacecraft millions of kilometers into an unknown part of the universe -- and perhaps ask it to land gently in a particular place -- will require extremely accurate clocks to synchronize its navigation equipment."

More accurate time measurement is also useful in testing the laws of physics, said Margolis. "Optical clocks will also provide a powerful tool... to explore questions such as 'are the fundamental physical constants really constant or do they change with time'."

The researchers' optical clock can be used now for fundamental science applications like testing the consistency of physical constants, said Margolis. It could be used for global positioning system ground stations in five years and on satellites in 10 years, she said.

Margolis's research colleagues were Geoffrey Barwood, Guilong Huang, Hugh Klein, Stephen Lea, Krzystof Szymaniec and Patrick Gill. The work appeared in the November 18, 2004 issue of Science. The research was funded by the UK Department of Trade and Industry.

Timeline:   Now, 5 years, 10 years
Funding:   Government
TRN Categories:  Physics; Applied Technology; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Hertz-level Measurement of the Optical Clock Frequency in a Single 88Sr + Ion," science italic, November 19, 2004


December 15/22, 2004

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