Application of Einstein’s theory of relatively to make a new type of clock may finally allow precise definition of the kilogram


A Rolex is nice, but this is a truly elegant timepiece. A new type of clock keeps time by weighing the smallest bits of matter, physicists report. Compared with standard atomic clocks, which work differently, the new clock keeps lousy time. However, by connecting mass and time the technique could lead to a quantum-mechanical redefinition of the kilogram.

“This gorgeous experiment shows that the road to redefining the kilogram is opening up,” says John Hall, a physicist at JILA, a laboratory run jointly by the University of Colorado, Boulder, and the National Institute of Standards and Technology.

A standard atomic clock takes advantage of the fact that an atom can absorb electromagnetic radiation such as light at certain frequencies as its internal structure jumps from one “quantum state” to another. The clock essentially exposes atoms to radiation tuned to such a frequency, which then serves as the ticking of the clock. The atomic clocks that keep official world time are accurate to 3 parts in 1016, so they would gain or lose less than a second in 100 million years.

It should be possible to keep time in a different way, says Holger Müller, a physicist at the University of California, Berkeley. Any massive particle must be described by a quantum wave that oscillates up and down even if the particle just sits there. The heavier the atom is, the higher the frequency of that flapping, which is known as the Compton frequency. In principle, the quantum oscillation can be used to keep time.

In practice, the Compton frequency for an atom is far too high to be measured by any electronic counter—something like a million-billion-billion cycles per second. So Müller, his student Shau-Yu Lan, and colleagues devised a way to track it in an experiment that exploits Albert Einstein’s theory of relativity, as they report online this week in Science.

The researchers start with a puff of cesium atoms that falls through space toward a detector. Along the way, the atoms encounter pulses of two opposing lasers with slightly different frequencies that gently nudge the atoms without making their inner structure change. The pulses split the cloud in two, and one half of the cloud falls as normal. The other gets pushed up away from the first half and then gets pushed back toward it to catch up.

Here’s where the relativity enters. From the perspective of the un-nudged half of the cloud, the second half moves away and then moves back. Because that second half is moving at a few centimeters per second, its time should appear to slow down just a bit thanks to the weird time dilation predicted by Einstein’s theory of special relativity. So the quantum wave for that half of the cloud oscillates slightly slower than the one for the first half of the cloud.

When the clouds recombine, that difference in oscillations affects how they overlap and “interfere.” If the researchers tune the difference in the two lasers’ frequency just right, the recombining waves will interfere “constructively” so that the cloud falls into the detector. And in that condition, a simple equation relates the “difference frequency,” which can be read out like a clock’s ticking, to the Compton frequency of the atoms and the much lower average frequency of the two lasers.

This is almost what the researchers want, except for the presence of the pesky average frequency of the lasers. So long as that frequency remains as an independent input, the whole scheme relies on whatever clock is used to set it, and is not itself an independent clock. To get around that problem, the researchers employ an elaborate feedback system called a frequency comb that fixes the average frequency of the lasers at a known multiple of the difference frequency. The average laser frequency then drops out of the equation, leaving the difference frequency set to a known fraction of the Compton frequency. The rate of the clock’s ticking is thus set by the cesium atom’s mass alone.

The experiment is a “tour de force,” says Hall, who shared the Nobel Prize in physics in 2005 for his role in developing the frequency comb. However, the rig’s precision is only a part in 100 million, he says, so any claim that it can compete with atomic clocks “leads one to believe that smoking a certain substance is legal in California.”

The real value of the approach may come in redefining the kilogram, Hall says. The kilogram is the last physical unit in the International System of Units defined by a physical artifact, a platinum-iridium cylinder kept by the International Bureau of Weights and Measures in Sèvres, France. But that standard has been getting steadily lighter over the decades as it is repeatedly cleaned, he says.

The measurement of the Compton frequency offers another way to define the unit. Researchers could simply define Planck’s constant, the number that needs to be multiplied by that frequency to get an atom’s mass. A measurement of an atom’s Compton frequency would then give an exact value of its mass in kilograms. In fact, Müller’s experiment can be reinterpreted as a measurement of Planck’s constant within the existing unit system, Hall says. A team led by François Biraben of the École Normale Supérieure in Paris has used a very similar technique to produce a better measurement, he adds.

Even if the kilogram is redefined in this way, it will still take work to translate it to the macroscopic scale of everyday life. “There’s a long way to go from a microscopic mass of an atom to something you can take to the supermarket to guarantee that when you buy a kilogram of sugar, it’s a kilogram,” says Steven Cundiff, a physicist at JILA. Researchers with a kilogram-definition initiative called the Avogadro Project are trying bridge this gap by fashioning spheres of silicon containing precise numbers of atoms. If scientists can nail down the mass of the silicon atom, the spheres would translate that mass to the macroscopic scale.

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