Using a single mercury atom, scientists develop the most precise atomic clock yet.

NIST physicist Jim Bergquist with the world’s most accurate clock. The silver cylinder in the foreground is a magnetic shield that surrounds a cryogenic vacuum system, which in turn holds the heart of the clock, a single mercury ion (electrically charged atom). The ion is brought to rest by laser-cooling it to near absolute zero. The optical oscillations of the essentially motionless ion are used to produce the “ticks” or “heartbeat” of the world’s most stable and accurate clock.  ©Geoffrey Wheeler

Scientists at the National Institute of Standards and Technology (NIST) have developed a prototype for the most precise clock ever made.

The researchers present their experimental clock in the July 14th issue of Physical Review Letters. Based on a single mercury ion, it’s fives times more precise than the current national standard clock, which is based on a fountain of cesium atoms. This cesium clock, which has been the standard for more than half a century, determines official US time and currently defines the second.

“[A mercury clock] allows us to have incredible precision and accuracy in a very short amount of time,” said Jim Bergquist, a principal investigator on the project. “Already it’s been demonstrated, in our device, that [these atoms] are, in fact, more stable and more accurate than the present standard.”

Like all clocks, the mercury atomic clock generates countable events on a regular time period. In a grandfather clock, for example, the event is the swing of a pendulum. In the mercury clock, the event is the laser-induced oscillation of a mercury ion between two transition states.

The mercury clock also operates at an optical frequency, which is much faster than the microwave frequency of the cesium clock—a million billion cycles per second versus ten billion cycles. This higher frequency divides time into smaller increments, improving the clock’s precision. It also makes the clock more stable: The cesium clock could operate for 70 million years without gaining or losing a second—the mercury clock can go about 400 million years.

“Optical standards will eventually replace the microwave standard of today,” Bergquist said.

Responding to that assertion, Patrick Gill, a senior fellow at the National Physical Laboratory in Middlesex, England, said not quite.

“Mercury is doing very well at the moment, but it’s a question of whether there’s a clear winner.” Gill said. “And that is not at all clear at the moment.”

There are other atoms in the running, like strontium and terbium, he noted. Deciding on a new standard will depend on a number of factors, like the accessibility of the atom in question and the level of difficulty of making the actual clock.

Bergquist and Gill both agree that it will be at least ten years before a decision is made.

“The next step is [then] to convince people politically that it’s wise and good to change,” Bernquist said. “It will take a while before it’s finally decided that it should be time to change.”

Originally published July 27, 2006


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