'Quantum logic clock' now rivals the world's most accurate clock

An atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world now rivals the world's most accurate clock, which is based on a single mercury atom.

The measurements were made in a yearlong comparison of the two next-generation clocks, both designed and built at the Commerce Department's National Institute of Standards and Technology (NIST) in the US.

"The aluminum clock is very accurate because it is insensitive to background magnetic and electric fields, and also to temperature," said Till Rosenband, the NIST physicist who built the clock. "It has the lowest known sensitivity of any atomic clock to temperature, which is one of the most difficult uncertainties to calibrate," he added.

The new atomic clock is referred to as the "quantum logic clock", because it is based on the logical reasoning process used in quantum computers. The clock is a spin-off of NIST research on quantum computers, which grew out of earlier atomic clock research.

The atomic and mercury clocks were compared with record precision, allowing scientists to measure the relative frequencies of the two clocks to 17 digits - the most accurate measurement of this type ever made.

The comparison produced the most precise results yet in the worldwide quest to determine whether some of the fundamental constants that describe the universe are changing slightly over time, a hot research question that may alter basic models of the cosmos.

The aluminum and mercury clocks are both based on natural vibrations in ions (electrically charged atoms) and would neither gain nor lose one second in over 1 billion years - if they could run for such a long time - compared to about 80 million years for NIST-F1, the U.S. time standard based on neutral cesium atoms.

The mercury clock was first demonstrated in 2000 and is now four times better than its last published evaluation in 2006, thanks to ongoing improvements in the clock design and operation.

The mercury clock continues its reign as the world's most accurate for now, by a margin of 20 percent over the aluminum clock.

But the quantum logic clock's tandem ion approach is unique among the world's atomic clocks and has a key advantage over the mercury clock.

"You can pick from a bigger selection of atoms," explains NIST physicist Jim Bergquist, who built the mercury clock. "And aluminum has a lot of good qualities - better than mercury's," he added