‘Quantum Logic Clock’ Rivals Mercury Ion as World’s Most Accurate Clock A new limit on change in fine-structure constant

NIST physicist Till Rosenband adjusts the quantum logic clock, which derives its “ticks” from the natural vibrations of an aluminum ion. The aluminum ion is trapped together with one beryllium ion inside the copper-colored chamber in the foreground. [Photo credit and copyright: Geoffrey Wheeler]

In a paper published in today's issue of journal 'Science', a team of scientists from the National Institute of Standards and Technology (NIST) reports the development of an atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world and which now rivals the world's most accurate clock, based on a single mercury atom, previously designed and built by NIST scientists (See our past posting). Both clocks are at least 10 times more accurate than the current U.S. time standard.

An optical clock can be evaluated precisely only by comparison to another clock of similar accuracy serving as a “ruler.” NIST scientists used the quantum logic clock to measure the mercury clock, and vice versa. The measurements were made in a yearlong comparison of the two next-generation clocks with record precision, allowing scientists to record the relative frequencies of the two clocks to 17 digits—the most accurate measurement of this type ever made.

The aluminum and mercury clocks are both based on ions vibrating at optical frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1, the U.S. time standard based on neutral cesium atoms, and other similar time standards around the world. The aluminum and mercury clocks 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 NIST quantum logic clock uses two different kinds of ions, aluminum and beryllium, confined closely together in an electromagnetic trap and slowed by lasers to nearly “absolute zero” temperatures. Aluminum is a stable source of clock ticks, but its properties cannot be detected easily with lasers. The NIST scientists applied quantum computing methods to share information from the aluminum ion with the beryllium ion, a workhorse of their quantum computing research. The scientists can detect the aluminum clock’s ticks by observing light signals from the beryllium ion.

Highly accurate clocks are used to synchronize telecommunications networks and deep-space communications, and for satellite navigation and positioning. Next-generation clocks may also lead to new types of gravity sensors, which have potential applications in exploration for underground natural resources and fundamental studies of the Earth. NIST scientists have several other optical atomic clocks in development, including one based on thousands of neutral strontium atoms. The strontium clock recently achieved twice the accuracy of NIST-F1, but still trails the mercury and aluminum clocks.

Cosmology Connection: The comparison of these clocks 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.

Based on fluctuations in the frequencies of the two clocks relative to each other over time, NIST scientists were able to search for a possible change over time in a fundamental quantity called the fine-structure constant. This quantity measures the strength of electromagnetic interactions in many areas of physics, from studies of atoms and molecules to astronomy. Some evidence from astronomy has suggested the fine-structure constant may be changing very slowly over billions of years. If such changes are real, scientists would have to dramatically change their theories of the fundamental nature of the universe. [Readers may refer to the article "Changing Constants, Dark Energy and the Absorption of 21 cm Radiation" by Prof. Ben Wandelt of University of Illinois, 2Physics.com, July 25, 2007]

The NIST measurements indicate that the value of the fine-structure constant is not changing by more than 1.6 quadrillionths of 1 percent per year, with an uncertainty of 2.3 quadrillionths of 1 percent per year (a quadrillionth is a millionth of a billionth). The result is small enough to be “consistent with no change,” according to the paper. However, it is still possible that the fine-structure constant is changing at a rate smaller than anyone can yet detect. The new NIST limit is approximately 10 times smaller than the best previous measurement of the possible present-day rate of change in the fine-structure constant. The mercury clock is an especially useful tool for such tests because its frequency fluctuations are magnified by any changes in this constant.

Reference
"Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place"
T. Rosenband, D.B. Hume, P.O. Schmidt, C.W. Chou, A. Brusch, L. Lorini, W.H. Oskay, R.E. Drullinger, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, W.C. Swann, N.R. Newbury, W.M. Itano, D.J. Wineland, and J.C. Bergquist
Science, Vol. 319. no. 5871, pp. 1808 - 1812 (28 March 2008). Abstract Link

We thank Media Relations, NIST for materials used in this posting