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, based on a single mercury atom. Both
clocks are at least 10 times more accurate than the current U.S. time
The measurements were made in a yearlong comparison of the two
next-generation clocks, both designed and built at the Commerce
Institute of Standards and Technology (NIST).
The 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 research is described in the March 6 issue of Science
Express. 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 designers say
both experimental clocks could be improved further.
"The aluminum clock is very accurate because it is insensitive
to background magnetic and electric fields, and also to temperature,"
says Till Rosenband, the NIST physicist who built the clock and is the
first author of the new paper. "It has the lowest known sensitivity of
any atomic clock to temperature, which is one of the most difficult
uncertainties to calibrate."
Both the aluminum clock and the mercury clock are based on
ions vibrating at optical frequencies, which are 100,000 times higher
than microwave frequencies used in NIST-F1 and other similar time
standards around the world. Because optical clocks divide time into
smaller units, they can be far more precise than microwave standards.
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.
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.
Laboratories around the world are developing optical clocks
based on a variety of different designs and atoms; it is not yet clear
which design will emerge as the best candidate for the next
The new paper provides the first published evaluation of the
operational quantum logic clock, so-named because it is based on the
logical reasoning process used in quantum computers (see sidebar for
details). The clock is a spin-off of NIST research on quantum
computers, which grew out of earlier atomic clock research. Quantum
computers, if they can be built, will be capable of solving certain
types of complex problems that are impossible or prohibitively costly
or time consuming to solve with today's technologies.
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.
NIST's tandem ion approach is unique among the world's atomic
clocks and has a key advantage: "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."
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. In addition, 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.
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.