to develop the standards that
serve as reference for time and frequency services and to
research advanced measurement systems.
INTENDED OUTCOME AND BACKGROUND
The Time and Frequency Division maintains standards with the accuracy, continuity, and stability essential for supporting
U.S. commerce and scientific research; provides an official source of time for U.S. civilian applications; and supports coordination of
international time and frequency standards, including realization of the SI second.
NIST time and frequency standards are based on the NIST time scale and the NIST primary frequency standard, NIST-F1.
NISTF1. The time scale is an ensemble of five hydrogen masers and eight cesium-beam clocks. The stability of the time scale is approximately
0.2 fs/s for thirty days of averaging, with a long-term frequency drift of less than 3 (fHz/Hz)/year. The frequency of the time scale is calibrated
by periodic comparisons to the NIST-F1 laser-cooled cesium primary frequency standard (9.2 GHz microwave frequency), with a fractional frequency
uncertainty Δf/f approaching 4 × 10-16 (0.4 fHz/Hz, as of October 2007).
The NIST time scale is the basis of NIST’s realization of Coordinated Universal Time (UTC), the international time scale. NIST is one of about 60
timing laboratories across the world continuously contributing to the realization of UTC. Through improvements to the NIST time scale, NIST’s
realization of UTC rarely differs from the international average by more than 10 nanoseconds. In addition, NIST is one of only seven laboratories
worldwide (as of late 2007) operating the highest accuracy primary frequency standards to determine the frequency (rate) of UTC.
The extraordinarily stable NIST time scale, coupled with world-leading performance of the NIST primary frequency standard
(as of late 2007), provides U.S. industry and science with a unique resource for the most demanding applications of accurate time and frequency.
However, commercial and scientific needs for even more accurate and stable time and frequency standards drive a vigorous NIST research program to
improve microwave frequency standards and to develop new, optical frequency standards.
Since the first atomic clock was invented at the National Bureau of Standards (NIST’s predecessor) in 1949, the performance of primary frequency
standards has consistently improved by about a factor of ten each decade—driven by, and enabling, advances such as telecommunications synchronization
and the Global Positioning System (GPS). NIST research on microwave and optical frequency standards strives to at least maintain this rate of
performance improvement.
Accomplishments
Primary Frequency Standards
© Geoffrey Wheeler
Figure 1.The NIST-F1 cesium-fountain primary frequency standard, the nation’s standard for frequency and the SI second.
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The NIST-F1 laser-cooled, cesium fountain primary frequency standard (Fig. 1) is the U.S. national standard for frequency and the realization of the
SI second. Since the first formal report of NIST-F1 frequency to the International Bureau of Weights and Measures (BIPM) in 1999, the NIST-F1
uncertainty has been reduced by about a factor of four.
NIST-F1 frequency evaluations reported to BIPM in 2007 included an “in-house” fractional frequency uncertainty of approximately 4 × 10-16 (0.4 fHz/Hz),
increasing to about 8 × 10-16 (0.8 fHz/Hz) as received at BIPM due to uncertainties in the satellite-transfer process. Both of these results were the
best ever reported to BIPM. NIST-F1 has for several years been the world’s best performing primary frequency standard, continuing to lead as the
performance of both NIST-F1 and other standards across the world improves.
While continuing to optimize NIST-F1, the Division is actively developing the next-generation primary frequency standard, NISTF2.
The ultimate goal for NIST-F2 is to approach an “in-house” fractional frequency uncertainty of 1 × 10-16 in the next few years.
NIST-F2 will use a multi-toss, multiple-velocity system, in which about ten low-density atom balls will be launched to different heights in rapid
succession, all coalescing in the detection zone without having crossed paths in the Ramsey interrogation region. This approach will minimize
spin-exchange shifts while still providing sufficient atom numbers at detection for good stability.
The second major improvement in NISTF2 will be to cool the drift tube and interrogation regions to cryogenic temperatures, vastly reducing the
blackbody shift. Use of different cryogens, and/or pumping on the cryogens, will also enable accurate measurement of the blackbody shift, the
value of which has been the subject of intense debate.
Research on Optical Frequency Standards
The ultimate accuracy limit of cesium microwave standards, which operate near 1010 Hz, is expected to be on the order of 10-16
Hz/Hz (their fractional frequency uncertainty). Optical frequency standards, operating on the order of 1015 Hz, have the potential for substantially
greater stability and accuracy. Optical frequency standards also have a potential for dissemination through optical fiber, which may be advantageous
in many applications. As of late 2007, several Division optical frequency standards are performing with fractional frequency uncertainties on a scale
of 10-17 Hz/Hz, with rapid progress continuing toward the expected achievement of fractional frequency uncertainties in the 10-18 Hz/Hz range.
The Division conducts a vigorous research program on prospective optical frequency standards, simultaneously pursuing several different approaches.
These include cold, trapped single ions; cold, neutral atom clouds; and “logic clocks,” using techniques of quantum information processing.
A crucial part of optical clock research is optical frequency synthesis using femtosecond laser frequency combs, described in a later section.
There are several reasons for studying multiple systems. It is too soon to predict which optical standard will have the lowest overall uncertainty
during a period of particular rapid progress; having multiple standards using different species and techniques enables intercomparisons that reveal
unforeseen uncertainties; different standards show promise for different applications such as higher stability or lower ultimate uncertainty,
just as hydrogen masers display greatest stability and cesium fountains lowest uncertainty among current microwave standards; and comparing different
frequency standards is a sensitive probe of fundamental physics, such as possible time variation in fundamental constants.
Mercury-Ion Optical Frequency Standard
A frequency standard based on optical transitions (282 nm, 1064.7 THz) in a single, laser-cooled, trapped mercury ion has potential for better
accuracy than cesium fountain standards by a factor of 100 or more. With a Q factor > 1014 and a transition that is relatively insensitive to
environmental factors, the potential fractional frequency uncertainty Δf/f for a mercury ion standard is as small as 10-18 Hz/Hz.
As of late 2007, continuing improvements in the mercury ion standard have reduced the fractional frequency uncertainty to
1.6 × 10-17 Hz/Hz—the world’s best result so far for any frequency standard. This result is about 25 times better than the current NIST-F1 performance.
However, frequency is defined by a microwave transition in cesium, so no standard based on another oscillator can have a smaller absolute frequency
uncertainty. (As further improvements are developed, there may ultimately be an international redefinition of frequency.)
For several years, NIST has been conducting intercomparisons between NIST-F1, the mercury-ion clock, and other optical frequency standards.
Measurements are referenced to NIST-F1 through a hydrogen maser, using a femtosecond-laser frequency comb to compare optical and microwave frequencies.
Such experiments demonstrate the frequency stability of the optical standards. They also set a limit on possible variations of fundamental constants
related to the fine-structure constant a, in particular the possible temporal variation of the quantity gCs( m / m ) α6.
Assuming any variation is due to α only, the most recent NIST result (early 2007) sets an upper bound for the fractional change in the fine-structure
constant as no greater than 1.3 × 10-16 per year. It is a tighter bound by about a factor of 300 than astronomical observations that suggested
variations in the fine-structure constant over periods comparable to the age of the universe, and better by a factor of 20 compared to previous
measurements based on frequency standards.
“Logic Clock” Optical Frequency Standard
© Geoffrey Wheeler
Figure 2.Till Rosenband adjusts the “logic clock,” an optical frequency
standard based on quantum information processing techniques.
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A new type of optical frequency standard uses quantum-information (QI) techniques to exploit previously inaccessible clock transitions.
The aluminum ion has a very narrow, doubly forbidden transition at 267 nm that is highly promising for a precise optical frequency standard.
However, the aluminum-ion laser-cooling transition at 167 nm is not accessible with current laser technology. The logic clock navigates around this
barrier by using a beryllium ion and an aluminum ion in tandem. Laser operations on the beryllium ion—the workhorse of the trapped-ion QI program—cool
and interrogate the aluminum ion.
Even in early-stage development, the aluminum-ion logic clock has relative uncertainties approaching 2 × 10-17 Hz/ Hz, with significant improvement
likely. This approach can be applied to nearly any ion, opening up a wide range of potential frequency standards that were previously unavailable.
Neutral Atom Optical Frequency Standards
The Division develops optical frequency standards based on clouds of cold neutral calcium atoms and lattices of cold ytterbium atoms.
The calcium optical standard, based on a 657 nm (456 THz) transition, is particularly robust and well suited as a frequency reference (“flywheel”)
for inter-comparisons of optical standards.
Division scientists have constructed a simple, robust, and potentially transportable version of the calcium optical standard, requiring less than 15
minutes warm-up time and remaining locked with no intervention for 10 or more hours. The calcium standard achieves a short-term fractional frequency
stability of 2 × 10-15 Hz/Hz with one second of averaging and 3 × 10-16 Hz/ Hz for 200 seconds.
The Division recently demonstrated an optical standard based on neutral ytterbium atoms confined to a lattice, produced by the electromagnetic
potential wells of standing laser beams. The lattice confines ultracold atoms to small spatial areas, enabling high signal-to-noise ratios while
suppressing motion-related effects such as Doppler shifts and cold collisions. The ytterbium lattice is the first example of using even-numbered
nuclei (174Yb) with a strictly forbidden 1S0 – 3P0 clock transition.
A small, external magnetic field provides enough dipole mixing to enable the transition, one insensitive to AC Stark shifts.
The 174Yb lattice standard demonstrates a fractional frequency stability of order 10-16 Hz/Hz
in early testing, with substantial improvements expected in the near future.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2005-2007" - Table of Contents |
