Time and Frequency Division
Division Overview |
Program Directions |
Major Technical Highlights
Major Technical Highlights
- A New Generation of Frequency Standards. Staff members of the
Physics Laboratory in Boulder, in collaboration with T. Udem of the Max
Planck Institute in Germany, have recently demonstrated an optical frequency
standard with a microwave output, which in effect signals the introduction of a
new generation of atomic clocks with potential performances well beyond those
in operation today. This first demonstration of an optical clock with a
microwave output, reported in the August 3 issue of Science, involves an
optical frequency standard, with a Q factor (frequency divided by linewidth) of
greater than 1014 coupled to a new optical-frequency synthesizer
that bridges an octave in the optical spectrum and provides for a direct
microwave output that is phase locked to the optical standard. The potential
uncertainty of the optical clock is 1 part in 1018, a factor of 1000
times better than today's best standards.
NIST staff members contributing to the development of this clock include
J. Bergquist, A. Curtis, S. Diddams, R. Drullinger,
L. Hollberg, W. Itano, D. Lee, C. Oates, K. Vogel, and
D. Wineland.
Essential components of this standard include a single trapped mercury ion in a
cryogenic trap, an ultra-stable laser used to probe the transition in the
mercury ion, and a femtosecond laser system that produces a frequency comb
spanning a full octave in frequency. The clock transition for the
199Hg+ ion is at
1.064 × 1015 Hz. The ultra-stable laser is first
locked to this mercury transition and then a single element in the comb is
locked to the stable laser. Finally, the repetition rate (at about 1 GHz)
of the femtosecond laser is stabilized by a self-referencing technique to
provide a phase coherent linkage between the optical and the microwave range
where cycle counting can be done. In comparisons with a laser-cooled calcium
optical standard, an upper limit for the fractional instability of
7 × 10-15 was measured in a 1 second averaging
interval, which is significantly better than that of the world's best microwave
atomic clock. (J. Bergquist)
- Further Improvements to NIST-F1. S. Jefferts and
T. Heavner have made several improvements to NIST's cesium-fountain
primary frequency standard NIST-F1, providing primarily for improved
reliability of operation, but also resulting in improved accuracy. Reliability
issues are important, since the value of NIST data submitted to the BIPM is
substantially higher if data is submitted at regular intervals. Furthermore,
the achievement of highly regular operation can offer the opportunity to use
the data from this standard directly in the NIST time scale.
The improvements made to the standard include (1) the installation of new
light shutters of substantially higher reliability, (2) the implementation
of a servo-control system on the number of atoms tossed in each ball,
(3) a lower noise quartz-crystal local oscillator, and (4) new
software for the main line-center servo-control system. A completely new laser
system (Ti:sapphire) has been acquired, and this will replace the current
systems as soon as final acceptance testing is completed.
With these changes, the uncertainty of the standard has been improved from
1.7 × 10-15 to 1.3 × 10-15.
This latter evaluation number is the best yet reported to the BIPM. Furthermore,
quantum-projection noise has now been observed at a level of 30 atoms.
This is lower by a factor of 2 to 3 than has been previously seen in a fountain
frequency standard. (S. Jefferts)
- International Frequency Comparisons. T. Parker and
J. Levine of the Division have improved both the two-way time transfer
system and the GPS carrier-phase time transfer system allowing for more-routine
international frequency comparisons with an added comparison uncertainty of
5 × 10-16 over intervals of 20 to 30 days. Continued
improvements in these comparisons are needed to support comparisons of the
fountain standards, which are clearly going to improve over the next few years.
The use of both of these systems has proven to be an essential aspect of this
effort, since, in combination with time-scale date, each of the systems tends
to uncover problems with the other. The day-to-day time stability for both
systems is about 300 ps. Time comparisons at each end of a fixed time
interval then result in a measurement of the relative average frequency over
that time interval.
The best comparison made to date between fountain frequency standards at NIST
and PTB show a frequency difference of 1.7 × 10-15
with a total uncertainty of 2.1 × 10-15. This is well
within the 1 uncertainties of the two
standards. (T. Parker and J. Levine)
- A Transportable Cesium Fountain. A small, transportable
cesium-fountain standard has recently been constructed and demonstrated in a
preliminary way by T. Heavner and S. Jefferts. The objective of this
work is a transfer standard of very high reproducibility that can be used to
compare widely separated primary standards. As primary standards have increased
in accuracy, the noise (and possibly biases) in satellite comparison methods
have risen in importance, and it is now essential that these be evaluated using
a transportable artifact.
Figure 1. Central portion of the Ramsey pattern for the small cesium
standard. The frequency offset is due to the relatively large C field. The
solid line is a fit to a sine wave. |
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The small toss height (30 cm) and large aperture of this small
standard assure very high signal-to-noise performance, assuring that
measurement time is limited solely by the primary standard, not the
transportable device. Figure 1 shows the central portion of the Ramsey
fringes from this device, which at this time is not fully shielded
magnetically. Additional improvements will be made during the next six months
before serious efforts are made to study the size and stability of systematic
effects.
The development of this small standard will also have impact on the next NIST
primary frequency standard, since some of the systems developed will be useful
in that development. These systems include a laser package that delivers
cooling, state-preparation, and state-detection radiations to the standard
through polarization-maintaining optical fibers; and collimators on the source
chamber designed to assure efficient trapping of larger numbers of atoms.
(T. Heavner) |
- Phase-Modulation Servos for Atomic Clocks. S. Jefferts and
F. Walls of the Physics Laboratory in Boulder, in collaboration with Bill
Klipstein and John Dick of the Jet Propulsion Laboratory (JPL), have developed
an improved modulation method for laser-cooled atomic clocks, providing for a
high level of immunity to vibrations and substantial reduction of a number of
systematic frequency shifts that can affect these clocks.
Figure 2. Schematic representation of the difference between frequency
modulation and phase modulation. As can be seen, the operating points for
frequency modulation are the steepest points on the resonance, whereas phase
modulation leads to operation on the peak of the resonance, where perturbations
have the least effect. |
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The concept involves phase modulation of the interrogating microwave field
rather than the traditional frequency modulation used in most atomic clocks. In
this new scheme, graphically shown in fig. 2, the phase of the microwave
field in the first portion of the Ramsey cavity is fixed and the phase in the
second Ramsey region is varied alternately between +90° (relative to the
phase in first region) and -90°. The advantage of the method is that the
frequency of the microwave field can be kept continuously on the center of the
resonance, rather than being stepped from one side of the resonance to the
other as is done using frequency modulation. At the peak of the resonance the
clock is substantially less sensitive to vibration and to systematic effects
such as line pulling and cavity pulling than it is when the system resides most
of the time on the steepest portion of the resonance curve.
The concept was initially developed to take care of the vibration sensitivity
of the laser-cooled clock on the Primary Atomic Reference Clock in Space
(PARCS). In this collaborative program, involving NIST, JPL, the University of
Colorado and the Harvard-Smithsonian Center for Astrophysics, a laser-cooled
cesium clock will be put aboard the International Space Station (ISS) in 2005
to perform certain tests on gravitational theory and to improve upon the
realization of the second. The modulation concept was tested on the NIST
cesium-fountain clock, NIST-F1, and found to work so well that it has become
the preferred mode of operation. It has also been picked up by others and is
being used on other fountain clocks around the world. (S. Jefferts). |
- PARCS Advances Through NASA Reviews. The NIST-led program entitled
Primary Atomic Reference Clock in Space (PARCS) successfully completed its
second NASA review, the Requirements Definition Review, in December 2000. This
review, which included both a science panel and an engineering panel,
considered both the scientific merits of the mission and the technical
feasibility of the proposed experiments. This was the final review of the
science involved. Subsequent reviews will focus on the engineering development
of the flight systems.
This NASA-funded mission, which is a collaboration among NIST, the Jet
Propulsion Laboratory, the University of Colorado, the Harvard-Smithsonian
Center for Astrophysics, and the Politecnico di Torino, is currently scheduled
to fly in 2005. The objectives are to test certain aspects of relativity theory,
to improve upon the realization of the second, to study the performance of GPS
clocks, and to study the dynamics of atoms in microgravity.
In its report to NASA, the Review Panel recommended that "...the PARCS
project should proceed with all possible speed and deliberation toward the
Preliminary Design Review, ..." The Panel also concluded that,
"...based on all material presented to date, the experiments should fly
and can succeed." A complete preliminary design of the system will be
considered in the next review, which is scheduled for late summer 2002.
(D. Sullivan)
- Chip-Scale Atomic Clocks. Following their development last year of a
very compact cesium frequency standard, J. Kitching and L. Hollberg
have been working on concepts that might someday allow atomic standards to be
reduced to chip scale. A particular focus of their studies has been the use of
dark states, which provide the means for interrogation of a microwave resonance
without need for a physical microwave cavity. In this scheme, the traditional
clock transition is probed through a microwave modulation imposed on a laser
signal tuned to an optical transition. Some effort has been made to develop at
least first-order models of the scaling laws appropriate to reduction to this
scale. For example, fig. 3 shows the scaling of the atomic Q factor with
size for three types of gas cell.
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While clock performance would clearly be degraded by reductions in size and
power requirements, there are many applications where such clocks might be
extremely useful. For example, electronic instrumentation (such as frequency
counters), where an accurate time base is needed, would benefit, since an
atomic standard would bring intrinsic accuracy to the instrument. Furthermore,
small and inexpensive devices of this sort would find application in GPS
receivers, where they would provide the accuracy needed to achieve more rapid
navigation solutions, and in telecommunications systems, where there is a need
for a very large number of clocks with performances better than are provided by
quartz oscillators.
The NIST work is very timely, since the Defense Advanced Research Projects
Agency (DARPA) has just initiated a call for proposals aimed at developing such
a clock, and NIST is thus well positioned to assist DARPA as it engages
industry in this development project. (L. Hollberg) |
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Figure 3. First-order scaling laws for atomic clocks based on gas cells.
The model is based on cubical cells (all dimensions equal). The horizontal line
is the Q of a typical mechanical microresonator of the sort that might be used
in a chip-scale clock. For very small gas cells, it is clear that wall-coated
cells will be the best choice. |
- Heating of Trapped Ions from the Quantum Ground State. Excess
heating, which has been limiting correlation times in quantum-logic experiments,
has recently been reduced substantially through shielding of trap electrodes
from beryllium deposition on the electrodes. This shielding was implemented
following heating studies, wherein Q. Turchette, D. Kielpinski,
B. King, D. Meekhof, M. Rowe, C. Sackett, W. Itano,
C. Monroe, D. Wineland, D. Leibfried, C. Myatt, and
C. Woods showed that the scaling of the heating with trap size was
inconsistent with the source being Johnson noise in external circuits, and
consistent with fluctuating patch-potential fields on the trap electrodes. If
patch effects could explain the heating, then it was clearly logical to do
something about the state of the electrode surfaces. The first step was to
shield the electrodes by masking the neutral beryllium atom source, preventing
deposition of beryllium on the electrode surfaces.
The observed reduction in heating was large, decreasing the level seen in
previous work by a factor of 200-300. While this will ultimately enable
increasing the number of quantum-logic operations by this same factor, Raman
laser intensity noise and noise caused by fluctuations in the background
magnetic field must be reduced before this benefit can be realized.
(D. Wineland).
- Entangled, Spin-Squeezed States. By applying coherent laser beams to
trapped ions, PL staff in Boulder have generated quantum-mechanically-entangled,
"spin-squeezed states and, for the first time, shown that such states can
be used to increase measurement precision beyond that which is possible without
the use of entanglement. V. Meyer, M. Rowe, D. Kielpinski,
C. Sackett, W. Itano, C. Monroe, and D. Wineland have
recently reported the results of these studies in the Physical Review Letters.
As a demonstration, they produced spin-squeezed states of two beryllium atomic
ions and showed that when the spins are rotated in a magnetic field, the
uncertainty in determining the rotation angle is smaller than can possibly be
obtained if the atoms are not entangled. The spin-squeezing result is shown in
fig. 4. Such techniques are an integral part of the emerging fields of
quantum logic and quantum information, but can also be used to improve
sensitivity in spectroscopy or reduce noise in atomic clocks.
Although spectroscopic precision can always be improved by increasing the
number of atoms (without the need for entanglement), in atomic clocks based on
ions for example, the quest for accuracy requires the use of only a small
number of ions. With a potential uncertainty of 1 part in 1018,
these techniques should actually find application in optical ion clocks.
(D. Wineland) |
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Figure 4. On the vertical scale we show the measured fluctuations of the
spin measured in a direction perpendicular to the direction of the net spin
vector. For an unsqueezed "coherent" spin state the fluctuations are
independent of the azimuthal angle  around
the direction of the spin (the "standard quantum limit," SQL). For a
squeezed entangled state these fluctuations are reduced for certain angles.
This "spin-squeezing" gives rise to increased sensitivity in
spectroscopy. |
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- Quenched Narrow-Line Cooling of Calcium. C. Oates and
L. Hollberg, along with A. Curtis of the University of Colorado have
recently achieved sub-Doppler cooling of 40Ca to near the recoil
limit using a quenched narrow-line laser-cooling method. Since the Doppler
limit is large, typically of the order of 1 mK, for alkaline-earth-metal
atoms, the uncertainty of optical standards based on these atoms has been
limited. In fact, the best frequency measurement (± 26 Hz) of the
657 nm (456 THz) transition in calcium, which was made recently at
NIST, was principally limited by residual atomic velocity.
The cooling process is illustrated in fig. 6. Because of the long lifetime
of the 3P1 state, the width and shape of the velocity
group excited from the 1S0 state to the
3P1 state can be readily controlled. In a step wise
fashion, a 657 nm laser is tuned slightly red of the transition to
transfer a velocity group of atoms to lower velocity through a one photon
recoil to the 3P1 state, and a subsequent pulse of
553 nm (propagating in the same direction) light moves the atoms to the
1S0 (4s5s) while giving them a second kick toward
zero velocity. These atoms return to the ground state through the two decays
(and their random recoils) shown in the figure. This process is then repeated
with pulses from the opposite direction, so that a second velocity group on the
opposite side of the distribution is moved toward zero velocity. This process
is repeated many times, driving all atoms toward zero velocity, where they have
a much smaller probability of being pumped away (since the pump laser is
preferentially pumping higher velocity atoms).
Using this method, calcium atoms were cooled in one dimension to as low as
4 µK. Further improvements on this cooling will be made, and it will
be extended to three dimensions. The objective is to improve upon the accuracy
of the calcium optical standard, which is the only optical standard for which
high-accuracy international comparisons have been made. The best comparison to
date is between NIST and PTB, where a difference of 30 Hz was measured.
This is well within the uncertainties of the individual measurements.
(C. Oates) |
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Figure 6. a) Energy level diagram for calcium showing levels used
in the narrow-line quenched-cooling technique. b) Velocity distribution of
atoms before and after quenched cooling. The broad curve is near the
Doppler-cooling limit and the narrowed one is near the recoil limit. |
- Diode Lasers Locked to Optical Cavities. R. Fox and
L. Hollberg have succeeded in controlling the fluctuations of diode lasers
with respect to the resonances of optical cavities, allowing them to build
systems that will be useful for analytical applications and length measurement.
In their experiments, they demonstrated locking to narrow modes (~5 kHz)
of very high-finesse optical cavities with a servo-control bandwidth of
3 MHz, followed by un-locking and re-locking to the same mode at rates as
high as 2 kHz. This capability is important to cavity ring-down
spectroscopy, an analytical technique used by many laboratories to measure
extremely small (trace) amounts of certain atoms and molecules. Usually, this
work is accomplished using poorly controlled pulsed laser sources. Better
sensitivity can be obtained using continuous-wave lasers, but to achieve higher
sensitivity it is necessary to frequency lock the laser to the ring-down cavity
as has been done here.
A second application of this work is to swept-wavelength laser interferometers
used for length measurement. Well-controlled diode lasers will be able to
measure 10-20 meter path lengths with micrometer uncertainty if the laser
can be swept over a precisely known wavelength interval, for instance by moving
the frequency of the laser precisely between two different modes of a stable
optical cavity. With the systems developed here, it should be possible to lock
the laser to a high-finesse laser mode, un-lock the laser, change the laser
wavelength by several nanometers, and then re-lock to another mode.
(R. Fox)
- Standards for Optical Communication. In collaboration with
S. Gilbert, N. Newbury, and K. Corwin of EEEL, L. Hollberg
and S. Diddams of the Division have initiated development of a mode-locked
femtosecond laser that will provide a comb of frequencies overlaying the
important optical-fiber communication bands. The objective of the project,
which is supported by ATP, is to establish extremely accurate measurements in
support of wave-division multiplexing (WDM), which is used extensively in
optical telecommunications systems. Because these optical systems are moving to
higher data rates through closer channel spacing, accurate measurements of
channel location and spacing are becoming increasingly important.
The new laser system, which uses an optical cavity with dispersion compensation
and a crystal of Cr:Forsterite, has been designed and all specialized
components for its construction have now arrived. The reference frequency for
these measurements will be the 657 nm line in calcium, since this has been
extremely well measured by several laboratories over the last few years. A
division by two of the calcium line produces a line in the vicinity of
1.3 µm, right within the region of interest for measurements
supporting WDM. This ATP-supported project also involves the application of
these new combs to length metrology. Jack Stone of MEL is collaborating with
the Time and Frequency Division on this part of the program. (S. Diddams)
- Further Growth and Improvements of the Network Time Service. Use of
the NIST Network Time Service (NTS) continues to grow at a monthly compounded
rate of about 8.5%, with current usage at over 200 M hits per day. To meet
this growing use, J. Levine has installed two additional servers. He has
also modified the code by sending special packets that allow users to obtain
either UTC or TAI. This addition of TAI was made in response to requests from
users who want a time scale that is absolutely continuous and free of leap
seconds. In order to deal with new operating systems he has written new client
software for Linux and Windows 2000/ME/XP.
Work also continues with three companies that are developing special-purpose
hardware and software to provide secure time services to financial markets,
where concerns for the legal authenticity for time/date stamps is driven by the
potential for expensive mistakes, or even fraud. (J. Levine)
- Microwave Synthesizers for Advanced Frequency Standards.
Low-phase-noise synthesizers play a critical role in the performance of primary
frequency standards, so the Division has been working to improve the
reliability and performance of these devices to support rapid advances in these
standards. C. Nelson and D. Howe of the Division, along with guest
researchers F. Garcia and A. Hati, have recently made substantial
improvements that are important, not only for fountain standards, but also for
other frequency standards such as the laser-cooled space clock for the PARCS
mission. Of particular significance to this latter project is the complete
re-engineering of the system to operate on the lower DC voltages that are used
on the space station. However, the most important improvement has been the
reduction in sensitivity to temperature variations. The synthesizers now
exhibit a temperature coefficient on the order of 0.1 ps/K. It is
important to minimize this coefficient, because this has an impact on phase
changes that occur during dead time in the standards.
NIST leadership in this area is highlighted by the fact that most new standards
in the world are or will be using the NIST synthesizer. In fact, there are very
few programs that are building their own synthesizers. NIST has now completed
and delivered ten synthesizers to other programs, primarily other primary
standards laboratories, and four more are under construction. Because the
operational mode and requirements of the various standards are different, some
custom design changes have been made with almost every one of these
synthesizers. (C. Nelson)
- Calibration System for Phase and Amplitude Noise at 100 GHz.
D. Howe and F. Walls have developed a system for calibrating the
level of phase-modulation (PM) noise and amplitude-modulation (AM) noise at
100 GHz in oscillators, amplifiers, and other components. At this time the
two key applications of the system are to the clocking of high-speed digital
processors and evaluation of the performance of broadband telecommunications
signals.
The system, shown schematically in fig. 7, relies for reference on a
cavity-stabilized Gunn oscillator that serves as a low-noise reference signal.
The oscillator is coupled and locked to an ultra-low-noise sapphire loaded
cavity oscillator (SLCO) through several regenerative frequency dividers, and
the broadband noise of the system is then limited by the mixer and divider
noise. Measurement of phase or amplitude noise at 100 GHz is by two nearly
identical phase-locked loops whose phase deviations are analyzed simultaneously
using a cross-correlation spectrum analyzer. Because most of the noise
generated in the components (mixers and detectors) in one channel is
independent of the noise generated in the same components in the other channel,
it does not show up in the correlation measurement. (D. Howe) |
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Figure 7. Simplified block diagram of the system used for measuring
PM and AM noise at 100 GHz. The DUT (device under test) is an oscillator
or some other component such as an amplifier. In the latter case the component
is fed a signal from the main 100 GHz reference oscillator. The dual-loop
configuration, along with the cross-correlation analyzer, provides the means
for canceling out the noise contributions from many of the elements (e.g.,
mixers and detectors) in the measurement circuit. |
- Repair of WWVB Antenna System. On February 28 of this year, a
400 foot tower of the 60 kHz antenna system for WWVB partially
collapsed, resulting in a reduction of broadcast power by nearly a factor of
two. Since there are a very large number of receivers, clocks, and watches that
are controlled by this broadcast service, there was an immediate reaction from
those located substantial distances from the transmitter (Florida, New
Hampshire, etc.). The signal-to-noise ratio in these areas was degraded to the
point where reception was marginal. In response to this mishap, the NIST Acting
Director directed the expenditure of special funds to bring the station back to
full power as quickly as possible. This turned out to be a complicated process,
however, full power was restored on June 15.
The WWVB broadcasts emanate from a pair of transmitters that are operated in
phase and radiate their respective signals from two closely spaced antenna
systems. This design allows for short-term maintenance of one system or the
other without shutting down the broadcasts and provides some longer-term
reliability. In order to minimize the impact on users during the repairs, the
radiating element for the damaged antenna was partially raised providing a
total power output of 38 kW, substantially below the normal operating
power of 50 kW, but better than the output of 28 kW from a single
transmitter system.
The cause of the antenna damage was structural failure in a metal component in
one of the insulator assemblies that isolate the antenna guy wires from ground.
This failure is being studied by the Materials Reliability Division of the
Materials Science and Engineering Laboratory. Their recommendation will form
the basis for replacement of similar insulator assemblies used on all the guy
wires. Repair of the antenna, led by M. Deutch and J. Lowe of the
division, involved custom fabrication of the upper third of the tower,
disassembly of the tower, and then reassembly with the replacement sections.
Since there are only two cranes in Colorado capable of handling components at
this height, scheduling of the repair was difficult. Furthermore, to prevent
damage to underground antenna components in the soft soil surrounding the
antennas, a short section of roadbed had to be laid to support the great weight
of this crane. (J. Lowe).
- WWVH Antennas. As part of a program to eventually replace all eleven
antenna masts at WWVH in Kauai, last year the Division replaced two of the
masts with fiberglass whip antennas (see fig. 8), and has initiated
replacement of two additional masts. D. Okayama and D. Patterson at
the station in Kauai and J Lowe and W. Hanson in Boulder are leading
efforts on this project. Despite vigorous maintenance efforts, these
30 year old antenna masts have suffered serious corrosion in the
salt-water environment, and, because they are becoming weakened structurally,
they pose risks to those maintaining them. The first two replacements were of
broadband antennas used as backups for the main antennas operating at
5 MHz, 10 MHz, and 15 MHz. The replacements this year is for the
primary antenna and backup antenna used for broadcasts at 2.5 MHz. These
are substantially taller antennas. As noted previously, the capital investment
in these new antennas will be recovered, in the form of reduced maintenance
costs, in about 7 years. (J. Lowe).
Figure 8. New whip antenna at WWVH on Kauai. Conventional antennas are
seen in the background. The fencing is a safety measure, preventing personnel
access to the high voltages on the antenna system. |
- Time-and-Frequency Users Survey.
A customer survey covering most
Division dissemination services was developed this year by J. Lowe and
J. Heidecker of the Division, and all customer survey input has now been
received. More than 18,000 responses to the survey have been received, a very
significant increase over the 7000 responses that were received during the last
survey in 1987. The objectives of the survey are to gauge customer use of the
various services, determine whether there are any problems with how the
services are provided, and elicit suggestions on user requirements that could
lead to modifications of existing services or the development of new services.
The focus of the survey is on the NIST radio broadcasts (WWV/WWVH/WWVB), the
telephone time service (Automated Computer Time Service), and Internet time
services (including the Network Time Service), the active Internet clock
(time.gov), and the informational content of the Division's web pages. The
results, which could take as long as 6 months to tabulate and analyze,
will guide the Division in both its near-term and long-term planning for the
future of its services. (J. Lowe)
- Time-and-Frequency Publication Data Base. M. Lombardi and
A. Novick, with support from G. Bennett have made a large number of
improvements in the Division's web site, with the key improvement in the area
of Division publications. The Division's database of more than
1500 publications is now on the web, and for more than 1/3 of these,
including most recent ones, the full paper is available on line. A plan is now
in place to systematically scan the older publications and make them accessible,
so that all papers are eventually available on line. This will reduce staff
time associated with sending out reprints, and will provide better support to
Division researchers, who must have ready access to all Division publications.
The Division web site, excluding the time services receives by far the largest
number of external hits, with over 300,000 page views per month.
(M. Lombardi)
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Inquiries or comments: Feedback
/2 pulse is first applied to the ion as it
sits in trap 1, and the ion is then shuttled to trap 2 where it is
subjected to a