to develop precision
measurement tools and
applications.
INTENDED OUTCOME AND BACKGROUND
The Quantum Physics Division continues
a leadership role in developing new
precision-measurement techniques and
applications. Precision measurement is
used for many industrial and scientific
purposes, and is central to many activities
of NIST and other standards laboratories.
Applications range from providing
the length scale for mechanical
measurements to providing a direct
connection between optical and radio frequencies.
The highly stabilized laser, a Division
specialty, is the workhorse of precision
measurement. Traditionally, this meant
continuous wave (CW) lasers that generated
light with a very precise and stable
single frequency. Recently, the
Division became an international leader
in adapting CW techniques to the stabilization
of mode-locked lasers. These
generate wideband "comb" spectra, consisting
of long series of sharp spectral
lines. Each such line can have individually
a very precise and stable frequency,
with their frequency separation constrained
to be the repetition rate of the
laser. This rate may be locked to a
radio frequency (RF) reference (e.g., microwave).
Frequency combs significantly advance
the art of measuring optical frequencies
and establish a direct connection
between optical and radio frequencies.
This connection enables optical atomic
clocks and absolute optical frequency
metrology.
Accomplishments
Advances in Frequency Comb Technology
Figure 1. The Quantum Physics Division's newest
hires, Ralph Jiminez (l) and Konrad Lehnert (r). |
Early progress in comb techniques was
enabled by the development of
microstructure optical fiber, which combines
high nonlinearity with low dispersion
to allow strong nonlinear broadening
of weak pulses. It broadened the
output of mode-locked lasers to the
point where the spectra included comb
lines that were twice the frequency of
other comb lines--a full octave, in
musical terms. However, microstructure
fiber has significant drawbacks, including
coupling into its 2 µm core, drift
of the coupling over time, facet
damage, and amplitude-to-phase-noise
conversion.
To eliminate the need for microstructure
fiber, we built a mode-locked
Ti:sapphire laser that directly generates
an octave-spanning spectrum. We
demonstrated that its offset frequency
could be locked using self-referencing.
Self-referencing has been the principal
technique used to control the offset frequency
of the comb. One compares the
high- and low-frequency wings of an
octave-spanning comb, and frequency-locks
a line with its double. It is typically
implemented as a bulk-optic
interferometer.
Recently, we have measured and controlled
the offset frequency using quantum
interference of injected photocurrents
in a semiconductor. This technique
uses quantum rather than optical
interference. It replaces a bulk optical
interferometer with a single semiconductor
chip, which is smaller, less
expensive, and less complicated. The
phase stability, as measured by an out-of-loop, standard self-referencing
interferometer, was as good, or better, than
that achieved using the standard self-referencing
technique.
New Implementation of an
Optical Molecular Clock
An optical "atomic" clock uses as its reference
an optical frequency transition
that has a lower relative frequency
uncertainty than does the standard,
cesium microwave frequency transition.
An optical clock phase-coherently
derives an RF signal from the optical
frequency standard, which can be done
with a mode-locked laser.
We have built an optical molecular
clock, based on difference-frequency
generation that does not rely on an
octave-spanning spectrum, microstructured
fiber, additional CW lasers, or
stabilization of the offset frequency. We
stabilize a HeNe laser at 3.39 µm wavelength
using the methane F2(2) line.
This laser serves as the infrared optical
frequency standard to which the
repetition rate of a Ti:sapphire laser is
phase-coherently locked. We use the
mode-locked laser's frequency comb for
difference-frequency generation to provide
an infrared comb at 3.39 µm with
a null carrier-envelope offset. This
infrared comb phase-coherently links
the 88 THz optical reference and the
repetition rate.
Comparison of the repetition rate signal
with a second frequency comb stabilized
to molecular iodine shows an instability
of 0.12 pHz/Hz at 1 s, limited by
microwave detection of the repetition
rates. The single sideband phase noise of
the microwave signal, for a 1 GHz carrier
frequency, is below -93 dBc/Hz at 1 Hz offset.
Novel Spectroscopy of Cold
Atoms
Historically, stabilized CW lasers have
been integral to spectroscopy; frequency
combs from mode-locked lasers served
as reference rulers for wavelength.
However, frequency combs also enable
vastly improved measurement of atomic
and molecular structural information.
The combination of frequency domain
precision with time domain dynamics
establishes a new paradigm for spectroscopy,
connecting the fields of precision
measurement and ultrafast science.
We recently demonstrated direct,
precision spectroscopy of global atomic
structure using a single, stabilized optical
frequency comb. This approach
exploits massively parallel spectral probing
in the frequency domain. The wide-bandwidth,
phase-coherent, absolute frequency-
referenced comb allowed us
to precisely measure atomic energy level
structure in the optical, terahertz, and
radio frequency domains at the same
time, with a systematic-error-free connection
among all spectral features.
Furthermore, the pulsed excitation permits
time-resolved studies of dynamics
and real-time monitoring and control
of both optical and quantum-coherent
interactions and state transfer.
In a real sense, we have merged the
fields of precision spectroscopy and
coherent control. At short times, we
monitor and control coherent accumulation
and population transfer. At long
times, we recover all the information
pertinent to atomic level structure at a
resolution limited only by the atomic
linewidth. The spectroscopic precision
we have achieved with a frequency
comb rivals that of the state-of-the-art
CW laser-based approaches, even
though our measurement covers a frequency
range of hundreds of terahertz
with a single laser.
Additionally, we have experimentally
demonstrated the frequency comb's
mechanical action on cold atoms.
Precise control of the frequency and
phase of the entire comb spectrum
allows us to apply a light force on
probed atoms without accumulating significant
effects from heating or Doppler
shifts. We exploit this mechanical cooling
for spectroscopy, significantly reducing
systematic errors. The flexibility in
the comb structure also allows us to
carefully measure and control the
AC Stark shift inevitably present in
light-atom interactions.
Our research on ultracold strontium
atoms has three main thrusts: optical
atomic clocks, ultracold collisions, and
quantum degeneracy. Thus far, we have
obtained new insights to laser-cooling
mechanisms by using the nuclear-spin
magnetic degeneracy of cold Sr atoms
to obtain strong sub-Doppler cooling,
despite a closely overlapped excited state
manifold.
Figure 2. Images showing strontium atoms forming a "cube" as
the frequency of the laser light used to manipulate them changes.
(a) Atoms become visible at the eight corners of a cube. (b) Atoms
also appear at the midpoints of the cube edges and begin appearing at the
center of each face. (c) Atoms appear more clearly at the centers of each
cube face.
|
We have demonstrated several unique
aspects of a magneto-optic trap supported
by narrow-line cooling, including
achieving a subrecoil temperature of
250 nK (the recoil temperature is
450 nK) for the first time without needing
atomic coherence in the ground
state. At such low temperatures, the
atom cloud displays interesting dynamics.
For example, if we tune the trapping
laser frequency above the atomic resonance,
the cloud divides into discrete
momentum packets resembling lattice
sites on a face-centered-cubic crystal.
(See Fig. 2.)
Precision spectroscopy on ultracold Sr
has also begun. We have determined the
absolute frequency of the Sr transition
to a 25 Hz uncertainty. We have also
observed a density-related frequency
shift and linewidth broadening for
alkaline earth atoms. This will be valuable
information for understanding the
collision dynamics of these atoms in a
quantitative manner.
Fast, Ultrasensitive
Bolometers and Calorimeters
The emerging fields of quantum computing
and quantum cryptography
require fast, energy-resolving photon
counters (calorimeters). At the same
time, future space-based observatories
will require imaging arrays of thousands
of ultrasensitive bolometers. It is
impractical to read-out each of these
sensors with its own amplifier. Thus,
the ability to multiplex the sensors, i.e.,
to read-out many sensors with one
amplifier, is paramount.
We have built sensors that can be configured
as either fast calorimeters or sensitive,
multiplexible bolometers. The
incident power or energy is inferred
from the resistance of a nanoscale thermistor
formed from normal metal-insulator-superconductor junctions.
Recent measurements show that our
sensor is fast enough to count photons
in submicrosecond times and sensitive
enough to detect an incident power of
7 × 10-17 watts in one second, when
operated at 270 mK.
The key innovation relies on measuring
the temperature-dependent resistance
using a microwave technique compatible
with subkelvin operating temperatures.
The sensors can be configured either for
maximum speed, in which case they are
calorimeters, or for slower, multiplexible
operation, in which case they are sensitive
bolometers. By optimizing the sensors
and operating them at a temperature
of 50 mK, we anticipate bolometric sensitivities of
10-20 W/Hz1/2, meeting
the requirement of the most demanding
future space missions.
Gravity Measurements
We measure the Newtonian constant
of gravity, G, and the acceleration of
gravity, g. To measure G, we directly
compare the attraction of a laboratory
mass to the attraction of the Earth. Our
experiment translates length changes
due to gravity into measurable beat frequency
changes between two lasers.
We expect our final measurements to
yield a relative uncertainty for G of
about 4 × 10-5.
We are also developing a new, compact
instrument to measure the acceleration
of gravity, g. The instrument employs a
novel cam-based mechanism to obtain
extremely fast and quiet control of the
entire release, freefall (measure), catch,
and lift cycle. It can be used for volcanology
and precision-measurement
studies, and has an uncertainty of
approximately 3 µGal (relative uncertainty
of 3 × 10-9).
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2004" - Table of Contents |
