Technical Highlights
- Femtosecond Laser Phase Control. An objective of coherent control
and quantum computing is to stimulate new research into phase control of atoms
and molecules. Phase and amplitude control are being investigated in the
preparation and probing of vibrational and rotational wave packets in a lithium
dimer electronically excited state. In new work, an eight-state phase control
process has been demonstrated, in which eight states are simultaneously created
in a superposition state with precise phase control of each state to about one
degree of phase. The ionization probability has been investigated to determine
the maximum and minimum ionization rates that are possible with variation of
the eight-state phase function. The experiments employ a three-photon
excitation sequence. Selection of an intermediate state from which to launch
the wave packet is accomplished with a single-mode cw laser. A 100 fs to 200 fs pulse
from a Ti:sapphire regenerative amplifier prepares the coherent superposition
of states from among eight rovibronic levels in the E electronic state. A
second, time-delayed, ultrafast pulse ionizes the molecules at a specific
localization of the wave packet (internuclear separation and rotational angle).
The dimer ions are detected as an electrical current by a pair of biased
electrodes in a lithium heat pipe. Phase and amplitude pulse shaping is
performed with a dual 128 pixel liquid crystal spatial light modulator to
alter the electric field of the laser pulse in either the wave packet
preparation (pump) step or the detection (probe) step. From the known
Franck-Condon factors, this permits precision assembly of the wave packet with
accurate knowledge of the amplitudes and phases in the coherent superposition.
This results in a focusing of the wave packet at a specific spatial location,
internuclear separation, and rotational angle, in time. Such focusing is
observed as maxima or minima in the probe ionization signal at specified times
after the pump step. With phase control of the vibrational and rotational
states in the superposition, and an arbitrary time chosen for target focusing,
a factor of 2.5 for the ratio of the maximum/minimum in the coherent control is
obtained. In new work, the decoherence rates of these phase-controlled
superpositions states are being explored to determine their suitability for
quantum computing aspects.
- "Quantized Tornados" in Bose-Einstein Condensates. One of
the most striking properties of a Bose-Einstein condensate (BEC) is its
response to an imposed rotation. Rather than accelerating smoothly into a
rotating state, the condensate supports rotation only through spawning
individual units of quantized rotation known as "vortices." As the
condensate is forced to rotate faster and faster, the body of the condensate
becomes pierced by more and more of these vortex lines, each resembling a tiny
tornado. The JILA BEC
collaboration has succeeded in creating the first vortex "in
captivity:" a condensate threaded by a single vortex and generated not
through physical rotation but through coherent control of the spinor wave
function of a rubidium gas. Each atom can be in either of two spin-states, and
the ability to coherently control the local direction of the spin of the Bose
condensed gas allows one to play various topological tricks on the wave
function. In particular, a vortex state, which would be a separate topological
state in a single-component condensate, can instead be continuously
"folded in" to a two-component condensate without anywhere disrupting
the continuity of the wave function. The existence of the vortex can be
verified by an interferometric technique that permits the quantum phase of the
wave function to be spatially imaged - the phase winds continuously from zero
to 2π as one circles the core of the vortex.
These techniques should lead eventually to a more intuitive understanding of
the origin of spurious dissipation in nominally lossless materials such as
superfluids and superconductors.
- Quantum Degenerate Gas of Fermionic Atoms. Four years ago,
experimentalists first realized Bose-Einstein condensation in a dilute gas by
cooling a gas of bosonic atoms to such low temperatures that quantum mechanical
effects became dominant. Now working with 40K, we have cooled a gas
of fermionic atoms to an ultralow temperature and observed the first quantum
degenerate Fermi gas of atoms. This work extends the field of quantum
degenerate gases to include fermions, the other class of quantum particles
found in nature. The behavior of fermions is fundamentally different from that
of bosons and is important throughout physics since the building blocks of
matter - electrons, protons, and neutrons - are all fermions. We have cooled a
gas of a million 40K atoms to below the Fermi temperature and have
observed the quantum nature of the gas with measurements of energy and the
momentum. We employ a new cooling strategy, which uses a two-component Fermi
gas held in a magnetic trap, and cools the gas of atoms to a temperature near
100 nK. At these ultralow temperatures, we find that the energy of the gas
stops decreasing linearly with the decreasing temperature and instead
corresponds to non-zero energy even at zero temperature. Accompanying this
excess energy, we observe that the momentum distribution of the gas is
distorted in a manner characteristic of fermions. These effects arise from the
Pauli exclusion principle that prohibits identical fermions from occupying the
same quantum state and thus forces our fermionic atoms to stack up in the trap,
filling higher and higher energy states. Finally, we also observe that
evaporative cooling of the gas becomes increasingly difficult, which suggests
that the quantum statistics have important effects of on the collisional
dynamics of the atoms. This experimental realization of a nearly ideal, quantum
degenerate Fermi gas of atoms introduces a new system for study. By exploiting
the weak and experimentally controllable interactions between the atoms in a
gas, one can explore Fermi-Dirac statistics in a novel regime that can lead to
new understanding of this important class of quantum systems. Other
possibilities for future work include studies of boson/ fermion mixtures and
searches for a fermionic superfluid state in which the fermionic atoms form
"Cooper pairs" similar to the physics that underlies
superconductivity in metals.
- Second Harmonic Generation from Si/SiO2. The
Si(100)/SiO2 interface is a structure that is critical to the
semiconductor industry. This interface occurs in the channel region of a
MOSFET and influences
device performance. Its importance is growing as the thickness of the
SiO2 gate dielectric shrinks with shrinking device size. As part of
an effort to understand why second harmonic generation is sensitive to
roughness at the Si/SiO2 interface, we have discovered that there is
a resonance in the response at a photon energy of about 1.8 eV. To
understand the origin of this resonance, we are studying the combined spectral
and symmetry properties. This requires careful normalization of the properties
due to variation with tuning in the power and width of the laser pulses used to
produce the second harmonic. We have implemented a reference arm that monitors
the second harmonic signal from quartz to compensate for this. The results
indicate that the resonance does have a one-fold symmetry, but higher
symmetries are inconsistent due to step edges, which are the most obvious
candidates for a one-fold symmetry. We speculate that a crystalline phase of
SiO2 at the interface may be responsible. Further work is in
progress to test this hypothesis.
Figure 1. Measured frequency of the a10 line of iodine
using the new technique using modelocked laser (right) compared to previous
discrete optical frequency measurements (left). The measurements made with a
modelocked laser show an average offset of 46.1 KHz from the CIPM
recommended value with a standard deviation of 2.8 KHz. Previous
measurements using discrete optical frequencies showed a smaller offset, but a
larger spread.
- Optical Frequency Measurement. Members of the Quantum Physics
Division have successfully used the frequency comb generated by an ultrafast
modelocked laser to directly measure large optical frequency spans. Optical
frequency measurements are made by comparison to a secondary optical standard
that in turn has been linked by workers at LPTF
to the 9.2 GHz cesium standard using a complex phase coherent chain.
However, making a connection between the secondary standard and the frequency
of interest has been challenging, requiring several phase-locked steps. By using
the broadband frequency comb generated by a modelocked laser, this comparison
has been reduced to a single step. Technology has been developed to stabilize
and control the modelocked laser’s comb spacing and position. Measurements have
been made across 104 THz to compare the frequency of an iodine stabilize
Nd:YAG laser to that of a Ti:sapphire laser stabilized to a rubidium two-photon
transition. Currently this approach is being extended to allow measurement of
the frequency span between the Nd:YAG laser and its second harmonic. Such
capability, when perfected, will be revolutionary because the results will
provide a measurement of an optical frequency directly referenced to the cesium
standard, thereby eliminating the need for the intervening phase coherent
chain.
- External Suppression of Laser Frequency Dither. One of the most
accurate methods of locking a laser to a quantum resonance involves the use of
a frequency "dither" to rapidly explore the shape of the resonance.
In this way the laser-centering information is presented at the fundamental as
well as other odd harmonics of the dither frequency. The third harmonic in
particular is interesting since it retains reasonable S/N, but is strongly
isolated from synchronous amplitude modulation effects. The stabilization is
good, but the laser’s output is then unfortunately modulated, typically by a
thousand times the actual noise associated with the average frequency. A new
idea conceived at JILA is to "de-dither" the laser’s frequency
modulation externally using a doubly passed acousto-optic modulator, driven by
a controlled rf source with a suitable "anti-dither" modulation. A
nice way has been found using direct digital frequency synthesis, and reduced
the linewidth from 6 MHz pk-pk down to the stabilization limit of about
7 kHz, nearly a thousand-fold reduction. NIST has filed
a provisional patent on the method.
- Observation of Saturated Absorption Signals in Weak Molecular Overtone
Absorption Bands. Building on the JILA-developed ultrasensitive rf/optical
method based on high-finesse cavities, the so-called "NICE-OHMS"
method, it has been possible to observe for the first time the narrow saturated
absorption lines in C2H2 (acetylene) at 1037 nm and
H2O (water) at 830 nm. The former will be of major interest in
building a new generation of optical frequency standards, that are simple but
of exceedingly high performance. This follows from the ~50-fold stronger
transition strength of the 1037 nm transition compared with the
1064 nm transition used in our previous studies. The huge dipole moment in
water vapor will make these H2O resonances of great interest in
collision studies, since these highly excited states have not been previously
accessible with such control and resolution. To indicate the extreme sensitivity
of the method, we note the water vapor overtone absorption was discovered
serendipitously in a low pressure sample of isotopically labelled acetylene,
prepared commercially under high purity conditions and sold as 99 % purity.
- Infrared Near Field Optical Microscopy of Photolithographic Films.
Polymer photoresist materials are tremendously important for the patterning of
semiconductors, flat panel displays, and data storage device components. The
rapidly decreasing line dimensions in these polymer photoresist materials,
especially with the advent of deep-ultraviolet-exposed, chemically amplified
acid-catalyzed photoresist chemistry, places new demands on measurement
characterization. In addition, this technology introduces important scientific
problems, such as the need for studies of the rates of acid diffusion in
polymers and the roughness of lithographic line features on nanometer length
scales. This project entails the development and use of a new method of
infrared near field scanning optical microscopy (IR NSOM) to probe
"spatially and chemically" the line dimensions patterned by
deep-ultraviolet (UV) photochemistry in polymer photoresist materials. The
initial experiments with IR NSOM demonstrate that this method is remarkably
powerful. Using tapered infrared transparent fiber optic tips and a
3 µm tunable color center laser, a spatial resolution of 300 nm
(wavelength/10) has been obtained, as well as an absorption sensitivity of
0.01 % transmittance. This is accomplished while probing the OH absorption
band modifications in patterned poly(t-butylmethacrylate) PTBMA polymer
containing 5 wt% of the photoacid generator (PAG) triphenylsulfonium
hexafluoroantimonate deposited on sapphire substrates and following the post
exposure bake. In fig. 2, the infrared microscope transmission at two
different wavelengths is shown along with the topographical images. A
remarkable change in contrast is observed with infrared wavelength. In this
figure the lines are 1 µm thick for each exposed and unexposed region,
however a doubling of the features occurs for the unexposed region due to the
interferometric lithography. The results of these studies offer the possibility
to probe the acid diffusion lengths for ever-decreasing line features in
semiconductor lithography.
Figure 2. Infrared transmission and topography of a
6.25 µm × 6.25 µm scan with the near field optical
microscope at two different wavelengths of 2.963 µm and 2.943 µm.
- Fundamental Constants and Tests of the Fundamental Postulates. A new
determination of G, the Newtonian constant of gravitation is being undertaken.
Though, at present, it is not connected to any of the other fundamental
constants by any accepted theories, as one of the fundamental constants of
Nature, G provides a valuable and idea-provoking challenge to precession
measurement techniques. Work is also progressing on a new absolute instrument
that will make the transfer standard g, the acceleration gravity, more
accessible for field measurements by the external research community where it
provides a valuable indicator of vertical height changes (e.g., observing post
glacial rebound) as well as subsurface mass movements (e.g., in
volcanology).
- New, Small, Absolute, and Highly Portable Gravity Instrument. Work
is progressing extremely well in the development of a new, cam-actuated
absolute gravimeter. This small, fast, portable, and as envisioned,
simple-to-use instrument uses a rotating cam to create a two-centimeter
free-fall drop followed by a soft catch, and then the return to the starting
position. This instrument inherently yields a high measurement rate
(3 drops per second). Furthermore, by employing a second cam that drives
an auxiliary mass, recoil effects having to do with the release of the mass,
the mass-carrying cart’s accelerations, and so forth can be eliminated. The
first data sets were obtained using an evacuated prototype double-cam dropping
chamber together with an FG-5 interferometer base (the latter was available and
suitable for this purpose). The results, though hardly perfect, were quite
encouraging. The absolute g data thus obtained is being examined in order to
flush out overlooked and/or unforeseen problems. This development is being
carried out as a part of an ongoing CRADA with Micro-g Solutions, the commercial
supplier of our previously (jointly) developed FG-5.
- Newtonian Constant of Gravitation. We are presently building a new
and dedicated G apparatus that will measure this fundamental constant by
sensing the change in the frequency of a laser that is locked to a Fabry-Perot cavity, each of whose mirrors is freely hanging.
This laser is then beat against another laser that is locked to a (fixed)
cavity monitoring the suspension point separation of the two hanging Fabry-Perot mirrors. When the available 500 kgm of
external masses are moved nearby, the free-hanging pendulums deflect and G can
be determined from the (small) changes in this cavity’s length. At the present
time, the uncertainty assigned to the value of this fundamental constant is
1.5 parts in 103. With this new apparatus, our aim is to reduce the
uncertainty in G to below 5 parts in 105.
- Strontium Atom Trapping. Strontium atom trapping offers some
outstanding opportunities for metrology and for studying the physics of cold
collisions. Since Sr singlet states have no fine structure, and the principle
isotope has no hyperfine structure, Sr provides exceptionally clean-cut tests
of trap collision theories. The triplet states of Sr offer an opportunity for
analysis of trap behavior, very-low-temperature cooling, metrology, and
possibly Bose condensation without evaporative cooling. We have developed a Sr
trap based on resonance-line trapping from a thermal vapor in a sapphire-window
cell. Trap loss due to leakage of excited 51P atoms (Sr*) to the
triplet manifold is prevented with lasers that recycle metastable atoms back to
the ground state, resulting in high trapped-atom densities and storage times.
The trap velocity distribution is nondestructively measured using
intercombination line fluorescence, which allows an exacting study of the
dependence of trap temperature and cloud-cooling rates on detuning and power of
the trapping beams. From the transient response of resonance-line and
intercombination-line fluorescence, we deduce the various radiative rates
within the lowest nine states of Sr. We have also measured the Sr-Sr*
collisional loss rate coefficient from trap-loading time dependence and power,
and density dependencies. This effect is dominated by excitation of the
attractive, nonradiating molecular state, which gains a dipole moment at large
atomic separation due to radiation retardation. This feature provides an
exceptionally sensitive test of retardation effects in molecular spectra, as
well as of trap loss theories, due to the exactly known molecular potentials.
Atoms trapped at mK temperatures in the resonance-line trap are also
transiently loaded into an intercombination-line trap, with ~50 %
efficiency using broad-band, red-detuned cooling. This yields high Sr densities
at K temperatures, as well as cooling rates and unique forms of Ramsey
fringes versus detunings.
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