Quantum Physics Division

Division Overview  |  Program Directions   |   Major Technical Highlights

Major Technical Highlights

Precision Spectroscopy in Cold, Dense, Trapped Atomic Samples. The technology developed for creating Bose-Einstein condensation has been used to create very dense and very cold samples of Rubidium atoms for studies of systematic effects in microwave spectroscopy. The densities of these samples are five orders of magnitude higher than in cold-atom fountain clocks, and the temperatures are within 40% of the transition temperature. The microwave linewidths are comparable to those in modern fountain clocks, but near the onset of quantum collective behavior the sources of systematic error associated are much larger. At these very high densities, atom-atom interaction terms are very large. Mean-field as well as exotic collective frequency shifts also become very large and are typically resolvable in just a few seconds of integration time. Quantitative and qualitative understanding of these errors in this high-density regime will allow very precise corrections to be applied in the low-density, precision regime of next-generation atomic clocks. (E.A. Cornell) Condensate Interferometry. A new apparatus capable of combining two atom-optic technologies -- Bose-Einstein condensation and lithographically patterned atomic wave-guides -- is nearing completion. Condensation has already been observed in the new machine and the wire-guiding technology is being integrated into the apparatus. The combined machine will be capable of measuring gravity gradients interferometrically, with applications to remote sensing, geodetics, and navigation. (E.A. Cornell) Exploding Solitons. Recent theoretical results suggest that soliton instabilities can exist in the presence of dissipation (gain and/or loss). These instabilities can result in the temporal length of the soliton suddenly increasing followed by the soliton returning to its steady state (an "explosion"). Modelocked lasers make an excellent test bed for soliton dynamics (in the dispersion-managed regime) because their output provides a sampling of the pulse after each round trip. Because capturing the actual temporal behavior at a high enough rate is impossible, we monitor the spectrum. Indeed, events are observed where the spectrum abruptly shift and narrow before returning to the steady state (see fig. 1). Depending on the dispersion, either single explosions or bursts of explosions occur. This work may have significant impact on long distance telecommunications where dispersion-managed solitons are a candidate format. The presence of such instabilities must be taken into account to avoid errors. (S.T. Cundiff)

Figure 1. Typical data showing soliton explosions. In the upper panel the dispersion is adjusted to yield solitary explosions, while in the lower it is adjusted to yield bursts of explosions. The resolution and position of spectral channels has been optimized in the lower panel.

• Coherent Response of Semiconductors. The coherent response of semiconductors is very sensitive to many-body interactions among the optically excited electrons and holes. A theoretical understanding of this coherent response has been developed over the last decade. Some approaches have developed a microscopic theory starting from the electron-hole Hamiltionian, while others have taken a phenomenological approach that is more appealing to one's physical intuition. We have discovered a new term in the phenomenological theories that is due to excitation induced shifts (EIS). Transient four-wave-mixing (TFWM) is the most common tool for probing the coherent response [see fig. 2(b).] We observe that the spectrum of the TFWM signal displays a split peak [see fig. 2 (c and d).]

Figure 2. (a) Linear absorption and laser spectrum. Experimental setup (b) shows the two-pulse configuration for TFWM and DT in transmission. Typical TFWM spectrum, in transmission (c) and in reflection (d), both for  = 0 delay.

Our phenomenological calculations only reproduce a split peak if EIS is included in addition to the well known phenomena of excitation induced dephasing and local fields (see fig. 3). To the best of our knowledge, the role of EIS has not been pointed out previously. The understanding of how many-body phenomena in semiconductors affect their interaction with light is crucial for developing a theoretical description of light-emitting diodes and laser diodes. (S.T. Cundiff)

Figure 3. Numerical calculation of a TFWM spectrum using the MOBE, with both EID and EIS present.

• Phase Stabilization of Modelocked Lasers. Stabilization of the carrier-envelope phase of modelocked lasers has recently resulted in a revolution in optical frequency metrology and is the enabling technology for the recent demonstration of optical atomic clocks in both the Time and Frequency Division and the Quantum Physics Division. We are currently focusing on understanding phase noise processes that introduce uncertainties into the results. One area of concern is the microstructure fiber used to broaden the pulse spectra. Because this broadening is a highly nonlinear process, it might be expected that small amplitude fluctuations in the laser power would be converted to significant phase fluctuations. As shown in fig. 4, we have measured this conversion factor and determined that it is insignificant. This is because it is the differential phase that matters, the phase between the pulse envelope and its carrier. Because the amplitude fluctuations have a common effect on both the pulse envelope and its carrier, the impact on the experiments is minimal. We are now turning to measuring the power spectrum of the frequency noise emitted by the laser, this which will allow us to determine the coherence time among the pulses in the train. (S.T. Cundiff)

  Figure 4. Measurement of the change in carrier-envelope phase (right axis) as function of amplitude modulation of the laser. A known modulation of variable amplitude is imposed on the laser to allow the conversion from amplitude to phase to be determined. The background phase noise is apparent at zero modulation. This adds in quadrature to the generated phase modulation to produce the curved line.

• Multiple Input AND Gate by Molecular Wave Packets. An AND gate represents a useful mathematical algorithm for computation: if all the inputs are 1, the output is 1; all other combinations of inputs give an output of 0. Using quantum interferences resulting from a coherent superposition of rovibrational molecular states in an ultrafast pump-probe experiment in molecular lithium dimers, a six-input AND has been demonstrated. The input values (0's or 1's) are imprinted into the phases of the eigenstates by a shaped femtosecond pulse. The computation is carried out by the quantum beat interferences during the field-free time evolution. The resultant output is probed at a specified delay time by ionization of the wave packet to determine the amplitude of the recurrences. A set of phases that obtains a global maximum output at a specific time delay is predetermined and defined as an output of 1; any other amplitude value, caused by different phases, is an output of 0. For the six-input AND gate studied here, a >99% success probability is achieved with a typical signal-to-noise ratio of 10. This can be improved considerably by additional signal averaging times or extended to hundreds of inputs. The results have applicability to problems such as determining whether a graph, consisting of points and edges connecting the points, has any isolated vertices. (S.R. Leone)

• Femtosecond Soft X-Ray Pump-Probe Dynamics. Ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy are important methods used to study the structure and energetics of neutral molecules, radicals, and semiconductor surface processes such as etching. Thus far it has not been experimentally feasible to study ultrafast processes of neutral molecules or materials surfaces with direct ultraviolet or soft x-ray photoelectron methods. In recent work a new method has been developed to probe camera-like snapshots of the ultraviolet photoelectron spectrum of bromine during dissociation. High order harmonics of a femtosecond Ti:sapphire laser are produced in a rare gas. A single harmonic (47 nm, 17th harmonic) is selected and used as the probe, while an ultrafast pump pulse at 400 nm is created by frequency doubling a portion of the 800 nm ultrafast laser. Figure 5 shows a time-resolved sequence of pump-probe spectra in bromine molecules following its photodissociation. The spectra reveal the clear formation of the photoelectron spectrum of the atoms, as well as the timescale for production of the free atoms. There are cross correlation (cc) features that occur because of above threshold ionization and permit a direct assessment of the time durations of the laser pulses and a weak feature at 8 eV that is attributed to the transient wave packet on the dissociative state prior to dissociation. An important result is the observation of an enhanced cross section for photoionization of the atomic species, compared to the molecules in the ground state. These results pave the way for a general new method to probe transient states in molecules, based on the familiar and powerful methods of photoelectron spectroscopy that have been so successfully employed for ground state analyses. (S.R. Leone)

  Figure 5. Femtosecond pump-probe spectra using soft x-ray radiation from a high order harmonic generation process as the probe on dissociating bromine molecules. The features labeled Br2+(X) and Br2+(A) are due to ionization of the Br2 parent molecules. The features labeled 2P2,1,0 are due to Br atoms upon dissociation. The cc are cross correlation features.

• Evolutionary Algorithms in Wave Packet Dynamics. An evolutionary algorithm has been developed to optimize wave packet dynamics in ultrafast pump-probe experiments. The computer is given multiple sets of random phases versus frequency that are applied to a femtosecond pump pulse. These phases are imprinted onto the wave packet, causing the wave packet recurrences to be optimized at various time delays. The computer is asked to optimize the wave packet recurrences at a specific time delay. Thus some individual phase patterns are found to be better than others and are selected for reproduction by simple mutation. Multiple sets of mutated phase patterns are again tested for the desired optimization and after many generations a final set of phase patterns is obtained that contains the physics of the wave packet optimization process. Several new aspects have been pioneered. These are the first investigations of the effects of evolutionary optimization of the phases of frequencies on and off specific transition resonances, i.e., the separation of effects due to both resonant and non-resonant frequencies. Non-resonant frequencies are found to play a dramatic role both in Raman pumping, which requires two or more photons, and in the time-dependent amplitude coefficients that occur within the time duration of the laser pulse. By programming the algorithm to optimize specific beat frequencies as well as the overall wave packet recurrences, it is possible to optimize specific high order processes, such as a fourth order Raman pumping process, to enhance single quantum beats. These results will be important for future work on manipulating molecular wave packets to create quantum computing algorithms, such as Controlled NOT. (S.R. Leone)

• Growth of Quantum Dot Materials. The formation of Ge nanodots on Si(100) occurs by strain-induced mechanisms (Ge is 4% larger than Si) and obeys the Stranski-Krastanov (SK) growth mode: a wetting layer (3-5 layers) is followed by the formation of three-dimensional Ge structures. Quantitative studies of Ge island size distributions and their shape transformations, including huts pyramids domes superdomes and shape changes due to annealing of the islands under the influence of surfactants, have been achieved by molecular beam epitaxial growth and atomic force microscopy (AFM) post-analysis. For device applications, it is important to attain control over the size and spatial distributions of self-assembled nanostructures. The Ge growth experiments are also carried out on patterned silicon substrates (mesas formed by electron beam lithography followed by etching), for specific positioning of the dots. In fig. 6, a two-dimensional array of aligned 85 nm diameter self-assembled dome-type Ge islands is shown on the tops of lithographically patterned Si(001) mesas with a 140 nm period. A "one island on one mesa" relationship is clearly achieved. This density of islands is higher than can normally be produced on unpatterned silicon, where island coalescence usually occurs well before this density is possible. Preferential growth on the tops of the mesas most likely occurs because the Si mesa tops are deformable, fulfilling a strain relaxation condition. In this work, pyramid-type islands as small as 25 nm are also aligned on the mesa tops, and no limit to the size reduction of the islands is apparent, being controlled mainly by the size of the etched features that can be introduced. (S.R. Leone)

  Figure 6. 2D array of self-assembled Ge dots on lithographically patterned Si(001). Deposition of 6 ML Ge at 850 K. (a) 2 µm × 2 µm AFM image, showing one island at the center of each mesa. (b) 3D AFM image showing the uniformity of the dome structures. (c) Contour map showing the square base of one island in the [110] direction. The base direction is independent of the lithographic grid direction selected. (d) Side facets [114] of a Ge island on a mesa.

• New Instrument to Study Growth of InGaN Materials. A new instrument has been fabricated and developed for the growth of InGaN materials. This apparatus has an in situ scanning tunneling microscope to interrogate the growing samples for the formation of three dimensional InN islands. The formation of these islands is crucial to the high efficiency of these materials for light emission, because the islands act as highly effective traps for carriers, despite large numbers of defects in the material itself. The ability to grow the material with uniform island sizes and spatial distributions will be a key aspect of new work to be undertaken. (S.R. Leone and S.T. Cundiff)

• Simultaneously Trapping and Cooling of Bosons and Fermions. An apparatus is being built to simultaneously trap and cool a mixture of atoms that are bosons and atoms that are fermions. With this capability we will be able to investigate new phenomena in a mixed Bose-Fermi quantum gas. We have demonstrated the first two-species ⁸⁷Rb-⁴⁰K magneto-optical trap. This will serve as the initial stage for cooling the mixture to quantum degeneracy. We have loaded the precooled atoms into a purely magnetic trap. In the near future, the magnetically confined gas will be moved to a higher-vacuum portion of the chamber where it will be further cooled via forced evaporation and sympathetic cooling. (D.S. Jin)

• Exploring a Quantum Degenerate Gas of Fermionic Atoms. An ultracold, degenerate Fermi gas of atoms provides a unique quantum system in which to explore the impact of Fermi-Dirac statistics. Using ⁴⁰K atoms in two different spin-states, we have experimentally realized an interacting Fermi gas and have begun exploring the dynamical behavior of this new system. An exciting prospect for the two-component Fermi gas is the possibility of a phase transition to a superfluid state, whose underlying physics is similar to superconductivity in metals. To explore this possibility we are investigating a predicted magnetic-field Feshbach resonance, which would allow experimental control of the interactions between atoms. The Feshbach resonance is predicted for spin states that cannot be held in a typical magnetic trap. Therefore we have loaded the ultracold gas into a far-off-resonant optical trap and have used RF adiabatic rapid passage to transfer the atoms to the appropriate spin states. We can now apply a uniform magnetic field of up to 250 × 10^-4 tesla (250 Gs) and have seen preliminary evidence for the Feshbach resonance. (D.S. Jin)

• Apertureless Near Field Scanning Optical Microscopy. With the ever decreasing scale of electronic components and chip design, there is an ever increasing need to develop efficient optical methods to measure properties of nanoscale objects with sizes well below the diffraction limit of light. There have been considerable breakthroughs in this area based on near field scanning optical microscopy (NSOM), which has conventionally been achieved by using metal coated optical fiber tips to confine the light in rapidly tapered optical fibers. However, this method is inherently limited by the skin depth of light in the metal cladding (12 nm for Al), which even for optimum cases yields only 20 nm to 30 nm resolution, and under typical operating conditions, more like 100 nm resolution. An alternative we actively explored was to develop new methods in apertureless near field scanning optical microscopy, which already have demonstrated resolution improvements down to the 2 nm to 3 nm length scale. This effort is based on a combination of (i) evanescent wave excitation of molecules/nanostructures on a prism surface, (ii) sharp Si or Ag coated Si structures guided by atomic force microscopy (AFM) to condense the evanescent electric fields in the vicinity of the tip, and (iii) sensitive resonant scattering light and/ or fluorescence detection of the molecules subsequent to the excitation event. (A.C. Gallagher and D.J. Nesbitt)

• Scattering/Extinction Near Field Microscopy. As a necessary first effort, these new apertureless NSOM methods have been used to evaluate the scattering cross sections of AFM tips in the presence of an evanescent wave as a function of tip distance above the surface. This has yielded the first absolute cross sections for near-to-far field scattering with well-characterized probe shapes, which are crucial in providing "benchmark" data for testing quantitative models of near field interactions. These methods were next used to obtain near field images of Au nanospheres at < 5 nm resolution by resonant scattering of 543 nm light near the plasmon resonance. These studies determined that the combination of AFM tip + particle leads to a scattering enhancement of over 4000-fold from that of the bare Au nanospheres. Such data again provides important benchmarks against which to test theoretical models. (D.J. Nesbitt)

• Fluorescence Near Field Microscopy. As a most recent achievement, these new apertureless NSOM methods have been extended into the domain of near field fluorescence microscopy domain, where the Si AFM tips are used to influence the near field excitation of dye doped nanospheres (on the 20 nm scale) or even down to semiconductor CdSe quantum dots (6 nm scale). The resulting fluorescence is imaged with a high numerical aperture microscope and fiber coupled avalanche photodiode combination. The data indicates substantial increase in the fluorescence from these molecules in the near field presence of the laser excited AFM tip, with spatial resolution that appears to be fully limited by the 5 nm tip radius of curvature. A simple electrostatic analysis of these data is consistent with a roughly 30-fold enhancement of electric field at the tip, which translates into a nearly 10³-fold enhancement of the near field laser intensity. (D.J. Nesbitt and A.C. Gallagher)

• Near-Field Theoretical Modeling. A new program in theoretical modeling of the apertureless NSOM data has obtained results based on modeling of the AFM tip, prism surface, and evanescent laser field by discrete electrostatic multipoles, and matching boundary conditions at the respective interfaces with least squares methods. Most importantly, these calculations can be converged for realistic elongated tip shapes that incorporate the lightening rod antenna effect and the actual exponential drop off of the evanescent fields. The results indicate (i) a significant enhancement of the fields near the tip, (ii) a strong sensitivity to the length of the tip elongation, and (iii) a limiting value of the field enhancement of   30 for tip lengths greater than the 1/e evanescent decay. This value is in remarkably good agreement with what is observed experimentally. Furthermore, this analysis indicates a significant contribution to near field enhancement from "image dipoles" generated in the prism when the laser polarized AFM tip approaches within approximately one tip radius (5 nm) of the surface ("Narscissus effect"). (D.J. Nesbitt)

• Single Molecule and Single Quantum Dot Confocal Microscopy. Spawned in conjunction with the NSOM efforts, we have been developing new capabilities in high sensitivity detection and spectroscopic characterization of single molecules based on laser excitation and fluorescence detection via scanning confocal microscopy, coupled with high sensitivity avalanche detection and/or a CCD array spectrometer. These methods have been used to investigate the photophysical dynamics of individual nanostructures, in particular focusing on ZnS overcoated CdSe and InP quantum dots on fused silica and glass surfaces. Particularly relevant has been the detailed studies of fluorescence intermittency or "blinking" of individual quantum dots, which begin to provide detailed kinetic information on electron hole pair ejection and recombination from single quantum dot structures. The distribution of time scales over which these ejection/recombination events occur spans an enormous dynamic range (microseconds to minutes), which can be statistically analyzed to show that the kinetics is intrinsically non-exponential, with "rates" varying over 5-6 orders of magnitude. (D.J. Nesbitt)

• Single Molecule Microscopy and Characterization of Biomolecules. The new single molecule microscopy capability described above has been exploited in new directions that involve biophysical applications. We have utilized confocal methods to study green fluorescent protein (GFP), and initiated studies of intercalation kinetics of fluorescent dye molecules into DNA strands tethered to surfaces by biotin-streptavidin interactions. We are also developing tools based on wide field illumination and cooled CCD array cameras to image electrophoresis of single biomolecules in agarose gels. Finally, we are building up new time resolved capabilities for single biomolecule fluorescence detection in liquids utilizing polarized fluorescence resonant energy transfer (FRET) and burst integrated fluorescence (BIFL) methods. Monitoring the frequencies, polarization, and fast time correlations between the fluorescence photons emitted will allow one to watch relative distance and orientation of fluorescent donor/ acceptor "tagged" biomolecules, and thereby probe kinetics of conformational changes in real time. (D.J. Nesbitt)

• Reaction Dynamics at the Gas-Liquid Interface. A major fraction of chemical reactions of industrial, commercial, and environmental importance occur at the gas-liquid interface, ranging from atmospheric processing of ozone to rapid detonation in internal combustion engines. Despite this importance, the tools for exploring reaction dynamics at the gas-liquid interface are remarkably limited. A new program of study based on direct IR laser absorption to investigate quantum-state-resolved dynamics of reactions at the gas-liquid interface has been initiated. The first test system has been F atom reactive scattering from long chain liquid hydrocarbons (essentially low vapor pressure pump oils), which leads to HF(v,J) formation that is detected via direct IR laser absorption. Since the F atom source is pulsed and the HF detection is time resolved, this permits the dynamics of "direct" reactive scattering vs slower temporarily trapped interfacial reactions to be distinguished with full quantum state resolution of the products. (D.J. Nesbitt)

• High Resolution IR Laser Spectroscopy in Plasmas. The overwhelming majority of all chemical processes occur via fast bimolecular reactions involving molecular species with unpaired electrons, i.e., free radicals. The reactivity of these species makes them especially hard to detect and study in detail. Of special interest are methods of spectroscopically characterizing these radicals at low temperature and high resolution, which is a crucial prerequisite for laser remote sensing of these species in the much more complicated "real world" environments of plasmas, combustion, etching, and so on. We are developing methods to combine the advantages of (i) low temperature expansions in slit supersonic jets with (ii) pulsed electrical discharges to produce unprecedentedly intense sources of molecular radicals under the low temperature (10 K to 20 K) and long absorption path environment ideally suited for direct absorption laser spectroscopy. We exploit this source for high resolution spectroscopic investigations of open shell hydrocarbon species (such as ethyl radical), as well as jet cooled molecular ions (such as protonated H₂, CH₄, and HCCH). As one example, vibrational spectra of H₃⁺, H₂D⁺, HD₂⁺, and D₃⁺ in the adiabatic approximation all derive from motion on the same Born Oppenheimer potential surface, which for a two-electron problem can be calculated to near spectroscopic accuracy. Comparison of high resolution spectra for these different mass combinations therefore provides direct evidence for non-adiabatic, Born Oppenheimer breakdown in the various isotopic mass combinations (D.J. Nesbitt)

• State-to-State Reaction Dynamics. Chemistry is a discipline of enormous technological and economic importance and yet a detailed understanding of how the simplest of chemical reactions occur still represents a state-of-the-art area of experimental and theoretical chemical physics research. We are making major advances in this area by exploiting (i) slit discharge technology as an intense source of jet cooled radicals and (ii) shot-noise-limited infrared laser absorption in order to explore quantum state-to-state reactive scattering studies in crossed supersonic jets. The initial focus has been on hydrogen abstraction reactions F + RH  HF(v,J) + R, where the nascent rovibrational product states of HF can be sensitively detected via direct absorption methods. The crucial advantage of such a high resolution laser-based approach is that it offers 10^-4 cm^-1 spectral resolution on the product states, which is > 10⁵ to 10⁶ fold better than previous crossed beam time-of-flight studies. These high resolution studies reveal major dynamical insights even for "simple" atom + diatom systems. For example, F + HD product state distributions have yielded direct evidence for Feshbach resonances corresponding to fleetingly bound vibrational states of the FHD complex with the H atom bouncing back and forth between the F and D atoms. These so called "transition state resonances" have been theoretically predicted but proven extremely elusive to detect experimentally without full quantum-state-resolved reactive-scattering methods. (D.J. Nesbitt)

• Atmospheric and Environmental Chemistry. There has been an ongoing effort towards reaction dynamics relevant to atmospheric chemistry, direct IR laser absorption of highly reactive radicals. One important target has been chemical chain reaction depletion of ozone in the lower stratosphere, based on the ubiquitous reactions between OH, HO₂ and O₃, which is studied by excimer laser photolytic generation of OH radicals in a fast flow reactor, with time-resolved high resolution IR laser absorption over a long flow path length as a direct measure of OH, HO₂ radical concentrations and therefore the relevant rate kinetics. Most importantly, these IR based methods completely circumvent problems due to UV photolysis of ozone, which plagues conventional LIF detection methods for OH radicals. Consequently, this IR laser apparatus uniquely permits such reactions to be explored down to temperatures crucially relevant to the stratospheric models of the atmosphere. Other projects on this apparatus include studies of formation and energy transfer of highly rotationally excited OH(v,N  30), whose remarkable and highly non-equilibrium presence has been recently detected via IR emission from the upper atmosphere. (D.J. Nesbitt)

  + Particle Growth in Deposition Discharges. Gas discharges are frequently used to deposit thin films, for purposes as diverse as making optical or protective coatings, or producing semiconductors. Silicon-based semiconducting films, used for liquid-crystal displays, copiers, paper readers, and photovoltaics, are normally deposited from discharges in (primarily) silane gas. Silicon particles are produced in the plasma region of these discharges, and they can have a major influence on the discharge and device properties. We have developed the first quantitative model for the cause, speed, and efficacy of this particle growth, as well as for how the growth depends on discharge parameters. This model has been guided by, and has explained, laboratory data on 10 nm to 50 nm diameter particles, as well as data from another laboratory that detected the very light negative ions (Si_nH_m- , n < 10) that initiate the particle growth. In both experiment and theory, thermophoresis has been shown to be a dominant mechanism controlling particle growth and survival in deposition discharges. This improved understanding has important implications for the design of thin-film coaters, especially those used for Si-based films. (A.C. Gallagher)

• Controlled Phase Coherence between Independent Femtosecond Lasers. This recent breakthrough has enabled us for the first time ever to phase-lock together two independent mode-locked lasers. By creating coherence between two mode-locked lasers, we open the door to a wide-reaching variety of applications. For example, in applications where quantum coherent control is desired, coherent light may be needed in several disparate regions of the optical spectrum. However, current broad-band ultrafast laser systems generate a fractional bandwidth on order of a mere 30%. This bandwidth may be increased using various non-linear techniques, but poor conversion efficiency limits its real-life application. Our approach enables the production of different wavelengths of light most efficiently and cleanly by using completely independent laser systems. The output from the laser systems can now be synchronized and phase-locked, resulting in a coherent composite field at exactly the frequencies of interest, with independent control of respective powers and relative optical phases, frequency chirps, and so on. One specific example of interesting science enabled by our breakthrough is to explore the interplay of two distinct regimes of quantum dynamics in a molecular system where one laser (red part of the spectrum) manipulates the ground state potential surface while another laser (blue part of the spectrum) controls the electronic wave function that would in turn favorably influence the ground state motion. Such exploration of two distinct regimes of quantum dynamics and of their mutual influence under a controlled fashion will surely be a most interesting part of quantum coherent control studies. (J. Ye)

• Pulse Shortening by Coherent Stitching of Separate Spectra for Different Lasers. Current pulse-shaping techniques are limited primarily by two things: available coherent bandwidth, and the bandwidth over which group-velocity dispersion may be managed. By using independent mode-locked lasers with different center wavelengths, dispersion in each laser system can be managed separately, and the various beams can then be combined, synchronized, and phase-locked to create the shortest pulse. Alternatively, dispersion over the bandwidth could be manipulated to create an arbitrarily shaped optical waveform, paving the way for an ultimate '"optical waveform synthesizer" machine. An important issue is to demonstrate control over the phase profile across the entire synthesized spectrum, namely pulse shaping. For example, a flat spectral phase profile is a prerequisite for generation of ultrashort pulses, while an arbitrary desired profile will be needed for coherent control applications. Under the current control scheme, we will be able to perform arbitrary pulse shaping over the entire phase coherent spectrum by controlling not only the phase slips between the two lasers but also their relative absolute phases. (J. Ye)

• Optical Frequency Metrology. Recent work has shown that by broadening the output of a Ti:sapphire laser with a microstructure optical fiber, a comb consisting of millions of stabilized CW lines can be generated. This comb is limited in extent because of the nature of the non-linear process used for its production. Our new phase-locking technology can be extended to coherently lock together as many of these laser combs as desired, using different laser mediums as required. Other applications include mid-infrared light generation through difference frequency mixing, novel pump-probe experiments requiring synchronized laser light, and x-rays or electron beams from synchrotrons, particle accelerators with phase locked pulsed laser arrays, and so on. A recent development in our lab has reduced the timing jitter between two independent lasers to be below 1 fs, an important benchmark for further work on phase coherence of fs lasers. (J. Ye)

• Demonstration of an Optical Clock. The first experimental success of a reliable clock operation based on an optical frequency standard has been demonstrated. This breakthrough is brought about by the recent merger of precision frequency metrology and ultrafast science. We have pushed the frontier to the next level where a single stabilized cw laser controls the entire optical comb over more than an octave bandwidth at a precision level of 10^-15. We have derived an rf clock signal of comparable stability to the optical standard itself (5 ´ 10^-14 at 1-s) over an extended period. We have effectively demonstrated that with an appropriately chosen optical standard, we can establish an optical frequency grid with lines repeating every 100 MHz over an octave bandwidth and with every line stable at the one Hz level. And it is clear now that with a mature technical solution to the "gearbox problem" at hand, all future progress in optical and rf domain standards can be utilized in both spectra. Optical frequency standards now have truly become general purpose laboratory tools for physicists across many different disciplines. Some specific applications include tests on gravitational physics, searches for time-dependent variation of fundamental physical constants, precision atomic and molecular spectroscopy, and the combined time and length metrology. Furthermore, our work shows the first experimental demonstration of true orthogonal control of the wide-bandwidth optical comb. This important technological achievement brings the entire femtosecond comb under tight control and makes it a reliable tool. This accomplishment in the clockwork mechanism will impact any future development of optical clocks. (J. Ye)

• Ultrahigh Resolution Spectroscopy Using Femtosecond Laser. Theoretical work has explored the aspect of novel high resolution spectroscopy employing a femtosecond laser. A phase-coherent wide-bandwidth optical comb is shown to induce the desired multi-path quantum interference effect for the resonantly enhanced two-photon transition rate. The analysis carried out in both the frequency and time domains makes the fundamental connection between the two physical pictures. Our calculation has provided a solid link between the time domain carrier-envelope phase and the frequency domain carrier-frequency offset. The consequence of these results was shown in terms of absolute control of both degrees of freedom for the fs comb, namely the comb spacing and the carrier offset frequency. The multi-pulse interference in the time domain gives an interesting variation and generalization of the two-pulse based temporal coherent control of the excited state wavepacket. (J. Ye)

• Manipulation of Molecular Wavepackets. We use precision control of independent femtosecond lasers to develop a general purpose laboratory tool for coherent quantum control in manipulation of molecular wavepackets. A project is underway to explore the use of cold molecules in ultrahigh resolution spectroscopy and in the study of molecular dynamics. Femtosecond-laser-based precision molecular spectroscopy is expected to yield information on molecular structure 1000 times more accurate than previous results. (J. Ye)

• Precision Spectroscopy of Atoms and Molecules. We are pursuing precision spectroscopy on cold samples of Rb atoms. An optical frequency standard based on a two-photon transition in the cold Rb atoms will be established. We are also investigating coherent interactions between a stable ultrafast laser and the cold atoms. A broadly tunable precision spectrometer has been built to study the unexplored regions of the iodine spectrum near its dissociation limit. We have discovered new and interesting behavior in the change of the hyperfine interactions of the iodine molecules. Many new transitions have been identified with excellent potential of being the next generation simple optical frequency standards. (J. Ye)

• Optical Frequency Measurement. During the past year the femtosecond method of optical frequency measurement has been brought to an astounding level of robustness, convenience, and accuracy. We have developed one excellent method for orthogonalizing the control of the fs laser's two frequency parameters, the pulse repetition rate and the carrier/envelope phase slip rate, otherwise called the carrier/envelope offset frequency. This method allows us to completely transfer to the fs laser comb the attained stability of a stable optical frequency reference source, in our case the Nd:YAG laser stabilized to a strong, narrow molecular Iodine resonance. With colleagues in NIST Time and Frequency Division, we have compared the frequency of an rf signal derived from our optical source directly with the NIST Hydrogen maser signal which reaches us from NIST by the Boulder Regional Area Network (the BRAN optical fiber). The frequency variations for times less than ~100 s are just equal to the stated uncertainty of the Hydrogen maser: thus we are entering the domain where optical sources provide frequency stability completely equal or superior to that obtainable with traditional high-end microwave approaches. (J.L. Hall)

• New, Small, Absolute, and Highly Portable Gravity Instrument. Dramatic progress was made during the past year in regards to this new, cam-based absolute gravimeter. This small, fast, portable, and simple-to-use instrument uses a rotating cam to create the following: a release, a 2 cm free-fall drop, a soft catch, and then a return to the starting position--with all of this requiring only 0.3 seconds. The instrument inherently yields a high measurement rate (3.3 drops per second) that compensates-in terms of measurement accuracy -for the relatively short dropping length. Furthermore, by employing a second co-rotating cam to drive an auxiliary mass it is possible to keep the instrumental center-of-mass fixed. In this way recoil effects having to do with both the release of the mass and the accelerations of the mass-carrying carts can be, in principle, completely eliminated. Measurement results obtained during this past year are extremely encouraging. Data taken at a good site (Table Mountain) was reproducible at the several times 10^-8 m/s of precision. Data taken at a poor site (bad floor and noisy environment) was reproducible at the 10^-7 m/s level. We are presently looking at the effects of misalignments in the mechanical system which might explain this performance. (J.E. Faller)

• Newtonian Constant of Gravitation. Our approach to measuring this fundamental constant involves measuring small length changes by sensing frequency changes in a laser that is locked to a Fabry-Perot cavity whose mirrors are located in two freely hanging masses. This laser is in turn beat with another laser that is locked to a fixed cavity that monitors the separation between the suspension points of the two hanging Fabry-Perot mirrors. When a 500 kg mass of tungsten is moved and appropriately positioned nearby, it deflects the freely hanging (70 cm long) pendulums. G can then be determined by combining the measured small frequency-sensed changes in this cavity's length with carefully made measurements of the experiment's geometry.

  During the past year, assembly of the apparatus was completed and a considerable amount of data was taken. Preliminary measurements indicate that -- subject to our understanding of all systematic errors -- an uncertainty of below 5 parts in 10⁵ can be obtained. A number of as yet to be understood systematic effects have however appeared in addition to strong evidence of a magnetic contaminant in one of the two freely hanging masses. The plan is first to address the systematic effects [which should produce an already interesting experimental accuracy for this constant (approaching 1 in 10⁴)] and then to replace the contaminated mass and take additional data with the anticipation that an additional factor of 2 or 3 improvement in accuracy will result. (J.E. Faller)