Recent Results
abstracts from recent papers, grouped by subject area
Astro-comb
A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s-1, which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earthmass planet in an Earth-like orbit, a precision of 5 cm s-1 is necessary. The combination of a laser frequency comb with a Fabry–Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (~1-Å) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-linespacing comb, or "astro-comb", is well matched to the resolving power of high-resolution astrophysical spectrographs. The astrocomb should allow a precision as high as 1 cm s-1 in astronomical radial velocity measurements.
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Atomic Clocks and Fundamental Symmetries
Comparison of 87Rb N-resonances for D1 and D2 transitionsWe report an experimental comparison of three-photon-absorption resonances (known as "N-resonances") for the D1 and D2 optical transitions of thermal 87Rb vapor. We find that the D2 N-resonance has better contrast, a broader linewidth, and a more symmetric lineshape than the D1 N-resonance. Taken together, these factors imply superior performance for frequency standards operating on alkali D2 N-resonances, in contrast to coherent population trapping (CPT) resonances for which the D2 transition provides poorer frequency standard performance than the D1 transition.
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Cancellation of light-shifts in an N-resonance clock
We demonstrate that first-order light-shifts can be cancelled for an all-optical, three-photon- absorption resonance ("N-resonance") on the D1 transition of 87Rb. This light-shift cancellation enables improved frequency stability for an N-resonance clock. For example, using a table-top apparatus designed for N-resonance spectroscopy, we measured a short-term fractional frequency stability (Allan deviation) 1.5 × 10-11 τ -1/2 for observation times 1s ≤ τ ≤ 50s. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated N-resonance clock.
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Three-photon-absorption absorption resonance for all-optical atomic clocks
We report an experimental study of an all-optical three-photon-absorption resonance known as an "N-resonance" and discuss its potential application as an alternative to atomic clocks based on coherent population trapping. We present measurements of the N-resonance contrast, width and light shift for the D1 line of 87Rb with varying buffer gases, and find good agreement with an analytical model of this resonance. The results suggest that N-resonances are promising for atomic clock applications.
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Modulation induced frequency shifts in a CPT-based atomic clock
We investigate systematic errors associated with a common modulation technique used for phase sensitive detection of a coherent population trapping (CPT) resonance. In particular, we show that modification of the CPT resonance lineshape due to the presence of off-resonant fields leads to frequency shifts which may limit the stability of CPT-based atomic clocks. We also demonstrate that an alternative demodulation technique greatly reduces these effects.
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Bound on Lorentz- and CPT-Violating Boost Effects for the Neutron
A search for a sidereal annual variation in the frequency difference between co-located 129Xe and 3He Zeeman masers sets a stringent limit on boost-dependent Lorentz and CPT violation involving the neutron, consistent with no effect at the level of 10-27 GeV.
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Testing Lorentz and CPT symmetry with hydrogen masers
We present details from a recent test of Lorentz and CPT symmetry using hydrogen masers. We have placed a new limit on Lorentz and CPT violation of the proton in terms of a recent standard model extension by placing a bound on sidereal variation of the F = 1 Zeeman frequency in hydrogen. Here, the theoretical standard model extension is reviewed. The operating principles of the maser and the double resonance technique used to measure the Zeeman frequency are discussed. The characterization of systematic effects is described, and the method of data analysis is presented. We compare our result to other recent experiments, and discuss potential steps to improve our measurement.
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Measurement of the 21Ne Zeeman frequency shift due to Rb-21Ne collisions
Using a 21Ne-3He Zeeman maser, we compared the frequency shift of the 21Ne nuclear Zeeman resonance induced by polarized Rb vapor to the shift induced in the 3He nuclear Zeeman resonance. The 3He-Rb shift has recently been measured with high precision [M.V. Romalis and G.D. Cates, Phys. Rev. A 58, 3004 (1998)], permitting the conversion of our differential measurement to an absolute value for the 21Ne-Rb shift. We report a value of 21 = 32.0±2.9 for the Rb-21Ne contact shift enhancement factor at a temperature of approximately 128 °C. We also propose high-precision differential contact shift measurements, the absolute accuracy of which could be limited by the error in the 3He contact shift.
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Limit on Lorentz and CPT violation of the proton using a hydrogen maser
We present a new measurement constraining Lorentz and CPT violation of the proton using a hydrogen maser double resonance technique. A search for hydrogen Zeeman frequency variations with a period of the sidereal day (23.93 h) sets a clean limit on violation of Lorentz and CPT symmetry of the proton at the 10-27 GeV level.
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Limit on Lorentz and CPT violation of the neutron using a two-species noble-gas maser
We searched for sidereal variations in the frequency difference between co-located 129Xe and 3He Zeeman masers, setting the most stringent limit to date on leading-order Lorentz and CPT violation involving the neutron, consistent with no effect at the level of 10-31 GeV.
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Double resonance frequency shift in a hydrogen maser
We used the dressed atom formalism to calculate the frequency shift in a hydrogen maser induced by applied radiation near the Zeeman frequency, and found excellent agreement with a previous calculation made in the bare atom basis. The maser oscillates on the F = 1, mF = 0 to F = 0, mF = 0 hyperfine transition, while the applied field is swept through the F = 1 Zeeman resonance. We determined the effect of the applied field on the Zeeman levels using the dressed atom picture, and then calculated the maser frequency shift by coupling the dressed states to the microwave cavity. Qualitatively, the dressed-atom analysis gives a new and simpler physical interpretation of this double resonance process, which has applications in precision hydrogen Zeeman spectroscopy, e.g., in fundamental symmetry tests.
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Slow and Stored Light
Optimizing slow and stored light for multidisciplinary applicationsWe present a preliminary experimental study of the dependence on optical depth of slow and stored light pulses in Rb vapor. In particular, we characterize the efficiency of slow and stored light as a function of Rb density; pulse duration, delay and storage time; and control field intensity. Experimental results are in good qualitative agreement with theoretical calculations based on a simplified three-level model at moderate densities.
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Slow light with integrated gain and large pulse delay
We demonstrate slow and stored light with a combination of desirable features: minimal loss and distortion of the pulse bandwidth and shape, and large fractional delay (> 10). This behavior is enabled by a medium with two key characteristics: (i) a group index that can be controllably varied during light pulse propagation, which allows large fractional pulse delay and correction for pulse distortion; and (ii) controllable gain integrated into the medium to compensate for pulse loss during propagation. The technique is general: any medium with the above two characteristics should be able to realize similarly high-performance slow light. The particular realization presented here involves a dynamic form of electromagnetically induced transparency (EIT) in warm Rb vapor, with the group index varied by an optical control field and gain provided by polarization self-rotation.
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Optimal control of light pulse storage and retrieval
We demonstrate experimentally a procedure to obtain the maximum efficiency for the storage and retrieval of light pulses in atomic media. The procedure uses time reversal to obtain optimal input signal pulse-shapes. Experimental results in warm Rb vapor are in good agreement with theoretical predictions and demonstrate a substantial improvement of efficiency. This optimization procedure is applicable to a wide range of systems.
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Optimization of slow and stored light in atomic vapor
We present a preliminary experimental study of optimized slow and stored light pulses in Rb vapor cells. We study the efficiency of light storage as a function of pulse duration, storage time, retrieval field intensity, etc. We demonstrate a procedure based on time reversal for the optimization of the efficiency for storage of light in atomic ensembles suggested in a recent theoretical paper [A.V. Gorshkov et al., e-print archive quant-ph/0604037 (2006)]. Experimental results are in a good qualitative agreement with theoretical calculations based on a simplified three-level model.
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Slow light in paraffin-coated Rb vapor cells
We present preliminary results from an experimental study of slow light in anti- relaxation-coated Rb vapor cells, and describe the construction and testing of such cells. The slow ground state decoherence rate allowed by coated cell walls leads to a dual-structured electromagnetically induced transparency (EIT) spectrum with a very narrow (< 100 Hz) transparency peak on top of a broad pedestal. Such dual-structure EIT permits optical probe pulses to propagate with greatly reduced group velocity on two time scales. We discuss ongoing efforts to optimize the pulse delay in such coated cell systems.
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Diffusion-induced Ramsey narrowing
Diffusion-induced Ramsey narrowing is characterized and identified as a general phenomenon, in which diffusion of coherence in and out of an interaction region such as a laser beam induces spectral narrowing of the associated resonance line shape. Illustrative experiments and an intuitive analytical model are presented for this spectral narrowing effect, which occurs commonly in optically interrogated atomic systems and may also be relevant to quantum dots and other solid-state spin systems.
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EIT and diffusion of atomic coherence
We study experimentally the effect of diffusion of Rb atoms on Electromagnetically Induced Transparency (EIT) in a buffer gas vapor cell. In particular, we find that diffusion of atomic coherence in-and-out of the laser beam plays a crucial role in determining the EIT resonance lineshape and the stored light lifetime.
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Optimizing stored light efficiency in vapor cells
We present a preliminary experimental study of slow and stored light in Rb vapor cells under the conditions of electromagnetically induced transparency (EIT). We study the efficiency of light storage as a function of pulse duration, storage time, retrieval field intensity, etc. We demonstrate that atomic diffusion in-and-out of the laser beam plays an important role not well described by previous analysis.
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Quantum control of light using electromagnetically induced transparency
We present an overview of recent theoretical and experimental work on the control of the propagation and quantum properties of light using electromagnetically induced transparency in atomic ensembles. Specifically, we discuss techniques for the generation and storage of few-photon quantummechanical states of light as well as novel approaches to manipulate weak pulses of light via enhanced nonlinear optical processes.
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Dynamic Optical Bistability in Resonantly Enhanced Raman Generation
We report observations of novel dynamic behavior in resonantly-enhanced stimulated Raman scattering in Rb vapor. In particular, we demonstrate a dynamic hysteresis of the Raman scattered optical field in response to changes of the drive laser field intensity and/or frequency. This effect may be described as a dynamic form of optical bistability resulting from the formation and decay of atomic coherence. We have applied this phenomenon to the realization of an all-optical switch.
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Atomic Memory for Correlated Photon States
We experimentally demonstrate emission of two quantum-mechanically correlated light pulses with a time delay that is coherently controlled via temporal storage of photonic states in an ensemble of rubidium atoms. The experiment is based on Raman scattering, which produces correlated pairs of spin-flipped atoms and photons, followed by coherent conversion of atomic states into a different photon beam after a controllable delay. This resonant nonlinear optical process is a promising technique or applications in quantum communication.
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Towards non-classical light storage via atomic-vapor Raman scattering
We present experimental work that investigates whether quantum information carried by light can be stored via recoverable mapping of the quantum state of such ligh onto a collective atomic coherence. Such a quantum memory could be utlizied to allow quantum communication over long, lossy channels. Current efforts concentrate on writing a photon-number-squeezed state of light onto a collective coherence between the ground-state hyperfine levels of 87Rb atoms in a warm vapor cell, and subsequent on-demand rerieval of this light. In this approach, intensity squeezing between the written and retrieved photon felds provides evidence for storage of a photon-number-squeezed state of light. The scheme is based on spontaneous Raman transitions that create the atomic coherence, and at the same time convert control fields into signal fields that propogate under conditions of electromagnetically induced transparency. We present experimental results demonstrating the storage and retrieval of light using this method, and discuss techniques for measuring intensity squeezing between these photon fields.
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Phase coherence and control of stored photonic information
We report the demonstration of phase coherence and control for the recently developed "light storage" technique. Specifically, we use a pulsed magnetic field to vary the phase of atomic spin excitations which result from the deceleration and storing of a light pulse in warm Rb vapor. We then convert the spin excitations back into light and detect the resultant phase shift in an optical interferometric measurement. The coherent storage of photon states in matter is essential for the practical realization of many basic concepts in quantum information processing.
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Storage of light in atomic vapor
We report an experiment in which a light pulse is effectively decelerated and trapped in a vapor of Rb atoms, stored for a controlled period of time, and then released on demand. We accomplish this "storage of light" by dynamically reducing the group velocity of the light pulse to zero, so that the coherent excitation of the light is reversibly mapped into a Zeeman (spin) coherence of the Rb vapor.
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MRI, Biomedical Imaging and Physiology
An Open-Access, Very-Low-Field MRI System for Posture-Dependent 3He Human Lung ImagingWe describe the design and operation of an open-access, very-low-field, magnetic resonance imaging (MRI) system for in-vivo hyperpolarized 3He imaging of the human lungs. This system permits the study of lung function in both horizontal and upright postures, a capability with important implications in pulmonary physiology and clinical medicine. The imager uses a bi-planar B0 coil design that produces an optimized 65 G (6.5 mT) magnetic field for 3He and 1H MRI at 210 kHz and 275 kHz, respectively. Three sets of bi-planar coils produce the x, y, and z magnetic field gradients while providing a 79-cm inter-coil gap for the imaging subject. We use solenoidal Q-spoiled RF coils for operation at low frequencies, and are able to exploit insignificant sample loading to allow for pre-tuning/matching schemes and for accurate pre-calibration of flip angles. We obtain sufficient SNR to acquire 2D 1H and 3He images with up to 2.8 mm resolution, as well as initial 2D and 3D 3He images of human lungs in both supine and upright orientations.
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Posture-Dependent Human 3He Lung Imaging in an Open Access MRI System: Initial Results
The human lung and its functions are extremely sensitive to orientation and posture, and debate continues as to the role of gravity and the surrounding anatomy in determining lung function and heterogeneity of perfusion and ventilation. However, study of these effects is difficult. The conventional high-field magnets used for most hyperpolarized 3He MRI of the human lung, and most other common radiological imaging modalities including PET and CT, restrict subjects to lying horizontally, minimizing most gravitational effects. In this paper, we briefly review the motivation for posture-dependent studies of human lung function, and present initial imaging results of human lungs in the supine and vertical body orientations using inhaled hyperpolarized 3He gas and an open-access MRI instrument. The open geometry of this MRI system features a "walk-in" capability that permits subjects to be imaged in vertical and horizontal positions, and potentially allows for complete rotation of the orientation of the imaging subject in a two-dimensional plane. Initial results include two-dimensional lung images acquired with ~ 4 mm in-plane resolution and three-dimensional images with ~ 1.5 cm slice thickness. Effects of posture variation are observed.
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A System for Open-Access 3He Human Lung Imaging at Very Low Field
We describe a prototype system built to allow open-access very-low field MRI of human lungs using laser-polarized 3He gas. The system employs an open four-coil electromagnet with a maximum B0 field of 7 mT, and planar gradient coils that generate gradient fields up to 0.18 G/cm in the x and y direction and 0.41 G/cm in the z direction. This system was used to obtain 1H and 3He phantom images and supine and upright 3He images of human lungs. We include discussion on challenges unique to imaging at 50–200 kHz, including noise filtering and compensation for narrow-bandwidth coils.
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3He Lung Imaging in an Open Access, Very-Low-Field Human MRI System
The human lung and its functions are extremely sensitive to gravity; however, the conventional high-field magnets used for most laser-polarized 3He MRI of the human lung restrict subjects to lying horizontally. Imaging of human lungs using inhaled laser-polarized 3He gas is demonstrated in an open-access very-low-magnetic-field (< 5 mT) MRI instrument. This prototype device employs a simple, low-cost electromagnet, with an open geometry that allows variation of the orientation of the imaging subject in a two-dimensional plane. As a demonstration, two-dimensional lung images were acquired with 4-mm in-plane resolution from a subject in two orientations: lying supine and sitting in a vertical position with one arm raised. Experience with this prototype device will guide optimization of a second-generation very-low-field imager to enable studies of human pulmonary physiology as a function of subject orientation.
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Reduced xenon diffusion for quantitative lung study - the role of SF6
In restricted environments, such as the lung, rapid gas diffusion can lead to violations of the narrow pulse approximation, a basic assumption of the standard Stejskal-Tanner NMR method of diffusion measurement. We therefore investigated the effect of a common, biologically inert buffer gas, sulfur hexafluoride (SF6), on 129Xe NMR and diffusion. We found modest contribution of SF6 to 129Xe T1 and T2 relaxation. We also measured the coefficient of 129Xe diffusion through SF6 to be approximately 4.6x10-6 m2/s for 1 bar pressure of SF6 and standard temperature. From these measurements we conclude that SF6 will not sufficiently reduce 129Xe diffusion to allow accurate surface-area/volume ratio measurements in human alveoli using time-dependent gas diffusion NMR.
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Magnetic resonance imaging of convection in laser-polarized xenon
We demonstrated NMR imaging of the flow and diffusion of laser-polarized xenon gas undergoing convection above evaporating laser-polarized liquid xenon. The large xenon NMR signal provided by the laser-polarization technique allows more rapid imaging than one can achieve with thermally polarized gas-liquid systems, permitting shorter time-scale events such as rapid gas flow and gas-liquid dynamics to be observed. Two-dimensional velocity-encoded imaging shows convective gas flow above the evaporating liquid xenon, and also permits the measurement of enhanced gas diffusion near regions of large velocity variation.
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A system for low field imaging of laser-polarized noble gas
We developed a device for MRI of laser-polarized noble gas at low magnetic fields (below 50 G). The system is robust, portable, inexpensive, and provides gas-phase imaging resolution comparable to that of high field clinical instruments. At 20.6 G, we imaged laser-polarized 3He in both sealed glass cells and excised rat lungs. In addition, we measured 3He T2 > 100 ms in excised rat lungs, which is roughly 20 times longer than typical values observed at high magnetic fields.
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Magnetic resonance imaging of laser polarized liquid xenon
We demonstrate magnetic resonance imaging (MRI) of laser polarized liquid xenon, and image exchange between the liquid and vapor phases. The exceptionally large magnetization density of this liquid should allow MRI with micron-scale spatial resolution without signal averaging. Applications may include imaging of density equilibration and convective flow near xenon's liquid-vapor critical point, low-field imaging of porous media microstructure, and mapping of the dynamics of two-phase (liquid-gas) flows.
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Low field MRI of laser polarized noble gas
NMR images of laser polarized 3He gas were obtained at 21 G using a simple, homebuilt instrument. At such low fields magentic resonance imaging (MRI) of thermally polarized samples (e.g., water) is not practical. Low-field noble gas MRI has novel scientific, engineering, and medical applications. Examples include portable systems for diagnosis of lung disease, as well as imaging of voids in porous media and within metallic systems.
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Porous and Granular Media studied by NMR
Noninvasive measurements of gas exchange in a three-dimensional fluidized bed using hyperpolarized 129Xe NMRWe present a novel NMR technique that provides a non-invasive, direct measurement of gas exchange in a three-dimensional gas-fluidized bed of solid particles. The NMR spectrum of hyperpolarized 129Xe gas in an Al2O3 particle bed displays three resolved peaks corresponding to xenon in bubbles, the interstitial spaces (emulsion), and adsorbed on particles. Modified NMR exchange and saturation-recovery sequences, together with data analysis based on an exchange-coupled set of Bloch equations, yield gas exchange rate constants between the emulsion and adsorbed phases, and between the bubble and emulsion phases. The results are in approximate agreement with previously unverified predictions from well-known models of fluidized bed behavior. Incorporation of NMR imaging methodologies would straightforwardly allow similar measurements on a spatially-resolved basis.
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Understanding the breakdown of Fourier’s law in granular fluids
In fluidized granular matter such as rapidly flowing sand heat can flow from colder to hotter granular temperatures, violating Fourier’s law. A simple heuristic explanation for this anomalous heat current is presented, based on the non-equilibrium nature of granular fluids. The heuristic explanation leads to a straightforward calculation of the heat current which is in good agreement with existing, more detailed calculations and with recent experiments.
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NMR measurements of grain and gas motion in a gas-fluidized granular bed
Nuclear magnetic resonance (NMR) experiments are described for gas-fluidized granular beds, which are important systems for many materials-processing operations. Using pulsed field gradient, magnetic resonance imaging, and hyperpolarized gas NMR, detailed information is obtained for the density and motions of both grains and interstitial gas. In particular, dynamic correlations in the grain density are used to measure the bubble velocity and hyperpolarized xenon gas NMR is used to measure the bubble-emulsion exchange rate. A goal of these measurements is to verify in earth gravity first-principles theories of granular flows.
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Xenon NMR Measurements of Permeability and Tortuosity in Reservoir Rocks
In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.
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Study of Gas-Fluidization Dynamics with Laser-Polarized 129Xe
We report initial NMR studies of gas dynamics in a particle bed fluidized by laser-polarized xenon (129Xe) gas. We have made preliminary measurements of two important characteristics: gas exchange between the bubble and emulsion phases; and the gas velocity distribution in the bed. We used T2* contrast to differentiate the bubble and emulsion phases by choosing solid particles with large magnetic susceptibility, in order. Experimental tests demonstrated that this method was successful in eliminating 129Xe magnetization in the emulsion phase, which enabled us to observe the time-dependence of the bubble magnetization. By employing the pulsed field gradient method, we also measured the gas velocity distribution within the bed. These results clearly show the onset of bubbling and can be used to deduce information about gas and particle motion in the fluidized bed.
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Simultaneous Measurement of Rock Permeability and Effective Porosity using Laser-Polarized Noble Gas NMR
We report simultaneous measurements of the permeability and effective porosity of oil-reservoir rock cores using one-dimensional NMR imaging of the penetrating flow of laser-polarized xenon gas. The permeability result agrees well with industry standard techniques, whereas effective porosity is not easily determined by other methods. This novel NMR technique may have applications to the characterization of fluid flow in a wide variety of porous and granular media.
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NMR Experiments on a Three-Dimensional Vibrofluidized Granular Medium
A three-dimensional granular system fluidized by vertical container vibrations was studied using pulsed field gradient (PFG) NMR coupled with one-dimensional magnetic resonance imaging (MRI). The system consisted of mustard seeds vibrated vertically at 50 Hz, and the number of layers Nl <= 4 was sufficiently low to achieve a nearly time-independent granular fluid. Using NMR, the vertical profiles of density and granular temperature were directly measured, along with the distributions of vertical and horizontal grain velocities. The velocity distributions showed modest deviations from Maxwell-Boltzmann statistics, except for the vertical velocity distribution near the sample bottom which was highly skewed and non-Gaussian. Data taken for three values of Nl and two dimensionless accelerations Γ=15,18 were fit to a hydrodynamic theory, which successfully models the density and temperature profiles away from the vibrating container bottom. A temperature inversion near the free upper surface is observed, in agreement with predictions based on the hydrodynamic parameter mu which is only nonzero in inelastic systems.
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Applications of Controlled-Flow Laser-Polarized Xenon Gas to Porous and Granular Materials Study
We report initial NMR studies of continuous flow laser-polarized xenon gas, both in unrestricted tubing, and in a model porous media. The study uses Pulsed Gradient Spin Echo-based techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients. Pulsed Gradient Echo studies of continuous flow laser-polarized xenon gas in unrestricted tubing indicate clear diffraction minima resulting from a wide distribution of velocities in the flow field. The maximum velocity experienced in the flow can be calculated from this minimum, and is seen to agree with the information from the complete velocity spectrum, or motion propagator, as well as previously published images. The susceptibility of gas flows to parameters such as gas mixture content, and hence viscosity, are observed in experiments aimed at identifying clear structural features from echo attenuation plots of gas flow in porous media. Gas-phase NMR scattering, or position correlation flow-diffraction, previously clearly seen in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack is not so clear in experiments using a different gas mixture. A propagator analysis shows most gas in the sample remains close to static, while a small portion moves through a presumably near-unimpeded path at high velocities.
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Diffusion NMR Methods Applied to Xenon Gas for Materials Study
We report initial NMR studies of i) xenon gas diffusion in model heterogeneous porous media, and ii) continuous flow laser-polarized xenon gas. Both areas utilize the Pulsed Gradient Spin Echo techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients - a brief overview of this area is provided in the introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t), (an indicator of mean squared displacement) to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. We find D(t) of imbibed xenon gas at short diffusion times is similar for the mixed bead pack and a pack of the smaller sized beads alone, hence reflecting the pore surface-area-to-volume-ratio of the smaller bead sample. The approach of D(t) to the long-time limit follows that of a pack of the larger sized beads alone, although the limiting D(t) for the mixed bead pack is lower, reflecting the lower porosity of the sample compared to that of a pack of mono-sized glass beads. The Pade approximation is used to interpolate D(t) data between the short and long time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20 - 200 mm/s). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm/s due to the high diffusivity of gases compared to liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data: namely, flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.
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The Narrow Pulse Approximation and Long Length Scale Determination in Xenon Gas Diffusion NMR Studies of Model Porous Media
We report a systematic study of xenon gas diffusion NMR in simple model porous media: random packs of mono-sized glass beads, and focus on three specific areas peculiar to gas-phase diffusion. These topics are: (i) diffusion of spins on the order of the pore dimensions during the application of the diffusion encoding gradient pulses in a PGSE experiment (breakdown of the 'narrow pulse approximation' and imperfect background gradient cancellation), (ii) the ability to derive long-length scale structural information, and (iii) effects of finite sample size. We find that the time-dependent diffusion coefficient, D(t), of the imbibed xenon gas at short diffusion times in small beads is significantly affected by the gas pressure. In particular, as expected, we find smaller deviations between measured D(t) and theoretical predictions as the gas pressure is increased, resulting from reduced diffusion during the application of the gradient pulse. The deviations are then completely removed when water D(t) is observed in the same samples. The use of gas also allows us to probe D(t) over a wide range of length scales, and observe the long-time asymptotic limit which is proportional to the inverse tortuosity of the sample, as well as the diffusion distance where this limit takes effect (~ 1 - 1.5 bead diameters). The Pade¢ approximation can be used as a reference for expected xenon D(t) data between the short and long time limits, allowing us to explore deviations from the expected behaviour at intermediate times as a result of finite sample size effects. Finally, the application of the Pade interpolation between the long and short time asymptotic limits yields a fitted length scale (the "Pade length"), which is found to be ~ 0.13b for all bead packs, where b is the bead diameter.
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Novel MRI Applications of Laser-Polarized Noble Gases.
Gas-phase NMR has great potential as a probe for a variety of interesting physical and biomedical problems that are not amenable to study by water or similar liquid. However, NMR of gases was largely neglected due to the low signal obtained from the thermally-polarized gases with very low sample density. The advent of optical pumping techniques for enhancing the polarization of the noble gases 3He and 129Xe has bought new life to this field, especially in medical imaging where 3He lung inhalation imaging is approaching a clinical application. However, there are numerous applications in materials science that also benefit from the use of these gases. We review primarily non-medical applications of laser-polarized noble gases for both NMR imaging and spectroscopy, and highlight progress with examples from our laboratory including high-resolution imaging at mT applied field strength and velocity imaging of convective flow. Porous media microstucture has been probed with both thermal and laser-polarized xenon, as xenon is an ideal probe due to low surface interaction with the grains of the porous media.
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Measuring surface-area-to-volume ratios in soft porous materials using laser-polarized xenon interphase exchange NMR.
We demonstrate a minimally invasive nuclear magnetic resonance (NMR) technique that enables determination of the surface-area-to-volume ratio (S/V) of soft porous materials from measurements of the diffusive exchange of laser-polarized 129Xe between gas in the pore space and 129Xe dissolved in the solid phase. We apply this NMR technique to porous polymer samples and find approximate agreement with destructive stereological measurements of S/V obtained with optical confocal microscopy. Potential applications of laser-polarized xenon interphase exchange NMR include measurements of in vivo lung function in humans and characterization of gas chromatography columns.
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Measurements of Grain Motion in a Dense, Three Dimensional Granular Fluid.
We have used NMR techniques to measure the short-time, three-dimensional displacement of grains in a system of mustard seeds vibrated vertically at 15g. The observation interval is in the ballistic regime, giving a direct measurement of the granular temperature profile. The dense, lower portion of the sample is well described by a recent hydrodynamic theory for inelastic hard spheres. Near the free upper surface the mean free path is much longer than the particle diameter and the hydrodynamic description breaks down.
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Tortuosity Measurement and the Effects of Finite Pulse Widths on Xenon Gas Diffusion NMR Studies of Porous Media
We have extended the utility of NMR as a technique to probe porous media structure over length scales of ~ 100 - 2000 μm by using the spin 1/2 noble gas 129Xe imbibed into the system's pore space. Such length scales are much greater than can be probed with NMR diffusion studies of water-saturated porous media. We utilized Pulsed Gradient Spin Echo NMR measurements of the time-dependent diffusion coefficient, D(t), of the xenon gas filling the pore space to study further the measurements of both the pore surface-area-to-volume ratio, S/Vp , and the tortuosity (pore connectivity) of the medium. In uniform-size glass bead packs, we observed D(t) decreasing with increasing t, reaching an observed asymptote of ~ 0.62 - 0.65D0 , that could be measured over diffusion distances extending over multiple bead diameters. Measurements of D(t)/D0 at differing gas pressures showed this tortuosity limit was not affected by changing the characteristic diffusion length of the spins during the diffusion encoding gradient pulse. This was not the case at the short time limit, where D(t)/D0 was noticeably affected by the gas pressure in the sample. Increasing the gas pressure, and hence reducing D0 and the diffusion during the gradient pulse served to reduce the previously observed deviation of D(t)/D0 from the S/Vp relation. The Pade approximation is used to interpolate between the long and short time limits in D(t). While the short time D(t) points lay above the interpolation line in the case of small beads, due to diffusion during the gradient pulse on the order of the pore size, it was also noted that the experimental D(t) data fell below the Pade line in the case of large beads, most likely due to finite size effects.
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Probing porous media with gas diffusion NMR
We showed that gas diffusion nuclear magnetic resonance (GD-NMR) provides a powerful technique for probing the structure of porous media. In random packs of glass beads, using both laser-polarized and thermally polarized xenon gas, we found that GD-NMR can accurately measure the pore space surface-area-to-volume ratio, S/Vp, and the tortuosity, (the latter quantity being directly related to the system's transport properties). We also showed that GD-NMR provides a good measure of the tortuosity of sandstone and complex carbonate rocks.
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Single-shot diffusion measurement
A single-shot pulsed gradient stimulated echo sequence is introduced to address the challenges of diffusion measurements of laser polarized 3He and 129Xe gas. Laser polarization enhances the NMR sensitivity of these noble gases by > 103, but createds an unstable nonthermal polarization that is not readily renewable. A new method is presented which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. The NMR characterization of a samples's diffusion behaviour can be accomplished in a single polarization step. As a demonstration, the diffusion coefficient of a sample of laser-polarized 129Xe gas is measured via this method.
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Pulsed field gradient measurements of time dependent gas diffusion
Pulsed-field-gradient NMR techniques are demonstrated for measurements of time dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 x 10-6 m2s-1 for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo or PGE, and the Pulsed Gradient-Multiple-Spin Echo or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times.
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Other Work
Quantum Physics Exploring Gravity in the outer Solary System: The Sagas ProjectWe summarise the scientific and technological aspects of the SAGAS (Search for Anomalous Gravitation using Atomic Sensors) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 timeframe. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements.
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Space-based research in fundamental physics and quantum technologies
Space offers unique experimental conditions and a wide range of opportunities to explore the foundations of modern physics with an accuracy far beyond that of ground-based experiments. Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for investigations in precision cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics. We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences’ Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee’s interagency “Fundamental Physics Task Force” to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA’s science objectives in space by including “laboratory research in fundamental physics” as an element in agency’s ongoing space research efforts.
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Tunable negative refraction without absorption via electromagnetically induced chirality
We show that negative refraction with minimal absorption can be obtained by means of quantum interference effects similar to electromagnetically induced transparency. Coupling a magnetic dipole transition coherently with an electric dipole transition leads to electromagnetically induced chirality, which can provide negative refraction without requiring negative permeability, and also suppresses absorption. This technique allows negative refraction in the optical regime at densities where the magnetic susceptibility is still small and with refraction/absorption ratios that are orders of magnitude larger than those achievable previously. Furthermore, the value of the refractive index can be fine-tuned via external laser fields, which is essential for practical realization of sub-diffraction-limit imaging.
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Interstitial gas and density-segregation of vertically-vibrated granular media
We report experimental studies of the effect of interstitial gas on mass-density-segregation in a vertically-vibrated mixture of equal-sized bronze and glass spheres. Sufficiently strong vibration in the presence of interstitial gas induces vertical segregation into sharply separated bronze and glass layers. We find that the segregated steady state (i.e., bronze or glass layer on top) is a sensitive function of gas pressure and viscosity, as well as vibration frequency and amplitude. In particular, we identify distinct regimes of behavior that characterize the change from bronze-on-top to glass-on-top steady-state.
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Measurement of persistence in 1-D diffusion
Using a novel NMR scheme we observed persistence in 1-D gas diffusion. Analytical approximations and numerical simulations have shown that for an initially random array of spins undergoing diffusion, the probability p(t) that the average spin orientation in a given region has not changed sign (i.e., "persists") up to time t follows a power law t-θ, where θ depends on the dimensionality of the system. The large nuclear spin polarization of laser-polarized 129Xe gas allowed us both to prepare an initial "quasi-random" 1-D array of spin orientations and then to perform real-time NMR imaging to monitor the spin diffusion. Our measurements are consistent with theoretical and numerical predictions of θ ~ 0.12. We also observed finite size effects for long time gas diffusion.
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Analytical estimate of the critical velocity for vortex pair creation in trapped Bose condensates
We use a Thomas-Fermi approximation that includes leading kinetic terms due to fluid motion to estimate analytically the critical velocity for the formation of vortex pairs in harmonically trapped Bose-Einstein condensates. We find rough agreement between this analytical estimate and recent experiments on trapped sodium gas condensates.
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Characterization of fiber-coupled Opto Power laser diode arrays
We have characterized the spectra and performance of an ensemble of 11 fiber-coupled laser diode arrays (LDAs) manufactured by Opto Power Inc. These high-powered LDA's operate near 795 nm and are of a type commonly used for spin-exchange optical pumping of noble gases. We find the Opto Power LDAs to vary significantly in output power, spectral width, and other important characteristics, in a manner not correlated with age, operating lifetime, or information supplied by the manufacturer. In addition we have developed a two-loop feedback techniue for use with LDAs that stabilizes the Rb magnetization in an optical pumping cell to better than one part in a thousand.
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