Papers by Pasad Kulatunga
Physical Review B, 1997
The melting and solidification of Pb thin films on pyrolytic graphite are investigated in situ by... more The melting and solidification of Pb thin films on pyrolytic graphite are investigated in situ by reflection high-energy electron diffraction. Thin films with thicknesses of 4-150 monolayers are investigated. The surface morphology of the thin films were studied by scanning electron microscopy. Superheating of the Pb thin films by 4+/-2 to 12+/-2 K is observed from diffraction intensity measurements. Upon cooling the substrate, the Pb on graphite is seen to supercool by ~69+/-4 K.
Biophysical Journal, 2009
membrane of specific types of carcinoma cells. Specifically, the over-expression in certain epith... more membrane of specific types of carcinoma cells. Specifically, the over-expression in certain epithelial carcinoma cells can be as high as a thousand fold and thus FR provides an important possible target in efficient drug therapy design. In this study, we are probing the dynamics and interactions of folate/FR receptor complexes in the plasma membrane and through each part of its endocytotic cycle at the single molecule level. We are employing single molecule fluorescence microscopy to track individual fluorophore-labeled folate molecules bound to cell surface FR. We found an average diffusion constant of D ¼ 2e-9 cm 2 /s on live human KB carcinoma cells when imaged at 30 fps. Trajectories of individual particles showed temporary stopping or confined motion. This anomalous diffusive behavior was quantified against Monte Carlo simulations of randomly diffusing particles. The frequency and duration of confinement was compared as the overall concentration of folate was incrementally increased from 1 fM to the physiological level of 1nM . The effect on the frequency and duration of single FR confinement due to cholesterol depletion, actin stabilization and actin depolymerization will be presented. During antigen recognition by T cells, signaling molecules on the T cell engage ligands on the antigen-presenting cell and organize into spatially distinctive patterns collectively known as the immunological synapse (IS). The spatial arrangement of proteins into well-defined zones within the IS is known to regulate T cell activation and signal transduction. The mechanisms by which this complex organization arises remain unclear. Here we alter the clustering state of the T cell costimulatory molecule, LFA-1, either by direct antibody crosslinking or by crosslinking its ligand, ICAM-1, displayed on the supported bilayer. Changes in receptor clustering lead to progressively more central localization of LFA-1 until it colocalizes with T cell receptors (TCR) at the center. The number of LFA-1 molecules within the resulting clusters is obtained by fluorescence correlation spectroscopy. Our results demonstrate that cluster size is a critical parameter in determining protein spatial positioning in the IS. We discuss a sorting mechanism, based on frictional coupling to the cytoskeleton, which is consistent with these observations and is, in principle, extendable to all cell surface proteins in the synapse. Furthermore, by presenting patterns of immobilized ICAM-1 within a fluid bilayer displaying the TCR ligand, peptide-loaded MHC, we investigate the importance of LFA-1 ring formation to T cell function and signaling.
Biophysical Journal, 2009
membrane of specific types of carcinoma cells. Specifically, the over-expression in certain epith... more membrane of specific types of carcinoma cells. Specifically, the over-expression in certain epithelial carcinoma cells can be as high as a thousand fold and thus FR provides an important possible target in efficient drug therapy design. In this study, we are probing the dynamics and interactions of folate/FR receptor complexes in the plasma membrane and through each part of its endocytotic cycle at the single molecule level. We are employing single molecule fluorescence microscopy to track individual fluorophore-labeled folate molecules bound to cell surface FR. We found an average diffusion constant of D ¼ 2e-9 cm 2 /s on live human KB carcinoma cells when imaged at 30 fps. Trajectories of individual particles showed temporary stopping or confined motion. This anomalous diffusive behavior was quantified against Monte Carlo simulations of randomly diffusing particles. The frequency and duration of confinement was compared as the overall concentration of folate was incrementally increased from 1 fM to the physiological level of 1nM . The effect on the frequency and duration of single FR confinement due to cholesterol depletion, actin stabilization and actin depolymerization will be presented. During antigen recognition by T cells, signaling molecules on the T cell engage ligands on the antigen-presenting cell and organize into spatially distinctive patterns collectively known as the immunological synapse (IS). The spatial arrangement of proteins into well-defined zones within the IS is known to regulate T cell activation and signal transduction. The mechanisms by which this complex organization arises remain unclear. Here we alter the clustering state of the T cell costimulatory molecule, LFA-1, either by direct antibody crosslinking or by crosslinking its ligand, ICAM-1, displayed on the supported bilayer. Changes in receptor clustering lead to progressively more central localization of LFA-1 until it colocalizes with T cell receptors (TCR) at the center. The number of LFA-1 molecules within the resulting clusters is obtained by fluorescence correlation spectroscopy. Our results demonstrate that cluster size is a critical parameter in determining protein spatial positioning in the IS. We discuss a sorting mechanism, based on frictional coupling to the cytoskeleton, which is consistent with these observations and is, in principle, extendable to all cell surface proteins in the synapse. Furthermore, by presenting patterns of immobilized ICAM-1 within a fluid bilayer displaying the TCR ligand, peptide-loaded MHC, we investigate the importance of LFA-1 ring formation to T cell function and signaling.
We report on an investigation of loading characteristics of deep microscopic dipole traps. The di... more We report on an investigation of loading characteristics of deep microscopic dipole traps. The dipole trap is loaded from a low density magneto optical trap (MOT) containing $\approx 5\times 10^{6}$ atoms. We determine the loading parameters that maximize the trapped atom number for a trap of waist 5 {\mu}m with trap depths ranging from 3.5 mK to 10 mK. We determine the optimal trap loading conditions and the loading rates, loss coefficients and temperature of the trapped atoms under these conditions. We show that it is possible to load a few hundred to thousand atoms in dipole traps of depth 3.5 mK to 8.5 mK under the optimal loading conditions.
As part of an experimental effort to demonstrate quantum logic gates using neutral atom hyperfine... more As part of an experimental effort to demonstrate quantum logic gates using neutral atom hyperfine qubits we present experimental results showing loading of atoms into two micron sized optical dipole traps separated by 8 μm. The trapping sites are optically resolved on a CCD camera using fluorescence imaging. We use the F=2, mF=0 and F=1, mF=0 clock states as the qubit basis. After optical pumping into the F=2, mF=0 state tightly focused beams are used to perform two-photon stimulated Raman rotations between the qubit states. This approach provides the capability for performing qubit rotations on a site specific basis, by spatial scanning of the Raman beams. These steps, together with work in progress on dipole-dipole interaction of atoms excited to high lying Rydberg levels, will form the basis for demonstration of a neutral atom CNOT gate. This work is funded by the NSF and the Army Research Office.
We present recent progress in loading and manipulation of neutral atoms in microscopic optical tr... more We present recent progress in loading and manipulation of neutral atoms in microscopic optical traps. Single Rb atoms are loaded into far off resonant optical traps from a background vapor of cold atoms. Tightly focused optical beams are used to perform two-photon stimulated Raman rotations between hyperfine qubit states. We demonstrate qubit rotations at a rate of 1.4 MHz, 1 ms coherence time, and individual site addressing with crosstalk at the level of 10-3. These results are a significant step towards quantum computing using optically trapped neutral atoms. We discuss work in progress aimed at observing strong, angle independent dipole-dipole interactions for fast two-qubit gates using microwave dressing of Rydberg states. We demonstrate two-photon coherent excitation of Rydberg levels by a 5s1/2- 5p3/2- nd5/2 sequence. The possibility of dipole-dipole interactions without angular zeroes will be important for gates, as well as for coupling to mesoscopic qubits to enable transmission of quantum states.
We present a design for implementation of a neutral atom quantum logic testbed based on Rydberg a... more We present a design for implementation of a neutral atom quantum logic testbed based on Rydberg atom interactions. Detailed calculations of expected decoherence rates and fidelities for 1 and 2 qubit logical operations are described. We discuss geometries for efficient implementation of crosstalk free row-parallel Rydberg gate operations. A novel solution for elimination of excited state motional decoherence is presented. We will report on experimental progress towards single atom loading into micron sized FORT traps.
Physical Review Letters, 2006
We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87 Rb atoms ... more We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87 Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by 8 m at the level of 10 ÿ3 . Ramsey spectroscopy is used to measure a dephasing time of 870 s, which is 5000 times longer than the time for a =2 pulse.
Physical Review Letters, 2006
We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87 Rb atoms ... more We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87 Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by 8 m at the level of 10 ÿ3 . Ramsey spectroscopy is used to measure a dephasing time of 870 s, which is 5000 times longer than the time for a =2 pulse.
This thesis reports on the experimental study of coherent radiative transport in an ultracold gas... more This thesis reports on the experimental study of coherent radiative transport in an ultracold gas of 85Rb atoms confined in a magneto-optic trap. Measurements are made of the polarization dependence of the spatial and spectral profile of light backscattered from the sample. The results shows an interferometric enhancement sensitive to coherent multiple scattering in the atomic gas, and strong variations with the polarization of the incident and detected light. Effects due to coherent enhancement of weak non-resonant transitions are also observed. Comparison of the measurements with realistic quantum Monte Carlo simulations of Kupriyanov, et al [1] yield very good agreement.
We report investigation of multiple coherent light scattering from ^85Rb atoms confined in a magn... more We report investigation of multiple coherent light scattering from ^85Rb atoms confined in a magneto-optic trap. In experimental studies, measurements are made of coherent backscattering of a low-intensity probe beam tuned near the F = 3 - F' = 4 transition in ^85Rb atoms. Polarization of backscattered light is determined by a backscattering polarimeter; the spatial distribution of light intensity is measured by a liquid-nitrogen cooled CCD camera set in the focal plane of the analyzing optics. The instrument has angular resolution of about 100 micro-radians, and a polarization analyzing power of roughly 1000. In this paper we describe the instrument details, including calibration procedures, and our measurements of atomic coherent backscattering. In a theoretical study of intensity enhancement of near-resonant backscattered light from cold ^85,87Rb atoms, we consider scattering orders up to 8 and a Gaussian atom distribution in the MOT. Enhancement factors are calculated for all D1 and D2 hyperfine components and for both isotopes.
We report investigation of multiple coherent light scattering from ^85Rb atoms confined in a magn... more We report investigation of multiple coherent light scattering from ^85Rb atoms confined in a magneto-optic trap. In a theoretical study of intensity enhancement of near-resonant backscattered light from cold ^85,87Rb atoms, we consider the dominant mode of double scattering only. Enhancement factors are calculated for all D1 and D2 hyperfine components and for both isotopes. In experimental studies, measurements are made of coherent backscattering of a low-intensity probe beam tuned near the F = 3 - F' = 4 transition in ^85Rb atoms. Polarization of backscattered light is determined by a backscattering polarimeter; the spatial distribution of light intensity is measured by a liquid-nitrogen cooled CCD camera set in the focal plane of the analyzing optics. The instrument has angular resolution of about 100 micro-radians, and a polarization analyzing power of roughly 1000. In this paper we describe the instrument details, including calibration procedures, and progress towards observation of atomic coherent backscattering.
We report on the progress towards observing coherent back-scattering from two ``sphere '' cluster... more We report on the progress towards observing coherent back-scattering from two ``sphere '' clusters of trapped ultra-cold ^85Rb atoms. The two clusters of atoms are composed of two microscopic dipole traps, each consisting of few hundred atoms. Each trap will be approximately 5 μm in waist and are individually and dynamically configurable. The Coherent back-scattered signal is observed in an angular width of the order 1/kd where k is the wavenumber and d is the cluster (trap) separation. Ideally the back scattered peak should be a factor 2 greater than the back ground, any deviation from this is an indication of near-field effects.
Physical Review A, 2003
... Thus the first and last scatterers in a multiple-scattering sequence lie to a good ... to the... more ... Thus the first and last scatterers in a multiple-scattering sequence lie to a good ... to the incident polarization direction, and the phase associated with the detected light will vary ... vary much more slowly in the vertical direction, in direct analogy to the interference fringes formed from ...
Journal of Modern Optics, 2005
We report experimental observations of polarization-dependent coherence loss occurring in strong-... more We report experimental observations of polarization-dependent coherence loss occurring in strong-field multiple scattering of light in ultracold atomic 85 Rb. A measure of coherence in multiple light scattering is the degree of contrast of the coherent backscattering enhancement from the vapour. For resonance saturation parameters up to 9, we see light-polarization-dependent modification of the backscattering enhancement, suggesting that inelastic atomic light scattering and dynamic atomic magnetization may play important roles in the multiple scattering process for 85 Rb.
Wave transport in mesoscopic systems can be strongly influenced by coherent multiple scattering,w... more Wave transport in mesoscopic systems can be strongly influenced by coherent multiple scattering,which can lead to novel magneto-optic, transmission, and backscattering effects of light in atomic vapors. Although related to traditional studies of radiation trapping, in ultracold vapors negligible frequency or phase redistribution takes place in the scattering, and high-order coherent light scattering occurs. Among other things, this leads to enhancement of the influence of otherwise small non-resonant terms in the scattering amplitudes. We report investigation of multiple coherent light scattering from ultracold Rb atoms confined in a magneto-optic trap (MOT). In experimental studies, measurements are made of the angular, spectral, and polarization-dependent coherent backscattering profile of a low-intensity probe beam tuned near the F = 3 - F' = 4 hyperfine transition. The influence of higher probe beam intensity is also studied. In a theoretical study of angular intensity enhancement of backscattered light, we consider scattering orders up to 10 and a realistic and asymmetric Gaussian atom distribution in the MOT. Supported by NSF, NATO, and RFBR.
Physical Review A, 2004
We report a combined theoretical and experimental study of the spectral and polarization dependen... more We report a combined theoretical and experimental study of the spectral and polarization dependence of near-resonant radiation coherently backscattered from an ultracold gas of 85 Rb atoms. Measurements in a ±6 MHz range about the 5s 2 S 1/2 → 5p 2 P 3/2 , F =3→ FЈ = 4 hyperfine transition are compared with simulations based on a realistic model of the experimental atomic density distribution. In the simulations, the influence of heating of the atoms in the vapor, magnetization of the vapor, finite spectral bandwidth, and other nonresonant hyperfine transitions are considered. Good agreement is found between the simulations and measurements.
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Papers by Pasad Kulatunga