Browsing by Author "Department of Physics, School of Science"

Now showing 1 - 10 of 51
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    Anomalous Edge State in a Non-Hermitian Lattice
    (APS, 2016-04) Lee, Tony E.; Department of Physics, School of Science
    We show that the bulk-boundary correspondence for topological insulators can be modified in the presence of non-Hermiticity. We consider a one-dimensional tight-binding model with gain and loss as well as long-range hopping. The system is described by a non-Hermitian Hamiltonian that encircles an exceptional point in momentum space. The winding number has a fractional value of 1/2. There is only one dynamically stable zero-energy edge state due to the defectiveness of the Hamiltonian. This edge state is robust to disorder due to protection by a chiral symmetry. We also discuss experimental realization with arrays of coupled resonator optical waveguides.
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    Approaching single temporal mode operation in twin beams generated by pulse pumped high gain spontaneous four wave mixing
    (OSA, 2016-01) Liu, Nannan; Liu, Yuhong; Guo, Xueshi; Yang, Lei; Li, Xiaoying; Ou, Z. Y.; Department of Physics, School of Science
    By investigating the intensity correlation function, we study the spectral/temporal mode properties of twin beams generated by the pulse-pumped high gain spontaneous four wave mixing (SFWM) in optical fiber from both the theoretical and experimental aspects. The results show that the temporal property depends not only on the phase matching condition and the filters applied in the signal and idler fields, but also on the gain of SFWM. When the gain of SFWM is low, the spectral/temporal mode properties of the twin beams are determined by the phase matching condition and optical filtering and are usually of multi-mode nature, which leads to a value larger than 1 but distinctly smaller than 2 for the normalized intensity correlation function of individual signal/idler beam. However, when the gain of SFWM is very high, we demonstrate the normalized intensity correlation function of individual signal/idler beam approaches to 2, which is a signature of single temporal mode. This is so even if the frequencies of signal and idler fields are highly correlated so that the twin beams have multiple modes in low gain regime. We find that the reason for this behavior is the dominance of the fundamental mode over other higher order modes at high gain. Our investigation is useful for constructing high quality multi-mode squeezed and entangled states by using pulse-pumped spontaneous parametric down-conversion and SFWM.
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    Atom-Light Hybrid Interferometer
    (APS, 2015-07) Chen, Bing; Qiu, Shuying; Guo, Jinxian; Chen, L. Q.; Ou, Z. Y.; Zhang, Weiping; Department of Physics, School of Science
    A new type of hybrid atom-light interferometer is demonstrated with atomic Raman amplification processes replacing the beam splitting elements in a traditional interferometer. This nonconventional interferometer involves correlated optical and atomic waves in the two arms. The correlation between atoms and light developed with the Raman process makes this interferometer different from conventional interferometers with linear beam splitters. It is observed that the high-contrast interference fringes are sensitive to the optical phase via a path change as well as the atomic phase via a magnetic field change. This new atom-light correlated hybrid interferometer is a sensitive probe of the atomic internal state and should find wide applications in precision measurement and quantum control with atoms and photons.
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    Atom-light superposition oscillation and Ramsey-like atom-light interferometer
    (OSA, 2016-07) Qiu, Cheng; Chen, Shuying; Chen, L. Q.; Chen, Bing; Guo, Jinxian; Ou, Z. Y.; Zhang, Weiping; Department of Physics, School of Science
    Coherent wave splitting is crucial in interferometers. Normally, the waves after this splitting are of the same type. But recent progress in interactions between atom and light has led to the coherent conversion of photon to atomic excitation. This makes it possible to split an incoming light wave into a coherent superposition state of atom and light and paves the way for an interferometer made of different types of waves. Here we report on a Rabi-like coherent-superposition oscillation observed between an atom and light in a Raman process. We construct a new kind of hybrid interferometer based on the atom–light coherent superposition state. Interference fringes are observed in both the optical output intensity and atomic output in terms of the atomic spin wave strength when we scan either or both of the optical and atomic phases. Such a hybrid interferometer can be used to interrogate atomic states by optical detection and will find its applications in precision measurement and quantum control of atoms and light.
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    Complete temporal mode analysis in pulse-pumped fiber-optical parametric amplifier for continuous variable entanglement generation
    (The Optical Society, 2015) Guo, Xueshi; Liu, Nannan; Li, Xiaoying; Ou, Z. Y.; Department of Physics, School of Science
    Mode matching plays an important role in measuring the continuous variable entanglement. For the signal and idler twin beams generated by a pulse pumped fiber optical parametric amplifier (FOPA), the spatial mode matching is automatically achieved in single mode fiber, but the temporal mode property is complicated because it is highly sensitive to the dispersion and the gain of the FOPA. We study the temporal mode structure and derive the input-output relation for each temporal mode of signal and idler beams after decomposing the joint spectral function of twin beams with the singular-value decomposition method. We analyze the measurement of the quadrature-amplitude entanglement, and find mode matching between the multi-mode twin beams and the local oscillators of homodyne detection systems is crucial to achieve a high degree of entanglement. The results show that the noise contributed by the temporal modes nonorthogonal to local oscillator may be much larger than the vacuum noise, so the mode mis-match can not be accounted for by merely introducing an effective loss. Our study will be useful for developing a source of high quality continuous variable entanglement by using the FOPA.
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    Conformational Changes in Two Inter-Helical Loops of Mhp1 Membrane Transporter
    (PLOS, 2015-07-17) Song, Hyun Deok; Zhu, Fangqiang; Department of Physics, School of Science
    Mhp1 is a bacterial secondary transporter with high-resolution crystal structures available for both the outward- and inward-facing conformations. Through molecular dynamics simulations of the ligand-free Mhp1 as well as analysis of its crystal structures, here we show that two inter-helical loops, respectively located at the extra- and intracellular ends of the “hash motif” in the protein, play important roles in the conformational transition. In the outward- and inward-facing states of the protein, the loops adopt different secondary structures, either wrapped to the end of an alpha-helix, or unwrapped to extended conformations. In equilibrium simulations of 100 ns with Mhp1 in explicit lipids and water, the loop conformations remain largely stable. In targeted molecular dynamics simulations with the protein structure driven from one state to the other, the loops exhibit resistance and only undergo abrupt changes when other parts of the protein already approach the target conformation. Free energy calculations on the isolated loops further confirm that the wrapping/unwrapping transitions are associated with substantial energetic barriers, and consist of multiple sequential steps involving the rotation of certain backbone torsion angles. Furthermore, in simulations with the loops driven from one state to the other, a large part of the protein follows the loops to the target conformation. Taken together, our simulations suggest that changes of the loop secondary structures would be among the slow degrees of freedom in the conformational transition of the entire protein. Incorporation of detailed loop structures into the reaction coordinate, therefore, should improve the convergence and relevance of the resulting conformational free energy.
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    Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving
    (Journal of Visualized Experiments, 2017-03-30) Li, Jiaming; de Melo, Leonardo F.; Luo, Le; Department of Physics, School of Science
    We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs the anharmonicity of the ODT, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, we selectively excite the hotter atoms out of the trap while keeping the colder atoms in the trap, generating parametric cooling. This experimental protocol starts with a magneto-optical trap (MOT) that is loaded by a Zeeman slower. The precooled atoms in the MOT are then transferred to an ODT, and a bias magnetic field is applied to create an interacting Fermi gas. We then lower the trapping potential to prepare a cold Fermi gas near the degenerate temperature. After that, we sweep the magnetic field to the noninteracting regime of the Fermi gas, in which the parametric cooling can be manifested by modulating the intensity of the optical trapping beams. We find that the parametric cooling effect strongly depends on the modulation frequencies and amplitudes. With the optimized frequency and amplitude, we measure the dependence of the cloud energy on the modulation time. We observe that the cloud energy is changed in an anisotropic way, where the energy of the axial direction is significantly reduced by parametric driving. The cooling effect is limited to the axial direction because the dominant anharmonicity of the crossed-beam ODT is along the axial direction. Finally, we propose to extend this protocol for the trapping potentials of large anharmonicity in all directions, which provides a promising scheme for cooling quantum gases using external driving.
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    Current Induced Vortex Wall Dynamics in Helical Magnetic Systems
    (Elsevier, 2015-03) Roostaei, Bahman; Department of Physics, School of Science
    Nontrivial topology of interfaces separating phases with opposite chirality in helical magnetic metals result in new effects as they interact with spin polarized current. These interfaces or vortex walls consist of a one dimensional array of vortex lines. We predict that adiabatic transfer of angular momentum between vortex array and spin polarized current will result in topological Hall effect in multi-domain samples. Also we predict that the motion of the vortex array will result in a new damping mechanism for magnetic moments based on Lenz’s law. We study the dynamics of these walls interacting with electric current and use fundamental electromagnetic laws to quantify those predictions. On the other hand discrete nature of vortex walls affects their pinning and results in low depinning current density. We predict the value of this current using collective pinning theory.
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    A data-driven alternative to the fractional Fokker–Planck equation
    (IOP, 2015-07) Pressé, Steve; Department of Physics, School of Science
    Anomalous diffusion processes are ubiquitous in biology and arise in the transport of proteins, vesicles and other particles. Such anomalously diffusive behavior is attributed to a number of factors within the cell including heterogeneous environments, active transport processes and local trapping/binding. There are a number of microscopic principles?such as power law jump size and/or waiting time distributions?from which the fractional Fokker?Planck equation (FFPE) can be derived and used to provide mechanistic insight into the origins of anomalous diffusion. On the other hand, it is fair to ask if other microscopic principles could also have given rise to the evolution of an observed density profile that appears to be well fit by an FFPE. Here we discuss another possible mechanistic alternative that can give rise to densities like those generated by FFPEs. Rather than to fit a density (or concentration profile) using a solution to the spatial FFPE, we reconstruct the profile generated by an FFPE using a regular FPE with a spatial and time-dependent force. We focus on the special case of the spatial FFPE for superdiffusive processes. This special case is relevant to, for example, active transport in a biological context. We devise a prescription for extracting such forces on synthetically generated data and provide an interpretation to the forces extracted. In particular, the time-dependence of forces could tell us about ATP depletion or changes in the cell's metabolic activity. Modeling anomalous behavior with normal diffusion driven by these effective forces yields an alternative mechanistic picture that, ultimately, could help motivate future experiments.
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    Direct affinity of dopamine to lipid membranes investigated by Nuclear Magnetic Resonance spectroscopy
    (Elsevier, 2016-04) Matam, Yashasvi; Ray, Bruce D.; Petrache, Horia I.; Department of Physics, School of Science
    Dopamine, a naturally occurring neurotransmitter, plays an important role in the brain’s reward system and acts on sensory receptors in the brain. Neurotransmitters are contained in lipid membraned vesicles and are released by exocytosis. All neurotransmitters interact with transport and receptor proteins in glial cells, on neuronal dendrites, and at the axonal button, and also must interact with membrane lipids. However, the extent of direct interaction between lipid membranes in the absence of receptors and transport proteins has not been extensively investigated. In this report, we use UV and NMR spectroscopy to determine the affinity and the orientation of dopamine interacting with lipid vesicles made of either phosphatidylcholine (PC) or phosphatidylserine (PS) lipids which are primary lipid components of synaptic vesicles. We quantify the interaction of dopamine's aromatic ring with lipid membranes using our newly developed method that involves reference spectra in hydrophobic environments. Our measurements show that dopamine interacts with lipid membranes primarily through the aromatic side opposite to the hydroxyl groups, with this aromatic side penetrating deeper into the hydrophobic region of the membrane. Since dopamine's activity involves its release into extracellular space, we have used our method to also investigate dopamine's release from lipid vesicles. We find that dopamine trapped inside PC and PS vesicles is released into the external solution despite its affinity to membranes. This result suggests that dopamine's interaction with lipid membranes is complex and involves both binding as well as permeation through lipid bilayers, a combination that could be an effective trigger for apoptosis of dopamine-generating cells.