The Experts below are selected from a list of 17226 Experts worldwide ranked by ideXlab platform
Daniel Loss - One of the best experts on this subject based on the ideXlab platform.
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hyperfine interaction and electron spin Decoherence in graphene and carbon nanotube quantum dots
Physical Review B, 2009Co-Authors: Jan Fischer, Bjorn Trauzettel, Daniel LossAbstract:We analytically calculate the nuclear-spin interactions of a single electron confined to a carbon nanotube or graphene quantum dot. While the conduction-band states in graphene are $p$-type, the accordant states in a carbon nanotube are $sp$-hybridized due to curvature. This leads to an interesting interplay between isotropic and anisotropic hyperfine interactions. By using only analytical methods, we are able to show how the interaction strength depends on important physical parameters, such as curvature and isotope abundances. We show that for the investigated carbon structures, the $^{13}\text{C}$ hyperfine coupling strength is less than $1\text{ }\ensuremath{\mu}\text{eV}$, and that the associated electron-spin Decoherence Time can be expected to be several tens of microseconds or longer, depending on the abundance of spin-carrying $^{13}\text{C}$ nuclei. Furthermore, we find that the hyperfine-induced Knight shift is highly anisotropic, both in graphene and in nanotubes of arbitrary chirality.
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nuclear spin state narrowing via gate controlled rabi oscillations in a double quantum dot
Physical Review B, 2006Co-Authors: D Klauser, W A Coish, Daniel LossAbstract:We study spin dynamics for two electrons confined to a double quantum dot under the influence of an oscillating exchange interaction. This leads to driven Rabi oscillations between the parallel to up arrow down arrow < state and the parallel to down arrow up arrow < state of the two-electron system. The width of the Rabi resonance is proportional to the amplitude of the oscillating exchange. A measurement of the Rabi resonance allows one to narrow the distribution of nuclear spin states and thereby to prolong the spin Decoherence Time. Further, we study Decoherence of the two-electron states due to the hyperfine interaction and give requirements on the parameters of the system in order to initialize in the parallel to up arrow down arrow < state and to perform a root SWAP operation with unit fidelity.
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Spin Relaxation and Decoherence of Holes in Quantum Dots
Physical review letters, 2005Co-Authors: D. V. Bulaev, Daniel LossAbstract:We investigate heavy-hole spin relaxation and Decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin Decoherence Time is 2 Times longer than the spin relaxation Time. We find that the spin relaxation Time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.
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phonon induced decay of the electron spin in quantum dots
Physical Review Letters, 2004Co-Authors: Vitaly N Golovach, Alexander Khaetskii, Daniel LossAbstract:We study spin relaxation and Decoherence in a GaAs quantum dot due to spin-orbit (SO) interaction. We derive an effective Hamiltonian which couples the electron spin to phonons or any other fluctuation of the dot potential. We show that the spin Decoherence Time T-2 is as large as the spin relaxation Time T-1, under realistic conditions. For the Dresselhaus and Rashba SO couplings, we find that, in leading order, the effective B field can have only fluctuations transverse to the applied B field. As a result, T-2=2T(1) for arbitrarily large Zeeman splittings, in contrast to the naively expected case T-2
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electron spin Decoherence in quantum dots due to interaction with nuclei
Physical Review Letters, 2002Co-Authors: Alexander Khaetskii, Daniel Loss, L I GlazmanAbstract:We study the Decoherence of a single electron spin in an isolated quantum dot induced by hyperfine interaction with nuclei. The decay is caused by the spatial variation of the electron wave function within the dot, leading to a nonuniform hyperfine coupling A . We evaluate the spin correlation function and find that the decay is not exponential but rather power (inverse logarithm) lawlike. For polarized nuclei we find an exact solution and show that the precession amplitude and the decay behavior can be tuned by the magnetic field. The decay Time is given by (h) over barN/A , where N is the number of nuclei inside the dot, and the amplitude of precession decays to a finite value. We show that there is a striking difference between the Decoherence Time for a single dot and the dephasing Time for an ensemble of dots.
D F Coker - One of the best experts on this subject based on the ideXlab platform.
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influence of site dependent pigment protein interactions on excitation energy transfer in photosynthetic light harvesting
Journal of Physical Chemistry B, 2013Co-Authors: Eva Rivera, D F Coker, Daniel Montemayor, Marco MasiaAbstract:A site-dependent spectral density system-bath model of the Fenna-Matthews-Olsen (FMO) pigment-protein complex is developed using results from ground-state molecular mechanics simulations together with a partial charge difference model for how the long-range contributions to the chromophore excitation energies fluctuate with environmental configuration. A discussion of how best to consistently process the chromophore excitation energy fluctuation correlation functions calculated in these classical simulations to obtain reliable site-dependent spectral densities is presented. The calculations reveal that chromophores that are close to the protein-water interface can experience strongly dissipative environmental interactions characterized by reorganization energies that can be as much as 2-3 Times those of chromophores that are buried deep in the hydrophobic protein scaffolding. Using a linearized density matrix quantum propagation method, we demonstrate that the inhomogeneous system-bath model obtained from our site-dependent spectral density calculations gives results consistent with experimental dissipation and dephasing rates. Moreover, we show that this model can simultaneously enhance the energy-transfer rate and extend the Decoherence Time. Finally, we explore the influence of initially exciting different chromophores and mutating local environments on energy transfer through the network. These studies suggest that different pathways, selected by varying initial photoexcitation, can exhibit significantly different relaxation Times depending on whether the energy-transfer path involves chromophores at the protein-solvent interface or if all chromophores in the pathway are buried in the protein.
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theoretical study of coherent excitation energy transfer in cryptophyte phycocyanin 645 at physiological temperature
Journal of Physical Chemistry Letters, 2011Co-Authors: Pengfei Huo, D F CokerAbstract:Recent two-dimensional photon-echo experiments suggest that excita- tion energy transfer in light harvesting systems occurs coherently rather than by incoherent hopping. The signature quantum beating of coherent energy transfer has been observed even at ambient temperatures. In this letter, we use an iterative linearized density matrix (ILDM) propagation approach to study this dynamics in a realistic multistate systembath model. Our calculations reproduce the observed 400 fs Decoherence Time, and studies that vary the system Hamiltonian and structured spectral density describing the chromophore networkprotein environment interac- tions give results that enable us to explore the role of initial coherence in energy transfer efficiency of the model network. Our findings suggest that the initial coherence has only a slight effect on energy transfer in this model system. We explore energy transfer optimization of different chromophores in the network by controlling environmentalproperties.Thisstudypointstotheimportanceofstochasticresonance behavior in determining optimal network throughput.
C Peralta - One of the best experts on this subject based on the ideXlab platform.
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gravitational radiation from hydrodynamic turbulence in a differentially rotating neutron star
The Astrophysical Journal, 2010Co-Authors: A Melatos, C PeraltaAbstract:The mean-square current quadrupole moment associated with vorticity fluctuations in high-Reynolds-number turbulence in a differentially rotating neutron star is calculated analytically, as are the amplitude and Decoherence Time of the resulting, stochastic gravitational wave signal. The calculation resolves the subtle question of whether the signal is dominated by the smallest or largest turbulent eddies: for the Kolmogorov-like power spectrum observed in superfluid spherical Couette simulations, the wave strain is controlled by the largest eddies, and the Decoherence Time approximately equals the maximum eddy turnover Time. For a neutron star with spin frequency νs and Rossby number Ro, at a distance d from Earth, the root mean square wave strain reaches hrms ≈ 3 × 10 −24 Ro 3 (νs/30 Hz) 3 (d/1 kpc) −1 . Ordinary rotation-powered pulsars (νs 30 Hz, Ro 10 −4 ) are too dim to be detected by the current generation of long-baseline interferometers. Millisecond pulsars are brighter; for example, an object born recently in a Galactic supernova or accreting near the Eddington rate can have νs ∼ 1k Hz, Ro 0.2, and hence hrms ∼ 10 −21 . A cross-correlation search can detect such a source in principle, because the signal decoheres over the Timescale τc ≈ 1 × 10 −3 Ro −1 (νs/30 Hz) −1 s, which is adequately sampled by existing long-baseline interferometers. Hence, hydrodynamic turbulence imposes a fundamental noise floor on gravitational wave observations of neutron stars, although its polluting effect may be muted by partial Decoherence in the hectohertz band, where current continuous-wave searches are concentrated, for the highest frequency (and hence most powerful) sources. This outcome is contingent on the exact shape of the turbulent power spectrum, which is modified by buoyancy and anisotropic global structures, such as stratified boundary layers, in a way that is understood incompletely even in laboratory situations.
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gravitational radiation from hydrodynamic turbulence in a differentially rotating neutron star
arXiv: High Energy Astrophysical Phenomena, 2009Co-Authors: A Melatos, C PeraltaAbstract:(Abridged.) The mean-square current quadrupole moment associated with vorticity fluctuations in high-Reynolds-number turbulence in a differentially rotating neutron star is calculated analytically, as are the amplitude and Decoherence Time of the resulting, stochastic gravitational wave signal. The calculation resolves the subtle question of whether the signal is dominated by the smallest or largest turbulent eddies: for the Kolmogorov-like power spectrum observed in superfluid spherical Couette simulations, the wave strain is controlled by the largest eddies, and the Decoherence Time approximately equals the maximum eddy turnover Time. For a neutron star with spin frequency $\nu_s$ and Rossby number $Ro$, at a distance $d$ from Earth, the root-mean-square wave strain reaches $h_{RMS} \approx 3\Times 10^{-24} Ro^3 (\nu_s / 30 Hz)^3 (d/1 kpc)^{-1}$. A cross-correlation search can detect such a source in principle, because the signal decoheres over the Time-scale $\tau_c \approx 10^{-3} Ro^{-1} (\nu_s / 30 Hz)^{-1} s$, which is adequately sampled by existing long-baseline interferometers. Hence hydrodynamic turbulence imposes a fundamental noise floor on gravitational wave observations of neutron stars, although its polluting effect may be muted by partial Decoherence in the hectohertz band, where current continuous-wave searches are concentrated, for the highest frequency (and hence most powerful) sources.
Bjorn Trauzettel - One of the best experts on this subject based on the ideXlab platform.
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ultralong spin Decoherence Times in graphene quantum dots with a small number of nuclear spins
Physical Review B, 2013Co-Authors: Moritz Fuchs, John Schliemann, Bjorn TrauzettelAbstract:We study the dynamics of an electron spin in a graphene quantum dot, which is interacting with a bath of less than ten nuclear spins via the anisotropic hyperfine interaction. Due to substantial progress in the fabrication of graphene quantum dots, the consideration of such a small number of nuclear spins is experimentally relevant. This choice allows us to use exact diagonalization to calculate the long-Time average of the electron spin as well as its Decoherence Time. We investigate the dependence of spin observables on the initial states of nuclear spins and on the position of nuclear spins in the quantum dot. Moreover, we analyze the effects of the anisotropy of the hyperfine interaction for different orientations of the spin quantization axis with respect to the graphene plane. Interestingly, we then predict remarkable long Decoherence Times of more than 10 ms in the limit of few nuclear spins.
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hyperfine interaction and electron spin Decoherence in graphene and carbon nanotube quantum dots
Physical Review B, 2009Co-Authors: Jan Fischer, Bjorn Trauzettel, Daniel LossAbstract:We analytically calculate the nuclear-spin interactions of a single electron confined to a carbon nanotube or graphene quantum dot. While the conduction-band states in graphene are $p$-type, the accordant states in a carbon nanotube are $sp$-hybridized due to curvature. This leads to an interesting interplay between isotropic and anisotropic hyperfine interactions. By using only analytical methods, we are able to show how the interaction strength depends on important physical parameters, such as curvature and isotope abundances. We show that for the investigated carbon structures, the $^{13}\text{C}$ hyperfine coupling strength is less than $1\text{ }\ensuremath{\mu}\text{eV}$, and that the associated electron-spin Decoherence Time can be expected to be several tens of microseconds or longer, depending on the abundance of spin-carrying $^{13}\text{C}$ nuclei. Furthermore, we find that the hyperfine-induced Knight shift is highly anisotropic, both in graphene and in nanotubes of arbitrary chirality.
Donald G Truhlar - One of the best experts on this subject based on the ideXlab platform.
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algorithmic Decoherence Time for decay of mixing non born oppenheimer dynamics
Journal of Chemical Physics, 2008Co-Authors: Shu Chun Cheng, Kuo Kan Liang, Donald G TruhlarAbstract:The performance of an analytical expression for algorithmic Decoherence Time is investigated for non–Born–Oppenheimer molecular dynamics. There are two terms in the function that represents the dependence of the Decoherence Time on the system parameters; one represents Decoherence due to the quantum Time-energy uncertainty principle and the other represents a back reaction from the decoherent force on the classical trajectory. We particularly examine the question of whether the first term should dominate. Five one-dimensional two-state model systems that represent limits of multidimensional nonadiabatic dynamics are designed for testing mixed quantum-classical methods and for comparing semiclassical calculations with exact quantum calculations. Simulations are carried out with the semiclassical Ehrenfest method (SE), Tully’s fewest switch version (TFS) of the trajectory surface hopping method, and the decay-of-mixing method with natural switching, coherent switching (CSDM), and coherent switching with rei...
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electronic Decoherence Time for non born oppenheimer trajectories
Journal of Chemical Physics, 2005Co-Authors: Ahren W Jasper, Donald G TruhlarAbstract:An expression is obtained for the electronic Decoherence Time of the reduced density electronic matrix in mixed quantum-classical molecular-dynamics simulations. The result is obtained by assuming that Decoherence is dominated by the Time dependence of the overlap of minimum-uncertainty packets and then maximizing the rate with respect to the parameters of the wave packets. The expression for the decay Time involves quantities readily available in non-Born-Oppenheimer molecular-dynamics simulations, and it is shown to have a reasonable form when compared with two other formulas for the decay Time that have been previously proposed.
