Browsing by Author "Brumer, Paul"

Now showing 1 - 8 of 8
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    Coherent one-photon phase control in closed and open quantum systems: A general master equation approach
    (2012-12) Pachón, Leonardo Augusto; Yu, Li; Brumer, Paul
    The underlying mechanisms for one photon phase control are revealed through a master equation approach. Specifically, two mechanisms are identified, one operating on the laser time scale and the other on the time scale of the system-bat interaction. The effects of the secular and non-secular Markovian approximations are carefully examined.
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    Direct experimental determination of spectral densities of molecular complexes
    (2014) Pachón, Leonardo Augusto; Brumer, Paul
    Determining the spectral density of a molecular system immersed in a proteomic scaffold and in contact to a solvent is a fundamental challenge in the coarse-grained description of, e.g., electron and energy transfer dynamics. Once the spectral density is characterized, all the time scales are captured and no artificial separation between fast and slow processes need to be invoked. Based on the fluorescence Stokes shift function, we utilize a simple and robust strategy to extract the spectral density of a number of molecular complexes from available experimental data. Specifically, we show that experimental data for dye molecules in several solvents, amino acid proteins in water, and some photochemical systems (e.g., rhodopsin and green fluorescence proteins), are well described by a three-parameter family of sub-ohmic spectral densities that are characterized by a fast.
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    Incoherent excitation of thermally equilibrated open quantum systems
    (2013-02-07) Pachón, Leonardo Augusto; Brumer, Paul
    Under natural conditions, excitation of biological molecules, which display nonunitary open system dynamics occurs via incoherent processes such as temperature changes or irradiation by sunlight or moonlight. The dynamics of such processes is explored analytically in a non-Markovian generic model Specifically, a system S in equilibrium with a thermal bath TB is subjected to an external incoherent perturbation BB (such as sunlight) or another thermal bath TB', which induces time evolution in (S + TB).Particular focus is on (i) the extent to which the resultant dynamics is coherent, and (ii) the role of "stationary coherences" established in the (S +TB) equilibration in response to the second incoherent perturbation. Results for systems with parameters analogous to those in light-harvesting molecules in photosynthesis show that the resultant dynamical behavior is incoherent beyond a very short response to the turn on of the perturbation.
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    Influence of Non-Markavian Dynamics in Thermal-Equilibrium Uncertainty-Relations
    (2017-07-08) Triana, Johan F.; Pachón, Leonardo Augusto; Brumer, Paul
    Contrary to the conventional wisdom that deviations from standard thermodynamics originate from the strong coupling to the bath, it is shown that in quantum mechanics, these deviations originate from the uncertainty principle and are supported by the non-Markovian character of the dynamics. Specifically, it is shown that the lower bound of the dispersion of the total energy of the system, imposed by the uncertainty principle, is dominated by the bath power spectrum and therefore, quantum mechanics inhibits the system thermal-equilibrium-state from being described by the canonical Boltzmann's distribution. We show that for a wide class of systems, systems interacting via central forces with pairwise-self-interacting environments, this general observation is in sharp contrast to the classical case, for which the thermal equilibrium distribution, irrespective of the interaction strength, is exactly characterized by the canonical Boltzmann distribution and therefore, no dependence on the bath power spectrum is present. We define an effective coupling to the environment that depends on all energy scales in the system and reservoir interaction. Sample computations in regimes predicted by this effective coupling are demonstrated. For example, for the case of strong effective coupling, deviations from standard thermodynamics are present and, for the case of weak effective coupling, quantum features such as stationary entanglement are possible at high temperatures.
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    Mechanisms in environmentally assisted one-photon phase control
    (2013-10-31) Pachón, Leonardo Augusto; Brumer, Paul
    The ability of an environment to assist in one-photon phase control relies upon entanglement between the system and bath and on the breaking of the time reversal symmetry. Here, one-photon phase control is examined analytically and numerically in a model system, allowing an analysis of the relative strength of these contributions. Further, the significant role of non-Markovian dynamics and of Moderate system-bath coupling in enhancing one-photon phase control is demonstrated, and an explicit role for quantum mechanics is noted in the existence of initial non-zero stationary coherences. Finally, desirable conditions are shown to be required to observe such environmentally assisted control since the system will naturally equilibrate with its environment at longer times, ultimately resulting in the loss of phase control.
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    Open system Perspective on Incoherent Excitation of Light Harvesting Systems
    (2017-05-12) Botero, Juan D.; Pachón, Leonardo Augusto; Brumer, Paul
    The nature of excited states of open quantum systems produced by incoherent natural thermal light is analyzed based on a description of the quantum dynamical map. Natural thermal light is shown to generate long-lasting coherent dynamics because of (i) the super-Ohmic character of the radiation, and (ii) the absence of pure dephasing dynamics. In the presence of an environment, the long-lasting coherences induced by suddenly turned-on incoherent light dissipate and stationary coherences are established. As a particular application, dynamics in a subunit of the PC-645 light-harvesting complex is considered where it is further shown that aspects of the energy pathways landscape depend on the nature of the exciting light and number of chromophores excited. Specifically, pulsed laser and natural broadband incoherent excitation induce significantly different energy transfer pathways. In addition, we discuss differences in perspective associated with the eigenstate vs site basis, and note an important difference in the phase of system coherences when coupled to blackbody radiation or when coupled to a phonon background. Finally an Appendix contains an open systems example of the loss of coherence as the turn on time of the light assumes natural time scales.
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    Quantum driven dissipative parametric oscillator in a blackbody radiation field
    (2017-06-28) Pachón, Leonardo Augusto; Brumer, Paul
    We consider the general open system problem of a charged quantum oscillator con- fined in a harmonic trap, whose frequency can be arbitrarily modulated in time, that interacts with both an incoherent quantized (blackbody) radiation field and with an arbitrary coherent laser field. We assume that the oscillator is initially in thermodynamic equilibrium with its environment, a non-factorized initial density matrix of the system and the environment, and that at t 0 the modulation of the frequency, the coupling to the incoherent and the coherent radiation are switched on. The subsequent dynamics, induced by the presence of the blackbody radiation, the laser field, and the frequency modulation, is studied in the framework of the influence functional approach. This approach allows incorporating, in analytic closed formulae, the non- Markovian character of the oscillator-environment interaction at any temperature as well the non-Markovian character of the blackbody radiation and its zero-point fluctuations. Expressions for the time evolution of the covariance matrix elements of the quantum fluctuations and the reduced density-operator are obtained.
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    Uncertainty Principle Consequences at Thermal Equilibrium
    (2013-05-02) Pachón, Leonardo Augusto; Triana, Johan F.; Zueco, David; Brumer, Paul
    Contrary to the conventional wisdom that deviations from standard thermodynamics originate from the strong coupling to the bath, it is shown that these deviations are intimately linked to the power spectrum of the thermal bath. Specifically, it is shown that the lower bound of the dispersion of the total energy of the system, imposed by the uncertainty principle, is dominated by the bath power spectrum and therefore, quantum mechanics inhibits the system thermal-equilibriun-state from being described by the canonical Boltzrhann's distribution. This is in sharp contrast to the classical case, for which the thermal equilibrium distribution of a system interacting via central forces with pairwise-self-interacting environment, irrespective of the interaction strength, is shown to be eractly characterized by the canonical Boltzmann distribution. As a consequence of this analysis we define an effective coupling to the environment that depends on all energy scales in the system and reservoir interaction. Sample computations in regimes predicted by this effective coupling are demonstrated. For example, for the case of strong effective coupling, deviations from standard thermodynamics are present and, for the case of weak effective coupling, quantum features such as stationary entanglement are possible at high temperatures.