arXiv subjects

Paul Brumer

Publications and source records attributed to Paul Brumer.

At least 55 records · Page 3Linked to original sources

arXiv2019

Considerations regarding one-photon phase control

The dynamics of a system interacting with an ultrashort pulse is known to depend on the phase content of said pulse. For linear absorption, phase control is possible over time-varying quantities, such as the population of metastable states, but not over time-independent quantities, such as the population of steady states. We derive here a strict upper bound for phase control that interpolates between these two cases --- the bound quantifies the approach to the steady state and resulting loss of one-photon phase control based on physical timescales. Significantly, this bound is violated by a number of numerical and experimental investigations. A careful analysis of the physical conditions underlying this result exposes multiphoton effects as a mechanism for these experiments.

physics.chem-ph↗

arXiv2019

Observation of persistent orientation of chiral molecules by laser field with twisted polarization

Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules ($\mathrm{CH_{3}CHCH_{2}O}$, or PPO) spun by an optical centrifuge - a laser pulse, whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a time scale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation opens new avenues for optical manipulation, discrimination, and, potentially, separation of molecular enantiomers.

physics.chem-ph↗

arXiv2019

Ultrashort pulse two-photon coherent control of a macroscopic phenomena: light-induced current from channelrhodopsin-2 in live brain cells

Coherent control is extended to macroscopic processes under continuous pulsed laser irradiation. Here, this approach is used to analyze the experimentally measured two-photon phase control of currents emanating from a living brain cells expressing channelrhodopsin-2, a light-gated ion channel. In particular, a mechanism is proposed that encompasses more than 15 orders magnitude in time, from the ultrafast dynamics of retinal in channelrhodopsin-2 to the slow dynamics of the neuron current. Implications for other photochemical processes are discussed.

physics.chem-ph↗

arXiv2019

Laser induced persistent orientation of chiral molecules

We show, both classically and quantum mechanically, enantioselective orientation of gas phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation, but stays approximately constant long after the end of the laser pulses, behavior unique to chiral systems. We computationally demonstrate this long-lasting orientation using propylene oxide molecules (${\rm CH_{3}CHCH_{2}O}$, or PPO) as an example, and consider two kinds of fields with twisted polarization: a pair of delayed cross-polarized pulses, and an optical centrifuge. This novel chiral effect opens new avenues for detecting molecular chirality, measuring enantiomeric excess and separating enantiomers with the help of inhomogeneous external fields.

physics.chem-ph↗

arXiv2019

An efficient spectral method for numerical time-dependent perturbation theory

We develop the Fourier-Laplace Inversion of the Perturbation Theory (FLIPT), a novel numerically exact "black box" method to compute perturbative expansions of the density matrix with rigorous convergence conditions. Specifically, the FLIPT method is extremely well-suited to simulate multiphoton pulsed laser experiments with complex pulse shapes. The $n$-dimensional frequency integrals of the $n$-th order perturbative expansion are evaluated numerically using tensor products. The $N$ points discretized integrals are computed in $O(N^2)$ operations, a significant improvement over the $O(N^n)$ scaling of standard quadrature methods.

physics.chem-ph↗

arXiv2019

Influence of Non-Markovian Dynamics in Thermal-Equilibrium Uncertainty-Relations

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 \emph{exactly} characterized by the canonical Boltzmann distribution and therefore, no dependence on the bath power spectrum is present. We define an \emph{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.

quant-ph↗

arXiv2018

An efficient approach to the quantum dynamics and rates of processes induced by natural incoherent light

In many important cases, the rate of excitation of a system embedded in an environment is significantly smaller than the internal system relaxation rates. An important example is that of light-induced processes under natural conditions, in which the system is excited by weak, incoherent (e.g., solar) radiation. Simulating the dynamics on the time scale of the excitation source can thus be computationally intractable. Here we describe a method for obtaining the dynamics of quantum systems without directly solving the master equation. We present an algorithm for the numerical implementation of this method, and, as an example, use it to reconstruct the internal conversion dynamics of pyrazine excited by sunlight. Significantly, this approach also allows us to assess the role of quantum coherence on biological time scales, which is a topic of ongoing interest.

physics.chem-ph↗

arXiv2018

Coherent Control of Penning and Associative Ionization: Insights from Symmetries

Coherent control of reactive atomic and molecular collision processes remains elusive experimentally due to quantum interference-based requirements. Here, with insights from symmetry conditions, a viable method for controlling Penning and Associative ionization in atomic collisions is proposed. Computational applications to He$^*({}^3\text{S})$-Li(${^2\text{S}}$) and Ne$^*{}(^3\text{P}_2$)-Ar($^1\text{S}_0$) show extensive control over the ionization processes under experimentally feasible conditions.

quant-ph↗

arXiv2018

Dimensionality and integrals of motion of the Trappist-1 planetary system

The number of isolating integrals of motion of the Trappist-1 system - a late M-dwarf orbited by seven Earth-sized planets - was determined numerically, using an adapted version of the correlation dimension method. It was found that over the investigated time-scales of up to 20 000 years the number of isolating integrals of motion is the same as one would find for a system of seven non-interacting planets - despite the fact that the planets in the Trappist-1 system are strongly interacting. Considering perturbed versions of the Trappist-1 system shows that the system may occupy an atypical part of phase-space with high stability. These findings are consistent with earlier studies.

astro-ph.EP↗

arXiv2017

Secular vs. Non-secular Redfield Dynamics and Fano Coherences in Incoherent Excitation: An Experimental Proposal

Two different Master Equation approaches have been formally derived to address the dynamics of open quantum systems interacting with a thermal environment (such as sunlight). They have led to two different physical results: non-secular equations that show noise-induced (Fano) coherences and secular equations that do not. An experimental test for the appearance of non-secular terms is proposed using Ca atoms in magnetic fields excited by broadband incoherent radiation. Significantly different patterns of fluorescence are predicted, allowing for a clear test for the validity of the secular and non-secular approach and for the observation of Fano coherences.

quant-ph↗

arXiv2017

Open System Perspective on Incoherent Excitation of Light Harvesting Systems

The nature of excited states of open quantum systems produced by incoherent natural thermal light is analyzed based on a description of the generator of the dynamics. 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.

quant-ph↗

arXiv2017

Laser-induced molecular alignment in the presence of chaotic rotational dynamics

Coherent control of chaotic molecular systems, using laser-assisted alignment of sulphur dioxide (SO$_2$) molecules in the presence of a static electric field as an example, is considered. Conditions for which the classical version of this system is chaotic are established, and the quantum and classical analogs are shown to be in very good correspondence. It is found that the chaos present in the classical system does not impede the alignment, neither in the classical nor the quantum system. Using the results of numerical calculations, we suggest that laser-assisted alignment is stable against rotational chaos for all asymmetric top molecules.

physics.chem-ph↗

arXiv2016

Spin-orbit interactions and quantum spin dynamics in cold ion-atom collisions

We present accurate ab initio and quantum scattering calculations on a prototypical hybrid ion-atom system Yb$^+$-Rb, recently suggested as a promising candidate for the experimental study of open quantum systems, quantum information processing, and quantum simulation. We identify the second-oder spin-orbit (SO) interaction as the dominant source of hyperfine relaxation and decoherence in cold Yb$^+$-Rb collisions. Our results are in good agreement with recent experimental observations [L. Ratschbacher et al., Phys. Rev. Lett. 110, 160402 (2013)] of hyperfine relaxation rates of trapped Yb$^+$ immersed in an ultracold Rb gas. The calculated rates are 4 times smaller than predicted by the Langevin capture theory and display a weak $T^{-0.3}$ temperature dependence, indicating significant deviations from statistical behavior. Our analysis underscores the deleterious nature of the SO interaction and implies that light ion-atom combinations such as Yb$^+$-Li should be used to minimize hyperfine relaxation and decoherence of trapped ions in ultracold atomic gases.

physics.atom-ph↗

arXiv2016

Localized operator partitioning method for electronic excitation energies in the time-dependent density functional formalism

We extend the localized operator partitioning method (LOPM) [J. Nagesh, A.F. Izmaylov, and P. Brumer, J. Chem. Phys. 142, 084114 (2015)] to the time-dependent density functional theory (TD-DFT) framework to partition molecular electronic energies of excited states in a rigorous manner. A molecular fragment is defined as a collection of atoms using Stratman-Scuseria-Frisch atomic partitioning. A numerically efficient scheme for evaluating the fragment excitation energy is derived employing a resolution of the identity to preserve standard one- and two-electron integrals in the final expressions. The utility of this partitioning approach is demonstrated by examining several excited states of two bichromophoric compounds: 9-((1-naphthyl)-methyl)-anthracene and 4-((2-naphthyl)-methyl)-benzaldehyde. The LOPM is found to provide nontrivial insights into the nature of electronic energy localization that are not accessible using simple density difference analysis.

physics.chem-ph↗

arXiv2016

Coherent dynamics of V-type systems driven by time-dependent incoherent radiation

Light induced processes in nature occur by irradiation with slowly turned-on incoherent light. The general case of time-dependent incoherent excitation is solved here analytically for V-type systems using a newly developed master equation method. Clear evidence emerges for the disappearance of radiatively induced coherence as turn-on times of the radiation exceed characteristic system times. The latter is the case, in nature, for all relevant dynamical time scales for other than nearly degenerate energy levels. We estimate that, in the absence of non-radiative relaxation and decoherence, turn-on times slower than 1 ms (still short by natural standards) induce Fano coherences between energy eigenstates that are separated by less than 0.9 cm$^{-1}$.

physics.chem-ph↗

arXiv2016

An Approach to "Quantumness" in Coherent Control

Developments in the foundations of quantum mechanics have identified several attributes and tests associated with the "quantumness" of systems, including entanglement, nonlocality, quantum erasure, Bell test, etc. Here we introduce and utilize these tools to examine the role of quantum coherence and nonclassical effects in 1 vs. N photon coherent phase control, a paradigm for an all-optical method for manipulating molecular dynamics. In addition, truly quantum control scenarios are introduced and examined. The approach adopted here serves as a template for studies of the role of quantum mechanics in other coherent control and optimal control scenarios.

quant-ph↗

arXiv2016

Quantum dynamics of incoherently driven V-type system: Analytic solutions beyond the secular approximation

We present closed-form analytic solutions to non-secular Bloch-Redfield master equations for quantum dynamics of a V-type system driven by weak coupling to a thermal bath. We focus on noise-induced Fano coherences among the excited states induced by incoherent driving of the V-system initially in the ground state. For suddenly turned-on incoherent driving, the time evolution of the coherences is determined by the damping parameter $ζ=\frac{1}{2}(γ_1+γ_2)/Δ_p$, where $γ_i$ are the radiative decay rates of the excited levels $i=1,2$, and $Δ_p=\sqrt{Δ^2 + (1-p^2)γ_1γ_2}$ depends on the excited-state level splitting $Δ>0$ and the angle between the transition dipole moments in the energy basis. The coherences oscillate as a function of time in the underdamped limit ($ζ\gg1$), approach a long-lived quasi-steady state in the overdamped limit ($ζ\ll 1$), and display an intermediate behavior at critical damping ($ζ= 1$). The sudden incoherent turn-on generates a mixture of excited eigenstates $|e_1\rangle$ and $|e_2\rangle$ and their in-phase coherent superposition $|ϕ_+\rangle = \frac{1}{\sqrt{2\bar{r}}}(\sqrt{r_1} |e_1\rangle + \sqrt{r_2}|e_2\rangle)$, which is remarkably long-lived in the overdamped limit (where $r_1$ and $r_2$ are the incoherent pumping rates). Formation of this coherent superposition {\it enhances} the decay rate from the excited states to the ground state. In the strongly asymmetric V-system where the coupling strengths between the ground state and the excited states differ significantly, we identify additional asymptotic quasistationary coherences, which arise due to slow equilibration of one of the excited states. Finally, we demonstrate that noise-induced Fano coherences are maximized with respect to populations when $r_1=r_2$ and the transition dipole moments are fully aligned.

physics.chem-ph↗

arXiv2015

Classical Approach to Multichromophoric Resonance Energy Transfer

A classical formulation of the quantum multichromophoric theory of resonance energy transfer is developed on the basis of classical electrodynamics. The theory allows for the identification of a variety of processes of different order-in-the-interactions that contribute to the energy transfer in molecular aggregates with intra-coupling in donors and acceptor chromophores. Enhanced rates in multichromophoric resonance energy transfer are shown to be well described by this theory. Specifically, in a coupling configuration between N_{\mathrm{A}}$ acceptors and $N_{\mathrm{D}}$ donors, the theory correctly predicts an enhancement of the energy transfer rate dependent on the total number of donor-acceptor pairs. As an example, the theory, applied to the transfer rate in LH~II, gives results in excellent agreement with experiment. Finally, it is explicitly shown that as long as linear response theory holds, the classical multichromophoric theory formally coincides with the quantum formulation.

physics.bio-ph↗