Transform-Limited-Pulse Representation of Excitation with Natural Incoherent Light

We study the natural excitation of molecular systems, applicable to, for example, photosynthetic light-harvesting complexes, by natural incoherent light. In contrast with the conventional classical models, we show that the light need not have random character to properly represent the resultant linear excitation. Rather, thermal excitation can be interpreted as a collection of individual events resulting from the system's interaction with individual, deterministic pulsed realizations that constitute the field. The derived expressions for the individual field realizations and excitation events allow for a wave function formalism, and therefore constitute a useful calculational tool to study dynamics following thermal-light excitation. Further, they provide a route to the experimental determination of natural incoherent excitation using pulsed laser techniques.Comment: 5 pages, 3 figures, 1 page supplementary information. Comments welcom

Decoherence Effects in Reactive Scattering

Decoherence effects on quantum and classical dynamics in reactive scattering are examined using a Caldeira-Leggett type model. Through a study of dynamics of the collinear H+H2 reaction and the transmission over simple one-dimensional barrier potentials, we show that decoherence leads to improved agreement between quantum and classical reaction and transmission probabilities, primarily by increasing the energy dispersion in a well defined way. Increased potential nonlinearity is seen to require larger decoherence in order to attain comparable quantum-classical agreement.Comment: 25 pages, 6 figures, to be published in J. Chem. Phy

Coherent Control of Quantum Chaotic Diffusion

Extensive coherent control over quantum chaotic diffusion using the kicked rotor model is demonstrated and its origin in deviations from random matrix theory is identified. Further, the extent of control in the presence of external decoherence is established. The results are relevant to both areas of quantum chaos and coherent control.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let

Coherent Control and Entanglement in the Attosecond Electron Recollision Dissociation of D2+

We examine the attosecond electron recollision dissociation of D2+ recently demonstrated experimentally [H. Niikura et al., Nature (London) 421, 826 (2003)] from a coherent control perspective. In this process, a strong laser field incident on D2 ionizes an electron, accelerates the electron in the laser field to eV energies, and then drives the electron to recollide with the parent ion, causing D2+ dissociation. A number of results are demonstrated. First, a full dimensional Strong Field Approximation (SFA) model is constructed and shown to be in agreement with the original experiment. This is then used to rigorously demonstrate that the experiment is an example of coherent pump-dump control. Second, extensions to bichromatic coherent control are proposed by considering dissociative recollision of molecules prepared in a coherent superposition of vibrational states. Third, by comparing the results to similar scenarios involving field-free attosecond scattering of independently prepared D2+ and electron wave packets, recollision dissociation is shown to provide an example of wave-packet coherent control of reactive scattering. Fourth, this analysis makes clear that it is the temporal correlations between the continuum electron and D2+ wave packet, and not entanglement, that are crucial for the sub-femtosecond probing resolution demonstrated in the experiment. This result clarifies some misconceptions regarding the importance of entanglement in the recollision probing of D2+. Finally, signatures of entanglement between the recollision electron and the atomic fragments, detectable via coincidence measurements, are identified

Dynamic interference of photoelectrons produced by high-frequency laser pulses

The ionization of an atom by a high-frequency intense laser pulse, where the energy of a single-photon is sufficient to ionize the system, is investigated from first principles. It is shown that as a consequence of an AC Stark effect in the continuum, the energy of the photoelectron follows the envelope of the laser pulse. This is demonstrated to result in strong dynamic interference of the photoelectrons of the same kinetic energy emitted at different times. Numerically exact computations on the hydrogen atom demonstrate that the dynamic interference spectacularly modifies the photoionization process and is prominently manifested in the photoelectron spectrum by the appearance of a distinct multi-peak pattern. The general theory is shown to be well approximated by explicit analytical expressions which allow for a transparent understanding of the discovered phenomena and for making predictions on the dependence of the measured spectrum on the properties of the pulse.Comment: 5 figure

Aspects of quantum coherence in the optical Bloch equations

Aspects of coherence and decoherence are analyzed within the optical Bloch equations. By rewriting the analytic solution in an alternate form, we are able to emphasize a number of unusual features: (a) despite the Markovian nature of the bath, coherence at long times can be retained; (b) the long-time asymptotic degree of coherence in the system is intertwined with the asymptotic difference in level populations; (c) the traditional population-relaxation and decoherence times, $T_1$ and $T_2$, lose their meaning when the system is in the presence of an external field, and are replaced by more general overall timescales; (d) increasing the field strength, quantified by the Rabi frequency, $\Omega$, increases the rate of decoherence rather than reducing it, as one might expect; and (e) maximum asymptotic coherence is reached when the system parameters satisfy $\Omega^2 = 1/(T_1 T_2)$.Comment: 18 pages, 6 figures; to appear in J Chem Phy

Chaos and Correspondence in Classical and Quantum Hamiltonian Ratchets: A Heisenberg Approach

Previous work [Gong and Brumer, Phys. Rev. Lett., 97, 240602 (2006)] motivates this study as to how asymmetry-driven quantum ratchet effects can persist despite a corresponding fully chaotic classical phase space. A simple perspective of ratchet dynamics, based on the Heisenberg picture, is introduced. We show that ratchet effects are in principle of common origin in classical and quantum mechanics, though full chaos suppresses these effects in the former but not necessarily the latter. The relationship between ratchet effects and coherent dynamical control is noted.Comment: 21 pages, 7 figures, to appear in Phys. Rev.

Directed deterministic classical transport: symmetry breaking and beyond

We consider transport properties of a double delta-kicked system, in a regime where all the symmetries (spatial and temporal) that could prevent directed transport are removed. We analytically investigate the (non trivial) behavior of the classical current and diffusion properties and show that the results are in good agreement with numerical computations. The role of dissipation for a meaningful classical ratchet behavior is also discussed.Comment: 10 pages, 20 figure

Intermittency of glassy relaxation and the emergence of a non-equilibrium spontaneous measure in the aging regime

We consider heat exchange processes between non-equilibrium aging systems (in their activated regime) and the thermal bath in contact. We discuss a scenario where two different heat exchange processes concur in the overall heat dissipation: a stimulated fast process determined by the temperature of the bath and a spontaneous intermittent process determined by the fact that the system has been prepared in a non-equilibrium state. The latter is described by a probability distribution function (PDF) that has an exponential tail of width given by a parameter $\lambda$, and satisfies a fluctuation theorem (FT) governed by that parameter. The value of $\lambda$ is proportional to the so-called effective temperature, thereby providing a practical way to experimentally measure it by analyzing the PDF of intermittent events.Comment: Latex file, 8 pages + 5 postscript figure

Entanglement and Timing-Based Mechanisms in the Coherent Control of Scattering Processes

The coherent control of scattering processes is considered, with electron impact dissociation of H$_2^+$ used as an example. The physical mechanism underlying coherently controlled stationary state scattering is exposed by analyzing a control scenario that relies on previously established entanglement requirements between the scattering partners. Specifically, initial state entanglement assures that all collisions in the scattering volume yield the desirable scattering configuration. Scattering is controlled by preparing the particular internal state wave function that leads to the favored collisional configuration in the collision volume. This insight allows coherent control to be extended to the case of time-dependent scattering. Specifically, we identify reactive scattering scenarios using incident wave packets of translational motion where coherent control is operational and initial state entanglement is unnecessary. Both the stationary and time-dependent scenarios incorporate extended coherence features, making them physically distinct. From a theoretical point of view, this work represents a large step forward in the qualitative understanding of coherently controlled reactive scattering. From an experimental viewpoint, it offers an alternative to entanglement-based control schemes. However, both methods present significant challenges to existing experimental technologies