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Alignment control of an ensemble of nonpolar molecules is numerically studied by means of optimal control simulation. A nitrogen molecule that is modeled by a quantum rigid rotor is adopted. Controlled rotational wave packets are created through nonresonant optical transitions induced by polarizability coupling. Optimal pulses are designed to achieve the alignment control at a specified time in the absence/presence of external static fields in zero- and finite-temperature cases, as well as to maintain an aligned state. When maintaining an aligned state over a specified time interval is chosen as a target, the control mechanism is primarily attributed to a dynamical one. Multiple optimal solutions that lead to virtually the same control achievement are found, which are consistent with the topology of the quantum control landscape. [ABSTRACT FROM AUTHOR]

We numerically propose a way to perform quantum computations by combining an ensemble of molecular states and weak laser pulses. A logical input state is expressed as a superposition state (a wave packet) of molecular states, which is initially prepared by a designed femtosecond laser pulse. The free propagation of the wave packet for a specified time interval leads to the specified change in the relative phases among the molecular basis states, which corresponds to a computational result. The computational results are retrieved by means of quantum interferometry. Numerical tests are implemented in the vibrational states of the B state of I2 employing controlled-NOT gate, and 2 and 3 qubits Fourier transforms. All the steps involved in the computational scheme, i.e., the initial preparation, gate operation, and detection steps, are achieved with extremely high precision. [ABSTRACT FROM AUTHOR]

Optimal control simulation is applied to the cis-trans photoisomerization of retinal in rhodopsin within a two-dimensional, two-electronic-state model with a conical intersection [S. Hahn and G. Stock, J. Phys. Chem. B 104, 1146 (2000)]. For this case study, we investigate coherent control mechanisms, in which laser pulses work cooperatively with a conical intersection that acts as a “wave-packet cannon.” Optimally designed pulses largely consist of shaping subpulses that prepare a wave packet, which is localized along a reaction coordinate and has little energy in the coupling mode, through multiple electronic transitions. This shaping process is shown to be essential for achieving a high target yield although the envelopes of the calculated pulses depend on the local topography of the potential-energy surfaces around the conical intersection and the choice of target. The control mechanisms are analyzed by considering the motion of reduced wave packets in a nuclear configuration space as well as by snapshots of probability current-density maps. [ABSTRACT FROM AUTHOR]

Optimal control within the density matrix formalism is applied to the creation of a specified superposition state in condensed phases. The primary system modeled by a displaced harmonic oscillator is surrounded by a boson heat bath, the dynamics of which is described by a non-Markovian master equation. A newly developed monotonically convergent algorithm is used to solve the pulse design equations. The control mechanisms are strongly dependent on the bath correlation time that is characterized by the inverse of an exponential decay constant, γ. If the correlation time is shorter than the temporal width of a typical subpulse involved in an optimal pulse, the solution is reduced to that in the Markovian case. If we assume a longer correlation time, by weighing less physical significance on the penalty due to pulse fluence, an optimal pulse with high intensity is obtained, the temporal width of which approaches ∼1/γ. We also see considerable changes in the shape of the optimal pulse with increasing intensity, suggesting that strong fields open up control mechanisms that are qualitatively different from those in weak fields. [ABSTRACT FROM AUTHOR]

Abstract: We extend our previous optimal control simulation to explicitly include the polarization effects of a laser pulse. The simulation is applied to a rigid rotor model of a nitrogen molecule with two illustrative objectives to evaluate its numerical performance and examine the role of optimal time-dependent polarization vectors. The first example aims at aligning a molecule along the space-fixed z-axis that is perpendicular to the polarization vectors on the xy-plane, which leads to a circularly polarized pulse as an optimal solution. The second objective is to create a rotational wave packet that aligns two directions specified by y =±x simultaneously. The optimal polarization direction changes with time from y =+x to y =−x, alternately. These examples illustrate the importance of examining an optimal pulse as a vector wave, and the present simulation provides a useful means to explore the best (time-dependent) polarization conditions of a laser pulse. [Copyright &y& Elsevier]

Abstract: Wave packet isotope separation using short intense laser pulses is examined by means of optimal control simulation through a case study of the separation of a 79Br2/81Br2 mixture. Both electronic and vibrational degrees of freedom are considered. A designed optimal pulse enhances the isotope-dependent phase shifts in the wave packets through multi-photon transitions, causes isotope-dependent quantum interferences, and transfers each isotope to its specified objective state. The present isotope separation scheme achieves a high degree of selectivity even in the presence of dephasing processes. [Copyright &y& Elsevier]

Optimal control within the density matrix formalism is applied to the production of desired non-equilibrium distributions in condensed phases. The time evolution of a molecular system modeled by a displaced harmonic oscillator is assumed to be described by the Markoffian master equation with phenomenological relaxation parameters. The physical objectives of concern are the creation of a specified vibronic state, population inversion and wave packet shaping. The effects of an initial thermal distribution and dissipation on these targets are examined. In order to transfer a large amount of population (i.e., the strong-field regime) to a target wave packet in an electronic excited state, it is shown that creating a shaped packet in the ground state is often required to achieve high yield. This control pathway cannot be taken into account within the weak-field approximation, and is especially important when the target state includes vibrational states that are weakly accessible from the initial state or that have preferential indirect excitation paths from the initial state. Although relaxation effects usually reduce the control efficiency, under certain conditions, the bath-induced dynamics can help to create an objective state. [Copyright &y& Elsevier]

Nonresonant optimal control simulation is applied to a CO moleculeto design two-color phase-locked laser pulses (800 nm + 400 nm) withthe aim of orienting the molecule under the field-free condition.The optimal pulse consists of two subpulses: the first subpulse alignsthe molecule and the second one orients it. The molecular alignmentinduced by the first subpulse considerably enhances the degree oforientation, the value of which is close to an ideal value at temperature T= 0 K. To confirm the effectiveness of this alignment-enhancedorientation mechanism, we adopt a set of model Gaussian pulses andcalculate the maximum degrees of orientation as a function of thedelay time and the intensity. In finite-temperature (T= 3.0 K and T= 5.0 K) cases, although the alignmentsubpulse can improve the degree of orientation, the control achievementdecreases with temperature rapidly; this decrease can be attributedto the initial-state-dependent (phase-shifted) rotational wave packetmotion. [ABSTRACT FROM AUTHOR]