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The Journal of Physical Chemistry A

Quantum Control of Molecular Wavepackets: An Approximate Analytic Solution for the Strong-Response Regime

Luı́s E. E. de Araujo, Ian A. Walmsley

Published Nov 19, 1999• 14 citations
Publisher

Abstract

We derive a simple approximate analytic solution for the driving field that will generate a desired target vibrational wavepacket in a diatomic molecule. This solution is valid for arbitrary population transfer from the initial to the final electronic states. Analytic solutions to the control problem are possible when the control field is of shorter duration than the characteristic time scale of the system being manipulated. For target states that are localized wavepackets, our technique yields multiple solutions for the control field. We describe a procedure to choose from the set of solutions one that achieves both the target state and population transfer with high accuracy, for a large class of target states. The fidelity of each solution is tested by comparing the state obtained from a direct numerical integration of Schrödinger's equation using the analytic solution with the specified target state.

DOI
10.1021/jp992323r

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•Nov 28, 2017•0 citations
We derive a simple approximate analytic solution for the driving field that will generate a desired target vibrational wavepacket in a diatomic molecule. This solution is valid for arbitrary population transfer from the initial to the final electronic states. Analytic solutions to the control problem are possible when the control field is of shorter duration than the characteristic time scale of the system being manipulated. For target states that are localized wavepackets, our technique yields multiple solutions for the control field. We describe a procedure to choose from the set of solutions one that achieves both the target state and population transfer with high accuracy, for a large class of target states. The fidelity of each solution is tested by comparing the state obtained from a direct numerical integration of Schrödinger's equation using the analytic solution with the specified target state.
In this article, we review a different approach to controlling quantum systems. We show that the quantum control problem can be greatly simplified by limiting the duration of the driving force to less than one characteristic period of the system. (For an atomic-electron Rydberg wavepacket this would be the Kepler period, for example, or the vibrational period in the case of a molecule.) If the target state is a bound state of the system, then for times less than the characteristic period, the particle does not have the opportunity to reach the system's boundary and acts essentially as a classical free particle. Such a restriction on the duration of the driving field allows an analytic solution to be found, even in the nonperturbative regime, helping clarify some of the differences between the perturbative and the nonperturbative regimes of excitation. We also show that our solution is nonunique, and the quantum controller has a multiplicity of solutions to chose from.
A simple analytic formula is derived for the impulse that will produce approximately, for arbitrarily large population transfer, a specified wave packet in a Rydberg atom. This solution to the nonlinear control problem is obtained in the limit that the duration of the pulsed control field is less than the Kepler period of the system. The validity of our solution is tested by comparison with a direct numerical integration of Schr\"odinger's equation, and it is found to yield the target state and population transfer with very high fidelity.
In this article, we describe an analytic approach to the quantum control of wave-packet dynamics. In particular, we show that the field that will produce a specified wave packet in a Rydberg atom can be found approximately from a simple analytic formula. This analytic solution is shown to hold even in the regime of strong excitation, and is found by limiting the duration of the pulsed control field to less than one Kepler period of the system. Comparison with a direct numerical integration of Schr\"odinger's equation shows that our solution yields the target state and population transfer with very high fidelity. The possibility of ionization to the continuum is also included in the model. The solution provides some insight into the multiplicity of solutions to the control problem and the relative complexity of the control-field shapes in the weak- and strong-response regimes.
A time-dependent formulation of quantum control is employed to investigate optimally controlled vibrational population transfer in a diatomic quantum system. The problem of finding the optimal laser field needed to achieve a specific quantum transition from an initial state to the desired target goal is formulated using an iterative method and the conjugate gradient method (CGM). The time-dependent Schrödinger equation is solved with interaction of laser radiation with matter included within a dipole approximation in the Hamiltonian. Appropriate boundary conditions are chosen for the evolution problem. The control objective is chosen as the value of transition probability from an initial state to a target state. A comparison is made between the results obtained using the iterative method and the CGM for optimization. Finally, quantum bits are encoded using the vibrational states of the diatomic in the regime of low-vibrational excitation.
The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation. Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
Quantum optimal control theory is applied to find optimal pulses for controlling the motion of an ion and a molecule for two different applications. Those optimal pulses enable the control of the dynamics of the system by driving the atom or the molecule from an initial state to desired states.The evolution equations obtained by means of the quantum optimal control theory are resolved iteratively using a monotonic convergent algorithm. A number of simulation parameters are varied in order to get the optimal pulses including the duration of the pulses, the time step of the time grid, a penalty factor that limits the maximal intensity of the fields, and a guess pulse which is used to start the optimal control.The optimal pulses obtained for each application are analyzed by Fourier transform, and also by looking at the time evolution of the populations that they generate in the system.The first application is the preparation of specific vibrational states of acetylene that are usually not reachable from the ground state, and that would remain unpopulated by usual spectroscopy. Relevant state energies and transition dipole moments are extracted from the experimental literature and especially from the global acetylene Hamiltonian conferring an uncommon precision to the control simulation. The control starts from the ground state. The target states belongs to the polyad Ns=1, Nr=5 of acetylene which includes two vibrational dark states and one vibrational bright state. First, the simulation is performed with the Schrödinger equation and in a second step, with the Liouville--von Neumann equation, as mixed states are prepared. Indeed, the control starts from a Boltzmann distribution of population in the rotational levels of the vibrational ground state chosen in order to simulate an experimental condition. But the distribution is truncated to limit the computational effort. One of the dark states appears to be a potential target for a realistic experimental investigation because the average population of the Rabi oscillation remains high and decoherence is expected to be weak. The optimal pulses obtained have a high fidelity, have a spectrum with well-resolved peak frequencies, and their experimental feasibility seems achievable within the current abilities of experimental laboratories.The second application is to propose an experimental realization of a microscopic physical device able to simulate quantum dynamics. The idea is to use the motional states of a Cd^+ ion trapped in an anharmonic potential to realize a quantum dynamics simulator of a single-particle Schrödinger equation. In this way, the motional states store the information and the optimal pulse manipulates this information to realize operations. In the present case, the simulated dynamics was the propagation of a wave packet in a harmonic potential. Starting from an initial quantum state, the pulse acts on the system to modify the motional states of the ion in such a way that the final superposition of motional states corresponds to the results of the dynamics. This simulation is performed with the Liouville--von Neumann equation and also with the Lindblad equation as dissipation is included to test the robustness of the pulse against perturbations of the potential. The optimal pulses that are obtained have a high fidelity which shows that the ion trap system has correctly realized the quantum dynamics simulation. The optimal pulses are valid for any initial condition if the potential of the simulation or the mass of the propagated wave packet is unchanged.