Stéphane Vranckx

Photodissociation and radiative association of HeH+ in the metastable triplet state.

We investigate the photodissociation of HeH(+) in the metastable triplet state as well as its formation through the inverse process, radiative association. In models of astrophysical plasmas, HeH(+) is assumed to be present only in the ground state, and the influence of the triplet state has not been explored. It may be formed by radiative association during collisions between a proton and metastable helium, which are present in significant concentrations in nebulae. The triplet state can also be formed by association of He(+) and H, although this process is less likely to occur. We compute the cross sections and rate coefficients corresponding to the photodissociation of the triplet state by UV photons from a central star using a wave packet method. We show that the photodissociation cross sections depend strongly on the initial vibrational state and that the effects of excited electronic states and nonadiabatic couplings cannot be neglected. We then calculate the cross section and rate coefficient for the radiative association of HeH(+) in the metastable triplet state.

Implementing quantum algorithms in hyperfine levels of ultracold polar molecules by optimal control

We numerically implement quantum algorithms in hyperfine levels of ultracold polar molecules. Logical operations are driven by pulses optimized by optimal control theory. All implementations take place in the lowest two rotational levels of the ground vibrational state of the ground (1)Σ(+) electronic state, exploiting the richness of the hyperfine energy structure and state mixing in static external fields. We show that it is possible to realize high fidelity complex logical operations with microsecond pulses. The possibility to run algorithms implemented on two interacting molecules is also demonstrated. (41)K(85)Rb and (41)K(87)Rb molecules are considered for the numerical simulations but the results are general and can be extended to other species.

Control of molecular dynamics with zero-area fields: Application to molecular orientation and photofragmentation

The constraint of time-integrated zero area on the laser field is a fundamental requirement, both theoretically and experimentally, in the control of molecular dynamics. By using techniques of local and optimal control theory, we show how to enforce this constraint in two benchmark control problems, namely, molecular orientation and photofragmentation. The origin and the physical implications for the dynamics of this zero-area control field are discussed.

External constraints on optimal control strategies in molecular orientation and photofragmentation: role of zero-area fields

We propose a new formulation of optimal and local control algorithms which enforces the constraint of time-integrated zero-area on the control field. The fulfillment of this requirement, crucial in many physical applications, is mathematically implemented by the introduction of a Lagrange multiplier aiming at penalizing the pulse area. This method allows one to design a control field with an area as small as possible, while bringing the dynamical system close to the target state. We test the efficiency of this approach on two control purposes in molecular dynamics, namely, orientation and photodissociation.

Determination of photodissociation and radiative association cross sections from the same time-dependent calculation

We illustrate some of the difficulties that may be encountered when computing photodissociation and radiative association cross sections from the same time-dependent approach based on wavepacket propagation. The total and partial photodissociation cross sections from the 33 vibrational levels of the b 3Σ+ state of HeH+ towards the nine other 3Σ+ and 6 3Π n = 2, 3 higher lying electronic states are calculated, using the autocorrelation method introduced by Heller (1978 J. Chem. Phys. 68 3891) and the method based on the asymptotic behaviour of wavepackets introduced by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741). The corresponding radiative association cross sections are extracted from the same calculations, and the photodissociation and radiative association rate constants are determined.

From atoms to biomolecules: A fruitful perspective

We present a summary of the research activities of the “Quantum Chemistry and Atomic Physics” theoretical group of the “Chimie Quantique et Photophysique” Laboratory at Université Libre de Bruxelles. We emphasize the links between the three orientations of the group: theoretical atomic spectroscopy, structure, and molecular dynamics and list the perspectives of our collaboration.

Radiative stabilization and photodissociation of HeH+ in its two lowest 3 sigma plus states

Although it is thought to play an important role in the chemistry of some extra-terrestrial environments, the HeH+ cation has not been detected in space so far. We suggest it could be observed in its triplets rather than singlet states and we study the formation by radiative stabiliation and the destruction by photodissociation of the two lowest states of this symmetry.

Implementing Quantum Gates and Algorithms in Ultracold Polar Molecules

We numerically investigate the implementation of small quantum algorithms, an arithmetic adder and the Grover search algorithm, in registers of ultracold polar molecules trapped in a lattice by concatenating intramolecular and intermolecular gates. The molecular states are modulated by the exposition to static electric and magnetic fields different for each molecule. The examples are carried out in a two-molecule case. Qubits are encoded either in rovibrational or in hyperfine states, and intermolecular gates involve states of neighboring molecules. Here we use π pulses (i.e., laser pulses such that the integral of the product of the transition dipole moment and their envelope is equal to π, thus ensuring a total population inversion between two states) and pulses designed by optimal control theory adapted to a multi-target problem to drive unitary transformations between the qubit states.

Local control of nonadiabatic photodissociation dynamics using Moller operators

We implement a local control strategy based on the use of Moller operators and use it to control the photodissociation of diatomic molecules in the presence of nonadiabatic interactions.

Implementing quantum algorithms in hyperfine levels of ultracold polar molecules

Using optimal control, we implement simple quantum algorithms using hyperfine states of ultracold polar molecules. The qubits are encoded in hyperfine states of isolated 41K85Rb or 41K87Rb molecules or in hyperfine states of two neighboring molecules.