Showing papers by "Stéphane Guérin published in 2005"

Control of field-free molecular alignment by phase-shaped laser pulses

Abstract: We report an experimental study of the control of molecular alignment of ${\mathrm{N}}_{2}$ by use of spectrally modulated pulses at an intensity regime below the intrinsic saturation of the alignment. By manipulating the relative timing of the alignment revival pattern arising from the even subset of the thermal ensemble as compared to the odd subset, we demonstrate that the angular distribution of the aligned molecule can be converted into planar delocalization at specific times. We also show that the angular focusing of the molecular axis can be switched off by applying a specific bipulse.

Efficient and long-lived field-free orientation of molecules by a single hybrid short pulse.

Abstract: We show that a combination of a half-cycle pulse and a short nonresonant laser pulse produces a strongly enhanced postpulse orientation. Robust transients that display both efficient and long-lived orientation are obtained. The mechanism is analyzed in terms of optimal oriented target states in finite Hilbert subspaces and shows that hybrid pulses can prove useful for other control issues. © 2005 The American Physical Society.

Atom-photon, atom-atom, and photon-photon entanglement preparation by fractional adiabatic passage

Abstract: We propose a relatively robust scheme to generate maximally entangled states of (i) an atom and a cavity photon, (ii) two atoms in their ground states, and (iii) two photons in two spatially separate high-$Q$ cavities. It is based on the interaction via fractional adiabatic passage of a three-level atom traveling through a cavity mode and a laser beam. The presence of optical phases is emphasized.

Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses.

Abstract: We show that a linear molecule subjected to a short specific elliptically polarized laser field yields postpulse revivals exhibiting alignment alternatively located along the orthogonal axis and the major axis of the ellipse. The effect is experimentally demonstrated by measuring the optical Kerr effect along two different axes. The conditions ensuring an optimal field-free alternation of high alignments along both directions are derived.

Fast SWAP gate by adiabatic passage

Abstract: We present a process for the construction of a SWAP gate which does not require a composition of elementary gates from a universal set. We propose to employ direct techniques adapted to the preparation of this specific gate. The mechanism, based on adiabatic passage, constitutes a decoherence-free method in the sense that spontaneous emission and cavity damping are avoided.

Decoherence-free creation of atom-atom entanglement in a cavity via fractional adiabatic passage

Abstract: We propose a robust and decoherence insensitive scheme to generate controllable entangled states of two three-level atoms interacting with an optical cavity and a laser beam. Losses due to atomic spontaneous transitions and to cavity decay are efficiently suppressed by employing fractional adiabatic passage and appropriately designed atom-field couplings. In this scheme the two atoms traverse the cavity-mode and the laser beam in opposite directions and become entangled in the free space outside the cavity. We also show that the coherence of a traveling atom can be transferred to the other one without populating the cavity mode.

Control of mixed-state quantum systems by a train of short pulses

Abstract: A density matrix approach is developed for the control of a mixed-state quantum system using a time-dependent external field such as a train of pulses. This leads to the definition of a target density matrix constructed in a reduced Hilbert space as a specific combination of the eigenvectors of a given observable through weighting factors related to the initial statistics of the system. A train of pulses is considered as a possible strategy to reach this target. An illustration is given by considering the laser control of molecular alignment and orientation in thermal equilibrium. © 2005 The American Physical Society.

Laser control for the optimal evolution of pure quantum states

Abstract: Starting from an initial pure quantum state, we present a strategy for reaching a target state corresponding to the extremum (maximum or minimum) of a given observable. We show that a sequence of pulses of moderate intensity, applied at times when the average of the observable reaches its local or global extremum, constitutes a strategy transferable to different control issues. Among them, postpulse molecular alignment and orientation are presented as examples. The robustness of such strategies with respect to experimentally relevant parameters is also examined.

State-selective chirped adiabatic passage on dynamically laser-aligned molecules

Abstract: We show that rovibrational state selectivity can be achieved by chirped adiabatic passage of molecules that are adiabatically aligned by a nonresonant laser field. We develop the tools to design the appropriate frequency and amplitude modulations that allow us to select a given route in the Hilbert space that leads to a final complete excitation of the chosen state, by infrared or by Raman processes. This method allows us to select a given vibrational state in a well-defined rotational $J$ state.

Quantum averaging and resonances: Two-level atom in a one-mode quantized field

Abstract: We construct a nonperturbative approach based on quantum averaging combined with resonant transformations to detect the resonances of a given Hamiltonian and to treat them. This approach, which generalizes the rotating-wave approximation, takes into account the resonances at low field and also at high field (nonlinear resonances). This allows us to derive effective Hamiltonians that contain the qualitative features of the spectrum, i.e., crossings and avoided crossings, as a function of the coupling constant. At a second stage the precision of the spectrum can be improved quantitatively by standard perturbative methods like contact transformations. We illustrate this method by determining the spectrum of a two-level atom interacting with a single-mode quantized field.