Recent advances in atomic/molecular physics have enabled unprecedented quantum control over molecules. However, quantum state preparation and detection of molecules remains a formidable challenge. Having demonstrated rovibronic ground state preparation of the aluminum monohydride cation in our lab, we now aim to develop a nondestructive method of detecting the internal quantum state of a single molecule using an extension of quantum logic spectroscopy. This technique will provide new avenues toward achieving high precision measurements and quantum information processing.
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How difficult is it to design the laser pulse which will transfer the quantum system to desired final state? We show that the natural laboratory limits on the maximum laser intensities give rise to complicated dependencies of the outcome on the pulse parameters even in the rather trivial case of a two-level system, and in general preclude the efficient usage of standard local search algorithms (such as the gradient descent methods). However, in the case of two-level systems, a simple mathematical trick allows us to drastically simplify the search for the best pulse shape.
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Can one coherently control the dynamics of quantum systems without lasers? We argue that the conventional quantum controls, such as shaped laser pulses, can be substituted with ordinary incoherent chemistry once the quantum system is put into the proper environment. As an example, we demonstrate that environmental engineering can dramatically enhance the efficiency of charge separation in donor-bridge-acceptor systems (which is a common model for light harvesting in organic solar cells) and even force the electronic energy flow in a counterintuitive direction, from a lower to a higher electronic level.

Torsions display fascinating dynamics for several reasons. In particular, their energy eigenvalue spectra, and hence also their laser-driven revival patterns, exhibit characteristics of both rotational and vibrational spectra, but yet are much richer than either of these previously intensively explored revival patterns. Furthermore, torsional modes play a critical role in a variety of molecular processes. An experimental handle on molecular torsions thus translates, potentially, into control over a wide variety of chemical phenomena. These include charge transfer events, energy flow, electric transport in molecular junctions, molecular chirality, the dynamics of nuclear spin isomers, and chemical reactions, including catalytic processes. In the past we used 1- and 2-dimensional models to explore the fundamental properties of torsional coherences, their controllability subject to a dissipative environment, the way in which they exchange phase information with a medium and several of their applications. Currently we develop and apply a full-dimensional (4D) model for control of coupled torsional-rotational wavepackets. We explore the interaction of coherently excited torsions with rotations and access the validity of low-dimensional models.

For molecules in which steric hindrance interferes with torsional motion but does not prevent it altogether, the torsion angle may be controlled with an external laser field. This is demonstrated on a model system consisting of biphenyl molecules adsorbed onto a silicon (111) surface. In an unperturbed monolayer, the planes of the biphenyl’s two rings are at an angle to each other (a). A moderately-intense nonresonant laser field induces dipoles in the molecules leading to the molecules’ rearrangement into the coplanar configuration to facilitate the induced dipole-laser field interaction (b). For this system there are two ways to achieve coplanarity in the biphenyl molecules. (i) The laser field is linearly polarized in a direction parallel to the silicon surface, and the field-matter interaction is maximized when both rings are parallel to the field polarization direction. (ii) The laser field polarization is perpendicular to the surface, and pi electrons can delocalize over a greater distance, resulting in a greater induced dipole, in the coplanar configuration.

Molecules have long been known to align in moderately intense, far off-resonance laser fields with a large variety of applications in physics, optics, chemistry and material research. In recent work we illustrate and explore the physical origin of a new phenomenon in the alignment dynamics of molecules, which is fundamentally interesting and has a useful potential implication. Namely, the intensity dependence of the degree of alignment exhibits a threshold behavior, below which molecules are isotropically distributed rotationally and above which the alignment is rapidly maximized, before reaching a plateau. Furthermore, we find that the intensity dependence of the alignment of all molecules is universal. Finally, we note that the alignment threshold occurs at a lower intensity than the off-resonance ionization threshold, a numerical observation that is readily explained. This finding illustrates that nonresonant alignment of molecules is a general method, applicable to all anisotropic molecules. The threshold behavior is attributed to a tunneling mechanism that rapidly switches off at the threshold intensity, where tunneling between the potential wells corresponding to the two orientations of the aligned molecules becomes forbidden. The rotational temperature of molecules determines the position of the alignment threshold, hence the critical role of rotational cooling found experimentally in the past. The universal threshold behavior of molecular alignment is a simple phenomenon, but is of interest to a broad variety of experiments.