Wave-mechanical phenomena such as resonance and interference, in both light and matter, are central to the principles of quantum coherent control over molecular processes. Focusing on the dynamical aspects, this dissertation is a compilation of studies on the interaction physics involving wave packets in molecules, the driving light field, and the underlying coherence and control. In each work, we will demonstrate interesting correlations between the properties of a carefully designed excitation light field and desirable outcomes of the molecules quantum dynamics. We will analyze the dynamical effect of a Feshbach resonance in the adiabatic Raman photoassociation for ultracold diatomic molecule formation from ultracold atoms. A narrow resonance is shown to be able to increase the effective number of collisions, in an ultracold atomic gas, that are available for photoassociation. This results in an optimal resonance width much smaller than the atomic collision energy bandwidth, due to the balance between the effective collision rate and single-collision transfer probability. Next, we demonstrate the linear molecular response to high-intensity, broadband, shaped optical fields. We show that this originates from interferences based on intra-pulse Raman excitations, and thus response linearity is not unique to the first-order perturbative limit and can not be used to infer the strength of the field. In the last study, we simulate the stochastic vibrational wave packet and dissociation-flux dynamics in a molecule excited by light with temporal and spectral incoherent properties. Between this case and that using a coherent pulse with the same spectral profile, we compare the vibrational wave functions and the loss of electronic and vibrational coherence, and demonstrate the qualitative difference between coherently and incoherently driven dynamics in molecules.
Wave-mechanical phenomena such as resonance and interference, in both light and matter, are central to the principles of quantum coherent control over molecular processes. Focusing on the dynamical aspects, this dissertation is a compilation of studies on the interaction physics involving wave packets in molecules, the driving light field, and the underlying coherence and control. In each work, we will demonstrate interesting correlations between the properties of a carefully designed excitation light field and desirable outcomes of the molecules quantum dynamics. We will analyze the dynamical effect of a Feshbach resonance in the adiabatic Raman photoassociation for ultracold diatomic molecule formation from ultracold atoms. A narrow resonance is shown to be able to increase the effective number of collisions, in an ultracold atomic gas, that are available for photoassociation. This results in an optimal resonance width much smaller than the atomic collision energy bandwidth, due to the balance between the effective collision rate and single-collision transfer probability. Next, we demonstrate the linear molecular response to high-intensity, broadband, shaped optical fields. We show that this originates from interferences based on intra-pulse Raman excitations, and thus response linearity is not unique to the first-order perturbative limit and can not be used to infer the strength of the field. In the last study, we simulate the stochastic vibrational wave packet and dissociation-flux dynamics in a molecule excited by light with temporal and spectral incoherent properties. Between this case and that using a coherent pulse with the same spectral profile, we compare the vibrational wave functions and the loss of electronic and vibrational coherence, and demonstrate the qualitative difference between coherently and incoherently driven dynamics in molecules.
In this work, the coherence properties of the matter is studied in the frame of photoassociation of ultracold alkali atoms by using short and ultrashort laser pulses. Ultracold can be described with temperatures below 100 microK. When energy approaches the ultracold regime, quantum properties of matter becomes visible. In this regime matter such as atoms and molecules show wave properties. Coherent control employs the wave properties of matter to steer a physical process toward a desired target. It can be implemented with coherent radiation such as laser light. In particular, femtosecond laser pulses can be shaped. They thus have become a standard tool for coherent control. Since matter shows its wave properties most clearly at ultralow temperatures, ultracold matter represents the perfect object for coherent control. In the other hand using incoherent fields on ultracold matter does not give us good opportunities to drive quantum properties of it. In this work, it is aimed to bring together ultracold and ultrafast regimes in the frame of photoassociation. Photoassociation, excitation of two colliding atoms by laser light creates very weakly bound molecules. In the first part of this study, femtosecond photoassociation experiments are simulated. In these experiments and simulations, the broad spectrum of the femtosecond pulses addressing both atomic and molecular transitions is manipulated in a way to prevent the atomic transitions and to promote the molecular transitions. Then the effect of chirp and the effect of additional phase to the pulse are examined. Excited and ground state population are monitored by carrying out quantum dynamical simulations. The driving ultrafast dipole dynamics are presented. The photoassociation rate is limited by the low initial atomic pair density at inter-nuclear separations where photoassociation is feasible. In the second part of this study, a new scheme is proposed in order to manage this low pair density by manipulating the shape resonances of the initial atom pair. A shape resonance which is a metastable state occurs when some part of the wavefunction of partial waves with J > 0 trapped by a rotational barrier shows a bound structure in the closer internuclear distances where the rest of it shows a scattering wavefunction behavior. Most of the shape resonances in alkaline and alkaline earth metals occur typically at temperatures of a few milli-Kelvin. Therefore, the thermal weight of a shape resonance at photoassociation temperatures is quite small. By applying a non-resonant laser to the system the energy of the resonant state is brought close to the energies those corresponding to the photoassociation temperatures. Thus, the thermal weight of a shape resonance is amplified and initial pair density is enhanced. This effect is studied in both adiabatic and non-adiabatic time scales with respect to the rotational period of the system, intensity effects on the resonance energy and lifetime are presented.
This thesis presents original work contributing to the understanding of coherent control of wave packet dynamics in atomic and molecular systems excited by sequences of ultrafast laser pulses. An introduction to coherent control and a review of the literature is presented in chapter 1. Different approaches to coherent control are considered from the perspective of the phase complexity of the light fields used to achieve them, starting with a single transform-limited pulse, advancing through two-colour and multi-pulse schemes to chirped pulses, and finally arbitrarily shaped pulses. The purpose of this literature review is to position the work described in this thesis within the context of contemporary coherent control. In particular, our approach is seen to be in line with the current trend towards understanding the physics underlying coherent control processes. In chapter 2, development of a sodium atom and dimer beam source for use in optical coherent control experiments is described. The concepts of molecular beams are introduced along with a brief review of the literature on sodium atom and dimer beams. The beam source is considered in two parts. Firstly the sodium oven and nozzle are described in detail, along with the vacuum chamber that houses them. This is followed by a description of the experimental chamber, which contains a time-of-flight mass spectrometer. Finally a number of potential improvements to the system are discussed. Experiments to control the angular momentum composition of Rydberg electronic wave packets in atomic sodium are reported in chapter 3. The control scheme uses phase-locked pairs of transform-limited picosecond laser pulses to create pairs of identical radial wave packets in the Rydberg states. Quantum interference between the wave packets, or the excitation pathways to them, is manipulated to control the total angular composition of the resultant wave packet, which is detected either by the optical Ramsey method or by state-selective field ionisation. The results are interpreted both from the perspective of time evolution of the wave packet and the frequency content of the laser fields. The control strategy is then extended theoretically by the inclusion of linear chirp in one of the laser pulses. Chapter 4 describes progress towards a novel experiment for optical coherent control of vibrational wave packet dynamics in the sodium dimer. A coherent superposition of high-lying Rydberg states of the Na2 is excited by the broad bandwidth of a femtosecond laser pulse. The Rydberg states converge to several vibrational ionisation limits of the dimer, so the excitation effectively forms a vibrational wave packet in the X+ 22g+ ionic potential. Excitation of several wave packets by a series of phase-locked transform-limited pulses allows control over the composition and dynamics of the wave packet, and its detection using a combination of the optical Ramsey method and pulsed-field ionisation techniques adapted from ZEKE spectroscopy. A simple genetic algorithm is presented in chapter 5. It has been implemented to investigate the suitably of this type of optimisation procedure in simulations of Rydberg electronic wave packet coherent control in atomic sodium. The operation of the algorithm is described in detail. It is then tested against known solutions for two experimental scenarios: coherent control based on pairs of transform-limited laser pulses and transform-limited first pulse followed by a chirped second pulse. The behaviour of the algorithm is discussed in detail for each of these cases. Finally, the potential for application of the algorithm to unknown problems is briefly discussed.
We investigate the dynamics of molecular photoexcitation by unchirped femtosecond laser pulses using RbCs as a model system. This study is motivated by a goal of optimizing a two-color scheme of transferring vibrationally excited ultracold molecules to their absolute ground state. In this scheme the molecules are initially produced by photoassociation or magnetoassociation in bound vibrational levels close to the first dissociation threshold. We analyze here the first step of the two-color path as a function of pulse intensity from the low-field to the high-field regime. We use two different approaches, a global one, the ``wave-packet'' method, and a restricted one, the ``level-by-level'' method, where the number of vibrational levels is limited to a small subset. The comparison between the results of the two approaches allows one to gain qualitative insights into the complex dynamics of the high-field regime. In particular, we emphasize the nontrivial and important role of far-from-resonance levels which are adiabatically excited through ``vertical'' transitions with a large Franck-Condon factor. We also point out the spectacular excitation blockade due to the presence of a quasidegenerate level in the lower electronic state. We conclude that selective transfer with femtosecond pulses is possible in the low-field regime only. Finally, we extend our single-pulse analysis and examine population transfer induced by coherent trains of low-intensity femtosecond pulses.
Feshbach resonances play a major role in translationally cold-molecule preparation. In this context, their laser control is of crucial importance. This work is devoted to the depiction of some basic mechanisms of such a control using intense, short laser pulses and referring to nonlinear multiphoton processes. Our goal is to adiabatically transport a Feshbach resonance onto a zero-width resonance, the characteristics of which have already been discussed in the literature. Three processes are then addressed: (i) during the rise of the pulse and its plateau, the preparation of a so-called laser bound molecule (LBM) still stable, but structurally different from the standard chemically bound molecule; (ii) during the pulse switching off, an adiabatic transport of this LBM on a very few excited vibrational levels, and (iii) concomitantly, a filtration strategy to photodissociate all these levels except one, giving thus rise to but a single field-free excited vibrational state. With or without an eventual stimulated Raman adiabatic passage technique to bring all the population to the ground rovibrational state, this opens an alternate for a full optical control of ultracold-molecule formation. The illustrative example, offering the potentiality to be transposed to other diatomics, is H2+.
Any real system, no matter how well isolated, unavoidably interacts with the environment. This interaction makes the evolution of a quantum system nonunitary and destroys the coherence of quantum superpositions. This process, known as decoherence, is widely regarded as the most important and fundamental obstacle to the practical realization of quantum information processing. On the other hand, the objectives of quantum control are predicated on the ability to manipulate the system coherently over a period of time. Quantum control has been applied successfully to several experimental systems and coupling such a controlled system to an environment makes the study of coherence loss possible. The standard model of decoherence is formulated in terms of a system, defined by the observables that can be measured, and a reservoir, de¯ned by those degrees of freedom that are not accessed by observation. The vibrational and rotational degree of freedom of diatomic molecules form an experimental model of such a system and bath 1. The nature of the ro-vibrational coupling does not allow a Decoherence Free Subspace, instead we try to identify a Decoherence Resistant Subspace using closed loop control. We set up a learning control loop experiment to monitor the coherence in a vibrational wavepacket, consisting of about 6 vibrational states of the A(1Σ+g ) electronic state, excited by a shaped, ultrashort optical pulse, and then to determine the pulse shape that yielded the longest coherence lifetime. The coherence of the vibrational wavepacket was monitored by measuring the fluorescence quantum beats from the outer turning point of the wave packet's trajectory. The closed loop controller altered the shape of the driving optical pulse to maximize the the depth of modulation of the quantum beats at a prescribed time after excitation, within the constraints of the laser system. The outcome of this closed loop control experiment shows an improvement of the beat decay constant of about 60%, as shown in Fig. (1), a result which occurs for several di®erent pulse shapes. Our experiments show that a number of interesting decoherence-resistant excitations exist. The nature of the dephasing was examined theoretically. Using measured optical pulse shapes, we simulated numerically the molecular wavepacket in the limit of perturbation theory. The experiment and simulations show that there are di®erent mechanisms contributing to the decoherence resistant excitation. The shaped pulse reduces the °uctuations in the wave packet spatial extent, which in turn changes the system reservoir coupling. Other components of the optimal pulse are a sequence of pulses that creates constructive interference of the vibrational wavepacket at the desired time, and chirp which compensates for the anharmonicity of the adiabatic potential. The experimental results show general agreement with numerical simulations. We conclude that wavepackets resistant to dephasing do exist,and that learning algorithms provide an effcient means for finding them.
The assembly and coherent control of ultracold molecules have opened new horizons in quantum science with applications in ultracold chemistry, quantum information, quantum simulation, precision metrology and fundamental physics. While considerable progress has been achieved in controlling diatomic molecules at ultracold temperatures, extending to atom-diatom collisions and triatomic systems introduces additional internal structure, anisotropy, and interaction complexity, enabling access to many-body phenomena. This dissertation presents a detailed theoretical study on the production and stabilization of ultracold alkali-metal triatomic molecules, with a particular focus on the NaK2 molecular species. First, long-range photoassociation in ultracold NaK(X^1Σ+) + K(4s) mixtures is investigated near the molecular transition X^1Σ+ → b^3Π. Photoassociation rates into weakly bound trimers in the electronically excited 3^2A' state are evaluated under experimentally relevant conditions. The calculated photoassociation rates indicate experimentally feasible and spectrally selective routes for detecting ultracold atom–diatom photoassociation resonances, while suppressing competing diatomic loss channels. Second, a microwave-assisted association mechanism is introduced for coherently binding ultracold atoms and polar molecules into weakly bound triatomic molecules on the ground-state 1^2A' potential energy surface. This process relies on the interaction between the electric component of the microwave field and the permanent electric dipole moment of the NaK molecule. In the presence of the anisotropic atom–diatom potential, which couples the molecular rotation to the relative angular motion, the microwave field can drive transitions from scattering states to weakly bound trimer states entirely in the electronic ground-state of the triatomic molecule, without involving spontaneous emission. This process is therefore intrinsically coherent and particularly suited to closed-shell polar molecules for which magnetically tunable Feshbach resonances are weak or absent. Finally, candidate pathways toward coherent population transfer to the absolute rovibronic ground state of NaK2 are explored using bridge states belonging to the excited 3^2A' electronic state, such as those identified in [Ref.1]. The relevant potential energy surfaces, transition dipole moments, and rovibrational structures are analyzed, revealing favorable conditions for future stimulated Raman adiabatic passage scheme. The theoretical modeling combines high-level electronic-structure calculations with nuclear-motion methods across multiple energy and length scales. Three-dimensional potential energy surfaces and electronic transition dipole moment surfaces were obtained from ab initio multireference configuration interaction calculations employing effective-core pseudopotentials and core-polarization potentials. Nuclear motion was treated using coupled-channel methods for describing weakly bound states and discrete variable representation methods for deeply bound rovibrational states. The resulting wavefunctions were subsequently used to evaluate Franck–Condon factors, photo- and microwave association rates, and transition strengths relevant to experimental implementation. Overall, this dissertation provides an extensive theoretical framework for the formation, detection and ground-state stabilization of ultracold NaK2 trimers. Reference: [1] J. Cao, B.-Y. Wang, H. Yang, Z.-J. Fan, Z. Su, J. Rui, B. Zhao, and J.-W. Pan, “Observation of photoassociation resonances in ultracold atom-molecule collisions,” Phys. Rev. Lett., vol. 132, p. 093403, 2024.