Quantum mechanics forbids us to describe a particle, once it correlates with another, as an individual physical object whose complete set of properties can be solely determined.Such correlations play one of the most important roles in understanding quantum phenomena such as non-adiabatic transitions in molecular dissociation, quantum teleportation, and Bose-Einstein condensate.This dissertation theoretically explores several phenomena to reveal the role of quantum correlations in molecular systems.Specifically, it studies the photoionization of H 2 and unambiguously evaluates the correlation between the photoelectron and H + 2 in terms of entanglement between the two particles.The correlation among the vibrational states of H + 2 is evaluated in terms of coherence and the relation with entanglement is studied.The ultrafast vibrational motion of H + 2 and its dependence on the photoelectron is analysed and the origin of this correlation is ascribed to the non-locality of the wave function.Moreover, the experimental scheme to extract the correlation effect from the vibrational motion of H + 2 using the sequence of ultrashort laser pulses is proposed.The correlation between a molecule and photons in a plasmonic nanocavity is also studied.This system has been attracting attention due to its potential use in quantum information and in the control of chemical reactions, and the analytical and numerical methods for describing the molecular dynamics in such a system are formulated.
Quantum mechanics forbids us to describe a particle, once it correlates with another, as an individual physical object whose complete set of properties can be solely determined.Such correlations play one of the most important roles in understanding quantum phenomena such as non-adiabatic transitions in molecular dissociation, quantum teleportation, and Bose-Einstein condensate.This dissertation theoretically explores several phenomena to reveal the role of quantum correlations in molecular systems.Specifically, it studies the photoionization of H 2 and unambiguously evaluates the correlation between the photoelectron and H + 2 in terms of entanglement between the two particles.The correlation among the vibrational states of H + 2 is evaluated in terms of coherence and the relation with entanglement is studied.The ultrafast vibrational motion of H + 2 and its dependence on the photoelectron is analysed and the origin of this correlation is ascribed to the non-locality of the wave function.Moreover, the experimental scheme to extract the correlation effect from the vibrational motion of H + 2 using the sequence of ultrashort laser pulses is proposed.The correlation between a molecule and photons in a plasmonic nanocavity is also studied.This system has been attracting attention due to its potential use in quantum information and in the control of chemical reactions, and the analytical and numerical methods for describing the molecular dynamics in such a system are formulated.
We make a theoretical investigation of photoionization of an H2 molecule, induced by the irradiation of an ultrashort extreme ultraviolet (XUV) laser pulse. We consider a system composed of a photoelectron ejected from H2 and the resultant H2+ as a bipartite system. In order to clarify how the interparticle correlations among two electrons and two protons in H2 are reflected to the bipartite system, we examine the entanglement between the photoelectron and the vibrational states of H2+ as well as the coherence in the vibrational states of H2+ by simulating the photoionization process of one-dimensional H2. In the simulation, we solve a time-dependent Schr\"odinger equation using a symmetry-adapted grid method. On the basis of the simulations with ten different sets of three parameters characterizing an ultrashort XUV laser pulse, i.e., the pulse duration, the wavelength, and the peak intensity, we show that the extent of the entanglement depends sensitively on the coherence in the vibrational states of H2+.
Attosecond physics enables the study of ultrafast coherent electron dynamics in matter upon photoexcitation and photoionization, revealing spectacular effects such as hole migration and coherent Auger dynamics in molecules. In the photoionization scenario, there has been a strong focus on probing the physical manifestations of internal quantum coherence within the individual parent ion and photoelectron systems. However, quantum correlations between these two subsystems emerging from attosecond photoionization events have thus far remained much more elusive. In this work, we design theoretically and model numerically a direct probe of quantum entanglement in attosecond photoionization in the form of a Bell test. We simulate from first principles a Bell test protocol for the case of noble gas atoms photoionized by ultrashort, circularly polarized infrared laser pulses in the strong-field regime predicting robust violation of the Bell inequality. This theoretical result paves the way for the direct observation of entanglement in the context of ultrafast photoionization of many-electron systems. Our work provides a novel perspective on attosecond physics directed toward the detection of quantum correlations between systems born during attosecond photoionization and unraveling the signatures of entanglement in ultrafast coherent molecular dynamics, including in the chemical decomposition pathways of molecular ions. Published by the American Physical Society 2024
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.
With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing. This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâ s transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 ± 0.025, violating the CHSH inequality, compatible with the non-locality of the state. The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.
We characterize both entanglement and quantum coherence in a molecular system by connecting the linear entropy of electronic-nuclear entanglement with Wigner-Yanase skew information measuring vibronic coherence and local quantum uncertainty on electronic energy. Linear entropy of entanglement and quantifiers of quantum coherence are derived for a molecular system described in a bipartite Hilbert space H=Hel⨂Hvib of finite dimension ${N}_{el}\ifmmode\times\else\texttimes\fi{}{N}_{v}$, and relations between them are established. For the specific case of the electronic-vibrational entanglement, we find the linear entropy of entanglement as having a more complex informational content than the von Neumann entropy. By keeping the information carried by the vibronic coherences in a molecule, linear entropy seizes vibrational motion in the electronic potentials as entanglement dynamics. We analyze entanglement oscillations in an isolated molecule, and show examples for the control of entanglement dynamics in a molecule through the creation of coherent vibrational wave packets in several electronic potentials by using chirped laser pulses.