In the present work we theoretically analyze the emergence appearance of entanglement between electronic and vibrational states in a system of two coupled molecules after switching on the interaction between them. In the framework of the adiabatic approach this effect appears because the equilibrium position of one of the molecules is sensitive to its electron occupation. The time evolution of the system shows multiple changes between its entangled and unentangled states. If this evolution starts from a pure coherent vibrational mode state and a state of a single electron localized in one of the molecules, the Schr\"odinger-cat states can appear during the system's time evolution. The Schr\"odinger-cat states can arise as a quantum superposition of distinct coherent states of molecule vibrational mode. Such states are entangled with empty and singly occupied electronic states of the molecule. The presence of entanglement between electronic and vibrational molecule states can be revealed by measuring the electron occupation (charge) of a particular molecule.
In the present work we theoretically analyze the emergence appearance of entanglement between electronic and vibrational states in a system of two coupled molecules after switching on the interaction between them. In the framework of the adiabatic approach this effect appears because the equilibrium position of one of the molecules is sensitive to its electron occupation. The time evolution of the system shows multiple changes between its entangled and unentangled states. If this evolution starts from a pure coherent vibrational mode state and a state of a single electron localized in one of the molecules, the Schr"odinger-cat states can appear during the system's time evolution. The Schr"odinger-cat states can arise as a quantum superposition of distinct coherent states of molecule vibrational mode. Such states are entangled with empty and singly occupied electronic states of the molecule. The presence of entanglement between electronic and vibrational molecule states can be revealed by measuring the electron occupation (charge) of a particular molecule.
Abstract The formation of entanglement between the electronic and vibrational subsystems of two interacting molecules localized between tunneling contact leads was theoretically analyzed using the Keldysh diagram technique. The time evolution of concurrence after ‘switching on’ the coupling between the molecules was investigated. It was revealed that non-zero concurrence can be present in the system in the resonant case, even if the molecules are connected by the leads. It was also shown that the stationary value of concurrence can be directly expressed by the stationary tunneling current. It reveals non-monotonic behavior with increasing coupling between the molecule’s electronic states. In the regime of small tunneling rates between the molecules and the leads, ‘switching on and off’ the coupling between molecules multiple times, while detecting one of the molecules’ charge states after each ‘on’ and ‘off’ cycle, results in the appearance of non-classical phonon statistics and opens the possibility of creating a vibrational mode in a Fock state.
In this thesis, we will analyze quantum entanglement phenomena in systems composed of quantum molecules, considering two different models. In the first, N molecules, each with a single excess electron, interact via Coulomb coupling. This interaction is restricted to the first neighbors. In the second system, electrons from distinct molecules interact with each other through the electron-phonon coupling. Considering the N-molecule system, we broadened our studies started in the master’s degree to verify the characteristics of the energy spectrum and eigenstates for the molecule system. We focus on the scalability of the system and its effect on entanglement, showing that some specific eigenstates are systematically associated with the maximally entangled GHZ states, regardless of the number of molecules. In the second part of the work, we explore the entanglement of both the electronic subsystem and the phonon subsystem, defining a new base composed of Bell’s electronic states associated with the excitation states in vibrational modes, which allows rewriting the Hamiltonian to the composite system. From this point of view, it is possible to determine a set of 4 ⊗ 4 matrix subspaces that elucidated the study of the entanglement behavior in both subsystems, allowing the determination of distinct sets of parameters effective in generating highly entangled states in the electronic subsystem and now in the phononic subsystem.
Macroscopic entangled cat states not only are significant in the demonstration of the fundamentals of quantum physics, but also have wide applications in modern quantum technologies such as continuous-variable quantum information processing and quantum metrology. Here we propose a scheme for generation of macroscopic entangled cat states in a longitudinal-coupled cavity-QED model. In particular, we introduce the scheme by using the terms of a molecular cavity-QED system, which is composed of an organic molecule (including electronic and vibrational states) coupled to a single-mode cavity field. In this system, the resonance frequencies of the molecular vibration and the cavity field are modulated simultaneously, then the molecular vibrational displacement is enhanced significantly and hence macroscopic entangled cat states between the molecular vibrational mode and the cavity mode can be created. We also study quantum coherence effects in the generated states by calculating the joint Wigner function and the degree of entanglement. The dissipation effects are included by considering the state generation in the open-system case. Our results will pave the way to the study of quantum physics and quantum chemistry in molecular cavity-QED systems. The physical mechanism behind this scheme is general and can be extended to other longitudinal-coupled cavity-QED systems.
Non-Markovian quantum evolution of the electronic subsystem in a laser-driven molecule is characterized through the appearance of negative decoherence rates in the canonical form of the electronic master equation. For a driven molecular system described in a bipartite Hilbert space H=Hel⨂Hvib of dimension $2\ifmmode\times\else\texttimes\fi{}{N}_{v}$, we derive the canonical form of the electronic master equation, deducing the canonical measures of non-Markovianity and the Bloch volume of accessible states. We find that one of the decoherence rates is always negative, accounting for the inherent non-Markovian character of the electronic evolution in the vibrational environment. Enhanced non-Markovian behavior, characterized by two negative decoherence rates, appears if there is a coupling between the electronic states g,e, such that the evolution of the electronic populations obeys d(PgPe)/dt>0. Non-Markovianity of the electronic evolution is analyzed in relation to temporal behaviors of the electronic-vibrational entanglement and electronic coherence, showing that enhanced non-Markovian behavior accompanies entanglement increase. Taking as an example the coupling of two electronic states by a laser pulse in the Cs2 molecule, we analyze non-Markovian dynamics under laser pulses of various strengths, finding that the weaker pulse stimulates the bigger amount of non-Markovianity. Our results show that increase of the electronic-vibrational entanglement over a time interval is correlated to the growth of the total amount of non-Markovianity calculated over the same interval using canonical measures and connected with the increase of the Bloch volume. After the pulse, non-Markovian behavior is correlated to electronic coherence, such that vibrational motion in the electronic potentials which diminishes the nuclear overlap, implicitly increasing the linear entropy of entanglement, brings a memory character to dynamics.
We consider the quantum entanglement of the electronic and vibrational degrees of freedom in molecules with tendencies towards double welled potentials. In these bipartite systems, the von Neumann entropy of the reduced density matrix is used to quantify the electron-vibration entanglement for the lowest two vibronic wavefunctions obtained from a model Hamiltonian based on coupled harmonic diabatic potential-energy surfaces. Significant entanglement is found only in the region in which the ground vibronic state contains a density profile that is bimodal (i.e., contains two separate local maxima). However, in this region two distinct types of density and entanglement profiles are found: one type arises purely from the degeneracy of energy levels in the two potential wells and is destroyed by slight asymmetry, while the other arises through strong interactions between the diabatic levels of each well and is relatively insensitive to asymmetry. These two distinct types are termed fragile degeneracy-induced entanglement and persistent entanglement, respectively. Six classic molecular systems describable by two diabatic states are considered: ammonia, benzene, BNB, pyridine excited triplet states, the Creutz-Taube ion, and the radical cation of the "special pair" of chlorophylls involved in photosynthesis. These chemically diverse systems are all treated using the same general formalism and the nature of the entanglement that they embody is elucidated.
A proposal for realizing one- and two-mode degenerate Raman coupled models, of the type proposed by Knight [Phys. Scr. T 12, 51 (1986)] in the context of quantum optics, for the motion of a trapped two-level ion, is presented. The interaction has a dispersive effect on the vibrational motion of the ion. For the one-mode case, an initial vibrational coherent state evolves into superpositions of coherent states (Schr\"odinger-cat states) correlated with the internal states of the ion. In the two-mode case, with initial coherent states in each mode, entanglements of coherent states between the modes correlated with the internal ionic states are obtained. Detection of such states is briefly discussed.