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The Journal of Chemical Physics

Quantum-phase and information-entropy dynamics of a two-state molecular system interacting with strongly amplitude- and phase-squeezed fields

Masayoshi Nakano, Kizashi Yamaguchi

Published Feb 8, 2000• 15 citations
Publisher

Abstract

We perform numerically exact calculations of quantum dynamics for a two-state molecular model system interacting with initially single-mode strongly amplitude- and strongly phase-squeezed fields (average photon number 〈n̂〉=8). The second-order correlation function for these fields are taken to be 2, which is equivalent to that for the chaotic field with the identical average photon number. Although the responses of the molecule for these two squeezed fields are relatively similar to those for the chaotic field as shown in a previous paper, there are found to be some significant differences among these fields in the dynamical behavior of quantum-phase distributions of photons, i.e., the Pegg–Barnett phase and the Q function distributions. These differences are also elucidated by the dynamics of the information entropy for the molecule, i.e., molecular entropy, which represents the degree of entanglement between the molecule and the photon field.

DOI
10.1063/1.480851

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理论物理通讯：英文版•Jan 1, 2004•0 citations
We perform numerically exact calculations of quantum dynamics for a two-state molecular model system interacting with initially single-mode strongly amplitude- and strongly phase-squeezed fields (average photon number 〈n̂〉=8). The second-order correlation function for these fields are taken to be 2, which is equivalent to that for the chaotic field with the identical average photon number. Although the responses of the molecule for these two squeezed fields are relatively similar to those for the chaotic field as shown in a previous paper, there are found to be some significant differences among these fields in the dynamical behavior of quantum-phase distributions of photons, i.e., the Pegg–Barnett phase and the Q function distributions. These differences are also elucidated by the dynamics of the information entropy for the molecule, i.e., molecular entropy, which represents the degree of entanglement between the molecule and the photon field.
From the viewpoints of photon-phase and information-entropy dynamics, we investigate the dynamical behavior of initially one-mode amplitude- and phase-squeezed photon fields interacting with a two-state molecular system. A peculiar behavior of these systems is known to be the collapses and revivals of Rabi oscillation of the molecular population. As shown in previous studies, significant differences are observed in the amplitude and the period of the collapse−revival behavior for these fields. These differences are found to be well-described by the dynamical behavior of the photon phase (the Pegg−Barnett phase and the quasiprobability ( Q function) distributions). The features of these photon-phase dynamics are also found to provide significant influence on the time evolution of the information entropy of the molecule, which characterizes the degree of the entanglement between the molecule and the field.
We investigate the quantum dynamics for a molecular-state-model system interacting with a resonant two-mode squeezed coherent field, in which each mode is initially correlated. As an example, we consider a three-state molecular model, which mimics the electronic excited states of trans-octatetraene obtained by a full-configuration-interaction calculation using the Pariser-Parr-Pople Hamiltonian, interacting with a two-mode squeezed coherent field. For comparison, we also perform parallel studies, in which the initial fields are prepared in uncorrelated two-mode states, i.e., a two-mode coherent state and a two-mode thermal state. It is well known that the case of two-mode coherent field leads to the usual collapse-revival phenomena of the Rabi oscillations, while the case of a two-mode thermal state leads to an irregular evolution of them. Although the two-mode squeezed coherent field exhibits similar collapse-revival behavior to that for the two-mode coherent field, some differences are detected in the amplitudes and periods of collapse-revival oscillations. Further, the dynamics of off-diagonal molecular density matrices between the ground and second excited states for these two fields are found to be distinctively different from each other. Such attractive behavior is found to be closely associated with the two-mode photon-phase dynamics obtained by the Pegg-Barnett phase operator and the initial quasiprobability (Q function) distribution of photons. These differences in phase properties are also shown to significantly affect the dynamics of the information entropy for a molecule, i.e., molecular entropy, which represents the degree of entanglement between the molecule and photons.
We investigate the quantum-phase dynamics of a dissipative molecular system interacting with a single-mode amplitude-squeezed field (with a sub-Poissonian photon-number distribution) using the Monte Carlo wave-function method. As quantum dissipation, we consider molecular coherent (phase) and population relaxations, which are caused by nuclear vibration, and cavity relaxation (the dissipation of an internal single mode to outer mode). In this study, we elucidate the effects of these dissipations on the unique quantum dynamical behaviors of this coupled system, e.g., collapse-revival behavior of Rabi oscillations, using the quasiprobability (Q function) distribution of a single-mode field and the off-diagonal molecular density matrix. It is found that although the amplitude-squeezed field exhibits a distinct revival with larger amplitudes of molecular population in the presence of the molecular phase and population relaxations as compared to the coherent field, a very slight cavity relaxation suppresses the revival of molecular population and the increase in the magnitude of off-diagonal molecular density matrices during the quiescent region in the amplitude-squeezed field case more significantly than in the coherent field case. These features are shown to be closely related to the difference in the dynamics of Q function distributions between amplitude-squeezed and coherent fields.
We investigate the intermolecular-interaction (dipole–dipole interaction) effects on the quantum dynamics of dimer density matrices and photon-phase distributions using several dimer models with different intermolecular distances in the presence of a two-mode squeezed vacuum field. In this photon field, each mode is initially correlated and the reduced one-mode photon distribution is equivalent with that of a thermal field. For comparison, we perform parallel studies, in which the initial fields are two types of noncorrelated two-mode fields, i.e., a two-mode coherent field and a two-mode thermal field. It is found that although the two-mode squeezed vacuum field causes the random oscillations of dimer populations in the noninteracting dimer, the periodic oscillations like the collapse–revival behavior emerge as the intermolecular distance decreases (the intermolecular interaction increases). Similar and dissimilar features among quantum dynamics caused by these three types of fields are investigated by analyzing the dynamical behavior of two-mode Pegg–Barnett photon-phase distributions and off-diagonal dimer density matrices, which indicate the coherency between dimer states. In addition to the quantum statistical properties of initial photon field, the change in the degree of contribution between one- and two-photon processes caused by the intermolecular interaction is found to be important to determine these features.
From a quantum information point of view we investigate the entropy squeezing properties for a two-level atom interacting with the two-mode coherent fields via the two-photon transition. We discuss the influences of the initial state of the system on the atomic information entropy squeezing. Our results show that the squeezed component number, squeezed direction, and time of the information entropy squeezing can be controlled by choosing atomic distribution angle, the relative phase between the atom and the two-mode field, and the difference of the average photon number of the two field modes, respectively. Quantum information entropy is a remarkable precision measure for the atomic squeezing.
From a quantum information point of view we investigate the entropy squeezing properties for a two-level atom interacting with the two-mode coherent fields via the two-photon transition. We discuss the influences of the initial state of the system on the atomic information entropy squeezing. Our results show that the squeezed component number,squeezed direction, and time of the information entropy squeezing can be controlled by choosing atomic distribution angle,the relative phase between the atom and the two-mode field, and the difference of the average photon number of the two field modes, respectively. Quantum information entropy is a remarkable precision measure for the atomic squeezing.