C.F. Lo

Quantum prisoner dilemma under decoherence

It has recently been established that quantum strategies are superior to classical ones for games such as the prisoners dilemma. However, quantum states are subject to decoherence. In this Letter, we investigate the effects of decoherence on a quantum game, namely the prisoner dilemma, through three prototype decoherence channels. We show that in the case of prisoner dilemma, the Nash equilibria are not changed by the effects of decoherence for maximally entangled states.

Experimental and Theoretical Demonstration on the Transport Properties of Fused Ring Host Materials for Organic Light-Emitting Diodes

The charge transport properties of three tertiary-butyl (t-Bu) substituted anthracene derivatives (ADN), critical blue host materials for organic light-emitting diodes (OLEDs), have been investigated experimentally and computationally. From time-of-flight (TOF) measurements, all ADN compounds exhibit ambipolar characters. The hole and electron mobilities are in the range (1–5)×10-7 cm2 V-1 s-1 under an external applied field of about 1 MV cm-1. Un-substituted ADN has the highest carrier mobilities while heavily t-Bu substituted ADN has the least. The electron and hole conducting properties of are consistent with ab initio calculation, which indicates that the frontier orbitals are localized mainly on the anthracene moiety. t-Bu substitutions in ADN increase the hopping path lengths among the molecules and hence reduce the electron and hole mobilities. The results demonstrate that t-Bu substitution is an effective means of engineering the conductivity of organic charge transporter for OLED applications.

Two-mode squeezing in two coupled time-dependent oscillators

Abstract In this paper we investigate the time evolution of a pair of coupled time-dependent oscillators and examine the possibility of two-mode squeezing in these oscillators. Using the Lie algebraic technique, we obtain an exact form of the time evolution operator which, in turn, enables us to find the exact wave functions and coherent state at any time t. Our analyses indicate that the time- dependent coherent state is equivalent to either a product of two single-mode squeezed states or a generalized two-mode squeezed state, depending on the character of the initial coherent state of the oscillators. Accordingly, two-mode squeezing can be generated by the coupled time-dependent oscillators.

Coherent-state propagator of two coupled generalized time-dependent parametric oscillators

Abstract We have derived the coherent-state propagator of a pair of coupled generalized time-dependent parametric oscillators using the Lie algebraic approach. The results are for the most general pair of coupled time-dependent oscillators, and thus will be useful for future studies in quantum optics as well as in atomic and molecular physics.

Eigenstates of the potential V(x) = A2x2 + A4x4 + A6x6: state-dependent diagonalization method

Abstract We apply the state-dependent diagonalization method to study the eigenstates of quartic-sextic anharmonic oscillators ( A 2 > 0, A 4 > 0 and A 6 > 0) and double-well oscillators ( A 2 A 4 > 0 and A 6 = 0). This method is shown to be efficient for calculating energy eigenvalues and eigenfunctions; in particular, for the highly excited states. For a wide range of parameters, each of the first one thousand eigenstates of these systems can be accurately determined by diagonalizing matrices of dimension less than 220.

Optimal coupled-cluster approximation for the linear E-e Jahn-Teller effect

Abstract By introducing a slight modification to the successive coupled-cluster approximation, we have developed an optimal coupled-cluster approximation, which can take care of both the accuracies of the ground-state energy and wavefunction estimates, for the ground state of the linear E - e Jahn-Teller effect. Over the whole range of the coupling parameter, our results up to the third level of approximation show good agreement with the exact numerical diagonalization results and are better than those from earlier analytical treatments. The mathematical treatment in this work is simple and could be easily extended to the studies of other fermion-boson interacting systems.

Ground state of a simple spin-boson system: exact results

Abstract The ground state of a two-level system coupled to a dispersionless phonon bath is studied. The exact ground-state solution to the problem is obtained via a combination of unitary transformations and numerical diagonalization. The impossibility of the discontinuous localization-delocalization transition of the tunneling particle is shown.

Strong two-mode squeezing in two coupled oscillators

Abstract In this paper we have investigated the possibility of generating strong two-mode squeezing in a system of two coupled oscillators by a series of sudden changes of the intermode coupling. A strong dependence of the squeezing on the time interval between the sudden changes of the intermode coupling as well as on the magnitude of the changes is found. It is shown that a high degree of two-mode squeezing can be achieved by a series of well-timed sudden changes of the intermode coupling.

Correlated squeezed phonon states

Abstract In this paper we have generalized the variational approach proposed by Zheng [J. Phys. Condens. Matter 1 (1989) 1641] and applied it to a model system with strong electron-phonon interaction. We have introduced a new variational ground state in which the phonon subsystem is in a correlated squeezed state and the electron subsystem is in a superconducting pairing state. The correlated squeezed state has built in the correlation between different phonon modes and thereby goes beyond earlier variational treatments. Also, with optimal values of the variational parameters, this trial state will by construction yield an energy lower than that obtained by Zheng. This means that our variational ansatz is more stable as the ground state of the system. Furthermore, in this new approximate ground state the reduction effect of phonons is much more alleviated, and thus the mass enhancement inherent to the polaron effect is considerably weakened. This, in turn, seems to suggest the possibility of higher critical temperatures for superconductivity.

Propagator of the n-dimensional generalization of the Fokker-Planck equation with a linear force: Lie-algebraic approach

Abstract We have investigated the algebraic structure of the Cauchy problem for a generalized Fokker-Planck equation. Using the Lie-algebraic approach we have obtained an exact form of the time evolution operator which, in turn, enables us to derive the corresponding propagator readily. Since the propagator is for the most general case, results for any special case can be easily deduced from it.