Orientation states of two coupled polar molecules controlled by laser pulses are studied theoretically. By varying the period of a series of periodically applied laser pulse, transition from regular to chaotic behavior may occur. Schmidt decomposition is used to measure the degree of entanglement. It is found that the entanglement can be enhanced by increasing the strength of laser pulse.PACS [5,6]. Experimentally, several methods have been used to control the orientations of molecules [7,8,9,10,11]. For example, by turning on a picosecond laser pulse adiabatically, the pendular stateshybrid of field-free molecular eigenstates [12,13,14] -can be created. A femtosecond laser pulse, like impulsive excitation, is found to be able to generate a field-free orientation [15,16].Since entangled states are fundamental for quantum information processing [17,18], many research works have been proposed to generate entanglement in quantumoptic and atomic systems [19,20]. In this Letter we propose a novel way to generate entanglement between two coupled identical polar molecules separated in a distance of tens of nanometers. Both molecules are irradiated by ultra-short pulses of laser light. Our study shows the entanglement induced by the dipole interaction can be enhanced by controlling the inter-molecule distance and the field strength of laser pulse.Consider now two identical polar molecules (separated by a distance of R ). There exists dipole-dipole interaction between these two molecules. Ultrashort halfcycle laser pulses are applied to both molecules [21]. The Hamiltonian of the system can be written aswhere L 2 j andh 2 2I (= B) are the angular momentum operator and rotational constant, respectively./R 3 is the dipole interaction between two molecules, where µ 1 and µ 2 are the dipole moments. For simplicity, the dipole moments of two molecules are assumed to be identical, i.e. µ 1 = µ 2 = µ. The field-molecule coupling can be expressed as H l = −µE (t) cos θ cos (ωt) − µE (t) cos θ ′ cos (ωt) ,where θ and θ ′ are the angles between the dipole moments and laser field. The laser profile is assumed to be Gaussian shape, i.e. E (t) = E 0 e − (t−to) 2 σ 2, where E 0 is the field strength, t 0 is the center of peak, and σ is the pulse duration. With these assumptions, the time-dependent Schrödinger equation can be solved by expanding the wave function in terms of a series of field-free spherical harmonic functions (2) where (θ, φ) and θ ′ , φ ′ are the coordinates of first and second molecule respectively. c lml ′ m ′ (t) are the timedependent coefficients corresponding to the quantum numbers (l, m; l ′ , m ′ ) and can be determined by solving the Schrödinger equations numerically. In above equation, total wavefunction has no spatial dependence since we keep inter-molecule separation R as a fixed parameter. One might argue that variation of R is inevitable because of the effects of laser fields or inter-molecule vibrations. However, recent experiments have shown that it is possible to resolve two individual molecules separ...
