Coherent states and a newly developed form of the density operator are used to relate the third-order susceptibility to the first-order susceptibility. The theoretical result is shown to be in fair agreement with the empirical relation.
A theoretical scheme for enhanced Kerr nonlinearity is proposed in a four-level ladder-type atomic system based on double dark resonances (DDRs). We solve the relevant density matrix equations in steady state and utilize the perturbation theory to obtain the analytical expressions for the third order susceptibility of the atomic system. The influence of system parameters on behavior of the first and third order susceptibilities is then discussed. In particular, it is found that an enhanced Kerr nonlinearity with reduced linear and nonlinear absorption is obtained around zero probe detuning under the slow light condition through proper adjusting the laser field intensity and frequency detuning of driving fields. The dressed state analysis is employed to explain the physical origin of the obtained result. The obtained results may be important for all-optical signal processing and quantum information technology.
Using the non-negativity of Gram determinants of arbitrary order, we derive higher-order uncertainty relations for the symmetric uncertainty matrices of corresponding order n > 2 to n Hermitean operators (n = 2 is the usual case). The special cases of third-order and fourth-order uncertainty relations are considered in detail. The obtained third-order uncertainty relations are applied to the Lie groups SU(1,1) with three Hermitean basis operators (K 1,K 2,K 0) and SU(2) with three Hermitean basis operators (J 1,J 2,J 3) where, in particular, the group-coherent states of Perelomov type and of Barut–Girardello type for SU(1,1) and the spin or atomic coherent states for SU(2) are investigated. The uncertainty relations for the determinant of the third-order uncertainty matrix are satisfied with the equality sign for coherent states and this determinant becomes vanishing for the Perelomov type of coherent states for SU(1,1) and SU(2). As an example of the application of fourth-order uncertainty relations, we consider the canonical operators (Q 1,P 1,Q 2,P 2) of two boson modes and the corresponding uncertainty matrix formed by the operators of the corresponding mean deviations, taking into account the correlations between the two modes. In two mathematical appendices, we prove the non-negativity of the determinant of correlation matrices of arbitrary order and clarify the principal structure of higher-order uncertainty relations.
In addition to the suppressed absorption in the atomic resonance region, an electromagnetically induced transparency (EIT) medium also possess giant nonlinear dispersion which significantly affects the optical properties of the medium. In this work, we study the influence of third-order nonlinear susceptibility on electromagnetically induced grating (EIG) in a three-level V-type atomic medium. By applying stationary perturbation theory to the density matrix equations describing the time evolution of the quantum states of the system, we have found density matrix solutions to the third-order perturbation corresponding to the third-order susceptibility, and found the transmission function for the probe light. Using the Fourier transform of the transmission function, we can obtain the intensity distribution function of the EIG diffraction spectrum as a function of laser parameters and the atomic medium. To see the influence of third-order susceptibility on EIG diffraction spectrum, we have simulated EIG spectra in two cases with and without third-order susceptibility. We found that the first-order diffraction efficiency was significantly enhanced when third-order susceptibility was introduced.