Quantum entangled states, especially those having particular properties, are key resources for quantum information and quantum computation. In this paper, we put forward a new scheme to produce 31 continuous-variable （CV） tripartite entanglement fields based on three optical frequency combs via cascade nonlinear processes in an optical parametric cavity, and investigate the spectral characteristics of three frequency combs. The center wavelengths of the three combs are designed as 852 nm, 780 nm （atomic transition lines）, and 1550 nm （fiber communication wavelength）. The positivity under partial transposition （PPT） criterion, which is sufficient and necessary, is used to evaluate the entanglement in each group of comb lines. This scheme is experimentally feasible and valuable for constructing quantum information networks in future.
A generation system of continuous-variable （CV） quadripartite entangled state based on two cascaded second- harmonic generation （SHG） cavities below the threshold is investigated. Two reflected fundamental bearias of the first cavity, the reflected second-harmonic beam and the output fourth-harmonic beam of the second cavity are proved to be entangled, and the dependence of the entanglement degree on the normalized frequency, pump parameter, fourth-harmonic loss parameter, and second-harmonic loss parameter is also analyzed. Due to the fact that the cavity parameters and the nonlinear crystals of the two SHG cavities can be freely chosen, the practicality of the proposed protocol is relatively perfect and the system can also be extended to the preparation of multicolor entangled states for a quantum network.
Spatial quantum optics based on the high-order transverse mode is important for the super-resolution measurement and quantum image beyond the shot noise level. Quantum entanglement of the transverse plane Hermite–Gauss TEM（01） mode has been demonstrated experimentally in this paper. Two squeezed TEM（01） modes, which are generated by a pair of degenerate optical parametric amplifiers（DOPA） with the nonlinear crystals of periodically poled KTi OPO4, have been combined to produce TEM（01） mode entanglement using a beam splitter. The 1.5 dB for the sum of amplitude and 1.2 dB for the difference of phase below shot-noise level is achieved with the measurement system of a Bell state detection.
