Highly efficient quantum dense coding for continuous variables has been experimentally accomplished by means of exploiting a bright Einstein-Podolsky-Rosen (EPR) beam with anticorrelation of amplitude quadratures and correlation of phase quadratures. Two bits of classical information are encoded on two quadratures of a half of a bright EPR beam, then are simultaneously decoded by the other half of the EPR beam with the sensitivities beyond that of the shot noise limit. A high degree of immunity to unauthorized eavesdropping has been experimentally demonstrated.

A high energy long pulse and low peak power at 365nm ultra-violet laser is described. The laser at this wavelength is produced by the second harmonic generation of a flash-lamp pumped solid state Alexandrite laser. At optimum temperature, coherent 365nm light of 186mJ/pulse with 220 gs width is obtained without Q-switching.

A compact LD pumped intracavity frequency-doubled and frequency-stabilized ring Nd: YAP/KTP laser with high output up to 1.1W at 540nm is achieved. In the design of the laser configuration, the anisotropic characteristics of Nd:YAP optical biaxial crystal are taken into account. A quarter-wave plate is placed between the Faraday rotator and the s-cut KTP to ensure the output fundamental wave from KTP in pure linear-polarization paralleling to that of the oscillating laser and unidirectional operation of laser. Due to rigorously temperature controlling on KTP crystals and optimizing design of resonator, the stable single-frequency green output is obtained. The intensity fluctuation is less than + 1.5%, and frequency stability is better than

A frequency-doubled and frequency-stabilized ring Nd: YAP laser with a six-mirror cavity is demonstrated. This laser satisfies both the thermal insensitivity and optimal frequency-doubling conditions. A second-harmonic output to 1W at 0.54mm is achieved. The intensity fluctuation is less than 61.5% and the frequency stability is better than 61MHz

The configuration of a semimonolithic continuous wave nondegenerate optical parametric oscillator (cw-NOPO) consisting of an a-cut KTP crystal pumped by an intracavity frequency-doubled and frequency-stabilized Nd: YAP laser is reported. Both two-mode quadrature-phase squeezing of 3:7dB and intensity-difference squeezing of 7dB have been generated from the cw-NOPO operating respectively below and above the oscillation threshold through the frequency down-conversion process.

A new quantum measurement scheme of intensity difference fluctuation between two light beams of equal mean intensity is presented. In this system a beam splitter is used as the coupling device and the twin beams with high quantum correlation are injected into its dark port as the input meter wave instead of the usual vacuum field. A measurement satisfying all the quantum nondemolition criteria is experimentally achieved. The measured sum of the transfer coefficients and the conditional variance are, respectively, Ts 1Tm-1.31 and Vsym-22.1dB. [S0031-9007(99)08445-8]

A strong quantum correlation between twin-beams at 1:08um wavelength was observed. The intensity difference noise of twin-beams, which is generated by a semimonolithic OPO cavity pumped by a frequency-doubled and stabilized Nd:YAP laser, is about 80% (7 dB) below the short noise limit at the measurement frequency of 1.5 MHz. The threshold of the optical parametric oscillator is about 20mW. The parametric dependence of the quantum correlation is experimentally investigated. The experimental results agree with the theory well.

The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.

We present a protocol for performing entanglement swapping with intense pulsed beams. In a first step, the generation of amplitude correlations between two systems that have never interacted directly is demonstrated. This is verified in direct detection with electronic modulation of the detected photocurrents. The measured correlations are better than expected from a classical reconstruction scheme. In an entanglement swapping process, a four-partite entangled state is generated. We prove experimentally that the amplitudes of the four optical modes are quantum correlated 3 dB below shot noise, which is consistent with the presence of genuine four-party entanglement.

The frequency stability of an all-solid-state Nd:YVO4 laser is significantly improved by means of a specially designed Fabry-Perot (F-P) interferometer used for the frequency standard in the frequency-stabilizing system. The temperature of the F-P cavity is accurately controlled by a set of thermoelectric cooler (TEC) modules attached on the body of the cavity and the electronic feedback circuit. We find that the long-term unidirectional frequency shift of the output laser, resulting from the slow increase of the cavity length under the effect of the temperature integration on the cavity body, is essentially eliminated. The frequency stability of the output laser with the power of 530mW is better than