We experimentally investigate quadrature correlations between pump, signal, and idler fields in an above-threshold optical parametric oscillator. We observe new quantum correlations among the pump and signal or idler beams, as well as among the pump and a combined quadrature of signal and idler beams. A further investigation of unforeseen classical noise observed in this system is presented, which hinders the observation of the recently predicted tripartite entanglement. In spite of this noise, current results approach the limit required to demonstrate three-color entanglement.

Quantum teleportation and quantum dense coding are two typical examples to exploit nonlocal quantum correlation of entangled states in quantum information to perform otherwise impossible tasks. Quantum teleportation is the disembodied transport of an unknown quantum state from one place to another 1 . Quantum dense coding provide a method by which two bits of classical information can be transmitted by sending one qubit of quantum information 2 . Discrete and continuous variable teleportations have been performed experimentally for single-photon polarization

We propose a scheme for transferring quantum states from the propagating light fields to macroscopic, collective vibrational degree of freedom of a massive mirror by exploiting radiation pressure effects. This scheme may prepare an Einstein-Podolsky-Rosen state in position and momentum of a pair of distantly separated movable mirrors by utilizing the entangled light fields produced from a nondegenerate optical parametric amplifier.

A tripartite entangled state of bright optical field is experimentally produced using an Einstein-Podolsky-Rosen entangled state for continuous variables and linear optics. The controlled dense coding among a sender, a receiver and a controller is demonstrated by exploiting the tripartite entanglement. The obtained three-mode position correlation and relative momentum correlation between the sender and the receiver and thus the improvements of the measured signal to noise ratios of amplitude and phase signals with respect to the shot noise limit are 3.28dB and 3.18dB respectively. If the mean photon number n equals 11 the channel capacity can be controllably inverted between 2.91 and 3.14. When n is larger than 1.0 and 10.52 the channel capacities of the controlled dense coding exceed the ideal single Quantum entanglement shared by more than two parties is the essential base for developing quantum communication network and quantum computation. The threeparticle entangled states for discrete variables, called also Greenberger-Horne-Zeilinger (GHZ) states, have been proposed [1] and then experimentally realized with optical system consisting of nonlinear (χ 2 ) crystal, pulse laser and linear optical elements [2] and with nuclear magnetic resonance [3]. The controlled dense coding for discrete variables using a three-particle entangled state has been proposed [4]. Recently, under the motivation of the successful experiments on continuous-variable quantum teleportation [5] and quantum dense coding [6], the schemes demonstrating quantum teleportation network [7] and controlled dense coding [8] for quantum variables with a continuous spectrum using multipartite entanglement have been theoretically proposed. So far to the best of our knowledges, the experimental report on the generation of multipartite entangled states for continuous variables and its application has not been presented.In this paper we report the first experimental demonstration of quantum entanglement among more than two quantum systems with continuous spectra. The tripartite entangled state is produced by distributing a twomode squeezed state light to three parties using linear optics. The obtained tripartite entangled optical beams are distributed to a sender (Alice), a receiver (Bob) and a controller (Claire) respectively. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity accomplished under Claire's help is always larger than that without his help. For the large mean photon number(n > 10.52), the channel capacity of the controlled dense coding communication exceeds that of ideal squeezed state communication. Fig.1 is the schematic of the experimental setup for tripartite entanglement generation and controlled dense coding. A semimonolithic nondegenerate optical parameter amplifier (NOPA) involving an α-cut type-II KTP crystal and pumped by an intracavity frequency-doubled and frequency-stabilized Nd:YAP/KTP laser serves as the initial bipartite entanglement source. The ...

The cluster states and Greenberger-Horne-Zeilinger (GHZ) states are two different types of multipartite quantum entangled states. We present the first experimental results generating continuous variable quadripartite cluster and GHZ entangled states of electromagnetic fields. Utilizing two amplitude-quadrature and two phase-quadrature squeezed states of light and linearly optical transformations, the two types of entangled states for amplitude and phase quadratures of light are experimentally produced. The combinations of the measured quadrature variances prove the full inseparability of the generated four subsystems. The presented experimental schemes show that the multipartite entanglement of continuous variables can be deterministically generated with the relatively simple implementation.