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.

Abstract:The quantum entanglement of amplitude and phase quadratures between two intense optical beams with the total intensity of 22mW and the frequency difference of 1nm, which are produced from an optical parametric oscillator operating above threshold, is experimentally demonstrated with two sets of unbalanced Mach-Zehnder interferometers.The measured quantum correlations of intensity and phase are in reasonable agreement with the results calculated based on a semi-classical analysis of the noise characteristics given by C. Fabre et al. OCIS codes: 190.4410, 270.6570 In recent years quantum information with continuous variables (CV) where f is the noise frequency, S 0 is the shot noise limit (SNL), B and ξ =T/(T+δ) are the cavity bandwidth and the output coupling efficiency of NOPO respectively (T -the transmission coefficient of the output coupling mirror; δ -extra intracavity losses), η is the detection efficiency,is the pump parameter (P -the pump power, P 0 -the threshold pump power of NOPO). The intensity difference quantum correlations of twin beams were experimentally measured with self-homodyne detectors by different groups and were effectively applied [7][8][9][10][11][12] . However, the phase correlation of the twin beams was not observed for a long time Almost at a parallel period we were also devoting our efforts to measure the quantum entanglement of twin beams from NOPO above threshold. The measurement scheme used by us is basically same with that presented by O. Glockl et al. inRef.[15], where they performed sub-shot-noise measurement of the phase quadratures of intense pulsed light 16 . Considering that the phase correlation will be significantly affected by the phase fluctuation of the pump laser 6 and the restricted condition deducing Eqs. (1) and (2) in Ref. [4] requires the finesse of the NOPO cavity for the pump laser much lower than that for the twin beams, in our design the ratio of the cavity finesses for the pump and the twin beams is 16/164 which is much smaller than that in Refs.[13] and [14]. Due to the lower finesse the resonant peak of the pump laser in the cavity is relatively flat and thus the threshold power is higher (~120mW).At first, using a pair of Mach-Zehnder (M-Z) interferometers with unbalanced arm-lengths we detected the amplitude and phase noise of signal and idler output fields from a NOPO above threshold at a certain analysis frequency (20MHz), respectively. Then, the quantum correlations were denoted by the noise levels of the intensity difference and the phase sum of the photocurrents measured by two unbalanced interferometers. wave plate P1 (P2). Rotating the polarization orientation of P1 (P2) we can conveniently switch between phase and amplitude measurements 15 . In our system, the distance difference of two arms ∆L is 7.5m which matches the analysis frequency of 20MHz to make θ=π. The difference of the 5 dc photocurrents of D1 and D2 (D3 and D4) serves as the error signal and is fed back onto the PZT mounted on one of mirrors of the interferometer to...

No abstract

One-way quantum computation based on measurement and multipartite cluster entanglement offers the ability to perform a variety of unitary operations only through different choices of measurement bases. Here we present an experimental study toward demonstrating the controlled-X operation, a two-mode gate in which continuous variable (CV) four-partite cluster states of optical modes are utilized. Two quantum teleportation elements are used for achieving the gate operation of the quantum state transformation from input target and control states to output states. By means of the optical cluster state prepared off-line, the homodyne detection and electronic feeding forward, the information carried by the input control state is transformed to the output target state. The presented scheme of the controlled-X operation based on teleportation can be implemented nonlocally and deterministically. The distortion of the quantum information resulting from the imperfect cluster entanglement is estimated with the fidelity.

We propose a generation system of continuous-variable (CV) three-color entangled state of bright optical beams based on two cascaded standard nondegenerate optical parametric oscillators (NOPOs) above the threshold. One of signal and idler beams produced by the first NOPO is used for the pump light of the second NOPO. The three-color entanglement among signal and idler beams produced by the second NOPO and the retained another beam of the first NOPO is theoretically demonstrated. The symplectic eigenvalues of the partial transposition covariance matrix of the generated optical entangled state are numerically calculated in terms of experimentally reachable system parameters. The optimal operation conditions of the cascaded NOPOs system for obtaining high entanglement are found. The calculated results explicitly demonstrate that the OPO action can transfer entanglement. Due to that the cavity parameters and the nonlinear crystals of the two NOPOs can be freely chosen, the flexibility of the proposed protocol is relatively good and the system can be also extended to prepare entangled states with more parts easily.

We experimentally prepare a new type of continuous variable genuine four-partite entangled states, the quantum correlation property of which is different from that of the four-mode GHZ and cluster states, and which has not any qubit counterpart to be proposed at present. In the criterion inequalities for the full inseparability of the genuine four-partite entangled states, the amplitude and phase quadrature correlation variances totally consisting of three-party combination from the four entangled modes are involved. The measured correlation variances among the quadratures of the prepared entangled states satisfy the sufficient requirements for the full inseparability.The type of entangled states has especially potential application in quantum information with continuous quantum variables.

We propose to integrate the electro-optic (EO) tuning function into on-chip domain engineered lithium niobate (LN) waveguide. Due to the versatility of LN, both the spontaneously parametric down conversion (SPDC) and EO interaction could be realized simultaneously. Photon pairs are generated through SPDC, and the formation of entangled state is modulated by EO processes. An EO tunable polarization-entangled photon state is proposed. Orthogonally-polarized and parallel-polarized entanglements of photon pairs are instantly switchable by tuning the applied field. The characteristics of the source are theoretically investigated showing adjustable bandwidths and high entanglement degrees. Moreover, other kinds of reconfigurable entanglement are also achievable based on suitable domain-design. We believe tailoring entanglement based on domain engineering is a very promising solution for next generation function-integrated quantum circuits.

No abstract