Xiaolong Su

Experimental preparation of quadripartite cluster and Greenberger-Horne-Zeilinger entangled states for continuous variables.

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.

Experimental Demonstration of Unconditional Entanglement Swapping for Continuous Variables

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 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.

Experimental demonstration of quantum entanglement between frequency-nondegenerate optical twin beams

The quantum entanglement of amplitude and phase quadratures between two intense optical beams with a total intensity of 22 mW and a frequency difference of 1 nm, 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 semiclassical analysis of the noise characteristics given by Fabre et al. [J. Phys. (France) 50, 1209 (1989)].

Experimental Realization of Three-Color Entanglement at Optical Fiber Communication and Atomic Storage Wavelengths

Entangled states of light including low-loss optical fiber transmission and atomic resonance frequencies are essential resources for future quantum information networks. We present the experimental achievement on the three-color entanglement generation at 852, 1550, and 1440 nm wavelengths for optical continuous variables. The entanglement generation system consists of two cascaded nondegenerated optical parametric oscillators (NOPOs). The flexible selectivity of nonlinear crystals in the two NOPOs and the tunable property of NOPO provide large freedom for the frequency selection of three entangled optical beams. The presented system will hopefully be developed as a practical entangled source to be used in quantum-information networks with atomic storage units and long fiber transmission lines.

Experimental preparation of eight-partite cluster state for photonic qumodes

The preparation of multipartite entangled states is the prerequisite for exploring quantum information networks and quantum computation. In this Letter, we present what we believe is the first experimental demonstration of an eight-partite spatially separated continuous variable (CV) cluster state of optical modes. Via the linearly optical transformation of eight squeezed states of light, the eight-partite cluster entangled state with amplitude and phase quadrature correlations are prepared. The generated eight entangled photonic qumodes are spatially separated, which provides valuable quantum resources for implementing CV quantum information protocols.

Gate sequence for continuous variable one-way quantum computation

Measurement-based one-way quantum computation using cluster states as resources provides an efficient model to perform computation and information processing of quantum codes. Arbitrary Gaussian quantum computation can be implemented sufficiently by long single-mode and two-mode gate sequences. However, continuous variable gate sequences have not been realized so far due to an absence of cluster states larger than four submodes. Here we present the first continuous variable gate sequence consisting of a single-mode squeezing gate and a two-mode controlled-phase gate based on a six-mode cluster state. The quantum property of this gate sequence is confirmed by the fidelities and the quantum entanglement of two output modes, which depend on both the squeezing and controlled-phase gates. The experiment demonstrates the feasibility of implementing Gaussian quantum computation by means of accessible gate sequences.

Experimental generation of 6 dB continuous variable entanglement from a nondegenerate optical parametric amplifier.

We experimentally demonstrated that the quantum correlations of amplitude and phase quadratures between signal and idler beams produced from a non-degenerate optical parametric amplifier (NOPA) can be significantly improved by using a mode cleaner in the pump field and reducing the phase fluctuations in phase locking systems. Based on the two technical improvements the quantum entanglement measured with a two-mode homodyne detector is enhanced from approximately 4 dB to approximately 6 dB below the quantum noise limit using the same NOPA and nonlinear crystal.

Experimental generation of genuine four-partite entangled states with total three-party correlation for continuous variables

We experimentally prepare a type of continuous variable genuine four-partite entangled states, the quantum correlation property of which is different from that of the four-mode Greenberger-Horne-Zeilinger 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.

Continuous variable quantum key distribution based on optical entangled states without signal modulation

In this paper, we present the first experimental demonstration on continuous variable quantum key distribution using determinant Einstein-Podolsky-Rosen entangled states of optical field. By means of the instantaneous measurements of the quantum fluctuations of optical modes, respectively, distributed at sender and receiver, the random bits of secret key are obtained without the need for signal modulation. The post-selection boundaries for the presented entanglement-based scheme against both Gaussian collective and individual attacks are theoretically concluded. The final secret key rates of 84 kbits/s and 3 kbits/s are completed under the collective attack for the transmission efficiency of 80% and 40%, respectively.

Quantum Entanglement Swapping between Two Multipartite Entangled States

Quantum entanglement swapping is one of the most promising ways to realize the quantum connection among local quantum nodes. In this Letter, we present an experimental demonstration of the entanglement swapping between two independent multipartite entangled states, each of which involves a tripartite Greenberger-Horne-Zeilinger (GHZ) entangled state of an optical field. The entanglement swapping is implemented deterministically by means of a joint measurement on two optical modes coming from the two multipartite entangled states respectively and the classical feedforward of the measurement results. After entanglement swapping the two independent multipartite entangled states are merged into a large entangled state in which all unmeasured quantum modes are entangled. The entanglement swapping between a tripartite GHZ state and an Einstein-Podolsky-Rosen entangled state is also demonstrated and the dependence of the resultant entanglement on transmission loss is investigated. The presented experiment provides a feasible technical reference for constructing more complicated quantum networks.