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arXiv:1205.0590 · Optics Letters

Experimental preparation of eight-partite cluster state for photonic qumodes

Xiaolong Su, Yaping Zhao, Shuhong Hao, Xiaojun Jia, Changde Xie, Kunchi Peng

Published Dec 12, 2012• 114 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/ol.37.005178

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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.
Multipartite entangled state is the basic resource for implementing quantum information networks and quantum computation. In this paper, we present the experimental demonstration of the eight-partite two-diamond shape cluster states for continuous variables, which consist of eight spatially separated and entangled optical modes. Eight resource squeezed states of light with classical coherence are produced by four nondegenerate optical parametric amplifiers and then they are transformed to the eight-partite two-diamond shape cluster states by a specially designed linear optical network. Since the spatially separated multipartite entangled state can be prepared off-line, it can be conveniently applied in the future quantum technology.
The generation of large-scale entangled states is crucial for quantum technologies, such as quantum computation1, communication2 and metrology3. Integrated quantum photonics that enables on-chip encoding, processing and detection of quantum light states offers a promising platform for the generation and manipulation of large-scale entangled states4,5. Generating entanglement between qubits encoded in discrete variables within single photons is challenging, owing to the difficulty of making single photons interact on photonic chips6–11. Devices that operate with continuous variables are more promising, as they enable the deterministic generation and entanglement of qumodes, in which information is encoded in light quadratures. Demonstrations so far have been limited to entanglement between two qumodes12–20. Here we report the deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip. The chip delivers a quantum microcomb that produces multimode squeezed-vacuum optical frequency combs below the threshold. We verify the inseparability of our eight-mode state and demonstrate supermode multipartite entanglement over hundreds of megahertz sideband frequencies through violation of the van Loock–Furusawa criteria. By measuring the full matrices of nullifier correlations with sufficiently low off-diagonal noises, we characterize multipartite entanglement structures, which are approximate to the expected cluster-type structures for finite squeezing. This work shows the potential of continuous-variable integrated photonic quantum devices for facilitating quantum computing, networking and sensing. Deterministic generation of a continuous-variable eight-mode entanglement on an integrated optical chip is reported.
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.
While a recent breakthrough has realized the first frequency-encoded multi-qumode entanglement and cluster states on CV-QPIC[1], here we present a scalable CV-QPIC architecture capable of generating, manipulating, and verifying path-encoded multi-qumode cluster states (Fig. 1a). Our design monolithically incorporates several key components: spectrally tunable high-purity squeezing sources adopting dual-microring dual-interferometer (DMDI) configuration [2]–[4], reconfigurable linear optical networks for single-qumode rotations and two-qumode entangling gate, and on-chip homodyne detection interferometers with phase-stable local oscillators.
Quantum entanglement shared by more than two partites is the essential bases for developing quantum communication networks and quantum computation. The multipartite entanglement of continuous variables can be generated by distributing or combining squeezed states of light using linear optics. We experimentally obtained a tripartite entangled state of bright optical field by means of a nondegenerate optical parametric amplifier (NOPA) and optical beam-splitters. The NOPA is operated in the state of parametric deamplification by locking the relative phase between the pump laser and the injected seed wave to (2n+1)π, so the generated Einstein-Podolsky-Rosen (EPR) beam for continuous variables is an entangled state of light with the quantum correlation of phase quadratures and the quantum anticorrelation of amplitude quadratures [1]. The output EPR beam consists of two frequency-degenerate optical modes with horizontal and vertical polarizations which are distributed to three parties with two linear beam-splitters. We have theoretically and experimentally demonstrated the three parties are in a tripartite entangled state with three-mode “position” (amplitude quadrature) correlation and relative “momentum” (phase quadrature) correlation [2]-[4]. The obtained GHZ-like state is a fully inseparable tripartite continuous-variables state violating the three inequalities used for the criterion of the inseparability [5][6]. The controlled dense coding quantum communication is experimentally accomplished by exploiting the tripartite entanglement. The three entangled modes are sent 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 completed under Claire’s help is always larger than that without her help.