Four-photon Polarization-entangled States Research Articles

On four-photon entanglement from parametric down-conversion process

We propose two schemes to generate four-photon polarization-entangled states from the second-order emission of the spontaneous parametric down-conversion process. By using linear optical elements and the coincidence-detection, the four indistinguishable photons emitted from parametric down-conversion source result in the Greenberger-Horne-Zeilinger (GHZ) state or the superposition of two orthogonal GHZ states. For this superposition state, under particular phase settings we analyze the quantum correlation function and the local hidden variable (LHV) correlation. As a result, the Bell inequality derived from the LHV correlation is violated with the visibility larger than 0.442. It means that the present four-photon entangled state is therefore suitable for testing the LHV theory.

Generation of an arbitrary four-photon polarization-entangled decoherence-free state with cross-Kerr nonlinearity

We present a new scheme to provide an arbitrary four-photon polarization-entangled state, which enables the encoding of single logical qubit information into a four-qubit decoherence-free subspace robustly against collective decoherence. With the assistance of the cross-Kerr nonlinearities, a spatial entanglement gate and a polarization entanglement gate are inserted into the circuit, where the X-quadrature homodyne measurement is properly performed. According to the outcomes of homodyne measurement in the spatial entanglement process, some swap gates are inserted into the corresponding paths of the photons to swap their spatial modes. Apart from Kerr media, some basic linear optical elements are necessary, which make it feasible with current experimental techniques.

Perfect entanglement concentration of an arbitrary four-photon polarization entangled state via quantum nondemolition detectors

We show how to concentrate an arbitrary four-photon polarization entangled state into a maximally entangled state based on some quantum nondemolition detectors. The entanglement concentration protocol (ECP) resorts to an ancillary single-photon resource and the conventional projection measurement on photons to assist the concentration, which makes it more economical. Our ECP involves weak cross-Kerr nonlinearities, X homodyne measurement and basic linear-optical elements, which make it feasible in the current experimental technology. Moreover, the ECP considers cyclic utilization to enhance a higher success probability. Thus, our scheme is meaningful in practical applications in quantum communication.

Generation of four-photon polarization entangled state based on Einstein-Podolsky-Rosen entanglers

We show how to prepare four-photon polarization entangled states based on some Einstein-Podolsky-Rosen (EPR) entanglers. An EPR entangler consists of two single photons, linear optics elements, quantum non-demolition measurement using a weak cross-Kerr nonlinearity, and classical feed forward. This entangler which acts as the most primary part in the construction of our scheme allows us to make two separable polarization qubits entangled near deterministically. Therefore, the efficiency of the present device completely depends on that of EPR entanglers, and it has a high success probability.

An economic and feasible scheme to generate the four-photon entangled state via weak cross-Kerr nonlinearity

We propose a novel scheme to generate four-photon polarization-entangled state with homodyne measurement via weak cross-Kerr nonlinearity. Compared with the former protocols, our protocol is more economic because we only use two weak cross-Kerr nonlinearities, and more feasible because we do not need minus conditional phase shift. All these advantages make our protocol more useful in quantum information processing.

Auxiliary entanglement in photon pairs for multi-photon entanglement

A growing number of experiments make use of multiple pairs of photons generated in the process of spontaneous parametric down-conversion. We show that entanglement in unwanted degrees of freedom can adversely affect the results of these experiments. We also discuss techniques to reduce or eliminate spectral and spatial entanglement, and we present results from two-photon polarization-entangled source with almost no entanglement in these degrees of freedom. Finally, we present two methods for the generation of four-photon polarization-entangled states. In one of these methods, four-photon can be generated without the need for intermediate two-photon entanglement.

Highly efficient scheme for the preparation of four-photon polarization entangled state via quantum non-demolition measurement with classical feed-forward

A simple and efficient scheme is proposed to prepare a four-photon entangled state, which can be used as a resource for realizing a controlled-NOT (CNOT) gate pointed out by Gottesman and Chuang [Nature 402 (1999) 390], with the help of the cross-Kerr nonlinearity. The distinct feature of the present scheme is its preparation with near unity success probability and near perfect fidelity. It is not necessary in the present scheme that the cross-Kerr nonlinearity is very large because the weak nonlinearity can be compensated by using a coherent light laser probe beam with very large amplitude. This makes us more confident in the feasibility of the proposed scheme.

Generation of arbitrary four-photon polarization-entangled decoherence-free states

We propose a scheme to generate four-photon polarization-entangled states, which enables the encoding of an arbitrary qubit into a four-qubit decoherence-free subspace robust against collective decoherence. The scheme is based on linear optical elements and postselection strategy, which is feasible with existing experimental technology.

Generation of a high-visibility four-photon entangled state and realization of a four-party quantum communication complexity scenario

We obtain a four-photon polarization-entangled state with a visibility as high as $(95.35\ifmmode\pm\else\textpm\fi{}0.45)%$ directly from a single down-conversion source. A success probability of $(81.54\ifmmode\pm\else\textpm\fi{}1.38)%$ is observed by applying this entangled state to realize a four-party quantum communication complexity scenario, which comfortably surpasses the classical limit of 50%. As a comparison, two Einstein-Podolsky-Rosen pairs are shown to implement the scenario with a success probability of $(73.89\ifmmode\pm\else\textpm\fi{}1.33)%$. This four-photon state can also be used to fulfill decoherence-free quantum information processing and other advanced quantum communication schemes.

Entanglement Persistency of Multiphoton Entangled States

We experimentally demonstrate the entanglement persistency when losing photons in three- and four-photon polarization-entangled states. The entanglement properties of the mixed states obtained from multiphoton spontaneous parametric down-conversion are studied via witness and positive partial transpose approaches. Together with a quantification of the bipartite entanglement such analysis enables intuitive understanding of novel multiparty quantum communication protocols.