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arXiv:1306.0050 · Physical Review A

Hyperentanglement concentration for two-photon four-qubit systems with linear optics

Bao‐Cang Ren, Fang‐Fang Du, Fu‐Guo Deng

Published Jul 8, 2013• 194 citations
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Abstract

Hyperentanglement, defined as the entanglement in several degrees of freedom (DOFs) of a quantum system, has attracted much attention recently. Here we investigate the possibility of concentrating the two-photon four-qubit systems in partially hyperentangled states in both the spatial mode and the polarization DOFs with linear optics. We first introduce our parameter-splitting method to concentrate the systems in the partially hyperentangled states with known parameters, including partially hyperentangled Bell states and cluster states. Subsequently, we present another two nonlocal hyperentanglement concentration protocols (hyper-ECPs) for the systems in partially hyperentangled unknown states, resorting to the Schmidt projection method. It will be shown that our parameter-splitting method is very efficient for the concentration of the quantum systems in partially entangled states with known parameters, resorting to linear-optical elements only. All these four hyper-ECPs are feasible with current technology and they may be useful in long-distance quantum communication based on hyperentanglement as they require only linear optical elements.

DOI
10.1103/physreva.88.012302

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Hyperentanglement has attracted considerable attention recently because of its high-capacity for long-distance quantum communication. In this study, we present a hyperentanglement concentration protocol (hyper-ECP) for nonlocal three-photon systems in the polarization, spatial-mode, and time-bin partially hyperentangled Greenberger–Horne–Zeilinger (GHZ) states using the Schmidt projection method. In our hyper-ECP, the three distant parties must perform the parity-check measurements on the polarization, spatial-mode, and time-bin degrees of freedom, respectively, using linear optical elements and Pockels cells, and only two identical nonlocal photon systems are required. This hyper-ECP can be directly extended to the N -photon hyperentangled GHZ states, and the success probability of this general hyper-ECP for a nonlocal N -photon system is the optimal one, regardless of the photon number N .
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We present two hyperentanglement concentration schemes for two-photon states that are partially entangled in the polarization and time-bin degrees of freedom. The first scheme distills a maximally hyperentangled state from two identical less-entangled states with unknown parameters via the Schmidt projection method. The other scheme can be used to concentrate an initial state with known parameters and requires only one copy of the initial state for the concentration process. Both these two protocols can be generalized to concentrate NNN-photon hyperentangled Greenberger-Horne-Zeilinger states that are simultaneously entangled in the polarization and time-bin degrees of freedom. Our schemes require only linear optics and are feasible with current technology. Using the time-bin degree of freedom rather than the spatial-mode degree of freedom can provide savings in quantum resources, which makes our schemes practical and useful for long-distance quantum communication.
We present two hyperentangled concentration schemes for two-photon systems in a partially hyperentangled Bell state that is simultaneously entangled in polarization, spatial mode and time-bin degrees of freedom (DOFs). The first scheme extracts a maximally hyperentangled Bell state using cross-Kerr nonlinearity, which is discussed in two cases of unknown and known coefficients. We construct quantum nondemolition detectors based on cross-Kerr nonlinearities to implement the polarization and spatial parity check. In this scheme, we only require a small phase shift, as long as it can be distinguished from zero. The second scheme is used to concentrate an initial partially hyperentangled state with only linear optical elements, where coefficients of the initial state are known. Both of the two schemes are feasible with the current experimental technology. Moreover, hyperentanglement DOFs can largely improve the channel capacity of long-distance quantum communication. All of these make our schemes more practical and useful in quantum communication.
Hyperentanglement, defined as the simultaneous entanglement in several independent degrees of freedom (DOFs) of a quantum system, is a fascinating resource in quantum information processing with its outstanding merits. Here we propose heralded hyperentanglement concentration protocols (hyper-ECPs) to concentrate an unknown partially less polarization-spatial hyperentangled Bell state with available linear optics and common single-photon detectors. By introducing time-delay DOFs, the schemes are highly efficient in that the success of the scheme can be accurately heralded by the detection signatures, and postselection techniques or photon-number-resolving detectors, necessary for previous experiments, are not required. Additionally, our linear optical architectures allow certain states, where concentration fails, to be recyclable, and a trick makes the success probabilities of our schemes higher than those of previous linear optical hyper-ECPs.
Hyperentanglement involves multiple degrees of freedom of a quantum system and has attracted a lot of attention recently because of its high efficiency in quantum applications. We propose some practical schemes using linear optics for partially entangled n-photon 2n-qubit systems with spatial and polarization degrees of freedom. The states involved are not equivalent to the general Bell states or GHz states under local quantum operations and classical communication. Our schemes are based on the parameter-splitting method, which can change different entanglement coefficients into equal coefficients. They are very efficient and practical as they use only linear-optical elements and do not require nonlinear optics.