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Journal of the Optical Society of America B

Hyperentanglement concentration for n-photon 2n-qubit systems with linear optics

Ming‐Xing Luo, Xiu‐Bo Chen, Yixian Yang, Zhiguo Qu, Xiaojun Wang

Published Dec 11, 2013• 9 citations
Publisher

Abstract

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.

DOI
10.1364/josab.31.000067

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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.
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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.
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.
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 .
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 has attracted much attention due to its fascinating applications in quantum communication. However, it is impossible to purify a pair of photon systems in a mixed hyperentangled state with errors in two degrees of freedom using linear optical elements only, far different from all the existing entanglement purification protocols in a degree of freedom (DOF) for quantum systems. Here, we investigate the possibility of purifying a spatial-polarization mixed hyperentangled Bell state with the errors in both the spatial-mode and polarization DOFs, resorting to the nonlinear optics of a nitrogen-vacancy (NV) center in a diamond embedded in a photonic crystal cavity coupled to a waveguide. We present the first hyperentanglement purification protocol for purifying a pair of two-photon systems in a mixed hyperentangled Bell state with the errors in two DOFs. We also propose an efficient hyperentanglement concentration protocol for a partially hyperentangled Bell pure state, which has the maximal success probability in principle. These two protocols are useful in long-distance quantum communication with hyperentanglement.