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Progress in Informatics

Generation and characterization of bound entanglement in optical qubits

Fumihiro Kaneda, Ryōsuke Shimizu, Yasuyoshi Mitsumori, Hideo Kosaka, Keiichi Edamatsu

Published Mar 1, 2011• 3 citations
Publisher

Abstract

For efficient quantum information processing (QIP), pure and strong entanglement in qubits has been thought to be indispensable.However, it was recently pointed out that bound entangled states, which involve undistillable entanglement between qubits, also have great potential for QIP.In this paper, we report the efficient generation of a four-qubit bound entangled state referred to as the Smolin state using photon polarization qubits.We observed the unlockable bound entanglement which is the specific property of the Smolin state.

DOI
10.2201/niipi.2011.8.3

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For efficient quantum information processing (QIP), pure and strong entanglement in qubits has been thought to be indispensable.However, it was recently pointed out that bound entangled states, which involve undistillable entanglement between qubits, also have great potential for QIP.In this paper, we report the efficient generation of a four-qubit bound entangled state referred to as the Smolin state using photon polarization qubits.We observed the unlockable bound entanglement which is the specific property of the Smolin state.
Bound entanglement is central to many exciting theoretical results in quantum information processing, but has thus far not been experimentally realized. In this work, we consider a one-parameter family of four-qubit Smolin states. We experimentally produce these states in the polarization of four optical photons produced from parametric down-conversion. Within a range of the parameter, we show that our states are entangled and undistillable, and thus bound entangled. Using these bound-entangled states we demonstrate entanglement unlocking.
Four qubit bound entangled Smolin states are generalised in a natural way to even number of qubits. They are shown to maximally violate simple correlation Bell inequalities and, as such, to reduce communication complexity, though they do not admit quantum security. They are also shown to serve for remote quantum information concentration like in the case of the original four qubit states. Application of the information concentration to the process of unlocking of classical correlations and quantum entanglement by quantum bit is pointed out.
Quantum entanglement exhibits various interesting features that emerge only in high-dimensional systems. One of the most fascinating is bound entanglement, entanglement that cannot be extracted using local operations and classical communication. This thesis describes our work towards an experimental realization of the four-qubit bound-entangled Smolin state, using the polarization and spatial mode of photon pairs. We describe a number of interesting experimental challenges that this work faced. We present our results on entangled two-qubit spatial mode states and hyperentangled four-qubit polarization-spatial-mode states generated by spontaneous parametric down-conversion from a second-order TEM mode pump. Some of the subtleties involved in preparing genuine mixed states in the lab are discussed.
A class of unlockable bound entangled states and their applications are presented. They can be considered as quasi generalized Smolin states [Phys. Rev. A 63 (2001) 032306], which are the states of N + N qubits. No pure entanglement can be distilled from this class of states by local quantum operations and classical communications. However, if certain parties group together, they become distillable. Although they are bound entangled states, they could be used to achieve some non-trivial tasks, such as quantum secret sharing shown in the study.