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Physical Review A

Direct measurement of the concurrence of two-photon polarization-entangled states

Li-Hua Zhang, Qing Yang, Ming Yang, Wei Song, Zhuo‐Liang Cao

Published Dec 31, 2013• 34 citations
Publisher

Abstract

We present three protocols for directly measuring the concurrence of two-photon polarization-entangled states, including pure states and mixed states. Two signal photon pairs in the same polarization-entangled states are needed in each detection round, and the signal photons will pass through polarization-independent beam splitters simultaneously. The concurrence of the photon pairs will be encoded in the total probability of picking up the balanced states in the two output modes of the beam splitter, which can be directly measured by single-photon detectors. The photon detection efficiency can be further enhanced with the help of the weak cross-Kerr nonlinearity, and the signal photons will not be absorbed. The third scheme using the bilateral nondemolition measurements can measure the concurrence of both pure and mixed states with high efficiency and high fidelity and is robust against the copy error of the state to be measured. The sophisticated controlled-not operation of the previous schemes is not required here, so our protocols are more feasible within the current experimental technology.

DOI
10.1103/physreva.88.062342

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We propose an effective protocol for preparation of four-photon polarization-entangled decoherence-free states with quantum nondemolition detectors. The protocol is based on optical elements, single polarization photons, and cross-Kerr nonlinearity, which are feasible with existing experimental technology. Compared with previous protocols, the present one is to replace the entangled-state resources with much simpler single-photon resources and has a higher success probability. All these advantages make this protocol more efficient and more convenient than others in the applications in quantum communication.
We present an experimental proof-of-principle for the generation and detection of pure two-qubit states that have been encoded in degrees of freedom that are common to both classical-light beams and single photons. Our protocol requires performing polarization tomography on a single qubit from a qubit pair. The degree of entanglement in the qubit pair is measured by concurrence, which can be directly extracted from intensity measurements-or photon counting-entering single-qubit polarization tomography.
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