Concurrence for infinite-dimensional quantum systems

Concurrence is an important entanglement measure for states in finite-dimensional quantum systems that was explored intensively in the last decade. In this paper, we extend the concept of concurrence to infinite-dimensional bipartite systems and show that it is continuous and does not increase under local operation and classical communication (LOCC). Moreover, based on the partial Hermitian conjugate (PHC) criterion proposed in [Chin. Phys. Lett. \textbf{26}, 060305(2009); Chin. Sci. Bull. \textbf{56}(9), 840--846(2011)], we introduce a concept of the PHC measure and show that it coincides with the concurrence, which provides another perspective on the concurrence.