  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.
