Reduction criterion for separability

We introduce a separability criterion based on the positive map: (Tr) -, where is a trace-class Hermitian operator. Any separable state is mapped by the tensor product of and the identity into a non-negative operator, which provides a simple necessary condition for separability. This condition is generally not sufficient because it is vulnerable to the dilution of entanglement. In the special case where one subsystem is a quantum bit, reduces to time reversal, so that this separability condition is equivalent to partial transposition. It is therefore also sufficient for 22 and 23 systems. Finally, a simple connection between this map for two qubits and complex conjugation in the ``magic'' basis Phys. Rev. Lett. 78, 5022 (1997) is displayed.