A quantum system consisting of two subsystems is separable if its density matrix can be written as where and are density matrices for the two subsystems, and the positive weights satisfy . In this Letter, it is proved that a necessary condition for separability is that a matrix, obtained by partial transposition of ρ, has only non-negative eigenvalues. Some examples show that this criterion is more sensitive than Bell's inequality for detecting quantum inseparability.
- Received 8 April 1996
- Published in the issue dated 19 August 1996
© 1996 The American Physical Society