Entanglement and polarization are mutually constrained by the relationship C2 + P2 = 1, which engages concurrence (C) of a pure, two-qubit state and the degree of polarization (P) of either of its two subsystems. How the above constraint generalizes for mixed states, is an open question. We address mixed, two-qubit states of the X type, i.e., those whose density matrix has nonzero elements only in the two main diagonals. We focus on mixed states that arise out of a pure, two-qubit state that is subjected to either the amplitude damping channel or the depolarizing channel. We derive alternative constraints for concurrence and polarization and test them experimentally with polarization-entangled photons. We argue that our theoretical results hold also for classical light, whenever two binary degrees of freedom can be entangled.

Entanglement and polarization are mutually constrained by the relationship C2 + P2 = 1, which engages concurrence (C) of a pure, two-qubit state and the degree of polarization (P) of either of its two subsystems. How the above constraint generalizes for mixed states, is an open question. We address mixed, two-qubit states of the X type, i.e., those whose density matrix has nonzero elements only in the two main diagonals. We focus on mixed states that arise out of a pure, two-qubit state that is subjected to either the amplitude damping channel or the depolarizing channel. We derive alternative constraints for concurrence and polarization and test them experimentally with polarization-entangled photons. We argue that our theoretical results hold also for classical light, whenever two binary degrees of freedom can be entangled.