Conditional states and entropy in qudit-qubit systems

BILKIS, M.; CANOSA, N.; ROSSIGNOLI, R.; GIGENA, N.

PHYSICAL REVIEW A - ATOMIC, MOLECULAR AND OPTICAL PHYSICS

AMER PHYSICAL SOC

Lugar: New York; Año: 2019 vol. 99 p. 621191 - 6211911

1050-2947

We examine in correlated qudit-qubit systems the set of all conditional qubit states arising after a general projective measurement at the qudit. While for a projective measurement at the qubit, the conditional post-measurement qudit states lie on the surface of an ellipsoid, for a measurement at the qudit we show that the set of post-measurement qubit states can form more complex solid regions. In particular, we show the emergence, for some simple classes of mixed states, of sets which are the convex hull of solid ellipsoids and which may lead to cone-like and triangle-like shapes in limit cases. We also analyze the associated measurement dependent conditional entropy, providing a full analytic determination of its minimum and of the minimizing local measurement at the qudit for the previous states. Separable rank $2$ mixtures are also discussed.