  Counting statistics of charge transfers in a point contact interacting with
an arbitrary quantum system is studied. The theory for the charge specific
density matrix is developed, allowing the evaluation of the probability of the
outcome of any joint measurement of the state of the quantum system and the
transferred charge. Applying the method of charge projectors, the master
equation for the charge specific density matrix is derived in the tunneling
Hamiltonian model of the point contact. As an example, the theory is applied to
a quantum measurement of a two-state system: The evolution of the charge
specific density matrix in the presence of Nyquist or Schottky noise is studied
and the conditions for the realization of a projective measurement are
established.
