  We consider the evolution of a spin 1/2 (qubit) under the simultaneous
continuous measurement of three non-commuting qubit operators sigma_x, sigma_y,
sigma_z. For identical ideal detectors the qubit state evolves by approaching a
pure state with a random direction in the Bloch vector space and by undergoing
locally isotropic diffusion in the perpendicular directions. The quantum state
conditioned on the complete detector record is used to assess the fidelity of
classically inspired estimates based on running time averages and discrete time
bin detector outputs.
