  We consider the problem of an electron tunneling between two coupled quantum
dots, a two-state quantum system (qubit), using a low-transparency point
contact (PC) or tunnel junction as a detector continually measuring the
position of the electron. We focus on the qubit dynamics conditioned on a
particular realization of the actual measured current through the PC device. We
illustrate the conditional evolutions by numerical simulations. The different
behaviors between unconditional and conditional evolutions are demonstrated.
The conditional qubit dynamics evolving from quantum jumps to quantum diffusion
is presented.
