  In the scheme of a quantum nondemolition (QND) measurement, an observable is
measured without perturbing its evolution. In the context of studies of
decoherence in quantum computing, we examine the `open' quantum system of a
two-level atom, or equivalently, a spin-1/2 system, in interaction with quantum
reservoirs of either oscillators or spins, under the QND condition of the
Hamiltonian of the system commuting with the system-reservoir interaction. The
propagators for these QND Hamiltonians are shown to be connected to the
squeezing and rotation operators for the two baths, respectively. Squeezing and
rotation being both phase space area-preserving canonical transformations, this
brings out an interesting analogy between the energy-preserving QND
Hamiltonians and the homogeneous linear canonical transformations.
