  This article describes the AC Stark, Stern-Gerlach, and Quantum Zeno effects
as they are manifested during continuous interferometric measurement of a
two-state quantum system (qubit). A simple yet realistic model of the
interferometric measurement process is presented, and solved to all orders of
perturbation theory in the absence of thermal noise. The statistical properties
of the interferometric Stern-Gerlach effect are described in terms of a
Fokker-Plank equation, and a closed-form expression for the Green's function of
this equation is obtained. Thermal noise is added in the form of a
externally-applied Langevin force, and the combined effects of thermal noise
and measurement are considered. Optical Bloch equations are obtained which
describe the AC Stark and Quantum Zeno effects. Spontaneous qubit transitions
are shown to be observationally equivalent to transitions induced by external
Langevin forces. The effects of delayed choice are discussed. Practical
experiments involving trapped ions are suggested. The results are relevant to
the design of qubit readout systems in quantum computing, and to single-spin
detection in magnetic resonance force microscopy.
