The AC Stark, Stern-Gerlach, and Quantum Zeno Effects in Interferometric Qubit Readout

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 closedform 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. The results are relevant to the design of qubit readout systems in quantum computing, and to single-spin detection in magnetic resonance force microscopy. ∗To whom correspondence should be addressed. Email: [email protected]