Time optimal control of coupled spin dynamics: A global analysis

Abstract

In this paper, we study some control problems that derive from time optimal control of coupled spin dynamics in NMR spectroscopy and quantum information and computation. Time optimal control helps to minimize relaxation losses. In a two qubit system, the ability to synthesize local unitaries much more rapidly than evolution of couplings, gives a natural time scale separation in these problems. The generators of unitary evolution g, are decomposed into fast generators k, (local Hamiltonians) and slow generators p, (couplings) as a Cartan decomposition g=p⊕k. Using this decomposition, we exploit some convexity ideas to completely characterize the reachable set and time optimal control for these problems. The main contribution of the paper is that we carry out a second order analysis of time optimality.