Switching of geometric phase in degenerate systems

The geometric and open path phases of a four-state system subject to time v arying cyclic potentials are computed from the Schrodinger equation. Fast o scillations are found in the non-adiabatic case. For parameter values such that the system possesses degenerate levels, the geometric phase becomes an omalous, undergoing a sign switch. A physical system to which the results a pply is a molecular dimer with two interacting electrons. Additionally, the sudden switching of the geometric phase promises to be an efficient contro l in two-qubit quantum computing. (C) 2000 Elsevier Science B.V. All rights reserved.