Dissipative dynamics of four-site tunneling of H

We investigate the dissipative properties of a quantum particle which can t unnel among four equivalent sites on a plane. Our four-state system model i s of experimental relevance to probe the quantum properties of H or D being trapped by substitutional Zr in Nb. Upon performing a unitary transformati on, the problem is reduced to that of effective spin-boson systems. Numeric al and analytical results for the population dynamics of the four localized states are obtained. Intriguing dissipation-assisted quantum coherent osci llations are predicted to occur even for strong damping.