Signatures of a quantum dynamical phase transition in a three-spin system in presence of a spin environment

We have observed an environmentally induced quantum dynamical phase transition in the dynamics of a two-spin experimental swapping gate [G.A. Alvarez, E.P. Danieli, P.R. Levstein, H.M. Pastawski, J. Chem. Phys. 124 (2006) 194507]. There, the exchange of the coupled states vertical bar up arrow, down arrow > and vertical bar up arrow, down arrow > gives an oscillation with a Rabi frequency b/h (the spin-spin coupling). The interaction, h/tau(SE) with a spin-bath degrades the oscillation with a characteristic decoherence time. We showed that the swapping regime is restricted only to b tau(SE)greater than or similar to h. However, beyond a critical interaction with the environment the swapping freezes and the system enters to a Quantum Zeno dynamical phase where relaxation decreases as coupling with the environment increases. Here, we solve the quantum dynamics of a two-spin system coupled to a spin-bath within a Liouville-von Neumann quantum master equation and we compare the results with our previous work within the Keldysh formalism. Then, we extend the model to a three interacting spin system where only one is coupled to the environment. Beyond a critical interaction the two spins not coupled to the environment oscillate with the bare Rabi frequency and relax more slowly. This effect is more pronounced when the anisotropy of the system-environment (SE) interaction goes from a purely XY to an Ising interaction form. (c) 2007 Elsevier B.V. All rights reserved.