Search

4 pages, 4 figures.-- PACS nrs.: 03.65.Yz; 64.70.Tg; 05.70.Fh. We analyze the decoherence induced on a single qubit by the interaction with a two-level boson system with critical internal dynamics. We explore how the decoherence process is affected by the presence of quantum phase transitions in the environment. We conclude that the dynamics of the qubit changes dramatically when the environment passes through a continuous excited state quantum phase transition. If the system-environment coupling energy equals the energy at which the environment has a critical behavior, the decoherence induced on the qubit is maximal and the fidelity tends to zero with finite size scaling obeying a power law. This work has been partially supported by the Spanish Ministerio de Educación y Ciencia and by the European Regional Development Fund (FEDER) under Projects No. FIS2005-01105, No. FIS2006-12783-C03-01, No. FPA2006-13807-C02-02, and No. FPA2007-63074, by Comunidad de Madrid and CSIC under Project No. 200650M012, and by Junta de Analucía under Projects No. FQM160, No.FQM318, No. P05-FQM437, and No. P07-FQM-02962. A.R.was supported by the Spanish program "Juan de la Cierva", and P. P-F. by a grant from the Plan Propio of the University of Sevilla.

6 pages, 6 figures.--PACS nrs.: 73.43.Nq; 03.67.Mn; 75.10.Jm.--ArXiv pre-print available at: http://arxiv.org/abs/0704.1597v1, Using a model of spinless fermions in a lattice with nearest-neighbor and next-nearest-neighbor interactions, we show that the entropy of the reduced two-site density matrix (the bond entropy) can be used as an extremely accurate and easy to calculate numerical indicator for the critical parameters of the quantum phase transition when the basic ordering pattern has a two-site periodicity. The actual behavior of the bond entropy depends on the particular characteristics of the transition under study. For the Kosterlitz-Thouless-type phase transition from a Luttinger liquid phase to a charge-density wave state, the bond entropy has a local maximum, while in the transition from the Luttinger liquid to the phase separated state, the derivative of the bond entropy has a divergence due to the cancellation of the third eigenvalue of the two-site reduced density matrix., This work is supported in part by the Spanish Government Grant No. FIS2006- 12783-C03-01 and joint CAM-CSIC Grant No. 200650M012.

10 pages, 1 table, 12 figures.-- PACS nrs.: 21.60.Fw; 73.43.Nq.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-th/0609001v1, The phase diagram of two-level boson Hamiltonians, including the interacting boson model (IBM), is studied beyond the standard mean field approximation using the Holstein-Primakoff mapping. The limitations of the usual intrinsic state (mean field) formalism concerning finite-size effects are pointed out. The analytic results are compared to numerics obtained from exact diagonalizations. Excitation energies and occupation numbers are studied in different model space regions (Casten triangle for IBM) and especially at the critical points., This work has been partially supported by the Spanish Ministerio de Educación y Ciencia and by the European regional development fund (FEDER) under project numbers BFM2003-05316-C02-02, FIS2005-01105, and FPA2003-05958.