Determination of ground state properties in quantum spin systems by single qubit unitary op.pdf

Determination of ground state properties in quantum spin systems by single qubit unitary operations and entanglement excitation energies S. M. Giampaolo,1,2 F. Illuminati,1, 2, 3, ∗ P. Verrucchi,4 and S. De Siena1, 2 1Dipartimento di Matematica e Informatica, Universit`a degli Studi di Salerno, Via Ponte don Melillo, I-84084 Fisciano (SA), Italy 2 CNR-INFM Coherentia, Napoli, Italy, CNISM Unit`a di Salerno, and INFN Sezione di Napoli, Gruppo collegato di Salerno, Baronissi (SA), Italy 3ISI Foundation for Scientific Interchange, Viale Settimio Severo 65, I-00173 Torino, Italy 4 CNR-INFM SMC, and Dipartimento di Fisica, Universit`a di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy (Dated: December 13, 2007) 7 We introduce a method for analyzing ground state properties of quantum many body systems, 0 based on the characterization of separability and entanglement by single subsystem unitary opera- 0 tions. We apply the method to the study of the ground state structure of several interacting spin-1/2 2 models, described by Hamiltonians with different degrees of symmetry. We show that the approach c based on single qubit unitary operations allows to introduce “entanglement excitation energies”, e a set of observables that can characterize ground state properties, i