title: Symmetry restoration and quantumness reestablishment.
abstract: A realistic quantum many-body system, characterized by a generic microscopic Hamiltonian, is accessible only through approximation methods. The mean field theories, as the simplest practices of approximation methods, commonly serve as a powerful tool, but unfortunately often violate the symmetry of the Hamiltonian. The conventional BCS theory, as an excellent mean field approach, violates the particle number conservation and completely erases quantumness characterized by concurrence and quantum discord between different modes. We restore the symmetry by using the projected BCS theory and the exact numerical solution and find that the lost quantumness is synchronously reestablished. We show that while entanglement remains unchanged with the particle numbers, quantum discord behaves as an extensive quantity with respect to the system size. Surprisingly, discord is hardly dependent on the interaction strengths. The new feature of discord offers promising applications in modern quantum technologies.
authors.affiliation: College of Physics, Jilin University, Changchun 130012, China.
authors.forename: Guo-Mo
authors.initials: GM
authors.lastname: Zeng
authors.affiliation: 1] Department of Theoretical Physics and History of Science, The Basque Country University (EHU/UPV), PO Box 644, 48080 Bilbao Spain [2] Ikerbasque, Basque Foundation for Science, 48011 Bilbao Spain.
authors.forename: Lian-Ao
authors.initials: LA
authors.lastname: Wu
authors.affiliation: College of Physics, Jilin University, Changchun 130012, China.
authors.forename: Hai-Jun
authors.initials: HJ
authors.lastname: Xing