391 research outputs found
The role of coherence entropy of physical twin observables in entanglement
The concept of physical twin observables (PTO) for bipartite quantum
states,introduced and proved relevant for quantum information theory in recent
work, is substantially simplified. The relation of observable and state is
studied in detail from the point of view of coherence entropy. Properties of
this quantity are further explored. It is shown that, besides for pure states,
quantum discord (measure of entanglement) can be expressed through the
coherence entropy of a PTO complete in relation to the state.Comment: 19 pages, Latex+Revtex
Necesary and sufficient range-dimension conditions for bipartite quantum correlations
Necessary and sufficient conditions for the existence of a composite-system
statistical operator, and, separately, for the possibility of its being
correlated or uncorrelated, are derived in terms of its range dimension and the
range dimensions of its reduced statistical operators.Comment: 6 pages, Latex 2
Theory of strongly phase fluctuating d-wave superconductors and the spin response in underdoped cuprates
General theory of d-wave quasiparticles coupled to phase fluctuations of
superconducting order parameter is discussed. In the charge sector the
superfluid density is found to conform to Uemura scaling, and the charge
renormalization factor to decrease with underdoping. The spin susceptibilty
exhibits four distinct regimes with increasing frequency and scales with the
superconducting Tc, as observed.Comment: LaTex, 3 pages, proceedings of the M2S-HTSC conference in Rio, May
200
SO(3) symmetry between Neel and ferromagnetic order parameters for graphene in a magnetic field
I consider the Hubbard model of graphene in an external magnetic field and in
the Hartree-Fock approximation. In the continuum limit, the ground state energy
at half filling becomes nearly symmetric under rotations of the three-component
vector (N1,N2,m), with the first two components representing the Neel order
parameter orthogonal to and the third component the magnetization parallel with
the external magnetic field. When the symmetry breaking effects arising from
the lattice, Zeeman coupling, and higher Landau levels are included the system
develops a quantum critical point at which the antiferromagnetic order
disappears and the magnetization has a kink. The observed incompressible state
at filling factor one is argued to arise due to a finite third component of the
Neel order parameter at these electron densities. Recent experiments appear
consistent with vanishing N1 and N2, and finite N3, at the filling factors zero
and one, respectively.Comment: 5 revtex pages: new figure, new eqs. 20-22, and the discussion of the
experiment of Jiang et al added (v2). Cosmetic changes (v3). Accepted in PR
Chains of Quasi-Classical Informations for Bipartite Correlations and the Role of Twin Observables
Having the quantum correlations in a general bipartite state in mind, the
information accessible by simultaneous measurement on both subsystems is shown
never to exceed the information accessible by measurement on one subsystem,
which, in turn is proved not to exceed the von Neumann mutual information. A
particular pair of (opposite- subsystem) observables are shown to be
responsible both for the amount of quasi-classical correlations and for that of
the purely quantum entanglement in the pure-state case: the former via
simultaneous subsystem measurements, and the latter through the entropy of
coherence or of incompatibility, which is defined for the general case. The
observables at issue are so-called twin observables. A general definition of
the latter is given in terms of their detailed properties.Comment: 7 pages, Latex2e, selected for the December 2002 issue of the Virtual
Journal of Quantum Informatio
Interactions and phase transitions on graphene's honeycomb lattice
The low-energy theory of interacting electrons on graphene's two-dimensional
honeycomb lattice is derived and discussed. In particular, the Hubbard model in
the large-N limit is shown to have a semi-metal - antiferromagnetic insulator
quantum critical point in the universality class of the Gross-Neveu model. The
same equivalence is conjectured to hold in the physical case N=2, and its
consequences for various physical quantities are examined. The effects of the
long-range Coulomb interaction and of the magnetic field are discussed.Comment: four pages, one figure; few typos corrected, references adde
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