357 research outputs found
A simple spectral condition implying separability for states of bipartite quantum systems
For two qubits and for general bipartite quantum systems, we give a simple
spectral condition in terms of the ordered eigenvalues of the density matrix
which guarantees that the corresponding state is separable.Comment: 5 pages Revised 31 May 200
The Free Energy of the Spin Boson Model
For n spins coupled linearly to a boson field in a volume v_n, the existence of the specific free energy in the limit nââ, v_nââ, with n/v_n = const., is proved under specified conditions on the Hamiltonian. A variational expression is obtained for the limiting specific free energy, and a critical temperature is identified, above which the system behaves as if there were no coupling at all
Entanglement in thermal equilibrium states
We revisist the issue of entanglement of thermal equilibrium states in
composite quantum systems. The possible scenarios are exemplified in bipartite
qubit/qubit and qubit/qutrit systems.Comment: 4 figure
Two level systems interacting with bosons: thermodynamic limit of thermodynamic functions
For a two-level system coupled linearly to bosons, we reduce the existence of the thermodynamic limit of the thermodynamic functions to that of the corresponding limit for the free bosons. The case where the interaction is with the radiation-field is treated as a particularly relevant example
Two-spin subsystem entanglement in spin 1/2 rings with long range interactions
We consider the two-spin subsystem entanglement for eigenstates of the
Hamiltonian
for a ring of spins 1/2 with
asssociated spin vector operator for the -th
spin. Here is the chord-distance betwen sites and . The case
corresponds to the solvable Haldane-Shastry model whose spectrum
has very high degeneracies not present for . Two spin subsystem
entanglement shows high sensistivity and distinguishes from . There is no entanglement beyond nearest neighbors for all eigenstates
when . Whereas for one has selective entanglement at
any distance for eigenstates of sufficiently high energy in a certain interval
of which depends on the energy. The ground state (which is a singlet
only for even ) does not have entanglement beyond nearest neighbors, and the
nearest neighbor entanglement is virtually independent of the range of the
interaction controlled by .Comment: 16 figure
de Finetti reductions for correlations
When analysing quantum information processing protocols one has to deal with
large entangled systems, each consisting of many subsystems. To make this
analysis feasible, it is often necessary to identify some additional structure.
de Finetti theorems provide such a structure for the case where certain
symmetries hold. More precisely, they relate states that are invariant under
permutations of subsystems to states in which the subsystems are independent of
each other. This relation plays an important role in various areas, e.g., in
quantum cryptography or state tomography, where permutation invariant systems
are ubiquitous. The known de Finetti theorems usually refer to the internal
quantum state of a system and depend on its dimension. Here we prove a
different de Finetti theorem where systems are modelled in terms of their
statistics under measurements. This is necessary for a large class of
applications widely considered today, such as device independent protocols,
where the underlying systems and the dimensions are unknown and the entire
analysis is based on the observed correlations.Comment: 5+13 pages; second version closer to the published one; new titl
Spectral Conditions on the State of a Composite Quantum System Implying its Separability
For any unitarily invariant convex function F on the states of a composite
quantum system which isolates the trace there is a critical constant C such
that F(w)<= C for a state w implies that w is not entangled; and for any
possible D > C there are entangled states v with F(v)=D. Upper- and lower
bounds on C are given. The critical values of some F's for qubit/qubit and
qubit/qutrit bipartite systems are computed. Simple conditions on the spectrum
of a state guaranteeing separability are obtained. It is shown that the thermal
equilbrium states specified by any Hamiltonian of an arbitrary compositum are
separable if the temperature is high enough.Comment: Corrects 1. of Lemma 2, and the (under)statement of Proposition 7 of
the earlier version
Geometric interpretation for A-fidelity and its relation with Bures fidelity
A geometric interpretation for the A-fidelity between two states of a qubit
system is presented, which leads to an upper bound of the Bures fidelity. The
metrics defined based on the A-fidelity are studied by numerical method. An
alternative generalization of the A-fidelity, which has the same geometric
picture, to a -state quantum system is also discussed.Comment: 4 pages, 1 figure. Phys. Rev.
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