676 research outputs found
Entanglement enhancement and postselection for two atoms interacting with thermal light
The evolution of entanglement for two identical two-level atoms coupled to a
resonant thermal field is studied for two different families of input states.
Entanglement enhancement is predicted for a well defined region of the
parameter space of one of these families. The most intriguing result is the
possibility of probabilistic production of maximally entangled atomic states
even if the input atomic state is factorized and the corresponding output state
is separable.Comment: accepted for publication in J. Phys.
Topology and Phases in Fermionic Systems
There can exist topological obstructions to continuously deforming a gapped
Hamiltonian for free fermions into a trivial form without closing the gap.
These topological obstructions are closely related to obstructions to the
existence of exponentially localized Wannier functions. We show that by taking
two copies of a gapped, free fermionic system with complex conjugate
Hamiltonians, it is always possible to overcome these obstructions. This allows
us to write the ground state in matrix product form using Grassman-valued bond
variables, and show insensitivity of the ground state density matrix to
boundary conditions.Comment: 4 pages, see also arxiv:0710.329
Operator monotones, the reduction criterion and the relative entropy
We introduce the theory of operator monotone functions and employ it to
derive a new inequality relating the quantum relative entropy and the quantum
conditional entropy. We present applications of this new inequality and in
particular we prove a new lower bound on the relative entropy of entanglement
and other properties of entanglement measures.Comment: Final version accepted for publication, added references in reference
[1] and [13
Covariant Counterterms and Conserved Charges in Asymptotically Flat Spacetimes
Recent work has shown that the addition of an appropriate covariant boundary
term to the gravitational action yields a well-defined variational principle
for asymptotically flat spacetimes and thus leads to a natural definition of
conserved quantities at spatial infinity. Here we connect such results to other
formalisms by showing explicitly i) that for spacetime dimension the
canonical form of the above-mentioned covariant action is precisely the ADM
action, with the familiar ADM boundary terms and ii) that for the
conserved quantities defined by counter-term methods agree precisely with the
Ashtekar-Hansen conserved charges at spatial infinity.Comment: 27 pages; Dedicated to Rafael Sorkin on the occasion of his 60th
birthday; v2 minor change
A relational quantum computer using only two-qubit total spin measurement and an initial supply of highly mixed single qubit states
We prove that universal quantum computation is possible using only (i) the
physically natural measurement on two qubits which distinguishes the singlet
from the triplet subspace, and (ii) qubits prepared in almost any three
different (potentially highly mixed) states. In some sense this measurement is
a `more universal' dynamical element than a universal 2-qubit unitary gate,
since the latter must be supplemented by measurement. Because of the rotational
invariance of the measurement used, our scheme is robust to collective
decoherence in a manner very different to previous proposals - in effect it is
only ever sensitive to the relational properties of the qubits.Comment: TR apologises for yet again finding a coauthor with a ridiculous
middle name [12
Minimum-error discrimination between three mirror-symmetric states
We present the optimal measurement strategy for distinguishing between three
quantum states exhibiting a mirror symmetry. The three states live in a
two-dimensional Hilbert space, and are thus overcomplete. By mirror symmetry we
understand that the transformation {|+> -> |+>, |-> -> -|->} leaves the set of
states invariant. The obtained measurement strategy minimizes the error
probability. An experimental realization for polarized photons, realizable with
current technology, is suggested.Comment: 4 pages, 2 figure
Manipulating quantum information by propagation
We study creation of bi- and multipartite continuous variable entanglement in
structures of coupled quantum harmonic oscillators. By adjusting the
interaction strengths between nearest neighbors we show how to maximize the
entanglement production between the arms in a Y-shaped structure where an
initial single mode squeezed state is created in the first oscillator of the
input arm. We also consider the action of the same structure as an approximate
quantum cloner. For a specific time in the system dynamics the last oscillators
in the output arms can be considered as imperfect copies of the initial state.
By increasing the number of arms in the structure, multipartite entanglement is
obtained, as well as 1 to M cloning. Finally, we are considering configurations
that implement the symmetric splitting of an initial entangled state. All
calculations are carried out within the framework of the rotating wave
approximation in quantum optics, and our predictions could be tested with
current available experimental techniques.Comment: 9 pages, APS forma
Schmidt balls around the identity
Robustness measures as introduced by Vidal and Tarrach [PRA, 59, 141-155]
quantify the extent to which entangled states remain entangled under mixing.
Analogously, we introduce here the Schmidt robustness and the random Schmidt
robustness. The latter notion is closely related to the construction of Schmidt
balls around the identity. We analyse the situation for pure states and provide
non-trivial upper and lower bounds. Upper bounds to the random Schmidt-2
robustness allow us to construct a particularly simple distillability
criterion. We present two conjectures, the first one is related to the radius
of inner balls around the identity in the convex set of Schmidt number
n-states. We also conjecture a class of optimal Schmidt witnesses for pure
states.Comment: 7 pages, 1 figur
Inverse Scattering Construction of a Dipole Black Ring
Using the inverse scattering method in six dimensions we construct the dipole
black ring of five dimensional Einstein-Maxwell-dilaton theory with dilaton
coupling a = 2(2/3)^(1/2).The 5d theory can be thought of as the NS sector of
low energy string theory in Einstein frame. It can also be obtained by
dimensionally reducing six-dimensional vacuum gravity on a circle. Our new
approach uses GL(4, R) integrability structure of the theory inherited from
six-dimensional vacuum gravity. Our approach is also general enough to
potentially generate dipole black objects carrying multiple rotations as well
as more exotic multi-horizon configurations
G2 Dualities in D=5 Supergravity and Black Strings
Five dimensional minimal supergravity dimensionally reduced on two commuting
Killing directions gives rise to a G2 coset model. The symmetry group of the
coset model can be used to generate new solutions by applying group
transformations on a seed solution. We show that on a general solution the
generators belonging to the Cartan and nilpotent subalgebras of G2 act as
scaling and gauge transformations, respectively. The remaining generators of G2
form a sl(2,R)+sl(2,R) subalgebra that can be used to generate non-trivial
charges. We use these generators to generalize the five dimensional Kerr string
in a number of ways. In particular, we construct the spinning electric and
spinning magnetic black strings of five dimensional minimal supergravity. We
analyze physical properties of these black strings and study their
thermodynamics. We also explore their relation to black rings.Comment: typos corrected (26 pages + appendices, 2 figures
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