4,229 research outputs found
Entanglement entropy of multipartite pure states
Consider a system consisting of -dimensional quantum particles and
arbitrary pure state of the whole system. Suppose we simultaneously
perform complete von Neumann measurements on each particle. One can ask: what
is the minimal possible value of the entropy of outcomes joint
probability distribution? We show that coincides with entanglement
entropy for bipartite states. We compute for two sample multipartite
states: the hexacode state () and determinant states (). The
generalization of determinant states to the case is considered.Comment: 7 pages, REVTeX, corrected some typo
Diffusive spreading and mixing of fluid monolayers
The use of ultra-thin, i.e., monolayer films plays an important role for the
emerging field of nano-fluidics. Since the dynamics of such films is governed
by the interplay between substrate-fluid and fluid-fluid interactions, the
transport of matter in nanoscale devices may be eventually efficiently
controlled by substrate engineering. For such films, the dynamics is expected
to be captured by two-dimensional lattice-gas models with interacting
particles. Using a lattice gas model and the non-linear diffusion equation
derived from the microscopic dynamics in the continuum limit, we study two
problems of relevance in the context of nano-fluidics. The first one is the
case in which along the spreading direction of a monolayer a mesoscopic-sized
obstacle is present, with a particular focus on the relaxation of the fluid
density profile upon encountering and passing the obstacle. The second one is
the mixing of two monolayers of different particle species which spread side by
side following the merger of two chemical lanes, here defined as domains of
high affinity for fluid adsorption surrounded by domains of low affinity for
fluid adsorption.Comment: 12 pages, 3 figure
Implications of Teleportation for Nonlocality
Adopting an approach similar to that of Zukowski [Phys. Rev. A 62, 032101
(2000)], we investigate connections between teleportation and nonlocality. We
derive a Bell-type inequality pertaining to the teleportation scenario and show
that it is violated in the case of teleportation using a perfect singlet. We
also investigate teleportation using `Werner states' of the form x P + (1-x)
I/4, where P is the projector corresponding to a singlet state and I is the
identity. We find that our inequality is violated, implying nonlocality, if x >
1/sqrt(2). In addition, we extend Werner's local hidden variable model to
simulation of teleportation with the x = 1/2 Werner state. Thus teleportation
using this state does not involve nonlocality even though the fidelity achieved
is 3/4 which is greater than the `classical limit' of 2/3. Finally, we comment
on a result of Gisin's and offer some philosophical remarks on teleportation
and nonlocality generally.Comment: 10 pages, no figures. Title changed to accord with Phys. Rev. A
version. A note and an extra reference have been added. Journal reference
adde
Dynamical Mean-Field Study of the Ferromagnetic Transition Temperature of a Two-Band Model for Colossal Magnetoresistance Materials
The ferromagnetic (FM) transition temperature (Tc) of a two-band
Double-Exchange (DE) model for colossal magnetoresistance (CMR) materials is
studied using dynamical mean-field theory (DMFT), in wide ranges of coupling
constants, hopping parameters, and carrier densities. The results are shown to
be in excellent agreement with Monte Carlo simulations. When the bands overlap,
the value of Tc is found to be much larger than in the one-band case, for all
values of the chemical potential within the energy overlap interval. A nonzero
interband hopping produces an additional substantial increase of Tc, showing
the importance of these nondiagonal terms, and the concomitant use of multiband
models, to boost up the critical temperatures in DE-based theories.Comment: 4 pages, 4 eps figure
Generalized quantum measurements and local realism
The structure of a local hidden variable model for experiments involving
sequences of measurements rigorously is analyzed. Constraints imposed by local
realism on the conditional probabilities of the outcomes of such measurement
schemes are explicitly derived. The violation of local realism in the case of
``hidden nonlocality'' is illustrated by an operational example.Comment: Revtex, 12 pages; Some modifications of introduction has been made; a
note stating that part of results had been obtained earlier by other authors,
has been added; one postscript figure available at request from
[email protected]
Quantum Thermalization With Couplings
We study the role of the system-bath coupling for the generalized canonical
thermalization [S. Popescu, et al., Nature Physics 2,754(2006) and S. Goldstein
et al., Phys. Rev. Lett. 96, 050403(2006)] that reduces almost all the pure
states of the "universe" [formed by a system S plus its surrounding heat bath
] to a canonical equilibrium state of S. We present an exactly solvable, but
universal model for this kinematic thermalization with an explicit
consideration about the energy shell deformation due to the interaction between
S and B. By calculating the state numbers of the "universe" and its subsystems
S and B in various deformed energy shells, it is found that, for the
overwhelming majority of the "universe" states (they are entangled at least),
the diagonal canonical typicality remains robust with respect to finite
interactions between S and B. Particularly, the kinematic decoherence is
utilized here to account for the vanishing of the off-diagonal elements of the
reduced density matrix of S. It is pointed out that the non-vanishing
off-diagonal elements due to the finiteness of bath and the stronger
system-bath interaction might offer more novelties of the quantum
thermalization.Comment: 4 pages, 2 figure
Mixed-state entanglement and distillation: is there a ``bound'' entanglement in nature?
It is shown that if a mixed state can be distilled to the singlet form, it
must violate partial transposition criterion [A. Peres, Phys. Rev. Lett. 76,
1413 (1996)]. It implies that there are two qualitatively different types of
entanglement: ``free'' entanglement which is distillable, and ``bound''
entanglement which cannot be brought to the singlet form useful for quantum
communication purposes. Possible physical meaning of the result is discussed.Comment: RevTeX, 4 page
Quantum privacy amplification and the security of quantum cryptography over noisy channels
Existing quantum cryptographic schemes are not, as they stand, operable in
the presence of noise on the quantum communication channel. Although they
become operable if they are supplemented by classical privacy-amplification
techniques, the resulting schemes are difficult to analyse and have not been
proved secure. We introduce the concept of quantum privacy amplification and a
cryptographic scheme incorporating it which is provably secure over a noisy
channel. The scheme uses an `entanglement purification' procedure which,
because it requires only a few quantum Controlled-Not and single-qubit
operations, could be implemented using technology that is currently being
developed. The scheme allows an arbitrarily small bound to be placed on the
information that any eavesdropper may extract from the encrypted message.Comment: 13 pages, Latex including 2 postcript files included using psfig
macro
Recovery of entanglement lost in entanglement manipulation
When an entangled state is transformed into another one with probability one
by local operations and classical communication, the quantity of entanglement
decreases. This letter shows that entanglement lost in the manipulation can be
partially recovered by an auxiliary entangled pair. As an application, a
maximally entangled pair can be obtained from two partially entangled pairs
with probability one. Finally, this recovery scheme reveals a fundamental
property of entanglement relevant to the existence of incomparable states.Comment: 4 pages, 2 figures, REVTeX; minor correction
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