33,786 research outputs found
Entanglement Swapping Chains for General Pure States
We consider entanglement swapping schemes with general (rather than
maximally) entangled bipartite states of arbitary dimension shared pairwise
between three or more parties in a chain. The intermediate parties perform
generalised Bell measurements with the result that the two end parties end up
sharing a entangled state which can be converted into maximally entangled
states. We obtain an expression for the average amount of maximal entanglement
concentrated in such a scheme and show that in a certain reasonably broad class
of cases this scheme is provably optimal and that, in these cases, the amount
of entanglement concentrated between the two ends is equal to that which could
be concentrated from the weakest link in the chain.Comment: 18 pages, 5 figure
Explaining Phenomenologically Observed Space-time Flatness Requires New Fundamental Scale Physics
The phenomenologically observed flatness - or near flatness - of spacetime
cannot be understood as emerging from continuum Planck (or sub-Planck) scales
using known physics. Using dimensional arguments it is demonstrated that any
immaginable action will lead to Christoffel symbols that are chaotic. We put
forward new physics in the form of fundamental fields that spontaneously break
translational invariance. Using these new fields as coordinates we define the
metric in such a way that the Riemann tensor vanishes identically as a Bianchi
identity. Hence the new fundamental fields define a flat space. General
relativity with curvature is recovered as an effective theory at larger scales
at which crystal defects in the form of disclinations come into play as the
sources of curvature.Comment: This article were already in 2011 published as Proceedings of the
14th Bled Conference on "What Comes Beyond the Standard Models" organized by
Norma Manko Borstnik, Dragan Lukman, Maxim Khlopov, and H.B. Nielse
Quantifying nonorthogonality
An exploratory approach to the possibility of analyzing nonorthogonality as a
quantifiable property is presented. Three different measures for the
nonorthogonality of pure states are introduced, and one of these measures is
extended to single-particle density matrices using methods that are similar to
recently introduced techniques for quantifying entanglement. Several
interesting special cases are considered. It is pointed out that a measure of
nonorthogonality can meaningfully be associated with a single mixed quantum
state. It is then shown how nonorthogonality can be unlocked with classical
information; this analysis reveals interesting inequalities and points to a
number of connections between nonorthogonality and entanglement.Comment: Accepted for publication in Phys. Rev.
When only two thirds of the entanglement can be distilled
We provide an example of distillable bipartite mixed state such that, even in
the asymptotic limit, more pure-state entanglement is required to create it
than can be distilled from it. Thus, we show that the irreversibility in the
processes of formation and distillation of bipartite states, recently proved in
[G. Vidal, J.I. Cirac, Phys. Rev. Lett. 86, (2001) 5803-5806], is not limited
to bound-entangled states.Comment: 4 pages, revtex, 1 figur
Quantum cryptography protocols robust against photon number splitting attacks for weak laser pulses implementations
We introduce a new class of quantum quantum key distribution protocols,
tailored to be robust against photon number splitting (PNS) attacks. We study
one of these protocols, which differs from the BB84 only in the classical
sifting procedure. This protocol is provably better than BB84 against PNS
attacks at zero error.Comment: 4 pages, 2 figure
Building multiparticle states with teleportation
We describe a protocol which can be used to generate any N-partite pure
quantum state using Einstein-Podolsky-Rosen (EPR) pairs. This protocol employs
only local operations and classical communication between the N parties
(N-LOCC). In particular, we rely on quantum data compression and teleportation
to create the desired state. This protocol can be used to obtain upper bounds
for the bipartite entanglement of formation of an arbitrary N-partite pure
state, in the asymptotic limit of many copies. We apply it to a few
multipartite states of interest, showing that in some cases it is not optimal.
Generalizations of the protocol are developed which are optimal for some of the
examples we consider, but which may still be inefficient for arbitrary states.Comment: 11 pages, 1 figure. Version 2 contains an example for which protocol
P3 is better than protocol P2. Correction to references in version
Unconditional security of coherent-state quantum key distribution with strong phase-reference pulse
We prove the unconditional security of a quantum key distribution protocol in
which bit values are encoded in the phase of a weak coherent-state pulse
relative to a strong reference pulse. In contrast to implementations in which a
weak pulse is used as a substitute for a single-photon source, the achievable
key rate is found to decrease only linearly with the transmission of the
channel.Comment: 4 pages, 3 figure
The Initial Value Problem For Maximally Non-Local Actions
We study the initial value problem for actions which contain non-trivial
functions of integrals of local functions of the dynamical variable. In
contrast to many other non-local actions, the classical solution set of these
systems is at most discretely enlarged, and may even be restricted, with
respect to that of a local theory. We show that the solutions are those of a
local theory whose (spacetime constant) parameters vary with the initial value
data according to algebraic equations. The various roots of these algebraic
equations can be plausibly interpreted in quantum mechanics as different
components of a multi-component wave function. It is also possible that the
consistency of these algebraic equations imposes constraints upon the initial
value data which appear miraculous from the context of a local theory.Comment: 8 pages, LaTeX 2 epsilo
Unconditional security at a low cost
By simulating four quantum key distribution (QKD) experiments and analyzing
one decoy-state QKD experiment, we compare two data post-processing schemes
based on security against individual attack by L\"{u}tkenhaus, and
unconditional security analysis by Gottesman-Lo-L\"{u}tkenhaus-Preskill. Our
results show that these two schemes yield close performances. Since the Holy
Grail of QKD is its unconditional security, we conclude that one is better off
considering unconditional security, rather than restricting to individual
attacks.Comment: Accepted by International Conference on Quantum Foundation and
Technology: Frontier and Future 2006 (ICQFT'06
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