465 research outputs found

### Local modes, local vacuum, local bogoljubov coefficients and the renormalised stress tensor

Local modes and local particles are defined at any point in curved space time
as those that most resemble Minkowsky modes at that point. It is shown that the
renormalised stress tensor is the difference of energy between the physical
vacuum and that defined by these local modes.Comment: (talk presented at Journees relativistes 93), 4 pages (LATEX),ULB-TH
09/9

### Gravitational Instanton for Black Hole Radiation

Hawking radiation is derived from the existence of a euclidean instanton
which lives in the euclidean black hole geometry. Upon taking into account the
gravitational field of the instanton itself, its action is exactly equal to one
quarter the change in the horizon area. This result also applies to the
Schwinger process, the Unruh process, and particle creation in deSitter space.
The implications for horizon thermodynamics are discussed.Comment: 10 pages, late

### Purifying noisy entanglement requires collective measurements

Known entanglement purification protocols for mixed states use collective
measurements on several copies of the state in order to increase the
entanglement of some of them. We address the question of whether it is possible
to purify the entanglement of a state by processing each copy separately. While
this is possible for pure states, we show that this is impossible, in general,
for mixed states. The importance of this result both conceptually and for
experimental realization of purification is discussed. We also give explicit
invariants of an entangled state of two qubits under local actions and
classical communication.Comment: 5 pages, Late

### No-go theorems for \psi-epistemic models based on a continuity assumption

The quantum state \psi is a mathematical object used to determine the
probabilities of different outcomes when measuring a physical system. Its
fundamental nature has been the subject of discussions since the inception of
quantum theory: is it ontic, that is, does it correspond to a real property of
the physical system? Or is it epistemic, that is, does it merely represent our
knowledge about the system? Assuming a natural continuity assumption and a weak
separability assumption, we show here that epistemic interpretations of the
quantum state are in contradiction with quantum theory. Our argument is
different from the recent proof of Pusey, Barrett, and Rudolph and it already
yields a non-trivial constraint on \psi-epistemic models using a single copy of
the system in question.Comment: Version 1 contains both theory and an illustrative experiment.
Version 2 contains only the theory (the experiment with expanded discussion
will be posted separatly at a later date). The main novelty of Version 2 is a
detailed comparison in appendix 2 with L. Hardy arXiv:1205.14396. Version 2
is 6 pages of text and 1 figure; v3: minor change

### A family of loss-tolerant quantum coin flipping protocols

We present a family of loss-tolerant quantum strong coin flipping protocols;
each protocol differing in the number of qubits employed. For a single qubit we
obtain a bias of 0.4, reproducing the result of Berl\'{i}n et al. [Phys. Rev. A
80, 062321 (2009)], while for two qubits we obtain a bias of 0.3975. Numerical
evidence based on semi-definite programming indicates that the bias continues
to decrease as the number of qubits is increased but at a rapidly decreasing
rate

### The Extent of Multi-particle Quantum Non-locality

It is well known that entangled quantum states can be nonlocal: the
correlations between local measurements carried out on these states cannot
always be reproduced by local hidden variable models. Svetlichny, followed by
others, showed that multipartite quantum states are even more nonlocal than
bipartite ones in the sense that nonlocal classical models with (super-luminal)
communication between some of the parties cannot reproduce the quantum
correlations. Here we study in detail the kinds of nonlocality present in
quantum states. More precisely we enquire what kinds of classical communication
patterns cannot reproduce quantum correlations. By studying the extremal points
of the space of all multiparty probability distributions, in which all parties
can make one of a pair of measurements each with two possible outcomes, we find
a necessary condition for classical nonlocal models to reproduce the statistics
of all quantum states. This condition extends and generalises work of
Svetlichny and others in which it was shown that a particular class of
classical nonlocal models, the ``separable'' models, cannot reproduce the
statistics of all multiparticle quantum states. Our condition shows that the
nonlocality present in some entangled multiparticle quantum states is much
stronger than previously thought. We also study the sufficiency of our
condition.Comment: 10 pages, 2 figures, journal versio

### Optimal Quantum Cloning Machines

We present Quantum Cloning Machines (QCM) that transform N identical qubits
into $M>N$ identical copies and we prove that the fidelity (quality) of these
copies is optimal. The connection between cloning and measurement is discussed
in detail. When the number of clones M tends towards infinity, the fidelity of
each clone tends towards the optimal fidelity that can be obtained by a
measurement on the input qubits. More generally, the QCM are universal devices
to translate quantum information into classical information.Comment: 4 pages, Latex, 1 postscript figure, (very) minor modification

### Bell inequalities for arbitrarily high dimensional systems

We develop a novel approach to Bell inequalities based on a constraint that
the correlations exhibited by local realistic theories must satisfy. This is
used to construct a family of Bell inequalities for bipartite quantum systems
of arbitrarily high dimensionality which are strongly resistant to noise. In
particular our work gives an analytic description of numerical results of D.
Kaszlikowski, P. Gnacinski, M. Zukowski, W. Miklaszewski, A. Zeilinger, Phys.
Rev. Lett. {\bf 85}, 4418 (2000) and T. Durt, D. Kaszlikowski, M. Zukowski,
quant-ph/0101084, and generalises them to arbitrarily high dimensionality.Comment: 6 pages, late

### Violation of local realism vs detection efficiency

We put bounds on the minimum detection efficiency necessary to violate local
realism in Bell experiments. These bounds depends of simple parameters like the
number of measurement settings or the dimensionality of the entangled quantum
state. We derive them by constructing explicit local-hidden variable models
which reproduce the quantum correlations for sufficiently small detectors
efficiency.Comment: 6 pages, revtex. Modifications in the discussion for many parties in
section 3, small erros and typos corrected, conclusions unchange

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