153 research outputs found
Kochen-Specker Sets and the Rank-1 Quantum Chromatic Number
The quantum chromatic number of a graph is sandwiched between its
chromatic number and its clique number, which are well known NP-hard
quantities. We restrict our attention to the rank-1 quantum chromatic number
, which upper bounds the quantum chromatic number, but is
defined under stronger constraints. We study its relation with the chromatic
number and the minimum dimension of orthogonal representations
. It is known that . We
answer three open questions about these relations: we give a necessary and
sufficient condition to have , we exhibit a class of
graphs such that , and we give a necessary and
sufficient condition to have . Our main tools are
Kochen-Specker sets, collections of vectors with a traditionally important role
in the study of noncontextuality of physical theories, and more recently in the
quantification of quantum zero-error capacities. Finally, as a corollary of our
results and a result by Avis, Hasegawa, Kikuchi, and Sasaki on the quantum
chromatic number, we give a family of Kochen-Specker sets of growing dimension.Comment: 12 page
A Generalization of Kochen-Specker Sets Relates Quantum Coloring to Entanglement-Assisted Channel Capacity
We introduce two generalizations of Kochen-Specker (KS) sets: projective KS
sets and generalized KS sets. We then use projective KS sets to characterize
all graphs for which the chromatic number is strictly larger than the quantum
chromatic number. Here, the quantum chromatic number is defined via a nonlocal
game based on graph coloring. We further show that from any graph with
separation between these two quantities, one can construct a classical channel
for which entanglement assistance increases the one-shot zero-error capacity.
As an example, we exhibit a new family of classical channels with an
exponential increase.Comment: 16 page
Quantum measurements with prescribed symmetry
We introduce a method to determine whether a given generalised quantum
measurement is isolated or it belongs to a family of measurements having the
same prescribed symmetry. The technique proposed reduces to solving a linear
system of equations in some relevant cases. As consequence, we provide a simple
derivation of the maximal family of Symmetric Informationally Complete
measurements (SIC)-POVM in dimension 3. Furthermore, we show that the following
remarkable geometrical structures are isolated, so that free parameters cannot
be introduced: (a) maximal sets of mutually unbiased bases in prime power
dimensions from 4 to 16, (b) SIC-POVM in dimensions from 4 to 16 and (c)
contextuality Kochen-Specker sets in dimension 3, 4 and 6, composed of 13, 18
and 21 vectors, respectively.Comment: 10 pages, 2 figure
Entanglement and nonclassical properties of hypergraph states
Hypergraph states are multi-qubit states that form a subset of the locally
maximally entangleable states and a generalization of the well--established
notion of graph states. Mathematically, they can conveniently be described by a
hypergraph that indicates a possible generation procedure of these states;
alternatively, they can also be phrased in terms of a non-local stabilizer
formalism. In this paper, we explore the entanglement properties and
nonclassical features of hypergraph states. First, we identify the equivalence
classes under local unitary transformations for up to four qubits, as well as
important classes of five- and six-qubit states, and determine various
entanglement properties of these classes. Second, we present general conditions
under which the local unitary equivalence of hypergraph states can simply be
decided by considering a finite set of transformations with a clear
graph-theoretical interpretation. Finally, we consider the question whether
hypergraph states and their correlations can be used to reveal contradictions
with classical hidden variable theories. We demonstrate that various
noncontextuality inequalities and Bell inequalities can be derived for
hypergraph states.Comment: 29 pages, 5 figures, final versio
On small proofs of Bell-Kochen-Specker theorem for two, three and four qubits
The Bell-Kochen-Specker theorem (BKS) theorem rules out realistic {\it
non-contextual} theories by resorting to impossible assignments of rays among a
selected set of maximal orthogonal bases. We investigate the geometrical
structure of small BKS-proofs involving real rays and
-dimensional bases of -qubits (). Specifically, we look at the
parity proof 18-9 with two qubits (A. Cabello, 1996), the parity proof 36-11
with three qubits (M. Kernaghan & A. Peres, 1995 \cite{Kernaghan1965}) and a
newly discovered non-parity proof 80-21 with four qubits (that improves work of
P. K Aravind's group in 2008). The rays in question arise as real eigenstates
shared by some maximal commuting sets (bases) of operators in the -qubit
Pauli group. One finds characteristic signatures of the distances between the
bases, which carry various symmetries in their graphs.Comment: version to appear in European Physical Journal Plu
Clifford group dipoles and the enactment of Weyl/Coxeter group W(E8) by entangling gates
Peres/Mermin arguments about no-hidden variables in quantum mechanics are
used for displaying a pair (R, S) of entangling Clifford quantum gates, acting
on two qubits. From them, a natural unitary representation of Coxeter/Weyl
groups W(D5) and W(F4) emerges, which is also reflected into the splitting of
the n-qubit Clifford group Cn into dipoles Cn . The union of the
three-qubit real Clifford group C+ 3 and the Toffoli gate ensures a orthogonal
representation of the Weyl/Coxeter group W(E8), and of its relatives. Other
concepts involved are complex reflection groups, BN pairs, unitary group
designs and entangled states of the GHZ family.Comment: version revised according the recommendations of a refere
- …