458 research outputs found
Parameters of Integral Circulant Graphs and Periodic Quantum Dynamics
The intention of the paper is to move a step towards a classification of
network topologies that exhibit periodic quantum dynamics. We show that the
evolution of a quantum system, whose hamiltonian is identical to the adjacency
matrix of a circulant graph, is periodic if and only if all eigenvalues of the
graph are integers (that is, the graph is integral). Motivated by this
observation, we focus on relevant properties of integral circulant graphs.
Specifically, we bound the number of vertices of integral circulant graphs in
terms of their degree, characterize bipartiteness and give exact bounds for
their diameter. Additionally, we prove that circulant graphs with odd order do
not allow perfect state transfer.Comment: 12 page
Maximal diameter of integral circulant graphs
Integral circulant graphs are proposed as models for quantum spin networks
that permit a quantum phenomenon called perfect state transfer. Specifically,
it is important to know how far information can potentially be transferred
between nodes of the quantum networks modelled by integral circulant graphs and
this task is related to calculating the maximal diameter of a graph. The
integral circulant graph has the vertex set and vertices and are adjacent if ,
where . Motivated by the result on
the upper bound of the diameter of given in [N. Saxena, S. Severini,
I. Shparlinski, \textit{Parameters of integral circulant graphs and periodic
quantum dynamics}, International Journal of Quantum Information 5 (2007),
417--430], according to which represents one such bound, in this paper
we prove that the maximal value of the diameter of the integral circulant graph
of a given order with its prime factorization
, is equal to or , where
, depending on whether
or not, respectively. Furthermore, we show that, for a given
order , a divisor set with can always be found such that
this bound is attained. Finally, we calculate the maximal diameter in the class
of integral circulant graphs of a given order and cardinality of the
divisor set and characterize all extremal graphs. We actually show
that the maximal diameter can have the values , , and
depending on the values of and . This way we further improve the upper
bound of Saxena, Severini and Shparlinski and we also characterize all graphs
whose diameters are equal to , thus generalizing a result in that
paper.Comment: 29 pages, 1 figur
Perfect state transfer, graph products and equitable partitions
We describe new constructions of graphs which exhibit perfect state transfer
on continuous-time quantum walks. Our constructions are based on variants of
the double cones [BCMS09,ANOPRT10,ANOPRT09] and the Cartesian graph products
(which includes the n-cube) [CDDEKL05]. Some of our results include: (1) If
is a graph with perfect state transfer at time , where t_{G}\Spec(G)
\subseteq \ZZ\pi, and is a circulant with odd eigenvalues, their weak
product has perfect state transfer. Also, if is a regular
graph with perfect state transfer at time and is a graph where
t_{H}|V_{H}|\Spec(G) \subseteq 2\ZZ\pi, their lexicographic product
has perfect state transfer. (2) The double cone on any
connected graph , has perfect state transfer if the weights of the cone
edges are proportional to the Perron eigenvector of . This generalizes
results for double cone on regular graphs studied in
[BCMS09,ANOPRT10,ANOPRT09]. (3) For an infinite family \GG of regular graphs,
there is a circulant connection so the graph K_{1}+\GG\circ\GG+K_{1} has
perfect state transfer. In contrast, no perfect state transfer exists if a
complete bipartite connection is used (even in the presence of weights)
[ANOPRT09]. We also describe a generalization of the path collapsing argument
[CCDFGS03,CDDEKL05], which reduces questions about perfect state transfer to
simpler (weighted) multigraphs, for graphs with equitable distance partitions.Comment: 18 pages, 6 figure
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