1. Metric graphs

2. Quantum graphs

3. Theorems

4. Applications

5. See also

6. References

In mathematics and physics, a quantum graph is a linear, network-shaped structure of vertices connected on edges (i.e., a graph) in which each edge is given a length and where a differential (or pseudo-differential) equation is posed on each edge. An example would be a power network consisting of power lines (edges) connected at transformer stations (vertices); the differential equations would then describe the voltage along each of the line, with boundary conditions for each edge provided at the adjacent vertices ensuring that the current added over all edges adds to zero at each vertex.

Quantum graphs were first studied by Linus Pauling as models of free electrons in organic molecules in the 1930s. They also arise in a variety of mathematical contexts, e.g. as model systems in quantum chaos, in the study of waveguides, in photonic crystals and in Anderson localization, or as limit on shrinking thin wires. Quantum graphs have become prominent models in mesoscopic physics used to obtain a theoretical understanding of nanotechnology. Another, more simple notion of quantum graphs was introduced by Freedman et al.^[1]

Aside from actually solving the differential equations posed on a quantum graph for purposes of concrete applications, typical questions that arise are those of controllability (what inputs have to be provided to bring the system into a desired state, for example providing sufficient power to all houses on a power network) and identifiability (how and where one has to measure something to obtain a complete picture of the state of the system, for example measuring the pressure of a water pipe network to determine whether or not there is a leaking pipe).

Metric graphs

A metric graph

is a graph consisting of a set of vertices and

a set of edges where each edge has been associated

with an interval so that is the coordinate on the

interval, the vertex corresponds to and

to or vice versa. The choice of which vertex lies at zero is

arbitrary with the alternative corresponding to a change of coordinate on the

edge.

The graph has a natural metric: for two

points on the graph, is

the shortest distance between them

where distance is measured along the edges of the graph.

Open graphs: in the combinatorial graph model

edges always join pairs of vertices however in a quantum graph one may also

consider semi-infinite edges. These are edges associated with the interval

attached to a single vertex at .

A graph with one or more

such open edges is referred to as an open graph.

Quantum graphs

Quantum graphs are metric graphs equipped with a differential

(or pseudo-differential) operator acting on functions on the graph.

A function on a metric graph is defined as the -tuple of functions

on the intervals.

The Hilbert space of the graph is

where the inner product of two functions is

may be infinite in the case of an open edge. The simplest example of an operator on a metric graph is the Laplace operator. The operator on an edge is where is the coordinate on the edge. To make the operator self-adjoint a suitable domain must be specified. This is typically achieved by taking the Sobolev space of functions on the edges of the graph and specifying matching conditions at the vertices.

The trivial example of matching conditions that make the operator self-adjoint are the Dirichlet boundary conditions, for every edge. An eigenfunction on a finite edge may be written as

for integer . If the graph is closed with no infinite edges and the

lengths of the edges of the graph are rationally independent

then an eigenfunction is supported on a single graph edge

and the eigenvalues are . The Dirichlet conditions

don't allow interaction between the intervals so the spectrum is the same as

that of the set of disconnected edges.

More interesting self-adjoint matching conditions that allow interaction between edges are the Neumann or natural matching conditions. A function in the domain of the operator is continuous everywhere on the graph and the sum of the outgoing derivatives at a vertex is zero,

where if the vertex is at and if is at .

The properties of other operators on metric graphs have also been studied.

• These include the more general class of Schrödinger operators,

where is a "magnetic vector potential" on the edge and is a scalar potential.

• Another example is the Dirac operator on a graph which is a matrix valued operator acting on vector valued functions that describe the quantum mechanics of particles with an intrinsic angular momentum of one half such as the electron.
• The Dirichlet-to-Neumann operator on a graph is a pseudo-differential operator that arises in the study of photonic crystals.

Theorems

All self-adjoint matching conditions of the Laplace operator on a graph can be classified according to a scheme of Kostrykin and Schrader. In practice, it is often more convenient to adopt a formalism introduced by Kuchment, see,^[2] which automatically yields an operator in variational form.

Let be a vertex with edges emanating from it. For simplicity we choose the coordinates on the edges so that lies at for each edge meeting at . For a function on the graph let

Matching conditions at can be specified by a pair of matrices

and through the linear equation,

The matching conditions define a self-adjoint operator if

The spectrum of the Laplace operator on a finite graph can be conveniently described

using a scattering matrix approach introduced by Kottos and Smilansky

.^[3][4] The eigenvalue problem on an edge is,

So a solution on the edge can be written as a linear combination of plane waves.

where in a time-dependent Schrödinger equation is the coefficient

of the outgoing plane wave at and coefficient of the incoming

plane wave at .

The matching conditions at define a scattering matrix

The scattering matrix relates the vectors of incoming and outgoing plane-wave

coefficients at , .

For self-adjoint matching conditions is unitary. An element of

of is a complex transition amplitude

from a directed edge

to the edge which in general depends on .

However, for a large class of matching conditions

the S-matrix is independent of .

With Neumann matching conditions for example

Substituting in the equation for

produces -independent transition amplitudes

where is the Kronecker delta function that is one if and

zero otherwise. From the transition amplitudes we may define a

matrix

is called the bond scattering matrix and

can be thought of as a quantum evolution operator on the graph. It is

unitary and acts on the vector of plane-wave coefficients for the

graph where is the coefficient of

the plane wave traveling from to .

The phase is the phase acquired by the plane wave

when propagating from vertex to vertex .

Quantization condition: An eigenfunction on the graph

can be defined through its associated plane-wave coefficients.

As the eigenfunction is stationary under the quantum evolution a quantization

condition for the graph can be written using the evolution operator.

Eigenvalues occur at values of where the matrix has an

eigenvalue one. We will order the spectrum with

.

The first trace formula for a graph was derived by Roth (1983).

In 1997 Kottos and Smilansky used the quantization condition above to obtain

the following trace formula for the Laplace operator on a graph when the

transition amplitudes are independent of .

The trace formula links the spectrum with periodic orbits on the graph.

is called the density of states. The right hand side of the trace

formula is made up of two terms, the Weyl

term

is the mean separation of eigenvalues and the oscillating part is a sum

over all periodic orbits on the graph.

is the length of the orbit and

is

the total length of the graph. For an orbit generated by repeating a

shorter primitive orbit, counts the number of repartitions.

is

the product of the transition amplitudes at the vertices of the graph around

the orbit.

Applications

Quantum graphs were first employed in the 1930s

to model the spectrum of free electrons in organic molecules like

Naphthalene, see figure. As a first approximation the

atoms are taken to be vertices while the

σ-electrons form bonds that fix a frame

in the shape of the molecule on which the free electrons are confined.

A similar problem appears when considering quantum waveguides. These

are mesoscopic systems - systems built with a width on the scale of

nanometers. A quantum waveguide can be thought of as a fattened graph

where the edges

are thin tubes. The spectrum of the Laplace operator on this domain

converges to the spectrum of the Laplace operator on the graph

under certain conditions. Understanding mesoscopic systems plays an

important role in the field of nanotechnology.

In 1997^[5] Kottos and Smilansky proposed quantum graphs as a model to study

quantum chaos, the quantum mechanics of systems that

are classically chaotic. Classical motion on the graph can be defined as

a probabilistic Markov chain where the probability of scattering

from edge to edge is given by the absolute value of the

quantum transition amplitude squared, . For almost all

finite connected

quantum graphs the probabilistic dynamics is ergodic and mixing,

in other words chaotic.

Quantum graphs embedded in two or three dimensions appear in the study

of photonic crystals {{fix |link=Wikipedia:Citation needed#Citation needed |text=citation needed |class=Template-Fact }}. In two dimensions a simple model of

a photonic crystal consists of polygonal cells of a dense dielectric with

narrow interfaces between the cells filled with air. Studying

dielectric modes that stay mostly in the dielectric gives rise to a

pseudo-differential operator on the graph that follows the narrow interfaces.

Periodic quantum graphs like the lattice in are common models of

periodic systems and quantum graphs have been applied

to the study the phenomena of Anderson localization where localized

states occur at the edge of spectral bands in the presence of disorder.

See also

• Event symmetry
• Schild's Ladder, a novel dealing with a fictional quantum graph theory
• Feynman diagram

References

• Saleh Abdulla
• Saleh a-Din
• Saleh Ahmad al-Ali
• Saleh Ahmed (cricketer, born 1997)
• Saleh Ahmed Pamba
• Salehak
• Saleh al Ali
• Saleh al-Arouri
• Saleh Al-Beraiky
• Saleh Al Buraiki
• Saleh Al-Buraiki
• Saleh Al-Dawod
• Saleh Al Haddad
• Saleh Al-Haddad
• Saleh Ali al-Sammad
• Saleh Al-Jawhari
• Saleh Al Maghamsi
• Saleh al-Maghamsi
• Saleh al-Mergani
• Saleh al-Merghani
• Saleh al-Ogaili
• Saleh Al-Qarawi
• Saleh al-Rashed
• Saleh Al-Shehri
• Saleh al-Sheikh