During the last years quantum graphs have become a paradigm of quantum chaos with applications from spectral statistics to chaotic scattering and wave function statistics. In the first part of this review we give a detailed introduction to the spectral theory of quantum graphs and discuss exact trace formulae for the spectrum and the quantum-to-classical correspondence. The second part of this review is devoted to the spectral statistics of quantum graphs as an application to quantum chaos. Especially, we summarise recent developments on the spectral statistics of generic large quantum graphs based on two approaches: the periodic-orbit approach and the supersymmetry approach. The latter provides a condition and a proof for universal spectral statistics as predicted by random-matrix theory. Chapter 1
IntroductionThe general mathematical concept of a graph (network) as a set of elements which are connected by some relation has found applications in many branches of science, engineering, and also social science. A street network of a traffic engineer, the network of neurons studied by a neuroscientist, the structure of databases in computer science can all be described by graphs.Recently the Laplacian on a metric graph has gained a lot of attention in physics and mathematics in terms of the diffusion equation or Schrödinger equation. They have now become known as quantum graphs but different aspects are studied under various names such as quantum networks or quantum wires. They have a long history in mathematics and physics.In physics, the first application has probably been in the context of free electron models for organic molecules about seventy years ago by Pauling [198], an approach which has been further developed in subsequent years [172,173,201,209,77,182,204]. Quantum graphs have also been applied successfully to superconductivity in granular and artificial materials [12], acoustic and electromagnetic waveguide networks [112,181], the Anderson transition in a disordered wire [18,222], quantum Hall systems [71,151,150], fracton excitations in fractal structures [22,190], and mesoscopic quantum systems [142,166,242,243,241]. Quantum graphs have also been simulated experimentally [141].The construction of self-adjoint operators, or wave equations with appropriate boundary conditions on graphs has first been addressed by Ruedenberg and Scherr [209] (see also [204]). They considered graphs as an idealisation of networks of wires or wave guides of finite cross-section in the limit where the diameter of the wire is much smaller than any other length scale. Similar approaches to graphs as networks of thin wires with a finite diameter, or fat quantum graphs as they are now called, have been a topic in mathematical physics recently [211,212,208,167,106,202].Another interesting aproach which has been discussed mainly by Exner and his coworkers is based on leaky graphs [101,100,102,103,105,104 , 108, 109]. Here, a finite attractive potential in the Schrödinger equation is centered on a metric graph. A leaky g...