Quantum Graphs and Random-Matrix Theory

TL;DR

This paper proves the Bohigas-Giannoni-Schmit conjecture for quantum graphs with incommensurate bond lengths and unitary symmetry, demonstrating universal spectral fluctuation properties via supersymmetry and random-matrix theory techniques.

Contribution

It establishes the conjecture in its most general form for quantum graphs, linking classical mixing properties to universal quantum spectral fluctuations.

Findings

Universal fluctuation properties for large quantum graphs.

Confirmation of the Bohigas-Giannoni-Schmit conjecture.

Demonstration that zero modes dominate in classically mixing graphs.

Abstract

For simple connected graphs with incommensurate bond lengths and with unitary symmetry we prove the Bohigas-Giannoni-Schmit conjecture in its most general form. Using supersymmetry and taking the limit of infinite graph size, we show that the generating function for every (P, Q) correlation function for both closed and open graphs coincides with the corresponding expression of random-matrix theory. We use that the classical Perron-Frobenius operator is bistochastic and possesses a single eigenvalue +1. In the quantum case that implies the existence of a zero (or massless) mode of the effective action. That mode causes universal fluctuation properties. Avoiding the saddle-point approximation we show that for graphs that are classically mixing (i.e., for which the spectrum of the classical Perron-Frobenius operator possesses a finite gap) and that do not carry a special class of bound states, the zero mode dominates in the limit of infinite grapg size.