## PT-symmetry and quantum graphs

Miloslav Znojil

A new exactly solvable model of a quantum system is proposed, living on an equilateral q-pointed star graph (q is arbitrary). The model exhibits a weak and spontaneously broken form of $${\cal PT}-$$symmetry, offering a straightforward generalization of one of the standard solvable square wells with $$q=2$$ and unbroken $${\cal PT}-$$symmetry. The kinematics is trivial, Kirchhoff in the central vertex. The dynamics is one-parametric (viz., $$\alpha-$$dependent), prescribed via complex Robin boundary conditions (i.e., the interactions are non-Hermitian and localized at the outer vertices of the star). The (complicated, trigonometric) secular equation is shown reducible to an elementary and compact form. This renders the model (partially) exactly solvable at any $$q \geq 2$$ — an infinite subset of the real roots of the secular equation proves q-independent and known (i.e., inherited from the square-well $$q=2$$ special case). The systems with $$q=4m-2$$ are found anomalous, supporting infinitely many (or, at m=1, one) additional real m-dependent and $$\alpha-$$dependent roots.

http://arxiv.org/abs/1205.5211
Quantum Physics (quant-ph)