Construction of graphs with equal eigenvalues (co-spectral graphs) is an interesting problem in spectral graph theory. Seidel switching is a well-known method for generating co-spectral graphs. From a matrix theoretic point of view, Seidel switching is a combined action of a number of unitary operations on graphs. Recent works [1] and [2] have shown significant connections between graph and quantum information theories. Corresponding to Laplacian matrices of any graph there are quantum states useful in quantum computing. From this point of view, graph theoretical problems are meaningful in the context of quantum information. This work describes Seidel switching from a quantum perspective. Here, we generalize Seidel switching to weighted directed graphs. We use it to construct graphs with equal Laplacian and signless Laplacian spectra and consider density matrices corresponding to them. Hence Seidel switching is a technique to generate cospectral density matrices. Next, we show that all the unitary operators used in Seidel switching are global unitary operators. Global unitary operators can be used to generate entanglement, a benchmark phenomena in quantum information processing.
Generalised dual Seidel switching and Deza graphs with strongly regular children
具有参数（n，k，b，a）的Deza图G是具有n个顶点的k-正则图，使得任意两个不同的顶点有b或一个公共邻居，其中b >= a. Deza图G的子图G_A和G_B定义在G的顶点集上，使得每两个不同的顶点在G_A或G_B中相邻当且仅当它们分别有a个或b个公共邻居。本文给出了对偶Seidel切换的一般方法，并研究了子图是强正则图的Deza图。
