Continuous-time quantum walks over connected graphs, amplitudes and invariants

Abstract

We examine the time dependent amplitude $ \phi_{j}\left( t\right)$ at each vertex $j$ of a continuous-time quantum walk on the cycle $C_{n}$. In many cases the Lissajous curve of the real vs. imaginary parts of each $ \phi_{j}\left( t\right)$ reveals interesting shapes of the space of time-accessible amplitudes. We find two invariants of continuous-time quantum walks. First, considering the rate at which each amplitude evolves in time we find the quantity $T = \displaystyle\sum_{j=0}^{n-1} \lvert\dfrac{d \phi_{j}\left( t\right)}{d t}\rvert^{2}$ is time invariant. The value of $T$ for any initial state can be minimized with respect to a global phase factor $e^{i \theta t}$ to some value $T_{min}$. An operator for $T_{min}$ is defined. For any simply connected graph $g$ the highest possible value of $T_{min}$ with respect to the initial state is found to be $T_{min}^{max}=\left( \frac{\lambda_{max}}{2}\right)^{2}$ where $\lambda_{max}$ is the maximum eigenvalue in the Laplace spectrum of $g$. A second invariant is found in the time-dependent probability distribution $P_{j}\left(t\right) = \lvert\phi_{j}\left(t\right)\rvert^{2}$ of any initial state satisfying $T_{min}^{max}$, with these conditions $\displaystyle\sum_{j=0}^{n-1}\left(P_{j}^{max} - P_{j}^{min}\right)^{2} = \dfrac{4}{n}$ for all simply connected graphs of $n$ vertices.