# Search by Lackadaisical Quantum Walk with Nonhomogeneous Weights

## TL;DR

This paper extends lackadaisical quantum walks by assigning different weights to self-loops on irregular graphs, analyzing their impact on spatial search efficiency on bipartite graphs, and identifying conditions for improved success probabilities.

## Contribution

It introduces weighted self-loops in lackadaisical quantum walks on bipartite graphs and provides analytical conditions for enhanced spatial search performance.

## Key findings

- Success probability improves with optimal self-loop weights when marked vertices are in one partite set.
- Stationary initial states yield improved success probabilities without ratio constraints on graph parts.
- Multiple configurations show no improvement from self-loops when marked vertices are in both sets.

## Abstract

The lackadaisical quantum walk, which is a quantum walk with a weighted self-loop at each vertex, has been shown to speed up dispersion on the line and improve spatial search on the complete graph and periodic square lattice. In these investigations, each self-loop had the same weight, owing to each graph's vertex-transitivity. In this paper, we propose lackadaisical quantum walks where the self-loops have different weights. We investigate spatial search on the complete bipartite graph, which can be irregular with $N_1$ and $N_2$ vertices in each partite set, and this naturally leads to self-loops in each partite set having different weights $l_1$ and $l_2$, respectively. We analytically prove that for large $N_1$ and $N_2$, if the $k$ marked vertices are confined to, say, the first partite set, then with the typical initial uniform state over the vertices, the success probability is improved from its non-lackadaisical value when $l_1 = kN_2/2N_1$ and $N_2 > (3 - 2\sqrt{2}) N_1$, regardless of $l_2$. When the initial state is stationary under the quantum walk, however, then the success probability is improved when $l_1 = kN_2/2N_1$, now without a constraint on the ratio of $N_1$ and $N_2$, and again independent of $l_2$. Next, when marked vertices lie in both partite sets, then for either initial state, there are many configurations for which the self-loops yield no improvement in quantum search, no matter what weights they take.