A simple coin for a $2d$ entangled walk

TL;DR

This paper investigates a simple Bell pair-based coin operator in a 2D discrete quantum walk, deriving analytical solutions and analyzing its entangling properties, which connect to fermionic models in the continuum limit.

Contribution

It introduces a specific Bell pair coin operator for 2D quantum walks, providing analytical solutions and detailed entanglement analysis, linking discrete walks to fermionic field theories.

Findings

Analytical solutions for the 2D quantum walk with the Bell pair coin.

The coin induces oscillating entanglement that reaches a steady state.

In the continuum limit, the walk maps to coupled 1D massive fermions with gauge fields.

Abstract

We analyze the effect of a simple coin operator, built out of Bell pairs, in a $2d$ Discrete Quantum Random Walk (DQRW) problem. The specific form of the coin enables us to find analytical and closed form solutions to the recursion relations of the DQRW. The coin induces entanglement between the spin and position degrees of freedom, which oscillates with time and reaches a constant value asymptotically. We probe the entangling properties of the coin operator further, by two different measures. First, by integrating over the space of initial tensor product states, we determine the {\it Entangling Power} of the coin operator. Secondly, we compute the {\it Generalized Relative R\'{e}nyi Entropy} between the corresponding density matrices for the entangled state and the initial pure unentangled state. Both the {\it Entangling Power} and {\it Generalized Relative R\'{e}nyi Entropy} behaves similar to the entanglement with time. Finally, in the continuum limit, the specific coin operator reduces the $2d$ DQRW into two $1d$ massive fermions coupled to synthetic gauge fields, where both the mass term and the gauge fields are built out of the coin parameters.