Diagonal unitary entangling gates and contradiagonal quantum states

TL;DR

This paper explores the nonlocal properties of diagonal random unitary matrices, revealing how their entangling power relates to contradiagonal quantum states and deriving key statistical properties of these states.

Contribution

It introduces a novel connection between diagonal unitary gates, contradiagonal states, and combinatorial objects, providing new insights into their entanglement and entropy characteristics.

Findings

Average Schmidt strength scales as log N

Entangling power relates to von Neumann entropy of a derived state

Exact mean von Neumann entropy for phase density matrices

Abstract

Nonlocal properties of an ensemble of diagonal random unitary matrices of order $N^2$ are investigated. The average Schmidt strength of such a bipartite diagonal quantum gate is shown to scale as $\log N$, in contrast to the $\log N^2$ behavior characteristic to random unitary gates. Entangling power of a diagonal gate $U$ is related to the von Neumann entropy of an auxiliary quantum state $\rho=AA^{\dagger}/N^2$, where the square matrix $A$ is obtained by reshaping the vector of diagonal elements of $U$ of length $N^2$ into a square matrix of order $N$. This fact provides a motivation to study the ensemble of non-hermitian unimodular matrices $A$, with all entries of the same modulus and random phases and the ensemble of quantum states $\rho$, such that all their diagonal entries are equal to $1/N$. Such a state is contradiagonal with respect to the computational basis, in sense that among all unitary equivalent states it maximizes the entropy copied to the environment due to the coarse graining process. The first four moments of the squared singular values of the unimodular ensemble are derived, based on which we conjecture a connection to a recently studied combinatorial object called the "Borel triangle". This allows us to find exactly the mean von Neumann entropy for random phase density matrices and the average entanglement for the corresponding ensemble of bipartite pure states.