Construction and local equivalence of dual-unitary operators: from dynamical maps to quantum combinatorial designs

TL;DR

This paper analyzes the construction and properties of dual-unitary operators in quantum circuits, introduces new methods for generating them, and extends classical combinatorial designs to quantum ones, with a focus on two-qubit systems.

Contribution

It provides an analytical study of a nonlinear map for constructing dual-unitary operators, introduces stochastic variants, and extends classical combinatorial designs to quantum designs for dual-unitary operators.

Findings

Analytical description of basins of attraction for dual-unitary operators in two-qubit systems

Explicit construction of new dual and 2-unitary operators using nonlinear and stochastic maps

Extension of classical Latin square designs to quantum designs for dual-unitary operators

Abstract

While quantum circuits built from two-particle dual-unitary (maximally entangled) operators serve as minimal models of typically nonintegrable many-body systems, the construction and characterization of dual-unitary operators themselves are only partially understood. A nonlinear map on the space of unitary operators was proposed in PRL.~125, 070501 (2020) that results in operators being arbitrarily close to dual unitaries. Here we study the map analytically for the two-qubit case describing the basins of attraction, fixed points, and rates of approach to dual unitaries. A subset of dual-unitary operators having maximum entangling power are 2-unitary operators or perfect tensors, and are equivalent to four-party absolutely maximally entangled states. It is known that they only exist if the local dimension is larger than $d=2$. We use the nonlinear map, and introduce stochastic variants of it, to construct explicit examples of new dual and 2-unitary operators. A necessary criterion for their local unitary equivalence to distinguish classes is also introduced and used to display various concrete results and a conjecture in $d=3$. It is known that orthogonal Latin squares provide a ``classical combinatorial design" for constructing permutations that are 2-unitary. We extend the underlying design from classical to genuine quantum ones for general dual-unitary operators and give an example of what might be the smallest sized genuinely quantum design of a 2-unitary in $d=4$.