Operator dynamics and entanglement in space-time dual Hadamard lattices

TL;DR

This paper introduces a class of space-time dual quantum models based on Hadamard matrices, exploring their operator dynamics, integrability, and entanglement properties, with connections to classical chaos and Clifford automata.

Contribution

It generalizes self-dual models using Hadamard matrices, establishing space-time duality, integrability, and entanglement protocols in a novel quantum lattice framework.

Findings

Models exhibit space-time duality with interesting entanglement features.

Established integrability and conserved charges for a large subfamily.

Connected quantum operator dynamics to classical chaos and cellular automata.

Abstract

Many-body quantum dynamics defined on a spatial lattice and in discrete time -- either as stroboscopic Floquet systems or quantum circuits -- has been an active area of research for several years. Being discrete in space and time, a natural question arises: when can such a model be viewed as evolving unitarily in space as well as in time? Models with this property, which sometimes goes by the name space-time duality, have been shown to have a number of interesting features related to entanglement growth and correlations. One natural way in which the property arises in the context of (brickwork) quantum circuits is by choosing dual unitary gates: two site operators that are unitary in both the space and time directions. We introduce a class of models with $q$ states per site, defined on the square lattice by a complex partition function and paremeterized in terms of $q\times q$ Hadamard matrices, that have the property of space-time duality. These may interpreted as particular dual unitary circuits or stroboscopically evolving systems, and generalize the well studied self-dual kicked Ising model. We explore the operator dynamics in the case of Clifford circuits, making connections to Clifford cellular automata [J. Math. Phys. 49, 112104 (2008)] and in the $q\to\infty$ limit to the classical spatiotemporal cat model of many body chaos [Nonlinearity 34, 2800 (2021)]. We establish integrability and the corresponding conserved charges for a large subfamily and show how the long-range entanglement protocol discussed in the recent paper [Phys. Rev. B 105, 144306 (2022)] can be reinterpreted in purely graphical terms and directly applied here.