Universal qudit Hamiltonians

TL;DR

This paper proves that various classes of qudit Hamiltonians, including generalizations of the Heisenberg model and projector-based interactions, are universal, enabling them to simulate any other finite-dimensional Hamiltonian and solving complex quantum problems.

Contribution

It characterizes which qudit interactions are universal, including Heisenberg models and projector interactions, expanding the understanding of universal quantum Hamiltonians.

Findings

Almost all $k$-qudit interactions are universal with 1-local terms, except a simple stoquastic class.

SU(d) and SU(2) Heisenberg interactions are universal for all $d \\geq 2$, making related problems QMA-complete.

Any interaction proportional to a projector onto a pure entangled state is universal.

Abstract

A family of quantum Hamiltonians is said to be universal if any other finite-dimensional Hamiltonian can be approximately encoded within the low-energy space of a Hamiltonian from that family. If the encoding is efficient, universal families of Hamiltonians can be used as universal analogue quantum simulators and universal quantum computers, and the problem of approximately determining the ground-state energy of a Hamiltonian from a universal family is QMA-complete. One natural way to categorise Hamiltonians into families is in terms of the interactions they are built from. Here we prove universality of some important classes of interactions on qudits ($d$-level systems): (1) We completely characterise the $k$-qudit interactions which are universal, if augmented with arbitrary 1-local terms. We find that, for all $k \geqslant 2$ and all local dimensions $d \geqslant 2$, almost all such interactions are universal aside from a simple stoquastic class. (2) We prove universality of generalisations of the Heisenberg model that are ubiquitous in condensed-matter physics, even if free 1-local terms are not provided. We show that the $SU(d)$ and $SU(2)$ Heisenberg interactions are universal for all local dimensions $d \geqslant 2$ (spin $\geqslant 1/2$), implying that a quantum variant of the Max-$d$-Cut problem is QMA-complete. We also show that for $d=3$ all bilinear-biquadratic Heisenberg interactions are universal. One example is the general AKLT model. (3) We prove universality of any interaction proportional to the projector onto a pure entangled state.