The Quantum and Classical Complexity of Translationally Invariant Tiling and Hamiltonian Problems

TL;DR

This paper proves the classical and quantum complexity of translationally invariant tiling and Hamiltonian problems, showing they are NEXP-complete and QMA_EXP-complete respectively, with the problem size as the sole input parameter.

Contribution

It establishes the complexity of fixed-parameter translationally invariant problems, extending known results to cases with constant-sized parameters and input solely from system size.

Findings

Classical tiling problem is NEXP-complete.

Quantum ground state energy problem is QMA_EXP-complete.

Complexity results hold even with fixed, constant-sized parameters.

Abstract

We study the complexity of a class of problems involving satisfying constraints which remain the same under translations in one or more spatial directions. In this paper, we show hardness of a classical tiling problem on an N x N 2-dimensional grid and a quantum problem involving finding the ground state energy of a 1-dimensional quantum system of N particles. In both cases, the only input is N, provided in binary. We show that the classical problem is NEXP-complete and the quantum problem is QMA_EXP-complete. Thus, an algorithm for these problems which runs in time polynomial in N (exponential in the input size) would imply that EXP = NEXP or BQEXP = QMA_EXP, respectively. Although tiling in general is already known to be NEXP-complete, to our knowledge, all previous reductions require that either the set of tiles and their constraints or some varying boundary conditions be given as part of the input. In the problem considered here, these are fixed, constant-sized parameters of the problem. Instead, the problem instance is encoded solely in the size of the system.