Theory of Computing ------------------- Title : The Quantum and Classical Complexity of Translationally Invariant Tiling and Hamiltonian Problems Authors : Daniel Gottesman and Sandy Irani Volume : 9 Number : 2 Pages : 31-116 URL : https://theoryofcomputing.org/articles/v009a002 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 $N \times N$ 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, the usual approach is to 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. A preliminary version of this paper was posted on the arXiv in 2009. An extended abstract appeared in FOCS'09.