General Conditions for Universality of Quantum Hamiltonians

Recent work has demonstrated the existence of universal Hamiltonians—simple spin-lattice models that can simulate any other quantum many-body system to any desired level of accuracy. Until now, proofs of universality have relied on explicit constructions, tailored to each specific family of universal Hamiltonians. In this work, we go beyond this approach and completely classify the simulation ability of quantum Hamiltonians by their complexity classes. We do this by deriving necessary and sufficient complexity-theoretic conditions characterizing universal quantum Hamiltonians. Although the result concerns the theory of analog Hamiltonian simulation—a promising application of near-term quantum technology—the proof relies on abstract complexity-theoretic concepts and the theory of quantum computation. As well as providing simplified proofs of previous Hamiltonian universality results and offering a route to new universal constructions, the results in this paper give insight into the origins of universality; for example, finally explaining the previously noted coincidences between families of universal Hamiltonian and classes of Hamiltonians appearing in complexity theory.