Doubling the Size of Quantum Simulators by Entanglement Forging

Quantum computers are promising for simulations of chemical and physical systems, but the limited capabilities of today's quantum processors permit only small, and often approximate, simulations. Here we present a method, classical entanglement forging, that harnesses classical resources to capture quantum correlations and double the size of the system that can be simulated on quantum hardware. Shifting some of the computation to classical postprocessing allows us to represent ten spin orbitals of the water molecule on five qubits of an IBM Quantum processor in the most accurate variational simulation of the H2O ground-state energy using quantum hardware to date. We discuss conditions for applicability of classical entanglement forging and present a roadmap for scaling to larger problems.