A variational eigenvalue solver on a photonic quantum processor

The efficient calculation of Hamiltonian spectra, a problem that is intractable on classical computers, is one of the most anticipated applications of quantum computing. We introduce the variational quantum eigensolver: an application of the quantum-classical hybrid variational principle to a photonic quantum processor. The quantum processor prepares a trial chemical ground state with a parametrized quantum circuit, and a classical computer optimizes the parameters to minimize the expectation value of the molecule's Hamiltonian. We experimentally demonstrate this technology for the case of the He-H+ ion, and compute its ground-state energy as a function of bond length. We demonstrate the potential of massively parallel parametric operations and show how the technique scales to larger systems.