General-Purpose Quantum Simulation with Prethreshold Superconducting Qubits

We introduce a protocol for the fast simulation of $n$-dimensional quantum systems on $n$-qubit quantum computers with tunable couplings. A mapping is given between the control parameters of the quantum computer and the matrix elements of an $n$- dimensional real (but otherwise arbitrary) Hamiltonian that is simulated in the $n$- dimensional {\it single-excitation subspace} of the quantum simulator. A time- dependent energy/time rescaling minimizes the simulation time on hardware having a fixed coherence time. We demonstrate how three tunably coupled superconducting phase qubits can simulate a three-channel molecular collision using this protocol. The method makes a class of general-purpose time-dependent quantum simulation practical with today's sub-thershold-fidelity qubits.