  Like a quantum computer designed for a particular class of problems, a
quantum simulator enables quantitative modeling of quantum systems that is
computationally intractable with a classical computer. Quantum simulations of
quantum many-body systems have been performed using ultracold atoms and trapped
ions among other systems. Superconducting circuits have recently been
investigated as an alternative system in which microwave photons confined to a
lattice of coupled resonators act as the particles under study with qubits
coupled to the resonators producing effective photon-photon interactions. Such
a system promises insight into the nonequilibrium physics of interacting bosons
but new tools are needed to understand this complex behavior. Here we
demonstrate the operation of a scanning transmon qubit and propose its use as a
local probe of photon number within a superconducting resonator lattice. We map
the coupling strength of the qubit to a resonator on a separate chip and show
that the system reaches the strong coupling regime over a wide scanning area.
