  Creating a train of single photons and monitoring its propagation and
interaction is challenging in most physical systems, as photons generally
interact very weakly with other systems. However, when confining microwave
frequency photons in a transmission line resonator, effective photon-photon
interactions can be mediated by qubits embedded in the resonator. Here, we
observe the phenomenon of photon blockade through second-order correlation
function measurements. The experiments clearly demonstrate antibunching in a
continuously pumped source of single microwave photons measured using microwave
beam splitters, linear amplifiers, and quadrature amplitude detectors. We also
investigate resonance fluorescence and Rayleigh scattering in
Mollow-triplet-like spectra.
