Microwave photons inside lattices of coupled resonators and superconducting qubits can exhibit surprising matterlike behavior. Realizing such open-system quantum simulators presents an experimental challenge and requires new tools and measurement techniques. Here, we introduce scanning defect microscopy as one such tool and illustrate its use in mapping the normal-mode structure of microwave photons inside a 49-site kagome lattice of coplanar waveguide resonators. Scanning is accomplished by moving a probe equipped with a sapphire tip across the lattice. This locally perturbs resonator frequencies and induces shifts of the lattice resonance frequencies, which we determine by measuring the transmission spectrum. From the magnitude of mode shifts, we can reconstruct photon field amplitudes at each lattice site and thus create spatial images of the photon-lattice normal modes.

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Virtually all sources of coherent optical radiation rely on laser oscillators driven by atomic population inversion. While their technological importance cannot be overstated, challenges remain in applications requiring high phase stability, laser phase locking and coherent combining, or in particular, difficult spectral ranges such as the "THz gap" (0.1-10 THz). Here, we propose and analyze a new way to produce coherent radiation to span the THz gap by efficient second-harmonic generation (SHG) in low-loss dielectric structures, starting from technologically-mature electronic oscillators (EOs) in the mm-wave spectrum. New THz-band dielectric cavity designs enable this approach by combining extreme field concentration with nonlinear materials enhanced by phonon resonances, all while retaining high quality factors at the fundamental and second harmonic frequencies. The resulting designs enable efficient, cascaded parametric frequency converters, representing a new generation of light sources that are extensible into the mid-IR spectrum and beyond.

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