Circuit quantum optics (quantum optics with microwave photons in electronic circuits) has allowed to solve several hard problems in traditional quantum optics and to explore new physics. However, so far only one of two regimes of circuit quantum optics has been explored, the circuit quantum electrodynamics regime, where photons reside in electrical resonators.In this project we want to develop the other regime of circuit quantum optics, the wideband regime, where photons are wave packets propagating along transmission lines. To do so we will build devices based on dynamical Coulomb blockade in Josephson junctions, a phenomenon relating tunneling of Cooper pairs to the emission and absorption of photons. This effect is well understood, but only DC current has been studied so far. We want to employ the photonic aspect of dynamical Coulomb blockade: Engineering the impedance seen by the junction and applying appropriate voltages allows to select specific single- or multi-photon processes that we want to use to build single photon sources, detectors and amplifiers and many other devices. Together they will fully enable wideband circuit quantum optics.The successful project will also extend the frequency range accessible to circuit quantum optics: Current quantum circuits can be operated only in a limited range around 5 GHz due to engineering constraints. Our approach lifts these constraints and the proposed devices should function in the range from a few GHz up to 1 THz. This extended frequency window will enable the development of hybrid quantum systems coupling quantum circuits to single dopants, molecules, quantum dots or other mesoscopic devices. The output of our project will also be helpful for other domains where radiation in the GHz to THz has to be measured at the single photon level, for example astronomy.