Quantum optical setups are never completely isolated from their environment, e.g., they always interact with the vacuum photonic bath that leads to photons escaping from their system. Despite being a hinderance for many tasks, this photon loss can also be used to learn about the system itself by measuring it in appropriate ways.

Along my career, we have developed several techniques to characterize the light emerging from such quantum optical setups, focusing on:

• A method to study the metrological power of the states emitted in complex non-linear emitters, which lead to the emission into multi-mode states.

• A method to study frequency-resolved multi-photon correlations, which allow to unravel strongly correlated emission processes hidden in other standard quantum optical observables such as the photoluminescence spectrum or standard second-order coherence measurements.