Atom-photon interaction is at the heart of quantum science and technology, and it can never be good enough. This is why cavity QED with optical microcavities has come to play a key role in this field: their small mode cross-section and low loss enhances the fidelity quantum interfaces, but also provides an effective coherent coupling that can be used to create entanglement between particles that are placed inside the cavity mode. Fiber Fabry-Perot (FFP) cavities with laser-machined, ultralow roughness micromirrors are one cavity type that is being used quite successfully with many different quantum emitters, ranging from ultracold atoms to diamond NV centers and carbon nanotubes. After a general introduction, I will focus on two experiments where FFP cavities are used to generate many-particle entanglement in ultracold atoms. The first is an experimental realization of quantum Zeno dynamics, a phenomenon which had been theoretically studied for more than a decade, but is used here for the first time in an experiment to generate entanglement in an ensemble of qubit atoms. Cavity measurement effectively blocks part of the qubits’ Hilbert space, and entanglement emerges when the coherently driven qubits approach this blocked part. The second experiment is a trapped-atom clock on a chip, where the FFP cavity has allowed us to generate long-lived spin squeezed states in a metrological environment.