Brune leads the Circular Rydberg Atom / Cavity QED group at LKB (Collège de France site), continuing the work of Serge Haroche (Nobel 2012). Note: Brune is employed by ENS, not Sorbonne Université; postdoc contracts are typically ENS/CNRS. Research directions: (1) Circular Rydberg atoms — atoms in extremely high principal quantum number states (n~50) with extremely long radiative lifetimes (~30 ms) and large dipole moments; (2) Cavity QED quantum sensing — single circular atoms probe the microwave field in a superconducting cavity photon-by-photon via quantum non-demolition measurement; (3) Quantum state engineering — generating Fock states, Schrödinger cat states, and entangled atom-field states in the cavity; (4) Tests of quantum complementarity — observing decoherence of mesoscopic superpositions in real time as a probe of quantum-to-classical transition. The 'quantum radio receiver' using single atoms to sense individual microwave photons is a landmark quantum sensing demonstration.