The simple resonant Rabi oscillation of a two-level system in a single-mode coherent field reveals complex features at the mesoscopic scale, with oscillation collapses and revivals. Using slow circular Rydberg atoms interacting with a superconducting microwave cavity, we explore this phenomenon in an unprecedented range of interaction times and photon numbers. We demonstrate the efficient production of 'cat' states, quantum superposition of coherent components with nearly opposite phases and sizes in the range of few tens of photons. We measure cuts of their Wigner functions revealing their quantum coherence and observe their fast decoherence. This experiment opens promising perspectives for the rapid generation and manipulation of non-classical states in cavity and circuit Quantum Electrodynamics.
PACS numbers:The Rabi oscillations of a two-level atom in a resonant, single-mode coherent field state is one of the simplest phenomena in quantum optics. Nevertheless, it exhibits surprisingly complex features at the mesoscopic scale (few tens of photons) [1][2][3][4]. The oscillations, at an angular frequency Ω 0 √ n, collapse and revive (n is the average photon number in the coherent state; Ω 0 is the vacuum Rabi frequency measuring the atom-field coupling). The collapse, occurring on a time scale T c = 2 √ 2/Ω 0 , results from the quantum field amplitude uncertainty and from the corresponding dephasing of the Rabi oscillations. The (first) revival, around T r = 4π √ n/Ω 0 , results from the rephasing of oscillations associated to different photon numbers [5]. This revival provides a landmark illustration of field amplitude quantization [6]. Between collapse and revival, the field evolves into an entangled atom-field state, involving two coherent states with different phases [7,11,[35][36][37]. It is called a "cat state" in memory of Schrödinger's metaphor. Close to t = T r /2, the atomic state factors out of a field "cat", superposition of coherent states with opposite phases [5].These phenomena can be observed in systems implementing the Jaynes and Cummings model, a spin-1/2 coupled to a one-dimensional harmonic oscillator [12]. Ions in traps [13,14], cavity quantum electrodynamics (CQED) [3,6] and circuit quantum electrodynamics (cQED) [16, 39] are thus ideal platforms for this observation.Nevertheless, revival observations have so far been limited to small photon numbers since experiments face formidable challenges. For microwave CQED with superconducting cavities crossed by fast Rydberg atoms, the interaction time is limited to a few vacuum Rabi periods, 2π/Ω 0 . Revivals have been observed only for n 1 [6,17]. Early revivals induced by a time-reversal of the collapse can be observed for larger n values (about 10), but the maximum separation of the cat compo-nents is small [18,19]. Ion traps have similar limitations [13,20]. In cQED, the limited coherence time of tunable superconducting qubits makes it difficult to observe long-term dynamics in the resonant regime [21]. Large cat-state preparation so far relie...