Synchronization of Bloch oscillations by a ring cavity.

We consider Bloch oscillations of ultracold atoms stored in a one-dimensional vertical optical lattice and simultaneously interacting with a unidirectionally pumped optical ring cavity whose vertical arm is collinear with the optical lattice. We find that the feedback provided by the cavity field on the atomic motion synchronizes Bloch oscillations via a mode-locking mechanism, steering the atoms to the lowest Bloch band. It also stabilizes Bloch oscillations against noise, and even suppresses dephasing due to atom-atom interactions. Furthermore, it generates periodic bursts of light emitted into the counter-propagating cavity mode, providing a non-destructive monitor of the atomic dynamics. All these features may be crucial for future improvements of the design of atomic gravimeters based on recording Bloch oscillations.

Observation of a cooperative radiation force in the presence of disorder.

Cooperative scattering of light by an extended object such as an atomic ensemble or a dielectric sphere is fundamentally different from scattering from many pointlike scatterers such as single atoms. Homogeneous distributions tend to scatter cooperatively, whereas fluctuations of the density distribution increase the disorder and suppress cooperativity. In an atomic cloud, the amount of disorder can be tuned via the optical thickness, and its role can be studied via the radiation force exerted by the light on the atomic cloud. Monitoring cold (87)Rb atoms released from a magneto-optical trap, we present the first experimental signatures of radiation force reduction due to cooperative scattering. The results are in agreement with an analytical expression interpolating between the disorder and the cooperativity-dominated regimes.

Three-dimensional Wigner-function description of the quantum free-electron laser.

A free-electron laser (FEL) operating in the quantum regime can provide a compact and monochromatic x-ray source. Here we present the complete quantum model for a FEL with a laser wiggler in three spatial dimensions, based on a discrete Wigner-function formalism taking into account the longitudinal momentum quantization. The model describes the complete spatial and temporal evolution of the electron and radiation beams, including diffraction, propagation, laser wiggler profile and emittance effects. The transverse motion is described in a suitable classical limit, since the typical beam emittance values are much larger than the Compton wavelength quantum limit. In this approximation we derive an equation for the Wigner function which reduces to the three-dimensional Vlasov equation in the complete classical limit. Preliminary numerical results are presented together with parameters for a possible experiment.

Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing.

Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process.