Experimental Apparatus

Preparation of an ultracold Rubidium cloud

cavity The design of our apparatus features two nested vacuum chambers which are connected by an in vacuo magnetic transport configuration. After a first magneto-optical trap (MOT) stage in the prechamber, cold Rubidium-87 atoms are captured in a magnetic quadrupole trap and transported into the ultra-high vacuum (UHV) chamber. In the UHV chamber, the atoms are loaded into a magnetic quadrupole-Ioffe trap (QUIC) where they are cooled close to quantum degeneracy using a radio-frequency based evaporative cooling technique.

Optical transport

transport Our research focuses on the novel states of matter that can be prepared by the interaction of a Bose-Einstein condensate (BEC) with the quantized light field of an optical cavity. The cavity is placed 3.6 cm below the QUIC trap. To transport the ultracold atoms in a controlled way into the cavity, we implemented an optical conveyor belt formed by two counter-propagating, red-detuned laser beams (852 nm). Thanks to the optical dipole force, atoms are loaded into the antinodes of the vertically aligned standing-wave potential. Applying a frequency difference between the upwards and the downwards propagating wave, the optical standing-wave pattern is brought into motion. Once the atoms have reached the position of the cavity mode, they are loaded into a crossed-beam dipole trap, where they are further cooled to quantum degeneracy.

The high-finesse optical cavity

cavity The heart of our experiment is given by an optical Fabry-Perot cavity which consists of two highly reflecting mirrors facing each other at a distance of only 180 microns. With a finesse of 3.4x105 at 780 nm, light travels about 105 times between the mirrors before it gets lost through on of the mirrors. The small cavity mode volume allows us to reach the single-atom strong coupling regime of cavity QED. The resonant interaction between the atoms and the cavity, as well as the dispersive limit of CQED have been explored in our experiments.

Pumping beam and 3D intra-cavity lattice

lattice Our recent experiments make use of a retro-reflected laser beam at 785 nm, oriented orthogonally to the cavity axes and red-detuned from atomic resonance. This transverse pump beam induces and enhances cavity-mediated long-range interactions between the atoms and also generates an optical lattice along the z direction. In some of our experiments, a 3D optical lattice was created by intersecting the z transverse pump, a red-detuned cavity standing wave along x and a blue-detuned (670 nm) lattice along y. This configuration allowed exploring a Bose-Hubbard system enriched with cavity-mediated long-range interactions.

Detection methods

We acquire information about the state of the system via two different techniques that are used in parallel. The first is time-of-flight imaging of the atomic ensemble, which gives direct access to the momentum distribution of the atoms. In addition to this more usual method, we utilize the light field leaking from the cavity as a versatile probe for nondestructively monitoring the state of the system. We detect the cavity output light by using either a single-photon counter or a heterodyne setup. The latter provides access not only to the amplitude and phase of the cavity field, but also to its frequency spectrum.

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