Cold atoms and QND in a cavity (BIARO)
In French, BIARO is an acronym for : "Condensation de Bose-Einstein et Interférométrie Atomique dans un Résonateur Optique de Grande Finesse"
Quantum mechanics predicted the existence of matter waves already in his early days. Only in 1991 though it was demonstrated with puzzling interference experiments with a double slit. Since then, matter wave optics rapidly evolved into a powerful tool for precision measurements, competing with state of the art optical interferometers and mechanical devices. Atom based inertial sensors have been used to measure fundamental constants and their time variation, test general relativity, as well as in more applied contexts, such as gravity survey for ore and oil prospecting. New experiments to test equivalence principle, measure matter neutrality, and detect gravitational waves require even higher sensitivity to inertial forces. One way to improve quantum sensors performance when sensitivity is at the shot noise level consists of increasing the number of atoms. On the other hand, we want to investigate the use of engineered sources of matter waves to beat the quantum projection noise limit towards Heisenberg limit for sensitivity.
To manipulate the initial atomic state we employ a non demolition measurement, realized with a heterodyne detection system [1,2]. The target is to reach sub-shot-noise uncertainty for the atomic state, or what is called a squeezed atomic state. A high finesse optical cavity, now used to trap ultracold samples of atoms, will be exploited to increase the SNR of the measurement: it will provide an increased coupling strength between the probing photons and the atoms, as well as a reduced optical shot noise on the photodetector.
The radiation of two extended cavity diode lasers is used to cool and trap rubidium atoms. A double vacuum chamber implements a system where a 2D MOT loads a 3D MOT held in UHV. A macroscopic ring optical cavity is mounted in the second chamber. The four mirrors forming the cavity are placed at the corners of a square with a 90 mm diagonal, and are oriented so as to cross two arms at the center of the configuration. The MOT is precisely positioned at the crossing region. The optical resonator is pumped with radiation at 1560 nm, and the intracavity power reaches more than 200 W. The resulting optical potential is used to trap rubidium atoms loaded from the MOT, and evaporate them to increase the phase space density. The atypical wavelength adopted for the trap has been exploited to realize tomographic measurements of the potential in the resonator, both for the fundamental transversal mode, and higher order ones .
We are now optimizing the transfer of atoms from the MOT to the cavity optical trap, and the successive evaporative cooling by ramping down the dipole trap depth. The target is to obtain a Bose-Einstein Condensate directly in the optical resonator. By locking the 1560 nm laser to the fundamental transversal mode (TEM00) one potential well is formed, whereas a multi-well trap is obtained using higher modes (4 wells for the TEM10, and 9 wells for the TEM20). We want to exploit this scalable architecture to realise a multinode quantum sensor, suitable for measurement parallelisation, or to map spatial dependence of physical quantities like magnetic fields or gravity. On the other side, we plan to improve the heterodyne detection already used to non destructively follow the state evolution of an atomic ensemble , by injecting the probe in the cavity to improve the SNR of the measurement by the square root of the finesse. In this way we want to obtain spin squeezed atom samples suitable to quantum metrology.
 Heterodyne non-demolition measurements on cold atomic samples: towards non-classical state preparation for atom interferometry
S. Bernon, T. Vanderbruggen, A. Bertoldi, R. Kohlhaas, A. Landragin, and P.Bouyer
submitted to New Journal of Physics.
 Spin-squeezing and Dicke state preparation by heterodyne measurement
T. Vanderbruggen, S. Bernon, A. Bertoldi, A. Landragin, and P. Bouyer
Phys. Rev. A 83 013821 (2011)
 In situ characterization of an optical cavity using atomic light shift
A. Bertoldi, S. Bernon, T. Vanderbruggen, A. Landragin, and P. Bouyer
Opt. Lett. 35 3769 (2010)
* Thierry Botter, now PhD Berkeley, USA
* Remi Geiger, now PhD on I.C.E. Experiment at Institut d'Optique,