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"

People:

Thomas Vanderbruggen, Simon Bernon, Ralf Kohlhaas, Andrea Bertoldi, Arnaud Landragin, Philippe Bouyer

The field

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 experiment

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 [3].

Current research

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 [1], 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.

Publications

[1] 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.

[2] 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)

[3] 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)

Offers of Projects for Undergraduates, and PhDs

Former members:

* Thierry Botter, now PhD Berkeley, USA
* Remi Geiger, now PhD on I.C.E. Experiment at Institut d'Optique,
Palaiseau, France

web page of this project