BIARO experiment

QND measurements and squeezing in a cavity

QND measurements and squeezing in an optical cavity

We are building an experimental apparatus to investigate quantum non-demolition measurements (QND) and measurement induced squeezing in a cold sample of Rubidium atoms optically confined in a high finesse optical cavity. An atomic beam produced by a 2D MOT is used to load a conventional MOT in a ultra high vacuum chamber. The atoms, after a phase of compression and molasse, are then transfered to a dipole trap, obtained injecting the folded optical cavity on its fundamental mode with radiation at 1560 nm. Lowering the optical potential the sample will be evaporatively cooled till condensation where the two cavity arms cross.

A laser beam at 780 nm resonant with the cavity will provide the tool for the quantum non-demolition measurement inducing spin squeezing. Using multi-frequencies schemes and heterodyne detection techniques it is possible to cancel out several uncertainty sources, as well as measuring both the absolute and the differential population on the two hyperfine levels of the ground state. A high degree of squeezing is expected, as a consequence of the high atomic optical depth determined by the finesse of the optical cavity.

 

Experimental apparatus

The setup is made of an ultra-high-vacuum chamber, where the high-finesse cavity is mounted on a titanium plate. The MOT transfered to the dipole trap at 1560 nm is loaded by a beam of cold atoms generated with a 2D MOT, whose chamber (low, left corner on the image) is kept at a higher pressure.

The optical cavity if made of 4 mirrors placed on a 12 cm side square. Two mirrors are fixed, whereas the other two are mounted on piezoelectric driven mounts, to finely align the cavity and eventually to lock it to an atomic reference.

 

Locking the laser to the cavity

The optical tweezer to trap rubidium atoms is generated injecting the folded cavity with an erbium doped fiber laser at 1560 nm (Koheras Adjustic system). The laser is locked on the fundamental longitudinal mode of the cavity using the Pound-Drever Hall technique. The correction of the frequency is done acting on the PZT device where the laser is mounted (low bandwidth), and on an AOM in double pass placed in the optical path high bandwith). The optical power of the beam injecting the cavity is raised till 1 W using a fiber amplifier. The coupling efficiency at the input mirror is about 30%. The image reports the reflection and trasmission signal when the length of the cavity is swept. The last trace shows the error signal used to lock the laser.

 

Evaporative cooling in the crossed dipole trap

A numerical simulation based on Monte Carlo method for the evaporative cooling of a cloud of cold atoms in a cavity dipole trap has been developed. The approach is similar to that described in [Phys. Rev. A 56, 560 (1997)]: the dynamics of each particle is calculated at each time step, without the need to keep close to quasi-equilibrium distributions for velocity and position because the method is intrinsically consistent with Boltzmann equation.

The dynamics and the interatomic interactions are considered separately: at first all the particles move for a distance in the phase space given by the particle position and velocity, the time interval considered, and the gradient of the optical potential at the particle position (uniform gravitational field is also taken into account). Second, the effect of the interactions is evaluated: the interatomic collisions are considered binary and instantaneous, whereas the two particles involved are taken pointlike. The conservation laws (energy and momentum) give four relations, and for the remaining two parameters isotropic scattering is considered. The adoption of a hard sphere model is justified because for bosons at low temperatures collisions have predominantly s-wave character. Collisions are then treated classically, and the uncertainty principle is not considered.

A forced evaporative cooling of the initial ensemble is obtained decreasing progressively the trap height, done lowering the beams intensity. This action determines two effects: on the one hand it eases the escape of the hottest atoms from the potential well, but on the other it decreases the trap frequencies and then the phase space density.

Cooling in higher-order cavity modes will allow a more favorable volume/surface ratio, bringing to a faster evaporation. The image on the left shows a section of the optical potential when the cavity is pumped in its first transversal excited mode (TEM10). Four potential wells form at the centre of the configuration.

 

News

  • BEC in the cavity
  • 25 Mars 2011

  • Light-shift tomography of higher modes
  • May 2010

  • QND signal of the MOT
  • March 2010

  • Atoms in TEM10 and TEM20 modes
  • 3 September 2009

  • Atoms in the dipole trap
  • 7 August 2009

  • Fibered Laser @ 1560 nm locked to the cavity
  • May 2009