Collective Rayleigh scattering in a Bose Einstein condensate – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2008 > Collective Rayleigh sc...

Collective Rayleigh scattering in a Bose Einstein condensate

My talk will describe the construction of a machine to generate Bose
Einstein condensates in Rubidium 87 and the first experiments performed
with this machine on superradiant Rayleigh scattering.

Bose Einstein condensates of Rb 87 are produced by evaporatively cooling
atoms in a magnetic trap of the quadrupole-Ioffe configuration. The
atoms are loaded into the magnetic trap in a region of ultra-high vacuum
from a double Magneto-Optical trap set-up. The evaporative cooling is
achieved by selectively driving radio-frequency transitions to untrapped
magnetic substates. During the evaporation, the magnetic trap is relaxed
so that density dependent heating does not substantially reduce the
number of atoms in the condensate. With a duty cycle of about a minute,
we produce pure, prolate condensates containing up to a few million atoms.

The application of an off-resonant beam of light along the long axis of
the condensate leads to a form of collective Rayleigh scattering
analogous to the superradiance that occurs in electronically inverted
samples. One can think of this process as the amplification of quantum
noise: photons are spontaneously scattered out of the pump beam, and due
to the extended optical depth along the long axis of the BEC, the modes
that propagate along this axis see the most gain. In the end-pumped
geometry, the strongest superradiant mode is the one where photons are
back-scattered by the atoms. The overlap of stationary and recoiling
atoms recoil produces a density modulation - a Bragg grating - which
amplifies the back-scattering.
We have performed a systematic study of the effects of pump detuning on
the process while keeping the single particle scattering rate constant.
In this way, we move between the case where the pump beam functions as a
reservoir of photons to the situation where superradiance is clamped by
a lack of photons in the pump beam. Our experimental results are
strongly supported by simulations of the system based on 1D Maxwell
Schrödinger equations. We demonstrate that the dynamics result from the
structures that build up in the light and matter fields along the long
axis of the condensate. In particular, we find that the emission of the
first superradiant pulse may be understood in terms of the overlap of
light and matter wave gratings. Finally, the random nature of the
spontaneous scattering that initiates the collective scattering is
manifest at later times in the distribution of arrival times and photon
numbers of the first superradiant pulse.