Download Atomic and Molecular Physics for Physicists Ben-Gurion University of the Negev

Ben-Gurion University of the Negev
Atomic
Atomic and
and Molecular
Molecular Physics
Physics
for
for Physicists
Physicists
Ron Folman
Chapter 11: New experimental procedures with atom-light
interaction (absorption and fluorescence imaging, magneto optical
trap).
Main Reference:
Any modern text book or
Review paper on atom physics
(also thesis from our laboratory)
Exercises:
Dudi Moravchik.
www.bgu.ac.il/atomchip
www.bgu.ac.il/nanofabrication
www.bgu.ac.il/nanocenter
Before we start looking at experiments, we need to know how to see atoms?
Namely how to be able to measure the result of the experiments.
IMAGING ATOMS
Fluorescence Imaging
Atom Chip
Detector
Cold atomic cloud
87Rb
atoms
Homework:
πhν3
1. Why is saturation light intensity defined as: Is ≡
2
3τc
2. the ccd has a chip the size of 1cm^2 at a distance of 50cm from 10^8
Rb Atoms. The atoms receive saturation light intensity. How many photons does the
ccd receive?
Absorption Imaging
Take an object e.g. a key, in a dark room i.e. with a very feeble source of light
And you will find out that the best way to know its shape is not to look at the
Reflected light but rather at the shadow…..
Resonant
light
f1
f1
Results from
our lab
Homework: the laser beam
has a size of 1cm^2 with
10mW of power illuminating
10^8 Rb atoms.
How many photons does
the ccd miss?
Detector
MOT: Magneto-Optical-Trap
Lets look in one dimension:
Mirror (atomchip) MOT
problem: the chip blocks two
of the 6 beams.
Solution: trick the atoms to think
there are 6 beams.
Namely we use only 4 beams.
Homework:
1. Why do the helicities change upon reflection (this enables the atomchip setup)?
2. Until now we saw that all sigma (circular polatization) signs and/or helicities
are the same for counter propagating beams in the MOT. Why is the above
picture (also explaining a Mirror MOT) different?
Hint: the energy level diagram will also look different, but its just different
notation explaining the same physics.
Explaining the MOT with a different notation
The quantum axis is now not an arbitrary axis in space like before but parallel
to the local magnetic field or the photon propagation direction.
Let look at the angular momentum conservation in a simplified J=1/2 system.
Magnetic field in the MOT
B
J in the lowest energy state of the
EXCITED atom (this will be
the one interacting with the
light as the light is red-detuned)
B
J
J
J
J
(or flipped depending on
the sign of the Lande factor)
(or flipped depending on
the sign of the Lande factor)
The atom ground state will
be opposite as the J transfer
from the photon is J=1.
Where should the photons’ J point to?
Photon P
Photon P
To conserve J the answer is:
Photon P
Photon P
Namely, relative to the magnetic field axis or the photon momentum axis, the counter
propagating photons in a MOT are both right handed or both left handed circ. polarized
Homework
1. Why cant we build an all optical trap from the 6 beams of a MOT (i.e. without
the magnetic field)?
2. Why can U shaped wire simulate a 3D quadrupole field?
Beyond the doppler limit
γ d
TD =
2k B
Sisyphus cooling
Final result: the recoil limit
Tr =
Homework: calculate both limits.
( hν )
m
2