Atom Cooling and Trapping
Page constructed by Andrew
This Page has been updated on the 12th January, 2007.
Happy New Year!
describes the atom trapping and cooling experiments that are currently
Other pages which may be of interest in these laboratories are given
at the bottom of this page, with links to the different pages.
The purpose of this page is to detail
trapping and Cooling experiments which are being conducted at
The new apparatus which has been developed is aimed at providing a high
density source of atoms for several different experiments in cold
atoms. The apparatus produces a bright, cold atom source which competes
with the best in the
world for speed of accumulation of cold atoms into a
Magneto-Optical Trap (MOT), and for the lifetime of atoms that are
subsequently cooled and trapped.
In the first of these experiments, new types of
collision processes are to be studied, including electron collisions
cold atoms, BEC's and from slow atomic beams. These experiments will
exploit the momentum transferred in the collision process to ascertain
information about the target interactions, and to study collisions with
cold and ultra-cold targets for the first time.
2.0 Overview of Cold Atom Experiments in
Atom Facility which has been developed in Manchester is a bright
source of cold atoms, and is shown in Figure
1. The apparatus consists of a recirculating oven source, which
produces a quasi-collimated beam of atoms into the laser collimation
stage. Initial collimation of the atomic beam is achieved in the
recirculating oven source through the use of small apertures as is
shown in figure 2. In the
first experiments using this Facility, potassium atoms have been cooled
The cold atom facility that has been developed at
Manchester, showing the various components of the apparatus and the
laser beams which are used to cool and trap the atoms emitted from the
Photograph of the
actual apparatus which has now been commissioned. The Zeeman slower is
shown without the cooling coils which surround the current carrying
coils, and the liquid nitrogen trap has yet to be installed onto the
Atoms emitted from the source can be
laser collimated in the second stage of the experiment, which consists
of a 6-way 200mm 316LN cross attached to the source chamber. Large
windows are attached to the cross to allow the collimating laser beams
to enter this region.
Laser collimation can be accomplished either using an optical molasses,
through a 2D MOT or by exploiting the transverse momentum transferred
to the atomic beam as it traverses this region using angled mirrors. In
the present setup, optical molasses has been used to increase the
collimation of the atomic beam prior to the Zeeman slower.
Atoms from the collimated source that enter the Zeeman slower (figure 5) are longitudinally cooled
and compressed in phase space using a blue detuned laser which passes
along the axis of the Zeeman slower. Atoms travelling at velocities up
to ~800m/s can be captured and cooled by the laser beam within the
Zeeman slower. The atoms exit the Zeeman slower all with a velocity
~30m/s - 50m/s (depending on the setup of the Zeeman slower magnetic
field conditions) and are then subjected to the laser and magnetic
fields in the Magneto-Optical Trap (MOT1) (figure 6). Once the atoms enter
MOT1, they experience frictional force due to the laser interaction and
a restoring force due to the magnetic field, so that they rapidly cool
to a temperature ~250 micro-Kelvin around the centre of the trap.
The design of the apparatus in Manchester allows atoms to be cooled,
trapped and accumulated very rapidly from the source, which operates
It has been demonstrated that the trap density reaches saturation
(where no more atoms can be trapped due to radiation trapping and other
restrictions) within 1 second. The density of trapped atoms in MOT1
saturates at ~ 1010 atoms/cm3, and the
atoms can be held in the trap for more than 400 seconds before
collisions with the background gas (typical pressures in the MOT1
chamber are ~7 x 10-11 torr) eliminate them from the trap.
The MOT1 chamber has been designed with numerous CF-70 ports to allow
additional experiments to be connected to the chamber. The aim of this
design is to facilitate rapid trapping of the cold atoms, then transfer
of these cold atoms to different experiments that are connected to the
MOT1 chamber via these ports. In this way, atoms accumulated in MOT1
can be transferred to several different experiments where they can be
recaptured for further study, or experiments can be directly conducted
on the trapped atoms in MOT1.
Since the ratio of accumulation time to trapping time is 400:1, the
apparatus will be able to continuously load different cold atom
experiments from the main source while maintaining the maximum trapping
density which can be achieved in each of these experiments. In this
way, experiments which have low cross sections (such as collisions
experiments with electrons, neutrals or ions) will be possible in the
3.0 The Atomic Beam Source
One of the goals in the design of
this apparatus was to produce a well-collimated beam of atoms from the
atomic beam source prior to laser collimation (see figure 1). By ensuring this criteria
is met, the vacuum chambers are far less polluted by unwanted atoms
that are emitted from the source.
As an example, the potassium source
used in these cold atom experiments in Manchester has been operating
for over 2 years without the need to refill the crucible.
There are several different types of atomic
beam oven that can be used in these types of experiments,
ranging from a simple crucible
that directs atoms through a hypodermic needle (and hence produces a
beam that is quite divergent) through capillary
array sources (which are unsuitable for metal vapour targets as
needed here), on through passively
collimated sources as is used in Manchester for super-elastic
scattering experiments from calcium and finally on to the most
sophisticated (and most complex) sources which use a recirculating arrangement for
collimation and recycling of the material under study.
It is the recirculating type of oven
that was chosen for these studies.
Recirculating oven sources have
several advantages over other sources (if they are suitable).
Firstly, since the source only
produces atoms which enter the atomic beam, there is very little waste
of material. This means that the source can be operated for very long
periods of time without the need to refill the crucible.
the use of a
condensation chamber surrounding the emitting nozzle means that vapour
from the oven is recondensed onto the inner walls of this chamber, to
be fed back to the crucible. The physical size of the condensation
crucible and the output aperture then defines the collimation of the
atomic beam that exits and therefore becomes the emerging atomic beam.
Since no atoms are emitted apart from those in the atomic beam,
contamination of the vacuum chamber is virtually eliminated.
2. The recirculating Oven source used in the Cold Atom
Experiments in Manchester.
shows the recirculating
that has been
designed and commissioned in Manchester. The crucible
is loaded with potassium
(melting temperature ~337K) and the temperature of different components
of the source are closely controlled. In this way, atoms are emitted
from the nozzle into the condensation
, where they are recondensed onto the walls of this
The condensation chamber
held at a temperature ~340K so that the potassium on the walls is in a
liquid form. Surface tension ensures that the liquid potassium runs
down the walls of the condensation
and is gravity fed back to the crucible
(which operates at a
temperature of ~650K).
Potassium vapour that is directed through the output aperture
of the condensation chamber
atomic beam which is used in the experiments.
The estimated density of the emerging beam is ~1011
, whereas the angular divergence of this beam
is ~ 2 degrees.
Close control of the recirculating
temperature is achieved by using several constant current supplies
the tungsten heaters
in figure 2
. The temperature
of the condensation chamber
accurately controlled by passing a heated
high temperature silicon oil
through a sealed chamber within the
walls of the chamber as shown. In this way, the complex temperature
conditions which are essential for correct operation of this source can
Although not yet tested with Rb, it is fully expected that this atomic
oven can be used with equal success using Rubidium
(melting temperature 312K)
as the chosen source.
Rubidium will be used for the production of Bose Einstein Condensates (BEC's)
and for other experiments in the near future.
Details of this recirculating beam source are currently being written
as a paper for submission to an appropriate journal.
4.0 The Zeeman Slower
Following laser collimation using an
optical molasses (figure 1
the atomic beam enters the Zeeman slower. Since atoms emerging from the
oven have a wide range of different velocities (as dictated by the
Maxwell Boltzmann distribution), it is necessary to slow these atoms
down so that they can be captured by the MOT. Further, it is important
to also compress the velocity distribution of these atoms so that they
have a much narrower distribution at the exit of the slowing tube.
The effective way to slow atoms down using lasers is to adopt the
process of laser cooling where the spontaneous emission process is used
as a mechanism to provide a dissipative force.
The cooling process used to slow atoms down.
shows the process which is used.
An atom initially travelling with momentum pA
is subjected to laser radiation directed in the opposite direction,
is the mass of the atom and v is the velocity of
the atom. The laser frequency (wavelength) is set to be resonant
with the atomic transition
so that there is a high probability of absorption of a photon from the
laser field by the atom, which is then promoted to an excited state.
The linear momentum of the photon
is given by p = h/lambda, where lambda is the wavelength of the
incident laser radiation, and h is Planck's constant.
For potassium, lambda ~ 766nm, whereas h = 6.626 x 10-34
linear momentum must be conserved, absorption of a photon from the
field means that the atom must slow down.
Once the atom is in an excited state, two possible processes can occur.
it is possible for a second photon from the laser field
the atom to decay
back to the ground state. In this case (stimulated
) the photon from the atom emerges in the same
direction as the photon in the laser field. In this case, the momentum
of the atom will return back to its original value pA
there is NO slowing of the atom by the laser beam.
Secondly, the atom may relax back to the ground state spontaneously (spontaneous emission process
this case, the atom has no 'memory' of how it was excited, and so does
not know the direction of the incident photon. Hence the spontaneously
emitted photon can be emitted in ANY direction.
IF the photon is emitted in the same
as the incident photon, then the momentum of the atom
will return back to it's initial value prior to the interaction pA
v, and so the atom velocity does not change.
IF the photon is emitted in the
to the incident photon, then the momentum of
the atom will be reduced further, so that after the interaction the
atom momentum will be pA
v - 2h/lambda. The
atom will SLOW DOWN.
Since the process of spontaneous emission is completely random, this
means that ON AVERAGE
photon that is absorbed, there is a NETT change of linear momentum of
the atom given by h/lambda.
By repeating this process many thousands of times, the atoms will slow
down until they are stopped.
The process of compression of the velocity distribution occurs since
atoms with different velocities will only come into resonance with the
laser beam at certain times during the cycling process. By ensuring
that either the laser frequency or the atomic transition frequency is
time (or space) dependent, compression can occur.
There are several methods to slow and compress the velocity
distribution of these atoms. One method that is used is to 'chirp' the
slowing laser beam (figure 1
so that as the atoms slow down they remain in resonance with the laser
This process can be used to produce a
pulsed source of slow atoms.
The method adopted here changes the internal states of the atoms by
application of a magnetic field, which perturbs the atomic transitions
which are coupled together by the laser radiation field. As the atoms
slow down, the magnetic field is then adjusted so that the atoms remain
in resonance with the laser beam (or they will Doppler shift out of
This process produces a CONTINUOUS
beam of slow atoms from the Zeeman slower.
The magnetic field profile required for the Zeeman
shows the type of
magnetic field that is required to ensure
the atoms can be effectively slowed and cooled. A set of 'disruption
coils' are used at the exit of the slower to ensure atoms can escape
into the MOT chamber.
Picture of the Zeeman slower shown with the cooling
coils surrounding the current carrying coils.
and figure 1
shows the Zeeman slower
that has been designed
for the experiments in Manchester.
3,500m of rectangular section aluminium wire is used to provide the
field, wound onto the flight tube as 5 individual coils.
are operated by 5 x 20A constant current supplies whose current can be
tailored so as to produce the required field profile along the flight
Four additional coils are located at the exit of the Zeeman slower
which disrupt the field so as to release atoms into the MOT chamber
These coils are also supplied by 20A constant current supplies.
~5,000W when operated at full current, and
cooling of the slower is implemented by the external copper coils and
by an internal water jacket
next to the flight tube.
5.0 The MOT Chamber.
Atoms from the Zeeman slower
enter the MOT chamber as seen
in Figure 1. Inside this
chamber is a
twin set of coils setup to produce zero field at the centre of the
coils (anti-Helmholtz configuration), as shown in figure 6.
The anti-Helmholtz coils setup in the MOT chamber at
Six laser beams of appropriate polarization are directed at the centre
of the coils (the trapping region) which act as an OPTICAL MOLASSES to the atoms which
enter this region, as shown in Figure
The laser interaction acts as the velocity dependent
frictional force that slows the atoms down, in the same way as inside
the Zeeman Slower.
Setup for the MOT showing the coils and laser beams that
Since the molasses component is velocity dependent, as the atoms slow
down they will gradually stop moving (within these
The magnetic field from the anti-Helmholtz coils in combination with
the laser beams ensures that the force acting on the atoms also has a restoring component that drives the
atoms back to the centre of the trapping region (this force depends on
the distance from the centre of the trap).
Since the restoring component is
distance dependent, atoms subjected to this component will perform
simple Harmonic motion (within these approximations).
The combination of restoring force and
frictional force means that the motion of the atoms will be damped,
leading to the atoms all stopping in the centre of the trap (with
velocity = 0 m/s).
This does not happen, as the atoms may still absorb and emit
photons from the laser field, leading to a minimum velocity that is
given by the recoil momentum of the atoms undergoing these absorption
and emission processes (the quantum limit).
For potassium atoms in the experiment in Manchester, the temperature of
the atoms in the trap has been measured to be ~250 micro-Kelvin.
Picture of the atoms trapped by the laser and magnetic
fields inside the MOT chamber
Trapped atoms showing the direction of atoms as they
enter the MOT from the Zeeman Slower.
FIG 10. Movie of the atoms being trapped
in the MOT
from the Zeeman slower
(first successful trapping with
an accumulation time ~1sec, and a lifetime ~10 seconds).
6.0 Current Collision Experiments.
The success of the apparatus in producing a bright rapidly accumulating
source of cold atoms means that new experiments now become possible.
The first experiments to be attempted will scatter low energy electrons
from the cold atoms either produced from the Zeeman slower
, or as
inside the MOT
For these experiments, a pulsed
has been constructed and
installed in the MOT chamber which can produce a low energy electron
beam of up to 10 microamps current at energies from 10eV to 100eV.
Electrons from this source are directed at the trapped or slow atoms,
so that they are ionized by the electron collision as shown in figure
Collisional ionization of atoms from the Zeeman slower
or within the MOT trap, showing the setup of the experiment which
detects the recoiling ions from the interaction.
In this case, the momentum of the incident electron beam is well known,
as is the momentum of the cooled or trapped atoms.
The collision process for ionization of cold atoms from
the Zeeman slower and those trapped in the MOT.
By measuring the recoiling ions, it is then possible to measure differential cross sections for ionization
of these cold targets over the complete scattering geometry
shown in Figure 12
The graph shows the momentum which is transferred from the electron to
the ionized potassium target as a function of the momentum of the
where both outgoing electrons are scattered symmetrically wrt the
incident beam direction. The incident energy is 30eV, and the curves
show the effect of the ionization process on the final ion momenta
(velocity and angle) as a function of the scattering angle.
detecting one electron in coincidence with the ion momentum, it is
possible to completely determine the cross section for all scattering
This technique is also used in COLTRIMS
experiments which exploit supersonically prepared target atoms, however
by using cold atoms the sensitivity
of the technique is enhanced many times.
Further, since the momentum of the initial target can be controlled by
the Zeeman slower
, it is
possible to take sensitive measurements at
different angles so as to ensure accuracy in the data.
is not possible in the COLTRIMS (reaction microscope) technique.
Other collision experiments being considered include super-elastic
scattering of electrons from trapped atoms
, the study of the
interaction of electrons with BEC's
, and the interaction of fermions
with ultra-cold ensembles
Watch this space for future results
and developments in these collision experiments!
7.0 Conclusions & Future work.
The Cold Atom Facility which
has been developed in Manchester has the
potential to deliver a high density of cold atoms to different
experiments so as to allow multiple experiments to be performed from
the one source. These experiments may be far ranging, depending on the
apparatus that is connected to the source.
Examples of experiments that might be performed include:
Experiments involving optical
Studies of BEC processes
Studies of ultra-cold Fermionic
ensembles (eg potassium 40)
Studies of cold and ultra-cold
atoms including measurements of
Studies of ultra-cold molecules
produced by photo-association
Studies of ultra-cold atoms
produced by sympathetic cooling
laser and electron
studies of cold targets
and many other experiments where cold atoms are required at high
Links to other pages which might be of interest:
at the (e,2e) Computer Controlled Spectrometer Hardware
at the Symmetric (e,2e) Data collected by this spectrometer
at the Symmetric data parameterisation
at the Data where the Ion is left in an Excited State
at the 64.6eV Data where the detected electrons have unequal energies
the results that were collected in the Perpendicular plane
Link to the Experiments conducted
in the Laser Collisions Laboratory at Manchester
theManchester Electron Scattering group Home Page
Link to the Atomic, Molecular
& Laser Manipulation Group Home Page
the Manchester Physics & Astronomy Department Home Page