Transcript pulsed magnetic focusing

Ultra-Cold Strontium Atoms in a
Pyramidal
Magneto-Optical Trap
A.J. Barker1, G. Lochead2, D. Boddy2, M. P. A. Jones2
1Ponteland High School, Newcastle, UK
2Department of Physics, Durham University, Durham, UK
The aim of the project is to cool and stabilise a cloud of strontium atoms, of which some occupy a Rydberg state, by influence of a laser and a magnetic field in a
Pyramidal Magneto-Optical Trap (P-MOT). The advantage of a P-MOT is that only one vertical beam is required as opposed to 6. The Pyramidal MOT has to be
mounted on a further octagonal support to accommodate new strontium dispensers, raising the whole system 11.2 mm and raising the atoms out of the zero
point in the B field. A current differential was required to raise the zero-point. A laser locked at 460.7nm was then applied to hit the 5s² 1S0 to 5s5p ¹P1 transition
in the strontium atoms to hold them in the trap.
Rydberg atoms are atoms with one or more electrons that possess a very high principal quantum number. The have very distinct properties in that they have
very long range interactions with other Rydberg atoms and have exaggerated response to electric and magnetic fields. Due to the strong dipole interactions,
Rydberg atoms cannot come within a certain distance of each other so are surrounded by a cloud of non-Rydberg atoms; this effect is called a Rydberg
Blockade. The distance between Rydberg atoms is called the blockade radius and is given by (C6/Ω)1/6 ~ 5µm (Where C6 is the interactions caused by Van de
Waals over distance and Ω is the Ravi frequency). The long term goals of Rydberg atom experiments is to use Rydberg atoms in quantum computing.
Theory of a Pyramid Magneto-Optical Trap
A P-MOT uses the momentum of photons in order to slow atoms down and
hence cool them. The momentum of a photon is inversely proportional to its
wavelength. To slow the atoms down, we probe the atomic transitions, or
more simply the frequencies of light required to excite electrons to higher
levels.
A false colour image showing the
pyramid, octagon, octagon mount
and base plate; with the 461nm laser
entering from the top.
Laser Cooling
The lasers emit photons which hit the atoms on all
sides. The photons transfer their momentum to the
atoms, gradually slowing the atoms down. The atoms
resonate with certain lasers depending on their
direction of movement hence pushing them back.
The speeds go from 550 m/s to around 0.2 m/s.
Problems to Overcome
The strontium dispensers were originally 20mm long, however they are now 40mm; this
results in them touching the walls of the chamber and shorting the electrical connection. The
task put forward was to insert a piece between the base plate and the octagon to raise the
assembly the required 11.2 mm. The base plate also had to be altered.
Final Preparations: Pumping Down and Baking Out
The Bake-Out Apparatus
Bake-out Oven
Residual Gas
Analyser
(RGA)
The Bake-Out is required to remove
any residues in the chamber and to
remove gases which may have
adsorbed onto the walls of the
chamber.
Thermocouple
Multimeters
Pumps
A graph showing the partial pressure and abundances of
gases bake-out procedure. The most prominent were H2
and H2O. The chamber reached 1.8 x 10-8 torr after
pumping down.
The Experiment: The First Ever Group 2 Element
Held In a Pyramid MOT
On Wednesday 24th August 2011, strontium was seen fluorescing in the P-MOT; this was
conclusive evidence that the dispensers contained strontium and emitted at ~11A.
Diagram of Apparatus
Laser fibre
Power
meter
The strontium can be
seen fluorescing as the
beams of light pass
through the chamber.
Beam
Block
Beam Splitter
λ/4 waveplate
Viewport
Convex
Lenses
There were many optics used in order to align the laser with the Pyramid; such as beam
splitters, convex lenses, wave plates and mirrors. The image below shows the optics with
the 461nm laser activated.
Pyramid Location
Magnet
Ion
Coils
Pump
The final checks were to test the
arrangement of the coils, check for short
circuits and set the polarisation of the light
into the chamber. The laser also had to be
locked and the dispensers activated.
Mirrors
Assembly and Final Calculations
Before assembly, all new or touched pieces were vacuum cleaned, this involved ultrasonically
bathing the components in different solvents such as water, acetone and methanol.
Over the course of the experiment, a few bright spots were
seen in the chamber which could have been MOTs. Due to
the observational limitations, it was not possible to conclude
that a MOT was formed however it is likely that there may
have been one.
A strontium MOT in a
quadrupole trap.
Conclusions
The connecting rods which attach the electrical
connections to the dispensers can be seen on the
left of the pyramid.
The Chamber and the magnet coils: The upper
coil operates at 3.6A, the lower at 6.5A in order
to move the zero-point to the required location.
The P-MOT is achievable once all of the magnetic field alignments have
been completed, other coils may be required for this process. The P-MOT
will have significant advantages over conventional MOTs as only one beam
is required and the MOT can be loaded without any ovens or feed through
systems.
Acknowledgements
The equation for the magnetic field produced
by a current carrying coil.
Another equation which I used to calculate the absolute
B field produced from the anti-Helmholtz coils; the
equation is a solution to the Biot-Savart Law
I would also like to thank Durham University,
Dr Matt Jones and his PhD students for
their support throughout the placement.