Ultra-Cold Strontium Atoms in a Ultra-Cold Strontium Atoms in a Pyramidal Pyramidal

Magneto-Optical TrapMagneto-Optical TrapA.J. BarkerA.J. Barker11, G. Lochead, G. Lochead22, D. Boddy, D. Boddy22, M. P. A. Jones, M. P. A. Jones22

11Ponteland High School, Newcastle, UKPonteland High School, Newcastle, UK22Department of Physics, Durham University, Durham, UKDepartment of Physics, Durham University, Durham, UK

Acknowledgements

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.

I would also like to thank Durham University, Dr Matt Jones and his PhD students for their support throughout the placement.

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.

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.

A false colour image showing the pyramid, octagon, octagon mount and base plate; with the 461nm laser entering from the top.

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 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

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.

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. 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

Final Preparations: Pumping Down and Baking Out

Assembly and Final Calculations

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.

Laser fibre

Beam Splitter

λ/4 waveplate

Convex Lenses

Magnet

Coils

Pyramid Location

Mirrors

Ion

Pump

Viewport

Power meter

Beam

Block

Diagram of Apparatus

Conclusions

The strontium can be seen fluorescing as the beams of light pass through the chamber.

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.

The Bake-Out Apparatus

Residual Gas Analyser (RGA)

Thermocouple

Multimeters

Pumps

Bake-out Oven

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.

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.

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.

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.