Atom Source
Having gone to all that effort obtaining and maintaining ultra high vacuum in our chambers we now need to fill them back up with atoms to cool. Different species of atoms have different properties with pros and cons. We focus on rubidium due to its ubiquity across most cold atom experiments, as well as a sideline in strontium for its use in future atomic clock standards. The atom source should have the following properties:
- Stability during chip fabrication/sealing (temperatures up to 400C).
- Low vapour pressure when 'off'.
- High vapour pressure when 'on' (but not too high).
- High purity
- Practical control parameters (temperature, current, etc).
Rubidium melts around 40°C and has a room temperature vapour pressure of 10-7mbar. This is an order of magnitude too high for trapping in a MOT due to collisions and excessive fluorescence. Strontium, on the other hand, needs to be heated by several hundreds of degrees to obtain the same pressure. Therefore one can use a pure sample of Strontium in the MicroMOT and just heat it (with a heating element, induction, or a laser), but rubidium needs to be 'packaged'. Several methods to insert a rubidium (or any alkali-metal) into chip-scale vapour cells have been developed, but most are inadequate for our system due to vacuum compatibility and stability.
We are currently using the method developed at the Institut FEMTO, in which a commercial alkali dispenser is cut into a small 'pill' and deposited in the chip. The dispenser includes a rubidium chromate compound and a Zr-Al getter/reduction agent. Up on heating to 500-600°C, the reducton reaction is initiated and pure rubidium is released from the dispenser. Additional gases are gettered away. We will begin with laser heating the dispensers, but will look for more integrated solutions such as Joule heating (which requires electrical feedthroughs) or induction heating.
Over time the rubidium vapour will saturate the cell's small internal volume and so we must include a method to pump away excess rubidium. This can simply be a reaction with a gold-getter at elevated temperatures, or via the natural reactivity of rubidium with the chamber surfaces. The latter has a limited capacity but can be increased with highly porous materials such as Aerogel. We would also like to control the vapour pressure in short timescales (sub-second) to provide suitable numbers of atoms to be trapped, but then to reduce the pressure for sensitive manipulation schemes to avoid collisions. We aim to do this with Light Induced Atomic Desorption (LIAD), and/or integrated Peltier 'cold fingers'.