Research
Research effort in the microwave frequency standards area is directed at realisation of the SI second with reduced uncertainty and improved stability compared with the current caesium fountain primary frequency standard NPL-CsF1.
One of the key properties characterising atomic fountain performance is signal to noise ratio. This quantity depends on the number of atoms launched in the fountain. In order to be able trap 109 atoms, a compact magneto-optical source of slow atoms has been developed.
The atomic fountain operates with the atoms in magnetic insensitive states. Precise control of the atomic state based on state selection enables the signal amplitude to be increased and the uncertainty in the collisional shift to be reduced.
For several operational primary frequency standards, the cold collisions constitute one of the major systematic effects limiting the performance of the standard. We have observed cancellation of the collisional shift for caesium atoms and are currently working on implementing this effect into the operation of the frequency standard.
Apart from interactions between the atoms, blackbody radiation contributes significantly to the uncertainty budget of primary frequency standards. Recently implemented changes to the NPL fountain design should enable operation of the standard at a well-controlled temperature.
Because of the lower collisional shift compared to caesium, rubidium fountains offer the prospect of operating with a higher number of atoms. Currently we are developing a rubidium fountain that will take advantage of efficient loading from a 2D magneto-optical trap.
Low phase noise microwave oscillators are also being developed to supply the Ramsey cavities used in the fountains with a stable frequency source.
Microwave Frequency Standards research
- A caesium fountain primary frequency standard is an apparatus that realises the SI definition of the second.
- Primary frequency standards operate with atoms in states which are insensitive to magnetic fields. There are several techniques that can be used to prepare an atomic sample in the proper internal state.
- The atomic fountain is given its name because atoms are launched upwards and fall back under gravity, spreading outwards in the horizontal plane.
- We are developing a rubidium fountain standard at NPL, the stability of which will exceed the stability of our caesium fountains.
- An efficient way to load the magneto-optical trap in a fountain is to use a beam of slow atoms generated from an additional magneto-optical source of cold atoms.
- NPL has developed a design of stable microwave oscillator derived from a cryogenic resonator containing a ring of high-purity mono-crystalline sapphire that supports a ‘whispering-gallery’ electromagnetic mode with a Q-factor on the order of one billion.
- Collisions contribute to the shift of the clock transition frequency. Research at NPL has opened up the possibility of cancelling the collisional shift, leading to an improvement in the performance of caesium fountain primary frequency standards.
- COMSOL, Matlab and Mathematica sources codes, plus a few extras.
- Atomic frequency standards, high-stability oscillators and GPS-disciplined oscillators.