Background
The device described here is best termed an optical frequency standard. This is a device that outputs a laser which has its frequency stabilised to a reference. This reference can either be an atomic/molecular resonance or a mechanical resonance in a macroscopic object. An atomic/molecular reference typically gives the optical frequency standard good long-term performance whereas a mechanical reference gives good short term performance. In the case of the Two-Photon Cell Clock the reference is a gas of rubidium atoms.
The performance of an optical frequency standard can be measured in two ways being:
- Accuracy – how well the frequency represents/measures the unit of the second- Precision – how reproducible the system is between successive measurements.
Depending on the application, either accuracy or precision are desired. The Two-Photon Cell Clock is targeting precision, also called stability. This is typically characterised by the fractional frequency stability of the device.
Our Concept
To produce an optical frequency standard we stabilise two lasers to a two-photon transition (5S1/2 → 5D5/2) within rubidium atoms. The two lasers that drive the two-photon transition are at 780nm and 776nm. These two lasers were chosen as they provide an enormous increase in signal by utilising the intermediate (5P3/2) energy level. Previous two-photon clocks have not utilised an intermediate transition in this way before.
One possible decay path the atoms can take from the excited state (5D5/2) back to the ground state (5S1/2) is via the 6P3/2 state which generates blue 420nm fluorescence. It is this blue fluorescence that is monitored to determine whether the lasers are in resonance with the two-photon transition. By monitoring the blue fluorescence the driving lasers can be tuned to maintain the two-photon transition. This means that it is the sum of the 776nm and 780nm energies that is stabilised, neither is individually stabilised. This is a major difference between most frequency standards and this device.
Currently the device is operating at a fractional frequency stability of 1 part in 1012. To put this in context this is within a factor of 3 of the best commercial frequency standard which is the Hydrogen Maser. The device is still in a preliminary setup and calculations indicate that the stability should be able to be improved such that its stability is at least a factor of 3 better than a Hydrogen Maser.