We are developing optical and microwave sources having extremely high frequency stability for high impact experiments. These include direct measurement of Einstein’s time dilation effect, as well as frequency references for: leading-edge atomic clocks, optical and radio astronomy and radar applications.
We are also developing a fibre dissemination network to allow fast and precise frequency comparison between frequency standards within different laboratories. This can be extended to time dissemination over large scales for applications such as the square kilometre array (SKA) radio telescope.
A highly stabilised frequency sources can be made by linking the frequency of electromagnetic radiation of a microwave field to the physical dimension of a cryogenically cooled mechanical resonator. In this project a cylindrical Sapphire oscillator is used (pictured) in which the microwave field bounces around the cylinders circumference in a “whispering gallery mode” configuration.
The oscillator is cooled to near 4 Kelvin to reduce the frequency standards dependence on temperature through utilising a minimum in the Sapphire’s thermal expansion coefficient. Our goal is to achieve a relative frequency instability of 10-17 during at 1s observation time, a factor of 10 better than any previous demonstration. This expected frequency stability corresponds to 0.1uHz at the optical frequency of 11GHz.
The definition of the second is at the centre of the International System of Units as the other units are almost all defined in terms of time and of universal constants. As our measurement tolerances become narrower, we find that in many cases we are limited by our definition of the second, leading to a global push towards the transition to a new second based on an optical transition. A likely choice for this role is a doubly-forbidden transition in Ytterbium atoms at a wavelength of 578nm – a bright yellow. When this change of definition happens, Australia will need a collection of optical clocks capable of realising the new second independently of foreign metrology institutes.
The platforms being built in our group will not only fill this role but will also focus on moving from scientific instruments the size of a room down to turn-key devices the size of a briefcase.
Atomic frequency standards currently find uses in the Global Positioning System (GPS), telecommunications, National facilities which disseminate time, and the definition of the second. Unfortunately, the most accurate and precise frequency standards occupy entire laboratories and cannot be easily deployed into the field for real world applications. Here we are using the technology of hollow core photonic crystal fibre to both shrink the size of the frequency standard and to produce efficient excitation of desirable atomic transitions.
For the Rubidium frequency standard we use a two-photon transition which is excited in a Doppler-free configuration to reveal the narrow linewidths of the excited state. This technique allows us to produce a frequency standard with a fractional stability in the 10^-13 range.
A major challenge in measuring the stability of a frequency standard is in transporting the stabilised signal between various locations to make the comparison. Lengths of optical fibre or electronic transmission lines all act in such a way that the stabilised signal deteriorates. This can be circumnavigated by stabilising the length of the link that is transporting the frequency standard signal.
Using state-of-the-art noise cancellation techniques we will deliver the high spectral purity of our laser and microwave sources and allow precision measurements between remote locations. This can be extended beyond the laboratory enabling end-users who require precise timing to benefit from the frequency standard developed within the laboratory. One such end-users would be the square kilometre array (SKA) radio telescope.