In the 1950s, time and frequency metrology experienced a revolution - it was then that the first atomic clock based on transitions between levels in caesium ¹³³Cs atoms, operating in the microwave regine, was built at NPL laboratories. The first practical implementation of an atomic clock achieved an uncertainty of 10^-10, but subsequent developments in technology have reduced this uncertainty to 10^-13 for commercial standards and 10^-16 for the caesium fountain - the current frequency standard. Today, atomic standards are widely used not only in time and frequency laboratories, but in wider economic and social life, such as in telecommunications and satellite navigation.
At the beginning of this century, we witnessed another breakthrough in this field. Gigantic advances in laser cooling and trapping of atoms and ions, as well as laser technology, resulted in the construction of atomic (ion) clocks operating in the optical range. So-called optical atomic clocks have been developed, achieving uncertainties of the order of 10-18. This makes it possible to work on the next redefinition of the second, which will provide significant advances in all fields of science and technology requiring precise time metrology.
Optical atomic clocks are not commercially available and currently only function in a few advanced metrology laboratories around the world. Thanks to the efforts of several scientific institutions that make up the National Laboratory of Atomic and Molecular Physics (KL FAMO) located in the Institute of Physics at the Faculty of Physics, Astronomy and Applied Computer Science of the Nicolaus Copernicus University, two such clocks using ⁸⁸Sr atoms are also in operation in Poland. However, their availability, especially for metrology applications, is limited, due to the fact that work using POZA clocks must be carried out at KL FAMO. The present distributed optical clock concept offers an opportunity to dramatically improve this situation.
In order to fully exploit the potential of the POZA clocks, it is necessary to develop a way to link local reference optical oscillators located in different cities in Poland to the POZA clocks. We propose to develop a concept and thoroughly test a system called “distributed optical atomic clock”. The aim of the project is to develop a concept for a method to link local oscillators to POZA clocks using a fibre-optic network. The short-term stability of the oscillators is to be ensured by their design, while the long-term stability of such an oscillator will be disciplined to one of the POZA clocks.
In KL FAMO, the reference frequency generated by the optical atomic clock (wavelength around 700 nm), will be transferred, via an optical frequency comb, to the transmission window of the optical fibre (around 1550 nm). A local optical oscillator, stabilised to a high-quality optical cavity, will ensure the short-time stability of the infrared signal. Fibre-optic links equipped with regenerative laser stations and bidirectional optical amplifiers, provided for the project by the Poznań Supercomputing and Networking Centre, will be used. This will create a loop of several hundred kilometres through which the reference optical signal will return to KL FAMO, reaching the remote terminal of the optical atomic clock. This terminal, equipped with a very similar local oscillator, will use the signal sent over a phase-stabilised optical fibre link (loop). The use of a looped fibre-optic link will allow the transmission quality of the reference signal to be tested at KL FAMO.
The tangible result of the project will be a concrete solution that can be implemented as a system for transferring the reference optical frequency between KL FAMO and the GUM campus in Kielce. In addition, the developed methods could be used for a nationwide system of transmitting the POZA clock signal (or other optical clock) to other points, thus implementing the concept of a nationwide distributed optical atomic clock.