Compact Terahertz Clock

In recent decades, atomic clocks have developed from being solely research instruments to indispensable and infrastructure-critical devices. Atomic clocks are now widely used in Global Navigation Satellite Systems (GNSS), data centres, power and mobile networks, financial markets for transaction time stamping, and research and development. Presently, many applications requiring high-precision timing rely on GNSS signals. However, this makes crucial infrastructure vulnerable to GNSS tampering and failure, with significant socio-economic consequences. Therefore, local high-performance atomic clocks are needed to safeguard against this. Other applications need to function in a GNSS-denied environment such as the navigation of submarines or electronic warfare and other security situations. Clock performance beyond GNSS capability is also required for state-of-the-art scientific research and advanced timekeeping.
Current portable clocks currently have limited stability and accuracy or are too large and sensitive for applications on mobile platforms. While there has been immense progress in the miniaturisation of the laser systems and spectroscopy units for high-precision optical atomic clocks there are still two main challenges to overcome: The reference laser that requires a high-finesse optical cavity and the optical-frequency comb (OFC) that is required to convert the optical reference signal to a usable electronic signal.
Here we propose to employ Raman transitions to create a highly stable and accurate atomic clock. In contrast to optical atomic clocks, the atomic reference stability is not transferred to the frequency of a single laser but is encoded in the frequency difference between two Raman lasers. This significantly relaxes requirements on the OFC and the optical cavity for the clock lasers.
For the realisation of a THz-clock, we propose using calcium ions trapped in an RF ion trap and the Raman transition between the D3/2-level and the D5/2-level. The frequency splitting between these two states is 1.819 THz and the expected fractional frequency accuracy of the clock is better than 10-14 (systematic accuracy better than 1e-15) with a 20-litre form factor, significantly smaller than current optical clock systems.
Due to its high accuracy in conjunction with small SWAP as well as robustness, this novel clock is exceptionally fit for applications on mobile platforms and in locations with low environmental control. Such portability, makes it particularly well suited for applications in the defence and security sector and as GNNS holdover clocks for telecom and utility networks as well as data centres and financial markets with holdover times of several months. Additionally, it enables novel schemes for frequency dissemination and synchronisation across large-scale telecom networks.
Within this project, we will set up the THz-clock with equipment provided by CPI, characterise its performance and test the system in some application-relevant scenarios. CPI will perform environmental testing in their test facility, Leonardo will test the clock's performance on a mobile platform, and BT will investigate next-generation schemes for frequency dissemination and synchronisation across large optical fibre networks.