The availability of precise and stable timing is the backbone of modern science, global communications, and quantum technologies. From synchronising radio telescopes and radar systems to enabling tests of fundamental physics and supporting quantum communication networks, time and frequency is critical to the modern world. While optical fibre links can deliver frequency transfer with fractional instabilities below 10⁻¹⁹, their reach is limited by physical infrastructure. Free-space optical links offer a route to extend high-accuracy timing and frequency references to remote or mobile platforms, including observatories, space systems, and field-deployed experiments [1].
This project aims to develop and characterise a high-performance optical time and frequency transfer system over free-space channels, capable of maintaining ultra-low phase noise and high stability in realistic environmental conditions. The overarching goal is to enable dissemination of traceable frequency standards — such as those derived from optical clocks at Strathclyde and NPL — to users beyond the fibre network, bridging the gap between laboratory precision and field applications.
The research will combine experimental optics, frequency metrology, and control engineering, proceeding through three main phases:
- System design and stabilisation: The first objective is to develop an optical link architecture capable of bidirectional time–frequency transfer. Techniques such as heterodyne phase comparison [2], two-way optical frequency combs [3], and active path-length stabilisation will be implemented to mitigate atmospheric turbulence and mechanical vibration effects.
- Free-space link characterisation: The system will be deployed over controlled laboratory paths and then extended to outdoor line-of-sight channels. Measurements will quantify fractional frequency stability, timing jitter, and phase noise under varying turbulence, temperature, and alignment conditions.
- Applications and integration: In the final phase, the system will be evaluated in scenarios relevant to very-long-baseline interferometry (VLBI), synchronised radar, or distributed quantum sensing, where precise timing enables coherent operation across separated nodes. The results will demonstrate the feasibility of extending metrologically traceable time–frequency references from NPL facilities to end users via compact, deployable optical systems.
By the end of the project, the candidate will have developed a fully characterised optical free-space time-frequency transfer system, contributing directly to the UK’s capabilities in precision timing, frequency metrology, and quantum technology.