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FACT Final – Publishable Summary

The Marie Curie Initial Training Network Future Atomic Clock Technology (FACT) is training a cohort of 14 ESRs. It is coordinated by The University of Birmingham (UoB), UK, with 13 academic and industrial Partners. In addition there is one Associated Partner, Entanglement Technologies (ET) from the USA.

The FACT programme promotes international and inter-sector collaboration for the advancement of science and the development of innovation in the area of atomic clocks based on cold atom quantum sensors. In particular it fosters a shared culture of research and innovation that turns the Nobel-prize winning ideas of cold atom research and precision measurement (Nobel prizes 1997 and 2005) into innovation and future expertise. The FACT programme pursues a holistic approach to the atomic clock sector at the cutting edge of research, innovation and training.

We have mainly advanced two advanced mobile systems based on Sr and Yb in order to explore a wide variety of operational parameters, and minimise the associated risk. The initial targeted frequency stability and accuracy was on the order of 5 in 1017. There has already been a tremendous progress on various fronts, e.g. Sr system has already surpassed its original goal by two orders of magnitude. These two systems have been developed as part of the Space-Optical Clock (SOC2) programme as a precursor to taking optical lattice clocks to space environment.

Sr mobile setup is modular in nature and consists of robust modules like atomics package, compact frequency stabilizing system (FSS), state-of-the art computer control and laser systems. The advanced atomics package consisting of a low energy, compact and high flux with low divergence atom source atom source, a small compact and zero power Zeeman slower (by NPL) with permanent magnets, very low power magnetic coils and an advanced vacuum chamber with arrangements for minimising thermal gradients leading to black body radiation contribution. The FSS (developed jointly by HHUD and NPL) acting as a single unit in order phase stabilise cooling and lattice lasers, laser systems and a computer control based on FPGA control. A compact frequency distribution module acts as an interface between the atomics package and laser sources. The entire system fits in two 19’’ type racks. One is dedicated to atomics package and other modules, while the other rack is dedicated to power supplies and control units. The clock laser (OP) is a separate portable module and is approx. 15 litres in volume. The entire setup is confined within a volume of couple of thousand litres, and consumes less than 1kW of power. The setup was integrated at the University of Birmingham (UoB, UK) and then, in May 2015, successfully transported to PTB (Germany) with functionality remaining intact. Since then the setup is being characterised and compared against the other stationary setup at PTB. Utilising one of the ultra stable lasers at PTB, the setup has reached a landmark stability of 3 parts in 10^18. The uncertainty is still within 1 part in 10^16 but more characterisation is still in progress.

Another advanced mobile system based on Yb has been further developed at INRIM (Italy). The fractional systematic uncertainty of this apparatus has been evaluated to below 3×10-17. The clock has been used for side-by-side clock comparisons against other atomic clocks at PTB, Braunschweig, INRIM, Turino, and SYRTE, Paris, as well as for remote comparisons via optical fibre links spanning height differences and distances of 1000 m and 690 km, respectively. The clock frequency has also been measured using the INRIM Cs fountain. The measurements have been published and included in the Consultative Committee of Time and Frequency in the Convention of the metre for the recommended Secondary Realization of the Second.

In the direction of further miniaturisation, low size, weight and power (SWaP), more advanced FSS have been developed at UoB (UK) and INRIM (Italy) for Sr and Yb respectively. Additionally, OP (Paris, France) work focused on comparing various laser sources to form a lattice, and on designing a filtering strategy to filter out the spurious background of compact sources. The results obtained show that these sources are compatible with an uncertainty budget of 10^-17

A frequency comb is a device that enables us to go from optical frequency domain to radio frequency domain. In this programme, we are following both fiber based combs and micro-combs. Keeping an eye on space, Menlo Systems have tested a fiber comb on board a sounding rocket (German Space Agency’s TEXUS 51) attaining 6 minutes of microgravity at an altitude of 260 km. The Menlo Systems Optical Frequency Comb was used to compare two clocks, one based on the optical D2 transition in Rb, and another on hyperfine splitting in Cs. This represents the first frequency comb based optical clock operation in space, which is an important milestone for future satellite-based precision metrology. Second types of combs based on micro-resonators are a new and very intriguing way to generate frequency combs. These combs are generated using only a low-power continuous-wave telecom type laser. This new technology is in principle extremely attractive for future use in space since it is potentially capable of being implemented as an integrated-optics element. At EPFL (Switzerland), fundamental properties of dissipative Kerr solitons (DKS) in optical microresonators have been investigated. The fundamental understanding of soliton states allowed to deterministically generate single DKS in silicon nitride microresonators and use them (in collaboration with the group of Prof. C. Koos, KIT) for massively parallel coherent communications reaching record-high data rates of 50 Tb/s using chip-scale sources, and dual-comb based ultrafast optical ranging.

More over, metrological applications of frequency combs require the carrier-envelope offset frequency to be stabilized, which is normally achieved through the standard f-to-2f scheme. At UNINE it has been shown that it is possible to characterize the carrier-envelope offset in an optical frequency comb without traditional f-to-2f interferometry, which constitutes an attractive method for initial investigations on novel frequency combs for which the f-to-2f method is not yet applicable.

Keeping the training of ESRs as the crucial aspect of the programme, we have also used other state-of-the art systems like transportable Sr clock at PTB (stability 7 parts in 10^17), one stationary Sr optical lattice clock at NPL, one stationary Sr optical lattice clock at PTB, two stationary Sr optical lattice clocks at OP and one stationary Mg clock at LUH (Germany). Using PTB Sr clock, frequency ratio between the magnesium lattice clock and the strontium clock was measured. Using Bloch band spectroscopy, tunnelling effect in an optical lattice and its influence on the clock transition was also investigated in the Mg clock. Understanding tunnelling effect in optical lattice clocks is important for operating these clocks in microgravity and space.

At the UNOTT (UK), a flexible and efficient cold atom source aimed to cover the requirements of a broad range of applications, with the inclusion of a mechanism to transport the atoms from the trap to an optical lattice, has been realised. It is appropriately suitable for applications in quantum sensors, optical clocks and microscopy of surfaces. The ESR at UNIFI (Italy) has worked on three separate experiments based on atom interferometry of strontium. In particular, the ESR has participated in experiments exploring large-momentum-transfer Bragged interferometry, interferometry of combining Bragg diffraction and Bloch oscillations, and Bragg interferometry based on the intercombination transition of strontium for use in the first strontium gravity gradiometer. The work has served as a precursor to being able to perform atom interferometry using the clock transition of strontium, which has highly consequential prospects in precision metrology and gravitational wave detection, and which was finally demonstrated in 2017 as a result of the work performed during the FACT project.

This project has focussed on the technological developments enhancing the technology readiness level of new optical atomic clocks by implementing a training programme covering all aspects from the atomic reference and ultrastable lasers to frequency comb synthesis, precision frequency distribution and commercial system technology. Initially, it is expected that these devices will be used in space technology.

As part of its dissemination strategy, among other routes, it has already published 34 articles in the international renowned journals, 21 oral presentations and more than 64 posters in conferences/schools/invited. As part of its outreach, FACT has taken up a wide range of outreach activities including science festivals, public lectures, and lectures/visits to schools.

FACT Network Coordinator, Professor Kai Bongs, +44 121 4148278
The FACT Website can be accessed at