COMPACT VISIBLE FREQUENCY COMBS: THE MISSING LINK IN A VISION OF PERVASIVE QUANTUM TIMEKEEPING

Informed by the requirements of future precision atomic clocks, this project targets the development of an "optical frequency comb" -- a laser providing a thousands of regularly spaced optical frequencies which form a ruler in frequency that is a critical component in quantum timekeeping devices.

Quantum technology research in the UK and internationally is developing small atomic clocks to which the frequency of a special laser (not a laser comb) can be locked with extremely high stability. Yet these clocks "tick too fast": the clock laser oscillates at about 500 trillion "ticks per second", far too quickly to allow it to be interfaced to real-world systems like computer networks and electronic navigation devices. The laser comb can be used like a gearwheel to reduce this rate to one more appropriate for everyday applications of about 10 billion ticks per second. In this sense the comb works exactly like the clockwork mechanism in a pendulum clock, reducing the faster ticks of the pendulum to less frequent increments in the positions of the minute and hour hands.

To date, practical laser combs with the right technical characteristics have been difficult to produce, even with lab-scale dimensions. This project will address the need for compact combs as sub-systems within a practical optical clock--and the current absence of such technology--by developing a disruptive laser-comb architecture. This will be compatible with visible clock transitions in new ion-based time standards, and will have a scale suitable for integrating into quantum timekeeping devices needed by sectors from security, energy, geodesy, finance and defence.

Our approach will leverage advances in ultrafast lasers and integrated nonlinear photonic devices, complementary technologies in which the investigators at Heriot-Watt and Glasgow Universities are world leaders. Areas of emphasis are the development of robustly packaged infrared pulsed lasers operating at around 10 GHz (10 billion "ticks per second"), and the efficient extension of these to the visible region by using chip-scale "super-continuum" devices prototyped in the material gallium arsenide and finally to be made from the material silicon nitride. The output of these lasers will be made into a frequency comb by using a combination of optical and electronic stabilization techniques.

The project will be developed in close association with several academic and industrial partners who will contribute resources and expertise in lasers (Laser Quantum Ltd.), optoelectronic manufacturing (Optocap Ltd.), optical frequency metrology (NPL), optical frequency standards (EPSRC UK Quantum Technology Hub in Sensors and Metrology), optical systems engineering (Fraunhofer Centre for Applied Photonics) and expertise in end-user applications of combs (Dstl).

Our partners have committed up to £527.5K cash and £182K in-kind support, and span the supply chain from devices and systems, to verification and end-users. This breadth and depth of commitment will ensure that the project gains real-world traction and will have an enduring impact.

The modular comb targeted by the project resonates strongly with EPSRC's Photonics for Future Systems priority and addresses key portfolio areas of Optical Devices & Subsystems, Optoelectronic Devices & Circuits, Quantum Devices, Components & Systems and RF & Microwave Devices.

By the end of the project we expect to have demonstrated and evaluated this novel laser-comb technology, as well as created considerable new knowledge and IP in the areas of ultrafast lasers and integrated nonlinear photonics. This will leave us in a strong position to translate the technology into systems of commercial and scientific benefit to our industrial and academic partners and wider society.

Our research will produce immediate academic impacts, with industrial, economic and societal impacts emerging over a longer timescale.

1. Impacts on Project Researchers
Through the training and development of 3 doctoral students aligned to the project, the research will support UK industry's need for highly skilled researchers in photonics. The named Researcher Co-I, Dr. Richard McCracken, will also be developed in his management skills through mentorship by Reid and Ferrera, his professional network with industry and access to staff development courses.

2. Impacts on Academic Researchers
We will generate new scientific and engineering knowledge in the areas of femtosecond laser technology, integrated nonlinear photonic devices, photonic materials and optical frequency metrology. By disseminating our research in peer-reviewed technical journals and conferences our results will influence the practice of other researchers in areas including laser development, frequency comb techniques, metrology and photonic device fabrication.

3. Impacts on Industry
In general terms the project partners span the supply chain needed to establish a UK source of modular frequency combs, which would drive industrial development of the technology. Specifically, the research could deliver impact across several sectors:
(a) Defence
Precision timing and synchronisation underpin many critical navigation and communication systems. The fragility of the GPS network and its absence in some domains (e.g. sub-sea) mean that autonomous systems with high precision and accuracy are needed. Compact optical clocks, linked to electronic networks by modular combs of the kind proposed, can provide the stability needed to operate for months or even years without re-synchronization.
(b) Metrology
Modular combs, either optically referenced for the highest precision or RF referenced via local atomic standards or GPS, will become increasingly prominent as sub-components in industrial systems where time distribution or optical frequency calibration is necessary. Precision interferometers for distance measurement and wavemeters for optical frequency measurement are examples of industrial instruments which would both immediately benefit from embedded comb technology.
(c) Manufacturing
Our partnerships with Optocap (an optoelectronic packaging company) and Laser Quantum (a laser manufacturer) offer an immediate exploitation channel for IP developed in the project. For example, we expect that the collaboration with Optocap will result in a new methodology for aligning and bonding the components of a solid-state laser in a mechanically robust and thermally insensitive configuration, opening immediate commercial opportunities for this new generic technology. The specific 10-GHz Er- and Yb-based fs lasers to be developed in the project are complementary to the Ti:sapphire system currently sold by Laser Quantum and therefore also present an immediate commercialization opportunity.
(d) Finance
The increasing dependence of the financial sector on high-frequency trading, in which the need for time-stamping to ns precision is forecast, means that embedded atomically traceable time references are needed. Again, modular combs will play a key role in such technologies, as outlined above in 3(b).

4. Socio-Economic Impacts
In our information society, time-distribution and synchronization are vital to the trustworthy exchange of data across sectors such as energy, e-commerce, banking, transport and healthcare. Atomically traceable time standards, as enabled by modular combs, have the potential to play an increasingly important role in ensuring the smooth running of these systems and the public's confidence in their security and reliability.

Our public engagement plan (see Pathways to Impact) includes activities which will reach the general public, increasing public awareness of the role of photonics and the value of precision timing to the UK.