As stated above, extensive investment and effort is being made in the UKRI/EPSRC CDTs and Quantum Hubs to develop some critical elements of the talent and skills infrastructure for the PNT community. Allied to this, the work of the national laboratories such as Dstl and the National Physical Laboratory (through the National Timing Centre Programme) are examples of where a publicly funded national resources are providing strategic infrastructure for the advancement of timing technology. Furthermore, NPL's Postgraduate Institute [6] provides an environment where talent development across a broad range of skills is having an impact on developing the supply of capable postdoctoral researchers in a variety of technical areas that are essential to support the timing ecosystem.

The development of talented scientist and engineers also requires experience in real-world environments which can sometimes be difficult to access, but initiatives such as the STEM Futures scheme [7] – hosted by the Government Office for Science – provides a highly efficient mechanism to enable staff mobility for the purpose of training and development across a broad range of subject areas, amongst a wide range of academic, industrial and public bodies able to host secondees in highly specialised environments.

Within the Defence Science and Technology Laboratory (Dstl), the Cross-Cutting Timing Consortium (CCTC) has been established to primarily support provision of timing and time transfer technologies for defence and security sector. It is designed to contribute to the successful implementation of Distributed Resilience Timing/Time Transfer (DRT), the Resilient Enhanced Time Scale Infrastructure (RETSI) and applications that are dependent upon accurate, resilient timing in the following ways:

* Ensuring UK sovereign development of assured components for Ministry of Defence (MOD) DRT, RETSI and time distribution systems.

* Aiding awareness, coherence, and health of the UK's Timing & Time Transfer research landscape.

* Driving Suitably Qualified and Experienced Personnel development within Dstl, MOD, UK government, industry and academia.

This will be implemented, where appropriate and possible, by: Communicating requirements for timing, DRT & RETSI in Dstl and government; Identify calls for proposals; timing/time-transfer systems/component development; Highlighting relevant UK legislation; Communicate technical work in MOD, industry, academia and UK government; Identifying collaboration and secondment opportunities; and Promoting awareness of timing as an underpinning and critical capability

The exercise to survey the timing community and confirm the need for a coherent and collaborative approach to deliver a talent and skills development system, also asked for interested stakeholders to identify key drivers and blockers for this endeavour. Some initial outcomes are provided in Table 1 below.

<table><caption>**Table 1.** Extract of processed feedback from NPL/DSTL hosted timing community event held on 10 March 2025.</caption><thead><tr><th></th><th>High popularity</th><th>Medium popularity</th><th>Niche but critical</th></tr></thead><tbody><tr><th>What is needed?</th><td>Career pathways and awareness</td><td>Funding and incentives</td><td>Policy, compliance and national security</td></tr><tr><th>Why it matters?</th><td>Lack of awareness cited as a barrier - sector needs a coherent, visible story</td><td>Many organisations can't invest due to cost/risk, especially SMEs</td><td>Timing is critical to national infrastructure, but poorly understood</td></tr><tr><th>Suggested Action(s)</th><td>Launch national awareness campaigns; central job hub; career videos in schools</td><td>Co-funded public/private training schemes; training tax incentives</td><td>Embed timing skills in national resilience frameworks; standards literacy across industry</td></tr></tbody></table>
**Concluding remarks**

The proposal set out above provides some of the salient points that will support the development of a
robust and enduring talent development structure for timing and PNT more generally and is set within the
wider quantum skills landscape.

The proposed roadmap is predominantly driven by the context within which it has been framed, namely
government strategy and policy directions and how this flows down to an infrastructure that has, and will
continue to, receive significant investment. The criticality of coordination and coherence across the
community to enable effective collaboration cannot be overstated. The current plan is time sensitive by
definition as it is evolving and is required to be agile in the face of rapidly changing or evolving technologies,
needs, and capabilities. They key conclusion, therefore, to ensure we have a strong community and enduring
talent and skills development ecosystem is that in terms of action, “the time is now!”

**Acknowledgements**

The authors would like to acknowledge the support of NPL (National Timing Centre Programme), Dstl (StratCom), The Royal Institute of Navigation, The Quantum Enabled PNT Hub, for the support and enabling the surveys at several community events. Michael Lingard (NPL) for the development of the roadmap framework.

**References**

1. "New framework for Greater Position, Navigation and Timing Resilience to underpin Critical National Infrastructure (CNI)", 18 October 2023, https://www.gov.uk/government/news/critical-services-to-be-better-protected-from-satellite-data-disruptions-through-new-position-navigation-and-timing-framework,

2. https://www.ukri.org/what-we-do/developing-people-and-skills/epsrc/studentships/centres-for-doctoral-training/

3. https://www.ukri.org/news/five-hubs-launched-to-ensure-the-uk-benefits-from-quantum-future/

4. UK Quantum Skills Taskforce report, June 2025, https://www.gov.uk/government/publications/uk-quantum-skills-taskforce-report/uk-quantum-skills-taskforce-report

5. Royal Institute of Navigation (RIN), https://rin.org.uk/page/RIN_Training

6. PostGraduate Institute Five-Year Review 2025, Chapman, L; Ellis-Gibbings, L; Fradet, L; Burguete, R (2025), DOI: https://doi.org/10.47120/npl.10148

7. STEM Futures, https://www.gov.uk/government/publications/stem-futures-developing-capability-through-knowledge-exchange
# Microcombs for Portable Optical Clocks

Jonathan Silver¹

¹ National Physical Laboratory, Teddington, United Kingdom

E-mail: jonathan.silver@npl.co.uk

## Status

Optical atomic clocks [1] are the state of the art in timekeeping, with systematic uncertainties and stabilities at the level of 1 part in 10¹⁸ or below demonstrated with averaging times of just a few hours. By comparison, caesium fountain clocks, the leading microwave atomic clocks that are used in national metrology institutes around the world to generate Coordinated Universal Time (UTC) based on the current definition of the second, typically have uncertainties and stabilities of a few parts in 10¹⁶ with averaging times of weeks [2]. Since optical clocks are able to measure time so much more accurately, work is underway to redefine the second in the coming years based on one or more optical atomic transitions [3]. This level of accuracy enables entirely new applications from Earth monitoring via measurement of the gravitational potential with centimetre height resolution to detection of drifts or local variation of fundamental constants caused for example by dark matter. Furthermore, portable optical clocks of various sizes are being developed that deliver better stability performance than microwave clocks of comparable size, weight and power consumption (SWaP) [4], which could eventually replace these in their current applications and unlock many new ones in areas such as telecommunications and positioning, naviagation and timing (PNT).

A central component in any optical clock is the frequency comb [5]. These enable coherent conversion of frequencies both between the optical and microwave/RF domains and across the optical domain, allowing the counting of optical clock frequencies via down-conversion to RF as well as direct optical frequency ratio measurements over fibre, both locally and remotely via telecom fibre links. For a comb to be used for frequency ratio measurements, it must be self-referenced, meaning that the carrier-envelope offset frequency is detected and/or locked. This is usually achieved by frequency-doubling the low-frequency end of an octave-spanning comb and taking the beat note of this with the other end of the comb. Current commercial self-referenced combs are based on femtosecond modelocked lasers that are broadened to an octave in highly nonlinear fibres or waveguides. Although these systems perform well and can be reasonably robust, they range from a few litres to about a cubic metre in total volume, are hand-built and typically cost at least £100,000, which limits their uses. Having a mass-producible self-referenced comb with a much smaller form factor is therefore key to realising compact optical clocks as well as other applications of coherent frequency conversion such as optically-derived ultrastable microwave sources for radar.

In recent years, microresonator-based frequency combs, or microcombs for short [6], have emerged as a very promising route to ultracompact combs. Based on ultrahigh-Q whispering-garden-mode microresonators that convert a monochromatic pump laser into an entire comb via their intrinsic Kerr optical nonlinearity, they have been demonstrated in many types of resonator including chip-based waveguide microrings created from a range of material platforms [7]. Various different types of phase-locked microcombs have been produced including single-soliton, multi-soliton and soliton crystal combs in resonators with anomalous dispersion as well as dark solitons, platicons and switching waves in the normal-dispersion regime [8]. In contrast to modelocked-laser-based combs, which typically have repetition rates of
hundreds of MHz, microcombs usually have spacings in the range 10 GHz to 1 THz due to their much smaller cavity sizes. With the rapid recent pace of development of microphotonics fabrication processes, there is now a clear pathway to realising mass-producible chip-based self-referenced frequency combs, as outlined in the following sections.

Current and future challenges

In the last decade or so, silicon nitride has emerged as the leading platform for integrated microcombs in the near-infrared, due to its favourable dispersion properties, CMOS-compatible fabrication processes, relatively high Kerr nonlinearity (10 times that of silica) and the ultrahigh Q factors of over 10 million that have been achieved for high-confinement ring resonators [9]. The high repetition rates of microcombs mean that external broadening to an octave in a single pass through a nonlinear fibre or waveguide requires amplification to average powers of several watts [10]. As this is not currently feasible on chip, the octave-spanning comb must be generated inside a microresonator if the entire optical circuit is to be on chip. Octave-spanning soliton microcombs with a 1 THz repetition rate have been produced in silicon nitride microring resonators with as low as 40 mW of pump power on chip [11] and with higher pump powers at repetition rates down to around 200 GHz. In these studies, the resonator's dispersion was engineered via the waveguide width and thickness to allow broadband comb generation and place dispersive wave peaks in the spectrum an octave apart from each other to increase the power available for self-referencing.

Octave-spanning microcombs with repetition rates below 200 GHz are difficult to produce in phase-locked states, as well as requiring pump powers on the level of 1 W or above due to the larger resonator mode volume, and so are impractical. On the other hand, the limited availability of microwave components above 200 GHz poses a challenge to measuring such high repetition rates, which can be overcome in a number of ways. One is to use a microcomb with a gigahertz repetition rate to subdivide the repetition rate of the octave-spanning comb [12], but more recently a Vernier technique has been demonstrated using two terahertz-spacing combs with slightly different spacings, which has the added advantage that it allows terahertz carrier-envelope offset frequencies to be measured at the same time [13].

Another challenge is the integration of the pump laser with the microcomb due to the fact that diode lasers are based on III/V semiconductors which cannot be directly grown on a silicon-based platform due to the lattice constant mismatch. However, much progress has been made recently with heterogeneous integration techniques such as butt-coupling, flip-chip bonding and wafer bonding, and these have been used to integrate distributed feedback (DFB) lasers and reflective semiconductor optical amplifiers (RSOAs) with silicon nitride photonic integrated circuits (PICs) for on-chip pumping of microcombs, such as in [14]. In this and other demonstrations, the pump laser was self-injection-locked to a resonant Rayleigh back-reflection from the ultrahigh-Q resonator that generates the comb. This meant that the laser-resonator detuning was hardly affected by thermal nonlinearities, circumventing the difficulties encountered with entering the single-soliton comb state when using an external pump laser [8] and instead allowing simple turnkey access to it upon switching on the pump current.

Single-bright-soliton combs have certain advantages compared to other types of phase-locked microcombs, including stability, reproducibility, the potential for very broad spectra and the ability to generate strong beat note signals such as the repetition rate due to the alignment of the comb lines' phases. However, they also have one notable disadvantage when generated in a single resonator pumped with continuous-wave (CW) light, namely their low pump-to-comb power conversion efficiency, which scales as the inverse of the number of comb lines within the 3 dB bandwidth of the spectral envelope [15]. Several solutions to this have been demonstrated including pulse pumping, multi-soliton and soliton crystal combs, dark solitons, laser cavity solitons and schemes involving dual resonators and photonic crystal resonators [16]. In one case, an auxiliary resonator was used to selectively shift the pump resonance frequency, allowing
the production of a single-bright-soliton comb with more than 50% conversion efficiency from a CW pump, a result that was later improved upon by achieving the same effect using a photonic crystal resonator.

One potential disadvantage of microcombs compared with modelocked-laser-based combs for optical frequency metrology applications is their high therm refractive noise due to their small mode volume, which adds phase noise to their repetition rate and carrier-envelope-offset frequencies. Although this noise can be effectively suppressed by applying feedback to the pump laser and resonator microheater currents, it is still difficult to reach the stabilities possible with modelocked-laser-based combs using this method. However, a remarkably good solution to this is to injection-lock a comb line far from the pump to a second laser coupled into the resonator [17].

**Advances in science and technology to meet challenges**

Although the challenges of ultrahigh-Q microring resonator fabrication, on-chip comb generation, pump laser integration, octave-spanning comb generation, terahertz repetition rate measurement, efficient comb generation and noise suppression have all separately been solved, there is still work to be done to combine these all to produce a low-noise, power-efficient, octave-spanning microcomb on a chip. Furthermore, second-harmonic generation (SHG) must also be performed on the same chip to self-reference the comb and potentially also to frequency-double specific comb lines to produce visible optical clock wavelengths, since silicon nitride microcombs generally occupy the 1-2 µm wavelength range. A promising route to achieving this is to use periodically-poled thin-film lithium niobate (PPTFLN) ring resonators, which can achieve extremely high SHG efficiencies through a combination of high confinement, resonant enhancement and artificial phase-matching [18]. Integration of lithium niobate onto silicon nitride is currently an area of active research, with recent progress made on both transfer printing of devices and wafer bonding of membranes. There has also been a recent demonstration of two-colour soliton microcomb generation in PPTFLN ring resonators [19], which suggests the possibility of generating a self-referenced comb directly in a PPTFLN resonator by making the fundamental and second-harmonic combs broad enough that their spectra overlap through careful dispersion engineering and increasing the pump power. Finally, high-speed photodiodes must also be integrated onto the chip for beat note detection, although this does not present a significant additional challenge as both the waveguide-based photodetectors and the techniques for their heterogeneous integration have already been developed.

**Concluding remarks**

Microcombs offer an extremely promising route to achieving a mass-producible self-referenced frequency comb on a chip, which could be truly revolutionary, enabling portable, compact or even fully chip-based optical clocks [20] as well as ultrastable microwave sources and other technologies. Most of the challenges to realising chip-based self-referenced combs have already been individually addressed, and with the current rate of progress in microphotonics fabrication techniques, it is highly possible that they will be produced in the next few years.

**Acknowledgements**

We acknowledge the support of the UK Government Department for Science, Innovation and Technology through the UK National Quantum Technologies Programme.

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