Time and Quantum Frequency Standards: Towards a Redefinition of the SI Second Programme

Transcription

1 Project Title Project Number Price Time and Quantum Frequency Standards: Towards a Redefinition of the SI Second Programme Co-funding target Start Date 01/04/2016 End Date 31/03/2019 Project Lead Patrick Gill Project Team Summary Staff: Geoffrey Barwood, Fred Baynes, Anne Curtis, John Davis, Sean Donnellan, Christopher Edwards, Rachel Godun, Vera Guerrera, Ian Hill, Guilong Huang, Steven King, Hugh Klein, Jochen Kronjaeger, Elizabeth Laier- English, Helen Margolis, Giuseppe Marra, Peter Nisbet-Jones, Yuri Ovchinnikov, Filip Ozimek, Antoine Rolland, Setnam Shemar, Krzysztof Szymaniec, Thomas Vanderbruggen, Peter Whibberley, Ross Williams PhD students: Charles Baynham, William Bowden, Miguel Dovale, Richard Hobson, Jonathan Jones, Stephen Kyriacou, Maurice Lessing, Marco Menchetti This project aims to develop a portfolio of highly competitive optical atomic clocks, together with related time and frequency comparison and dissemination technologies, enabling NPL to play a leading role in international efforts directed at a future redefinition of the second. The limits to the stability and accuracy of our trapped ion and optical lattice clocks will be explored, their frequencies will be determined as accurately as possible relative to the present definition of the second, and high accuracy international optical clock comparisons will be performed to validate their estimated uncertainty budgets. These steps are essential prerequisites for a future redefinition of the second, since they will allow optical atomic clocks to be integrated into the international time scale UTC. The work will also support the development and characterization of miniature atomic clocks and sensors developed as part of the UK quantum technologies initiative. The Need Need for the proposed research Time and frequency play a central role within the SI because they can be measured more accurately than any other physical quantities. The realization of the SI unit of time is already used in the realisations of the metre and the ampere, and will play an even more prominent role in the new SI, which will be based on fundamental physical constants and other constants of nature. In this revised system, the definitions of all SI base units except for the mole will make explicit reference to the definition of the second. Since 1967 the SI second has been defined in terms of the ground-state hyperfine splitting in the 133 Cs atom at Hz. This definition has endured by virtue of the progress that has been made over this period in atomic and quantum techniques such as laser cooling and atom trapping, which have led to a parallel evolution in the accuracy of caesium primary frequency standards. Typically, the rate of improvement has been a factor of ten per decade, to the point where the current systematic uncertainties of the best caesium fountains are 1 2 parts in However over recent years a new generation of optical atomic clocks based on laser-cooled trapped ions or atoms have shown even greater rates of improvement, and have now reached levels of stability and accuracy that significantly surpass the performance of the best caesium primary standards. As a result of these developments, an optical redefinition of the SI second is being actively considered by the

2 international metrology community. However a number of different clocks are currently being investigated worldwide and most still show excellent prospects for further improvement. This fact, together with associated technological challenges, means that a significant body of work remains to be carried out before a redefinition can be implemented: The atomic and ionic species that are being studied as potential candidates for a redefinition of a second need to be fully investigated to explore their limits in terms of achievable accuracy. Improved local oscillators must be developed to probe the clock transitions if optical atomic clocks are to reach their full potential in terms of achievable stability. New methods need to be developed for comparing optical clocks in different NMIs, including techniques that can be used on an intercontinental scale. A programme of clock frequency comparisons must be carried out, to validate the uncertainty budgets of the optical clocks and to anchor their frequencies to the present definition of the second. The robustness and reliability of optical atomic clocks needs to be improved. A framework and procedures need to be established whereby optical atomic clocks can be integrated into the international timescales TAI and UTC. Beyond the metrology community, optical atomic clocks offer opportunities to push tests of fundamental physics to new levels. For example progress in experiments designed to search for violations of the Einstein Equivalence Principle by looking for temporal changes in fundamental physical constants is directly dependent on atomic clocks with improved stability and accuracy. Optical clocks could also find future application in making direct measurements of the Earth s gravity potential with high spatial and temporal resolution. To support such a programme of research and development it is essential to maintain a national time scale at the leading edge, with high stability maser oscillators steered using caesium primary frequency standards, and linked to other NMIs by satellite-based time and frequency transfer techniques. This national time scale provides the UK contribution to the global time scale UTC and the top-level reference for time and frequency measurement in the UK, underpinning sectors from space to finance. It also provides the top-level traceability for dimensional metrology, since the metre is linked to the second by its definition, and is realized by optical frequency standards. In December 2013, the UK government announced a 270 million initiative to support the translation of quantum-physics research into commercial products. Clocks are seen as the quantum technology most likely to demonstrate a return on investment within five years, with widespread impact expected on both defence and civil applications. A significant UK effort has been initiated to develop miniature atomic clocks (both optical and microwave) with accuracy comparable to that of national standards, and a number of university research groups are getting involved in this activity. To characterize the performance of these clocks, it is essential that NPL s time and frequency metrology infrastructure is maintained at the leading edge. Current state of the art The most advanced optical atomic clocks have reached levels of stability and estimated systematic uncertainty that surpass the performance of the best caesium fountain primary standards. The Sr optical lattice clock at JILA and the Al + optical clock at NIST have estimated systematic uncertainties of 2.0 parts in and 8.6 parts in respectively, and several other optical clocks have also reached uncertainties in the parts range. However, very few direct comparisons of optical clocks have been carried out to date to verify these estimated uncertainties. Frequency comparisons between two Sr optical lattice clocks at RIKEN have shown agreement at the level of 4.8 parts in 10 18, and several other local frequency comparisons between optical clocks of the same type (both atoms and ions) have demonstrated agreement at the few parts in level (including a comparison between two Sr + optical clocks at NPL). Remote comparison of two 87 Sr optical lattice clocks in two Japanese laboratories 24 km apart (NICT and the University of Toyko) has been demonstrated using an optical fibre link, with the frequencies of the two clocks agreeing to within the measurement uncertainty of 7.3 parts in However almost all information about the reproducibility of optical clocks currently comes from independent absolute frequency measurements made in different laboratories. Direct comparisons of remotely located optical clocks, free of the limitations imposed by

3 caesium fountain primary frequency standards, are required to validate the uncertainty budgets of the optical clocks. Track record of the research team NPL s time and frequency group has a long heritage in developing microwave and optical atomic clocks and delivering the national time scale, and maintains an internationally leading position in these fields. For example, recent significant achievements include: The systematic uncertainty of the NPL caesium fountain primary frequency standard NPL-CsF2 has been reduced to 2 parts in 10 16, which is one of the leading capabilities worldwide. This is one of the few fountains that contribute data regularly to BIPM for steering of UTC. Our absolute frequency measurements of the 88 Sr + and 171 Yb + E3 optical clock transitions are amongst the most accurate that have been performed worldwide, and have contributed to these transitions being accepted as secondary representations of the second. Agreement between two nominally identical 88 Sr + optical clocks has been demonstrated at the level of Our recent absolute frequency measurements of the E2 and E3 optical clock transitions in 171 Yb + have been used to set improved limits on present-day variation of fundamental physical constants (the fine structure constant and the electron to proton mass ratio). We have recently reported the first ever direct optical frequency ratio measurement between the two optical clock transitions in 171 Yb +. We have demonstrated capability to compare optical frequencies with an accuracy of , well below even the projected uncertainty limits of the optical clocks. Our results represent the highest reported level of agreement to date between Ti:sapphire and Er-doped fibre-based femtosecond combs. Leading results have been established for transfer of frequency combs over optical fibre, which allows optical and microwave frequencies to be transmitted simultaneously. Optical transfer has been demonstrated with a fractional stability of for averaging times of a few thousand seconds, while the mode spacing stability after optical-microwave conversion is better than over the same time scale. The group s credibility in this area is science area is demonstrated by a strong track record of publication in high-impact scientific journals, high citation rates and external funding from ESA, the EU and DSTL, as well as awards from external bodies such as the IoP and the Royal Institute of Navigation. Case for government funding This project is targeted on the long-term development of the SI and the measurement infrastructure of the UK, and as such is suitable for government funding. The research and development programme outlined here requires access to extensive time and frequency metrology infrastructure which, within the UK, is only available at NPL. This work could therefore not be undertaken by academia or industry, although we will collaborate with academic groups on some aspects of the programme. The work involves a requirement to provide traceability to primary standards on a long term and enduring basis, which is not a primary objective of university research. Finally, the facilities and capabilities developed within this NMS project will be essential to characterizing the performance of miniature clocks developed within the UK quantum technologies initiative. Vision Long-term vision The long-term vision for this project is to develop a portfolio of highly competitive optical atomic clocks, together with related comparison and dissemination technologies, enabling NPL to play a leading role in international efforts directed at a future redefinition of the second. As a spin-off benefit, the development of these state-of-the-art time and frequency metrology capabilities will also enable us to make significant contributions to fundamental science, since high accuracy comparisons between optical clocks can be

4 interpreted to set new limits on any possible time-variation of the fundamental physical constants. To underpin this work we will maintain a UK time scale at the leading edge, ensuring its international compatibility and underpinning industry sectors from space to finance. The capabilities developed will also support the development of a new generation of quantum sensors and compact atomic clocks for practical applications as part of the NPL Quantum Metrology Institute (QMI), which is expected to attract high calibre scientists from both academia and industry. Vision for this three-year project segment The major goals for this three-year phase of the larger development cycle are: To improve the stability and accuracy of our trapped ion optical clocks, targeting fractional instabilities at the / level and estimated systematic uncertainties below 1 part in ; To develop an internationally competitive optical lattice clock based on an ensemble of neutral strontium atoms held in an optical lattice; To develop improved optical and microwave local oscillators that are necessary to support stability improvements of our optical clocks and caesium fountain primary frequency standards; To assess the performance of our optical clocks via local frequency comparisons, either by direct beat frequency measurements (for clocks of the same type) or by using femtosecond optical clocks to measure optical frequency ratios; To determine the absolute frequencies of our optical clocks with the lowest possible uncertainty given the current definition of the SI second; To compare our optical clocks with those in other European NMIs, using optical fibre links and enhanced versions of satellite-based time and frequency transfer techniques; To investigate a novel technique for long-haul optical frequency transfer over fibre, which is compatible with existing fibre networks and could potentially be applied on an intercontinental scale; To enhance the generation of the UK time scale to achieve UTC-UTC (NPL) < 5 ns (1 ) for 95% of the time, with the necessary redundancy for near-continuous operation of caesium fountains to derive steering corrections. Contributions to Metrology This phase of the project will advance metrology by helping to build confidence in the new generation of optical clocks prior to a redefinition of the second. Specifically, we will explore the limits to the stability and accuracy of particular types of clocks and perform high accuracy international clock comparisons necessary to validate their estimated uncertainty budgets. The frequencies of the optical clocks will also be determined as accurately as possible relative to the present definition of the second. These steps will allow optical atomic clocks to be integrated into the international time scale UTC, which is an essential prerequisite for a redefinition of the second. Project Scope Primary frequency standards and the national time scale The national time scale UTC (NPL) plays a core role in the NMS, providing the UK contribution to the global time scale UTC and the top-level reference for time and frequency measurement in the UK. To support the rest of the work in this project, and to maintain our international reputation, it is essential that this capability remains at the leading edge, with a maser-based time scale closely steered to UTC using caesium primary frequency standards, and linked to other NMIs by both two-way satellite time and frequency transfer and carrier-phase GNSS methods. Optical frequency standards and clocks Optical clocks based on two different technologies, laser-cooled trapped ions and laser-cooled atoms trapped in an optical lattice, will be investigated to explore the limits to their stability and accuracy and to assess their potential as candidates for a future redefinition of the SI second.

5 Local clock comparisons As a first stage in validating the uncertainty budgets of our optical clocks, local comparisons between the clocks will be carried out. Comparing the clocks under local laboratory conditions will lead to the highest possible levels of stability and accuracy for the measurements. A spin-off from this work will be the possibility of setting new constraints on present-day time-variation of fundamental physical constants. Optical and microwave local oscillators A single system (universal synthesizer) will be developed to meet the requirements for improved local oscillators for the NPL optical clocks and caesium fountain primary frequency standards. This will be based on a dedicated femtosecond optical frequency comb that is referenced to a single state-of-the-art optical local oscillator (master oscillator). The comb acts to transfer the stability of the initial optical reference not only to other optical frequencies (different comb modes) for the optical clocks but also to the microwave region of the spectrum (comb mode spacing) for use with the caesium fountain. The system will also be used to transfer the stability of the optical atomic clocks to the 1.5 µm region for remote frequency comparisons using optical fibre networks. High-accuracy time and frequency transfer techniques New techniques and infrastructure to compare optical atomic clocks in different laboratories with high accuracy will be investigated, including techniques that could be applied on an intercontinental scale. These approaches will include the use of optical fibres linking European NMIs and enhanced versions of microwave satellite-based methods. International clock comparisons We will perform high accuracy comparisons between optical clocks developed in different NMIs. The target is to obtain a set of frequency ratio measurements between all high accuracy optical clocks being developed within European NMIs and potentially beyond, including sufficient redundancy for vital self-consistency checks to be carried out. In some cases, these comparisons may require an improved understanding of relativistic effects in time and frequency transfer, as well as improved knowledge of the gravity potential at the clock sites. Realisation of the metre We will maintain the traceability for the metre from the second, underpinning all NMS work on dimensional metrology. This involves ongoing work to maintain our UKAS-accredited interferometer and laser frequency calibration services, including periodic re-calibration of sensors and minor equipment upgrades and repairs as required. We will also act as a host laboratory for the CCL-K11 key comparison of optical frequency/wavelength standards. Quantum sensing This part of the project is exploratory in nature and aims to exploit some of the techniques and expertise developed within the core time and frequency metrology research to develop compact quantum sensors for ultraprecise measurements of inertial and electromagnetic forces. Benefits Benefits of the current phase of the project The most direct impact of this project will be on the top-level realization and dissemination of the SI unit of time; however there will also be immediate spin-off benefits to fundamental physics, as well as to the UK quantum technologies initiative. Improved frequency values for secondary representations of the second: The results from absolute frequency measurements and direct optical frequency ratio measurements will be submitted to the CCL-CCTF Frequency Standards Working Group (WGFS). When combined with similar input from other groups worldwide, this work will lead to significantly improved recommended frequency values for secondary representations of the second.

6 Independently verified optical clock performance: The programme of optical clock comparisons being carried out in collaboration with European partners will lead to detailed information about the consistency of the optical clocks within Europe. In this way it will provide independent verification of the uncertainty evaluations for each individual clock. Through publications, conference presentations and the submission of reports to the WGFS, this will help to build confidence in the new generation of optical clocks both within the international metrology community and beyond. NPL input to discussions concerning a redefinition of the SI second: The verification of optical clock performance levels will allow the international metrology community to make better informed decisions regarding a future redefinition of the second. It will also potentially allow the community to identify the most promising candidates for a future redefinition of the second. This project will ensure that NPL maintains a prominent role and influence within these international discussions. Tests of fundamental physical theories: The international scientific community will benefit from validated clock comparisons as a basis for tests of fundamental physical theories. One example is the search for temporal variation of the fine structure constant and the proton-to-electron mass ratio by comparing transition frequencies in different atomic clocks. Such tests will emerge naturally from the clock comparisons performed within this project. High accuracy ground-based optical clocks will also benefit space missions designed to test fundamental physical theories. One important example is the ACES (Atomic Clock Ensemble in Space) mission, scheduled to be launched in Facilities to support UK quantum technologies initiative: The leading-edge facilities, capabilities and expertise developed within this NMS project will be essential to characterizing the performance of miniature clocks and quantum sensors developed within the UK quantum technologies initiative. Benefits of the completed programme (3+ project cycles) Improved stability of international time scales: The uncertainty of the improved recommended frequency values for the secondary representations of the second will be at the limit set by primary standards. This will enable optical clocks to be used on a regular basis to provide steering corrections to UTC, along with the caesium primary standards. Increasing the number of high accuracy clocks used to determine the steering corrections will have immediate benefits to users of international time scales, since it will improve their stability. Examples of users who can be expected to benefit from the availability of higher stability reference time scales include the European Space Agency (ESA) and the European VLBI Network (EVN), both of which have major facilities (ESTEC, JIVE) that require accurate time and frequency signals. Clock-based geodesy: The extremely high accuracy of optical clocks means that they can be used to make direct measurements of the Earth s gravity potential over medium to long baselines. High-resolution measurements made at selected well-defined locations using transportable optical clocks could bring significant benefits to the geodesy community in terms of achieving a consistent alignment of national height systems within Europe, as well as checks of global and regional geoid models established by alternative means. Such measurements would complement the data obtained from satellite missions such as GOCE or GRACE, which provide global coverage but give values that are spatially averaged over length scales of about 100 km. Collaborators NPL s Time and Frequency group is collaborating with a wide range of external partners on aspects of this project, as well as with the NPL s Quantum Detection group on ion trap development. NMIs: We are collaborating with all the other European NMIs engaged in developing high accuracy optical atomic clocks (PTB, LNE-SYRTE, INRIM, MIKES and CMI), both on optical clock development and on optical clock comparisons. We also collaborate with NMIs outside Europe, having recently hosted a guest worker from NIST to work on femtosecond combs, and with secondees from KRISS, NIM and NMIJ having spent periods working on the NPL caesium fountains in recent years. Academic (UK): Our collaborative links with UK academic research groups provide us with access to high quality PhD and EngD students, with current students coming from the University of Oxford, the University of

7 Birmingham, the University of St Andrews, University College London, the University of York and the University of Liverpool. We also maintain strong links with the University of Southampton and Heriot-Watt University. Academic (International): Within the framework of the EMRP ITOC project we have established a collaboration with the Institut für Erdmessung (IfE) at the University of Hannover. This provides us with the geodesy expertise that is essential for evaluating the gravitational redshift corrections to be applied to optical clock frequencies. We are also collaborating with Pennsylvania State University on the modelling and evaluation of systematic frequency shifts in caesium primary frequency standards. Industry (UK): Industrial collaborators within the UK include Chronos Technologies and the General Lighthouse Authorities (GNSS and LF signal monitoring), Airbus (optical cavity development) and Renishaw (laser interferometer development). Additional support A major source of additional support will be the EMRP and EMPIR programmes. The current EMRP project in which we are involved (and for which the final stages of co-funding will be provided by this project) is: International timescales with optical clocks (ITOC), 1 st May th April Two further EMPIR proposals are currently being prepared, and would start in spring 2016 for three years if successful: Optical clocks with uncertainty (OC18); Optical frequency transfer a European network (OFTEN). The first proposal is being led by NPL. This source of funding provides additional value by allowing us to establish strong collaborations with other European NMIs, in particular to target comparison of optical clocks developed independently by different NMIs. Other sources of UK and European funding will also be sought to allow further such comparisons to be carried out, as well as to contribute to the optical clock development. For example we have recently won funding from The UK Space Agency for a dark fibre link between NPL and Harwell ( 500k) and to procure an ACES Microwave Link ground terminal for NPL ( 400k); The Pan-European Research and Education Network GÉANT via the GÉANTplus Open call, to establish a fibre link for optical clock comparisons between NPL and LNE-SYRTE (ICCOF project, now finished). The European Space Agency for a project Technical assessment of a single Yb + dual transition optical clock, 1 st November st October The FACT Marie Curie Training network for a student to work on the strontium optical lattice clock. Co-funding will be sought to increase links to users of the national time scale. Recent examples include Co-funding from INRIM (~ 25k per annum) for provision of UTC(NPL) data to the Galileo Time Validation Facility. A TSB project (SISTALS) which aims to use resilient positioning methods to enhance transport systems. Co-funding will also be sought for the exploratory work on quantum sensors, e.g. from Innovate UK. Knowledge Transfer and Exploitation Knowledge transfer Scientific publications: The proposed research offers excellent scope for high quality scientific publications in journals such as Physical Review Letters, Physical Review A, Optics Letters, Optics Express and Metrologia. For any particularly high impact results journals such as Nature Photonics, Science or Nature will be targeted. Conference presentations: Results from the project will be presented at leading international conferences such as the European Frequency and Time Forum (EFTF), the IEEE International Frequency Control

8 Symposium (IFCS), the Conference on Lasers and Electro Optics (CLEO), the International Conference on Atomic Physics (ICAP) and the International Conference on Laser Spectroscopy (ICOLS). This broad range of conferences reflects the fact that we expect results from the project to attract attention and interest from beyond the frequency metrology community. In conjunction with partners within the EMRP ITOC project, conferences held by the geodesy community will also be targeted. Committee representation: A key dissemination route for the results of this project will be input to the CCTF and its associated working groups. For example, the results of absolute frequency measurements and optical frequency ratio measurements will be provided to the CCL-CCTF Frequency Standards Working Group, as will the results of comparisons between our own optical clocks and those of other NMIs. The project also supports NPL representation on a range of other working groups including the CCTF Working Group on Coordination of the Development of Advanced Time and Frequency Transfer Techniques, the CCTF Working Group on Primary and Secondary Frequency Standards and the CCTF Working Group on Two-Way Satellite Time and Frequency Transfer (TWSTFT) as well as other committees such as the Consultative Committee for Length (CCL), ITU committees discussing the future of the leap second, the EURAMET Technical Committee on Time and Frequency and the EFTF Executive Committee. Conferences and topical workshops: The University of York has recently been selected as the venue for EFTF 2016, and NPL staff will form a large part of the local organizing committee for this meeting. We will also contribute to the organization of an associated workshop related to the EMRP ITOC project. Training: The Time and Frequency group maintains strong links to academic research groups within the UK and a number of PhD and EngD students will participate in this project. Members of the group are also regularly invited to present at summer schools. Web presence: The time and frequency pages receive by far the largest number of hits on the NPL website, and will be updated regularly. These will be linked to the web pages for NPL s Quantum Metrology Institute (QMI). Dedicated websites will also be set up and updated regularly for EMRP-funded and EMPIR-funded projects that NPL leads. Broader dissemination activities: NPL s time and frequency activities attract a high level of interest from the media and from numerous visitors to NPL, including high profile individuals from government and industry. We participate in NPL Open Days and members of the group are also regularly invited to give presentations to non-specialist audiences, e.g. IoP branch lectures and meetings of local scientific societies. Exploitation plans The main scientific outputs from this project will be results and methods necessary to underpin a future redefinition of the second, and thus will be exploited by providing input to the CCTF and its associated working groups as described above. However the leading-edge facilities, capabilities and expertise developed within the project will also be exploited to support the UK quantum technologies initiative by using them to characterize the performance of miniature atomic clocks and quantum sensors developed under that framework. The expertise developed will also be used in spin-off activities and projects for the UK Space Agency, future ESA missions, navigation etc. Dissemination of precise T&F in the UK is achieved through the MSF radio time signal, internet and dial-up computer time services, and UKAS-accredited services for frequency standard characterization and GPS common-view time and frequency comparisons. Dissemination also extends to dimensional metrology performed throughout the UK since the SI metre is realized by optical frequency standards traceable to the SI second. A much broader range of users of precise frequency and time will also benefit indirectly from NPL s regular contribution to the generation of UTC. In addition, NPL is exploring provision of traceable time to the financial services sector via optical fibre links. This should cater for new users for whom the MSF signal does not provide sufficient precision. Risks 1. The masers forming the national time scale are all old (9, 12, 19 and 24 years as of 2014) and there is therefore a significant risk of multiple failures, which would result in a requirement to purchase replacements

9 at short notice. The time and frequency transfer links are based on one TWSTFT system and one dualfrequency GPS receiver, with no redundancy against hardware faults. To mitigate these risks a plan for capital investment to improve the resilience of the timescale has been developed and is being implemented. 2. Robust operation of our optical clocks, caesium fountains and femtosecond optical frequency combs is dependent on maintaining good levels of temperature stability in our laboratories. Failures of these control systems could lead to significant downtime, resulting in delays in meeting project goals. 3. Operating optical atomic clocks with their optimum performance in several NMIs simultaneously for the international frequency comparison campaigns will be challenging. However this risk is mitigated by the fact that there is more than one type of optical clock available in three out of the four European NMIs involved in the planned experiments. 4. Comparing optical clocks via optical fibre requires access to links between NMIs, and maintaining this access is paramount to maintain NPL s profile in this area. A link between NPL and LNE-SYRTE has been secured via the GÉANT network, but its long-term future is not yet secure. Possible routes to continued funding of this link are currently being investigated to mitigate the risk that we lose this link part way through the project. Planned Outcomes 1 Start: 01/04/2016 End: 31/03/2019 Outcome 1: Position the UK to deliver optical atomic clocks for the redefinition of the SI second that will lead to a future higher precision timescale, fundamental scientific discoveries and improved navigation, communications and other high frequency commercial systems, by: 1. Developing internationally competitive optical atomic clocks, as demonstrated by their published uncertainties and stabilities. 2. Exploring fully the limits to the stability and accuracy of these clocks, and validating their uncertainty budgets by comparing them both with each other and with optical clocks in other NMIs. 3. Making significant contributions to the CIPM list of recommended values of standard frequencies by measuring the absolute frequencies of our optical atomic clocks and the direct optical frequency ratios between them and publishing the results in peer-reviewed scientific papers. 4. Actively engaging with the BIPM consultative committee for time and frequency and its working groups to determine the best candidate optical atomic clocks for the redefinition of the SI second. 2 Start: 01/04/2016 End: 31/03/2019 Outcome 2: Enable the UK to engage in inter-nmi high accuracy clock comparisons, international science projects and experiments, and to demonstrate the feasibility of extending precision traceability from local scale to a global scale as outlined in NPLs Metrology 2020s vision: 1. Investigating new techniques for high accuracy time and frequency transfer, including techniques that could be used on an intercontinental scale, e.g. optical fibre links and enhanced versions of microwave satellitebased methods. 2. Establishing and maintaining a link to a pan-european optical fibre network linking NMIs, and using this to validate the uncertainties of UK developed high accuracy clocks by comparing them with similar systems developed by other NMIs. 3. Developing a universal synthesizer to provide high stability local oscillators for NPL s optical atomic clocks and caesium fountains, enabling high accuracy clock comparisons to be carried out faster and more efficiently. 4. Developing an improved understanding of relativistic effects influencing comparisons between the next generation clocks, including the gravitational redshift of the clock frequencies. 5. Developing the expertise to enable the delivery of separately funded working systems and demonstrators with external partners (for example the SKA and ESA s Atomic Clock Ensemble in Space)

10 3 Start: 01/04/2016 End: 31/03/2019 Outcome 3: Maintain a national timescale at the leading edge to support the effective and efficient operation of the UK National Infrastructure, transport systems, trading, comms systems, and the general economy by: 1. Upgrading the facilities used to generate the national timescale UTC(NPL) to provide increased robustness and resilience to meet existing needs. 2. Developing caesium fountain primary frequency standards with optimised stability and internationally competitive accuracy, and using these to improve the medium- to long-term stability of UTC(NPL) by steering the high stability maser oscillator used to generate the timescale. 3. Demonstrating internationally the stability and accuracy of UTC(NPL) by contributing clock and time transfer data regularly to BIPM to generate International Atomic Time (TAI) and Coordinated Universal Time (UTC) 4. Providing time dissemination services (e.g. the MSF radio time signal and NTP services for computers), and monitoring time signals available in the UK. 5. Developing, commissioning and validating new dissemination techniques and systems to underpin innovation that critically depends on time (e.g. trading, transport systems, smart cities, national security). 6. Providing the traceability for the top-level realisation of the metre within the UK. 4 Start: 01/04/2016 End: 31/03/2019 Outcome 4: Active participation in developing and validating technologies with partners for a future UK Quantum industry in support of the governments Quantum Technology programme, Industry requirements and key science missions by: 1. Carrying out exploratory work on next generation compact quantum clocks, sensors and devices for use by NPL in future projects and for development in collaboration with partners in separately funded projects 2. Underpinning the commercial viability of UK-developed commercial clock systems and frequency-critical sensor and communications systems by providing the facilities and expertise necessary to validate the devices against internationally accepted standards at the highest levels of accuracy. 3. Exploiting the opportunities that arise from the development of new clocks and frequency metrology techniques to making new measurements of fundamental scientific interest, e.g. measurements of the Earth s gravity potential and searches for time variation of fundamental physical constants.