International Timescales with Optical Clocks (ITOC) H. S. Margolis, R. M. Godun, P. Gill, L. A. M. Johnson, S. L. Shemar, P. B. Whibberley National Physical Laboratory (NPL) Teddington, UK helen.margolis@npl.co.uk D. Calonico, F. Levi, L. Lorini, M. Pizzocaro Istituto Nationale di Ricerca Metrologica (INRIM) Torino, Italy P. Delva, S. Bize, J. Achkar LNE-SYRTE, Observatoire de Paris, CNRS and UPMC Paris, France H. Denker, L. Timmen, C. Voigt Institut für Erdmessung (IfE) Gottfried Wilhelm Leibniz Universität Hannover, Germany S. Falke, D. Piester, C. Lisdat, U. Sterr, S. Vogt, S. Weyers Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, Germany J. Gersl Cesky Metrologicky Institut (CMI) Brno, Czech Republic T. Lindvall, M. Merimaa Mittatekniikan Keskus (MIKES) Espoo, Finland Abstract—A new collaborative European project “International timescales with optical clocks” (ITOC) aims to tackle the key challenges that must be addressed prior to a redefinition of the SI second. A coordinated programme of comparisons will be carried out between European optical clocks developed in five different laboratories, enabling their performance levels to be validated at an unprecedented level of accuracy. Supporting work will be carried out to evaluate relativistic effects that influence the comparisons, including the gravitational redshift of the clock transition frequencies. A proof-of-principle experiment will also be performed to demonstrate that optical clocks could be used to make direct measurements of the Earth’s gravity potential with high temporal resolution. Keywords-optical clock; redefinition of the second; international timescales; time and frequency transfer; geodesy I. INTRODUCTION The SI second is currently defined in terms of the frequency between the two ground state hyperfine levels of the 133 Cs atom. However the most advanced optical atomic clocks have now reached levels of stability and accuracy that significantly surpass the performance of the best caesium primary frequency standards. For example, the Al + optical clock at NIST has an estimated systematic uncertainty below 1 part in 10 17 [1], and several others have reached uncertainties below 1 part in 10 16 [2–4]. In contrast, the accuracies of the best caesium fountain primary standards are in the range 2 – 5 parts in 10 16 [5–9]. As a result, an optical redefinition of the SI second is being considered by the international community [10]. The key prerequisite for a redefinition of the second is the integration of optical atomic clocks into the international timescales TAI (International Atomic Time) and UTC (Coordinated Universal Time). This requires a coordinated programme of clock frequency comparisons to be carried out, to validate the uncertainty budgets of the optical clocks, to anchor their frequencies to the present definition of the second, and to establish the leading contenders for a new definition. Such a comparison programme is planned within a new collaborative European project “International Timescales with Optical Clocks” (ITOC). Since the current satellite link infrastructure used to compare microwave clocks in spatially separated laboratories is inadequate to match the increased stability of optical clocks, several alternatives will be explored, with the emphasis on techniques that could be applied on an intercontinental scale. To support the comparison programme a complete evaluation will also be made of all relativistic effects influencing time and frequency comparisons at the 10 –18 level of accuracy, including the gravitational redshifts of the clock transition frequencies. II. CLOCK COMPARISON PROGRAMME To date, very few direct comparisons of optical clocks have been carried out to verify their estimated performance levels. Frequency comparisons between two Al + optical clocks at NIST demonstrate agreement at the 1.8 parts in 10 17 level, consistent with the estimated uncertainty of the older of the two clocks [1], whilst a recent comparison of two strontium optical 908 978-1-4799-0342-9/13/$31.00 ©2013 IEEE 2013 Joint UFFC, EFTF and PFM Symposium