Advances in atomic physics, such as cooling and trapping of atoms and molecules and developments in frequency metrology, have added orders of magnitude to the precision of atom-based clocks and sensors. Applications extend beyond atomic physics and this article reviews using these new techniques to address important challenges in physics and to look for variations in the fundamental constants, search for interactions beyond the standard model of particle physics, and test the principles of general relativity.

Search for New Physics with Atoms and Molecules

Atomic clocks have been instrumental in science and technology, leading to innovations such as global positioning, advanced communications, and tests of fundamental constant variation. Timekeeping precision at 1 part in 10 18 enables new timing applications in relativistic geodesy, enhanced Earth- and space-based navigation and telescopy, and new tests of physics beyond the standard model. Here, we describe the development and operation of two optical lattice clocks, both using spin-polarized, ultracold atomic ytterbium. A measurement comparing these systems demonstrates an unprecedented atomic clock instability of 1.6 × 10 –18 after only 7 hours of averaging.

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An atomic clock with $10^{-18}$ instability

Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6 × 10 -16. Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, mu-dot/mu = 0.2(1.1) × 10-16 yr-1, along with an improved constraint on time variation of the fine structure constant, alpha-dot/alpha = -0.7(2.1) x 10-17 yr-1.

Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time variation of fundamental constants.

Progress in realizing the SI second had multiple technological impacts and enabled further constraint of theoretical models in fundamental physics. Caesium microwave fountains, realizing best the second according to its current definition with a relative uncertainty of 2-4 Â 10 A 16 , have already been overtaken by atomic clocks referenced to an optical transition, which are both more stable and more accurate. Here we present an important step in the direction of a possible new definition of the second. Our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.5 Â 10 A 16. Their comparison with three independent caesium fountains shows a degree of accuracy now only limited by the best realizations of the microwave-defined second, at the level of 3.1 Â 10 A 16 .

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Experimental realization of an optical second with strontium lattice clocks.

We present a new measurement of the $1S\ensuremath{-}3S$ two-photon transition frequency of hydrogen, realized with a continuous-wave excitation laser at 205 nm on a room-temperature atomic beam, with a relative uncertainty of $9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$. The proton charge radius deduced from this measurement, ${r}_{p}=0.877(13)\text{ }\text{ }\mathrm{fm}$, is in very good agreement with the current CODATA-recommended value. This result contributes to the ongoing search to solve the proton charge radius puzzle, which arose from a discrepancy between the CODATA value and a more precise determination of ${r}_{p}$ from muonic hydrogen spectroscopy.

New Measurement of the 1S-3S Transition Frequency of Hydrogen: Contribution to the Proton Charge Radius Puzzle.

We give an overview of the work done with the Laboratoire National de Metrologie et d'Essais-Systemes de Reference Temps-Espace (LNE-SYRTE) fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift, which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the 87Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

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Progress in atomic fountains at LNE-SYRTE

The new realization of UTC(OP) is described. The steering algorithm and the prediction of UTC(OP) are presented in details. The results of the first semester of operation are reported.

https://ieeexplore.ieee.org/iel7/6687306/6702044/06702167.pdf

The new UTC(OP) based on LNE-SYRTE atomic fountains

We propose a new simple technique to improve the short term stability and the robustness of Common-Views (CV) of Global Positioning System (GPS) satellites. We use the ionosphere free linear combination of P-code data broadcast on both GPS carriers, called TAIP3, the data files being built according to the Common GNSS Time Transfer Standard (CGTTS) format. Instead of averaging the outliers out by using a given filter around a mean value computed for each CGTTS sampling epoch, we average the outliers out from a linear fit on daily CV files. We analyze the results over more than 200 d of data. Over a short baseline between remote stations, we see that the average number of satellites left by the daily filtering for the CV computation is almost twice as large as the number obtained from the epoch filtering, increasing that way the robustness of the time transfer. On the short term stability of the time transfer, the white phase noise modulation is improved by a factor larger than the square root of 2, above what was expected. Over a long baseline, the improved short term stability is about twice smaller between 10 000 s and 1 d, together with an average increase of more than 20 per cent on the number of satellites left after filtering for the CV computation. We study the singular event which causes the discrepancy between these two criteria, and conclude that the daily approach is providing the best results in all cases. This automated filtering technique might be useful for any GNSS time transfer based on code data.

A simple computation technique for improving the short term stability and the robustness of GPS TAIP3 common-views

We present the current developments performed at LNE-SYRTE, to produce a new time reference, called UTC(OP)_Maser. This time reference aims at taking benefits of our clock ensemble: the short term stability of a hydrogen maser, the mid term stability of our ensemble of industrial cesium clocks, and the accuracy of our three cold atom fountains. This paper describes an algorithm that produces a timescale optimised in terms of stability over averaging periods of 1-2 months, with a weighted average of the cesium clocks data. We also present the steering processes, involving the fountain calibrations, to drive the timescale towards UTC, and to control the hydrogen maser. The simulations, based on past data, show an improvement of a factor of 5, in terms of time accuracy, compared to the current UTC(OP). The first experimental results demonstrate a gain of more than one order of magnitude on the short term stability.

Ongoing improvements of the time and frequency references at LNE-syrte

We present a direct comparison between two different techniques for the relative calibration of time transfer between remote time scales when using the signals transmitted by the Global Positioning System (GPS). In the remote sites, the local measurements are driving either the computation of the hardware delays of the local GPS equipment with respect to a given reference GPS station, a “receiver” calibration, or the computation of a global hardware offset between two distribution reference points of the remote time scales, a “link” calibration. Both techniques do not require the same measurements on site, and we discuss the uncertainty budget computation differences. We report on one calibration campaign organized during Autumn 2013 between Observatoire de Paris (OP), Paris, France, Observatoire de la Cote d'Azur (OCA), Plateau de Calern, France, and NERC Space Geodesy Facility (SGF), Herstmonceux, United Kingdom. We show the different ways to compute uncertainty budgets, leading to improvement factors of 1.2 to 1.5 on the hardware delay uncertainties when comparing the relative link calibration to the relative receiver calibration.

M. Abgrall

Papers

Progress in atomic fountains at LNE-SYRTE

The new UTC(OP) based on LNE-SYRTE atomic fountains

A simple computation technique for improving the short term stability and the robustness of GPS TAIP3 common-views

Ongoing improvements of the time and frequency references at LNE-syrte

Link calibration against receiver calibration time transfer uncertainty when using the Global Positioning System