Accurate Optical Lattice Clock withSr87Atoms

Abstract

We report a frequency measurement of the 1 S0 − 3 P0 transition of 87 Sr atoms in an optical lattice clock. The frequency is determined to be 429 228 004 229 879 (5) Hz with a fractional uncertainty that is comparable to state-of-the-art optical clocks with neutral atoms in free fall. Two previous measurements of this transition were found to disagree by about 2 × 10 −13 , i.e. almost four times the combined error bar, instilling doubt on the potential of optical lattice clocks to perform at a high accuracy level. In perfect agreement with one of these two values, our measurement essentially dissipates this doubt.PACS numbers: 06.30. Ft,42.50.Hz,42.62.Fi Recent advances in the field of optical frequency metrology make measurements with a fractional accuracy of 10 −17 or better a realistic short term goal [1]. Among other possible applications (e.g. a redefinition of the S.I. second, optical very long baseline interferometry in space, direct mapping of the earth gravitational using the Einstein effect,...), a very interesting prospect with measurements at that level is a reproducible test of Einstein Equivalence Principle by the repeated determination of the frequency ratio of different atomic and molecular transitions [2,3,4,5,6,7]. The topicality of such a test has recently been renewed by measurements at the cosmological scale which seem to indicate a slow variation of the electron to proton mass ratio [8]. The richness of the test directly depends on the performance of the clocks that are used but also on the variety of clock transitions and atomic species on which high accuracy frequency standards are based.In that context, optical lattice clocks are expected to play a central role in the future of this field. They use a large number of atoms confined in the Lamb-Dicke regime by an optical lattice in which the first order perturbation of the clock transition cancels [9]. Due to the lattice confinement motional effects, which set a severe limitation to standards with neutral atoms in free fall [10,11], essentially vanish [12]. This gives hope for a ultimate fractional accuracy better than 10 −17 . On the other hand, the large number of atoms in an optical lattice clock in principle opens the way to a short term fractional frequency stability significantly better than 10 −15 τ −1/2 with τ the averaging time in seconds. In this regime the coherence time of the laser frequency locked to the clock transition would be several seconds, possibly tens of seconds [25]. Such a long coherence time could for instance be used to reduce the width of the optical resonances in single ion clocks down to or below the 0.1 Hz range opening new prospects for these devices also. Finally, the optical lattice clock scheme is in principle applicable to a large number of atomic species (Sr, Yb, Hg, Ca, Mg,...) which is a key feature for the fundamental test discussed above. It is then particularly problematic that the frequency delivered by the first two evaluated optical lattice clocks, which both use 87 Sr, disagree by about 2 × ...

Experimental realization of an optical second with strontium lattice clocks

“…We could thus resolve the second-order light shift, equal to 0.45 (10) mHz/E 2 r for a lattice polarization parallel to the quantization axis. These large depths, equivalent to temperatures of 0.8 mK, enable the capture of 10 4 atoms in 500 ms directly from the first stage 2 mK magneto-optical trap based on the 1 S 0 -1 P 1 transition at 461 nm 12 . An extra cooling step is performed on the narrow line 1 S 0 -3 P 1 , before the potential is ramped down adiabatically to 100 E r .…”

“…We have built two OLCs 12,22,30 based on the transition at 698 nm between the ground state 1 S 0 and the metastable state 3 P 0 of neutral 87 Sr atoms, with a natural linewidth of 1 mHz. The atoms are tightly confined in a few thousand wells of an optical lattice formed by a standing wave in an optical resonator.…”

“…8). Research on optical lattice clocks (OLCs) is comparatively recent [9][10][11][12][13] , but in less than 10 years they have reached the 10 À 16 level [14][15][16] , with excellent prospects towards the 10 À 17 level. The large number of probed atoms, on the order of 10 4 , is a major asset: it already allows unprecedented statistical resolutions that translate into frequency stabilities of a few 10 À 16 at one second, demonstrated in some groups [17][18][19][20] .…”

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