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LETTERS
PUBLISHED ONLINE: 9 FEBRUARY 2015 | http://www.nature.com/doifinder/10.1038/nphoton.2015.5
Web End =DOI: 10.1038/NPHOTON.2015.5
Ichiro Ushijima1,2,3, Masao Takamoto1,2,4, Manoj Das1,2,4, Takuya Ohkubo1,2,3
and Hidetoshi Katori1,2,3,4*
The accuracy of atomic clocks relies on the superb reproducibility of atomic spectroscopy, which is accomplished by careful control and the elimination of environmental perturbations on atoms. To date, individual atomic clocks have achieved a 1018 level of total uncertainties1,2, but a two-clock comparison at the 1018 level has yet to be demonstrated. Here, we demonstrate optical lattice clocks with 87Sr atoms interrogated in a cryogenic environment to address the blackbody radiation-induced frequency shift3, which remains the primary source of systematic uncertainty2,46 and has initiated vigorous theoretical7,8 and experimental9,10 investigations. The systematic uncertainty for the cryogenic clock is evaluated to be7.2 1018, which is expedited by operating two such cryo-clocks synchronously11,12. After 11 measurements performed
over a month, statistical agreement between the two cryo-clocks reached 2.0 1018. Such clocks reproducibility is a major step towards developing accurate clocks at the low 1018 level, and is directly applicable as a means for relativistic geodesy13.
The accuracy of an atomic clock relies on the presumed constancy of fundamental constants14 and the decoupling of the electronic states from ambient electromagnetic perturbations15. Clocks
based on single ions, trapped near the zero of an electric quadrupole eld, are promising candidates as they do not suffer from trapping perturbations1,16, but the quantum projection noise (QPN)17 requires days of averaging time to achieve the anticipated accuracy. In contrast, optical lattice clocks exploit well-engineered electromagnetic perturbation with the magic wavelength protocol4,18 to
facilitate the observation of many atoms (N) simultaneously, thus reducing the averaging time by a factor of N. Clock comparison stabilities at the level of 1016 (/s)1/2 have been demonstrated by rejecting the laser noise11,12 or by applying lasers stabilized to
long2,6 or cryogenic19 cavities. Such stabilities enable 1018 uncertainty to be achieved in a few hours of clock operation, allowing extensive studies on systematic uncertainties, such as collisional shifts between spin-polarized fermions12, light shifts due to hyper-polarizability effects20 and multipolar interactions of atoms with optical lattices20,21. Although the magic wavelength protocol allows the cancellation of lattice-related Stark shift perturbations, the Stark shift caused by the ambient electric eld22, including blackbody radiation (BBR), remains a major perturbation in...