Optical lattice clocks [1] benefit from a low quantum-projection noise by simultaneously interrogating a large number of atoms trapped in the standing wave of light (optical lattice) tuned to the “magic frequency” that mostly cancels out the light shift perturbation in the clock transition [2]. About a thousand atoms enable such clocks to achieve 10-18 instability in a few hours of operation [3-5], allowing intensive investigation and control of systematic uncertainties, such as multipolar and higher order light shifts [6] and the blackbody radiation shift [5]. It is now the uncertainty of the SI (International System of Units) second (~10-16) itself that restricts the absolute frequency measurements of such optical clocks [7, 8]. Direct comparisons of optical clocks are, therefore, the only way to demonstrate and utilize their superb performance beyond the SI second.
In this presentation, we report on frequency comparisons of optical lattice clocks with neutral strontium (87Sr), ytterbium (171Yb) and mercury (199Hg) atoms. By referencing cryogenic Sr clocks [5], we have determined the frequency ratios, R = νYb/νSr and νHg/νSr of a Yb clock and a Hg clock with uncertainty at the mid 10-17 [9]. Such ratios provide an access to search for temporal variation of the fundamental constants [10]. We also present remote comparisons of cryogenic Sr clocks located at RIKEN and the University of Tokyo over a 30-km-long phase-stabilized fiber link. The gravitational red shift Δν/ν0 ≈ 1.1×10-18 Δh cm-1 reads out the height difference of Δh~15 m between the two clocks with uncertainty of 5 cm [11], which demonstrates a step towards relativistic geodesy [12]. Finally, we mention our ongoing experiments that reduce clock uncertainty to 10-19 by applying “operational magic frequency” [13] that effectively cancels out higher-order light shifts arising from the dipole, multipolar, and hyper-polarizability effects for a certain range of lattice intensity.
References:
[1] H. Katori, Optical lattice clocks and quantum metrology, Nature Photon. 5, 203 (2011).
[2] H. Katori et al., Ultrastable optical clock with neutral atoms in an engineered light shift trap, Phys. Rev. Lett. 91, 173005 (2003).
[3] N. Hinkley et al., An atomic clock with 10-18 instability, Science 341, 1215 (2013).
[4] T. L. Nicholson et al., Systematic evaluation of an atomic clock at 2×10-18 total uncertainty, Nature Commun. 6, 6896 (2015).
[5] I. Ushijima et al., Cryogenic optical lattice clocks, Nature Photon. 9, 185 (2015).
[6] P. G. Westergaard et al., Lattice-Induced Frequency Shifts in Sr Optical Lattice Clocks at the 10-17 Level, Phys. Rev. Lett. 106, 210801 (2011).
[7] R. Le Targat et al., Experimental realization of an optical second with strontium lattice clocks, Nature Commun. 4, 2109 (2013).
[8] C. Grebing et al., Realization of a timescale with an accurate optical lattice clock, Optica 3, 563 (2016).
[9] N. Nemitz et al., Frequency ratio of Yb and Sr clocks with 5 × 10−17 uncertainty at 150 seconds averaging time, Nature Photon. 10, 258 (2016).
[10] J.-P. Uzan, The fundamental constants and their variation: observational and theoretical status, Rev. Mod. Phys. 75, 403 (2003).
[11] T. Takano et al., Geopotential measurements with synchronously linked optical lattice clocks, Nature Photon. 10, 662 (2016).
[12] C. Lisdat et al., A clock network for geodesy and fundamental science, Nature Commun. 7, 12443 (2016).
[13] H. Katori et al., Strategies for reducing the light shift in atomic clocks, Phys. Rev. A 91, 052503 (2015).
