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:
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[13] H. Katori et al., Strategies for reducing the light shift in atomic clocks, Phys. Rev. A 91, 052503 (2015).