1208.4654 (Kensuke Matsubara et al.)

Direct comparison of a Ca+ single ion clock against a Sr optical lattice
clock
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Kensuke Matsubara, Hidekazu Hachisu, Ying Li, Shigeo Nagano, Clayton Locke, Asahiko Nogami, Masatoshi Kajita, Kazuhiro Hayasaka, Tetsuya Ido, Mizuhiko Hosokawa