Optical frequency comparison of the ⁴⁰Ca⁺ clock transition ν_Ca (²S_1/2-²D_5/2, 729nm) against the ⁸⁷Sr optical lattice clock transition ν_Sr (¹S₀-³P₀, 698nm) has resulted in a frequency ratio ν_Ca / ν_Sr = 0.957 631 202 358 049 9(2 3). The rapid nature of optical comparison allowed the statistical uncertainty of frequency ratio ν_Ca / ν_Sr to reach 1 × 10⁻¹⁵ in 1000s and yielded a value consistent with that calculated from separate absolute frequency measurements of ν_Ca using the International Atomic Time (TAI) link. The total uncertainty of the frequency ratio using optical comparison (free from microwave link uncertainties) is smaller than that obtained using absolute frequency measurement, demonstrating the advantage of optical frequency evaluation. We note that the absolute frequency of ⁴⁰Ca⁺ we measure deviates from other published values by more than three times our measurement uncertainty.

© 2012 OSA

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Appl. Phys. B

• K. Hayasaka, “Synthesis of two-species ion chains for a new optical frequency standard with an indium ion,” Appl. Phys. B107(4), 965–970 (2012). [CrossRef]

Appl. Phys. Express

• K. Matsubara, K. Hayasaka, Y. Li, H. Ito, S. Nagano, M. Kajita, and M. Hosokawa, “Frequency measurement of the optical clock transition of 40Ca+ ions with an uncertainty of 10−14 level,” Appl. Phys. Express1, 067011 (2008). [CrossRef]
• A. Yamaguchi, M. Fujieda, M. Kumagai, H. Hachisu, S. Nagano, Y. Li, T. Ido, T. Takano, M. Takamoto, and H. Katori, “Direct comparison of distant optical lattice clocks at the 10−16 uncertainty,” Appl. Phys. Express4(8), 082203 (2011). [CrossRef]
• A. Yamaguchi, N. Shiga, S. Nagano, Y. Li, H. Ishijima, H. Hachisu, M. Kumagai, and T. Ido, “Stability transfer between two clock lasers operating at different wavelengths for absolute frequency measurement of clock transition in 87Sr,” Appl. Phys. Express5(2), 022701 (2012). [CrossRef]

Eur. Phys. J. D

• X. Baillard, M. Fouche, R. Le Targat, P. G. Westergaard, A. Lecallier, F. Chapelet, M. Abgrall, G. D. Rovera, P. Laurent, P. Rosenbusch, S. Bize, G. Santarelli, A. Clairon, P. Lemonde, G. Grosche, B. Lipphardt, and H. Schnatz, “An optical lattice clock with spin-polarized 87Sr atoms,” Eur. Phys. J. D48(1), 11–17 (2008). [CrossRef]

IEEE Trans. Instrum. Meas.

• C. Degenhardt, T. Nazarova, C. Lisdat, H. Stoehr, U. Sterr, and F. Riehle, “Influence of chirped excitation pulses in an optical clock with ultracold calcium atoms,” IEEE Trans. Instrum. Meas.54(2), 771–775 (2005). [CrossRef]
• F. Nakagawa, M. Imae, Y. Hanado, and M. Aida, “Development of multichannel dual-mixer time difference system to generate UTC(NICT),” IEEE Trans. Instrum. Meas.54(2), 829–832 (2005). [CrossRef]

IEEE Trans. Ultrason., Ferroelectr., Freq. Cont.

• J. Guéna, M. Abgrall, D. Rovera, P. Laurent, B. Chupin, M. Lours, G. Santarelli, P. Rosenbusch, M. E. Tobar, R. Li, K. Gibble, A. Clairon, and S. Bize, “Progress in atomic fountains at LNE-SYRTE,” IEEE Trans. Ultrason., Ferroelectr., Freq. Cont.59, 391–420 (2012).

J. Opt. Soc. Am. B

• P. A. Williams, W. C. Swann, and N. R. Newbury, “High-stability transfer of an optical frequency over long fiber-optic links,” J. Opt. Soc. Am. B25(8), 1284–1293 (2008). [CrossRef]

Jpn. J. Appl. Phys.

• Y. Li, S. Nagano, K. Matsubara, H. Ito, M. Kajita, and M. Hosokawa, “Narrow-line and frequency tunable diode laser system for S–D transition of Ca+ ions,” Jpn. J. Appl. Phys.47(8), 6327–6332 (2008). [CrossRef]
• S. Nagano, H. Ito, Y. Li, K. Matsubara, and M. Hosokawa, “Stable operation of femtosecond laser frequency combs with uncertainty at the 10−17 level toward optical frequency standards,” Jpn. J. Appl. Phys.48(4), 042301 (2009). [CrossRef]

Metrologia

• St. Falke, H. Schnatz, J. S. R. V. Winfred, T. Middelmann, S. Vogt, S. Weyers, B. Lipphardt, G. Grosche, F. Riehle, U. Sterr, and C. Lisdat, “The 87Sr optical frequency standard at PTB,” Metrologia48(5), 399–407 (2011). [CrossRef]
• G. K. Campbell, A. D. Ludlow, S. Blatt, J. W. Thomsen, M. J. Martin, M. H. G. de Miranda, T. Zelevinsky, M. M. Boyd, J. Ye, S. A. Diddams, T. P. Heavner, T. E. Parker, and S. R. Jefferts, “The absolute frequency of the 87Sr optical clock transition,” Metrologia45(5), 539–548 (2008). [CrossRef]

Nature

• M. Takamoto, F.-L. Hong, R. Higashi, and H. Katori, “An optical lattice clock,” Nature435(7040), 321–324 (2005). [CrossRef] [PubMed]

Opt. Express

• M. Fujieda, M. Kumagai, S. Nagano, A. Yamaguchi, H. Hachisu, and T. Ido, “All-optical link for direct comparison of distant optical clocks,” Opt. Express19(17), 16498–16507 (2011). [CrossRef] [PubMed]

Opt. Lett.

• L.-S. Ma, P. Jungner, J. Ye, and J. L. Hall, “Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Opt. Lett.19(21), 1777–1779 (1994). [CrossRef] [PubMed]
• F.-L. Hong, M. Musha, M. Takamoto, H. Inaba, S. Yanagimachi, A. Takamizawa, K. Watabe, T. Ikegami, M. Imae, Y. Fujii, M. Amemiya, K. Nakagawa, K. Ueda, and H. Katori, “Measuring the frequency of a Sr optical lattice clock using a 120 km coherent optical transfer,” Opt. Lett.34(5), 692–694 (2009). [CrossRef] [PubMed]

Phys. Rev. A

• Y. Huang, J. Cao, P. Liu, K. Liang, B. Ou, H. Guan, X. Huang, T. Li, and K. Gao, “Hertz-level measurement of the 40Ca+ 4s2S1/2–3d2D5/2 clock transition frequency with respect to the SI second through the Global Positioning System,” Phys. Rev. A85(3), 030503 (2012). [CrossRef]
• B. Arora, M. S. Safronova, and C. W. Clark, “Blackbody-radiation shift in a 43Ca+ ion optical frequency standard,” Phys. Rev. A76(6), 064501 (2007). [CrossRef]

Phys. Rev. Lett.

• G. P. Barwood, H. S. Margolis, G. Huang, P. Gill, and H. A. Klein, “Measurement of the electric quadrupole moment of the 4d2D5/2 level in 88Sr+.,” Phys. Rev. Lett.93(13), 133001 (2004). [CrossRef] [PubMed]
• M. Chwalla, J. Benhelm, K. Kim, G. Kirchmair, T. Monz, M. Riebe, P. Schindler, A. S. Villar, W. Hänsel, C. F. Roos, R. Blatt, M. Abgrall, G. Santarelli, G. D. Rovera, and Ph. Laurent, “Absolute frequency measurement of the 40Ca+ 4s2S1/2-3d2D5/2 clock transition,” Phys. Rev. Lett.102(2), 023002 (2009). [CrossRef] [PubMed]
• C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, “Frequency comparison of two high-accuracy Al+ optical clocks,” Phys. Rev. Lett.104(7), 070802 (2010). [CrossRef] [PubMed]

Science

• K. Predehl, G. Grosche, S. M. F. Raupach, S. Droste, O. Terra, J. Alnis, Th. Legero, T. W. Hänsch, Th. Udem, R. Holzwarth, and H. Schnatz, “A 920-kilometer optical fiber link for frequency metrology at the 19th decimal place,” Science336(6080), 441–444 (2012). [CrossRef] [PubMed]
• T. Rosenband, D. B. Hume, P. O. Schmidt, C. W. Chou, A. Brusch, L. Lorini, W. H. Oskay, R. E. Drullinger, T. M. Fortier, J. E. Stalnaker, S. A. Diddams, W. C. Swann, N. R. Newbury, W. M. Itano, D. J. Wineland, and J. C. Bergquist, “Frequency ratio of Al+ and Hg+ single-ion optical clocks; Metrology at the 17th decimal place,” Science319(5871), 1808–1812 (2008). [CrossRef] [PubMed]
• H. S. Margolis, G. P. Barwood, G. Huang, H. A. Klein, S. N. Lea, K. Szymaniec, and P. Gill, “Hertz-level measurement of the optical clock frequency in a single 88Sr+ ion,” Science306(5700), 1355–1358 (2004). [CrossRef] [PubMed]

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