Abstract

Optical clocks are not only powerful tools for prime fundamental research, but are also deemed for the redefinition of the SI base unit “second,” as they now surpass the performance of cesium atomic clocks in both accuracy and stability by more than an order of magnitude. However, an important obstacle in this transition has so far been the limited reliability of optical clocks, which made a continuous realization of a timescale impractical. In this paper, we demonstrate how this situation can be resolved and show that a timescale based on an optical clock can be established that is superior to one based on even the best cesium fountain clocks. The paper also gives further proof of the international consistency of strontium lattice clocks on the 1016 accuracy level, which is another prerequisite for a change in the definition of the second.

© 2016 Optical Society of America

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2016 (1)

N. Huntemann, C. Sanner, B. Lipphardt, C. Tamm, and E. Peik, “Single-ion atomic clock with 3 × 10−18 systematic uncertainty,” Phys. Rev. Lett. 116, 063001 (2016).
[Crossref]

2015 (10)

I. Ushijima, M. Takamoto, M. Das, T. Ohkubo, and H. Katori, “Cryogenic optical lattice clocks,” Nat. Photonics 9, 185–189 (2015).
[Crossref]

A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, and P. O. Schmidt, “Optical atomic clocks,” Rev. Mod. Phys. 87, 637–701 (2015).
[Crossref]

F. Riehle, “Towards a redefinition of the second based on optical atomic clocks,” C.R. Physique 16, 506–515 (2015), Special Issue on The Measurement of Time/La Mesure Du Temps.
[Crossref]

S. Häfner, S. Falke, C. Grebing, S. Vogt, T. Legero, M. Merimaa, C. Lisdat, and U. Sterr, “8 × 10−17 fractional laser frequency instability with a long room-temperature cavity,” Opt. Lett. 40, 2112–2115 (2015).
[Crossref]

A. Al-Masoudi, S. Dörscher, S. Häfner, U. Sterr, and C. Lisdat, “Noise and instability of an optical lattice clock,” Phys. Rev. A 92, 063814 (2015).
[Crossref]

E. Benkler, C. Lisdat, and U. Sterr, “On the relation between uncertainties of weighted frequency averages and the various types of Allan deviations,” Metrologia 52, 565–574 (2015).
[Crossref]

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

H. Hachisu and T. Ido, “Intermittent optical frequency measurements to reduce the dead time uncertainty of frequency link,” Jpn. J. Appl. Phys. 54, 112401 (2015).
[Crossref]

T. Tanabe, D. Akamatsu, T. Kobayashi, A. Takamizawa, S. Yanagimachi, T. Ikegami, T. Suzuyama, H. Inaba, S. Okubo, M. Yasuda, F.-L. Hong, A. Onae, and K. Hosaka, “Improved frequency measurement of the 1S0–3P0 clock transition in 87Sr using a Cs fountain clock as a transfer oscillator,” J. Phys. Soc. Jpn. 84, 115002 (2015).
[Crossref]

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
[Crossref]

2014 (9)

D. Akamatsu, H. Inaba, K. Hosaka, M. Yasuda, A. Onae, T. Suzuyama, M. Amemiya, and F.-L. Hong, “Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb,” Appl. Phys. Express 7, 012401 (2014).
[Crossref]

H. Hachisu, M. Fujieda, S. Nagano, T. Gotoh, A. Nogami, T. Ido, S. Falke, N. Huntemann, C. Grebing, B. Lipphardt, C. Lisdat, and D. Piester, “Direct comparison of optical lattice clocks with an intercontinental baseline of 9000  km,” Opt. Lett. 39, 4072–4075 (2014).
[Crossref]

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
[Crossref]

N. Huntemann, B. Lipphardt, C. Tamm, V. Gerginov, S. Weyers, and E. Peik, “Improved limit on a temporal variation of mp/me from comparisons of Yb+ and Cs atomic clocks,” Phys. Rev. Lett. 113, 210802 (2014).
[Crossref]

R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M. Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K. Bongs, and P. Gill, “Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time-variation of fundamental constants,” Phys. Rev. Lett. 113, 210801 (2014).
[Crossref]

H. Margolis, “Timekeepers of the future,” Nat. Phys. 10, 82–83 (2014).

S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
[Crossref]

G. Petit, F. Arias, A. Harmegnies, G. Panfilo, and L. Tisserand, “UTCr: a rapid realization of UTC,” Metrologia 51, 33–39 (2014).
[Crossref]

C. Hagemann, C. Grebing, C. Lisdat, S. Falke, T. Legero, U. Sterr, F. Riehle, M. J. Martin, and J. Ye, “Ultra-stable laser with average fractional frequency drift rate below 5 × 10−19/s,” Opt. Lett. 39, 5102–5105 (2014).
[Crossref]

2013 (5)

S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
[Crossref]

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

P. Dubé, A. A. Madej, Z. Zhou, and J. E. Bernard, “Evaluation of systematic shifts of the 88Sr+ single-ion optical frequency standard at the 10−17 level,” Phys. Rev. A 87, 023806 (2013).
[Crossref]

N. Hinkley, J. A. Sherman, N. B. Phillips, M. Schioppo, N. D. Lemke, K. Beloy, M. Pizzocaro, C. W. Oates, and A. D. Ludlow, “An atomic clock with 10−18 instability,” Science 341, 1215–1218 (2013).
[Crossref]

Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
[Crossref]

2012 (9)

T. E. Parker, “Invited review article: the uncertainty in the realization and dissemination of the SI second from a systems point of view,” Rev. Sci. Instrum. 83, 021102 (2012).
[Crossref]

A. Bauch, S. Weyers, D. Piester, E. Staliuniene, and W. Yang, “Generation of UTC(PTB) as a fountain-clock based time scale,” Metrologia 49, 180–188 (2012).
[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. Express 5, 022701 (2012).
[Crossref]

K. Matsubara, H. Hachisu, Y. Li, S. Nagano, C. Locke, A. Nohgami, M. Kajita, K. Hayasaka, T. Ido, and M. Hosokawa, “Direct comparison of a Ca+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement,” Opt. Express 20, 22034–22041 (2012).
[Crossref]

J. Levine, “Invited review article: the statistical modeling of atomic clocks and the design of time scales,” Rev. Sci. Instrum. 83, 021101 (2012).
[Crossref]

S. Weyers, V. Gerginov, N. Nemitz, R. Li, and K. Gibble, “Distributed cavity phase frequency shifts of the caesium fountain PTB-CSF2,” Metrologia 49, 82–87 (2012).
[Crossref]

T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
[Crossref]

G. P. Barwood, P. Gill, G. Huang, and H. A. Klein, “Automatic laser control for a 88Sr+ optical frequency standard,” Meas. Sci. Technol. 23, 055201 (2012).
[Crossref]

J. G. Hartnett, N. R. Nand, and C. Lu, “Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators,” Appl. Phys. Lett. 100, 183501 (2012).
[Crossref]

2011 (2)

M. J. Thorpe, L. Rippe, T. M. Fortier, M. S. Kirchner, and T. Rosenband, “Frequency-stabilization to 6 × 10−16 via spectral-hole burning,” Nat. Photonics 5, 688–693 (2011).
[Crossref]

S. Falke, H. Schnatz, J. S. R. Vellore 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,” Metrologia 48, 399–407 (2011).
[Crossref]

2010 (3)

V. Gerginov, N. Nemitz, S. Weyers, R. Schröder, R. D. Griebsch, and R. Wynands, “Uncertainty evaluation of the caesium fountain clock PTB-CSF2,” Metrologia 47, 65–79 (2010).
[Crossref]

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, 070802 (2010).
[Crossref]

G. Panfilo and T. E. Parker, “A theoretical and experimental analysis of frequency transfer uncertainty, including frequency transfer into TAI,” Metrologia 47, 552–560 (2010).
[Crossref]

2009 (2)

2008 (2)

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[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,” Metrologia 45, 539–548 (2008).
[Crossref]

2007 (2)

M. M. Boyd, A. D. Ludlow, S. Blatt, S. M. Foreman, T. Ido, T. Zelevinsky, and J. Ye, “87Sr lattice clock with inaccuracy below 10−15,” Phys. Rev. Lett. 98, 083002 (2007).
[Crossref]

D.-H. Yu, M. Weiss, and T. E. Parker, “Uncertainty of a frequency comparison with distributed dead time and measurement interval offset,” Metrologia 44, 91–96 (2007).
[Crossref]

2001 (1)

S. Weyers, U. Hübner, R. Schröder, C. Tamm, and A. Bauch, “Uncertainty evaluation of the atomic caesium fountain CSF1 of the PTB,” Metrologia 38, 343–352 (2001).
[Crossref]

1998 (1)

Abgrall, M.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Akamatsu, D.

T. Tanabe, D. Akamatsu, T. Kobayashi, A. Takamizawa, S. Yanagimachi, T. Ikegami, T. Suzuyama, H. Inaba, S. Okubo, M. Yasuda, F.-L. Hong, A. Onae, and K. Hosaka, “Improved frequency measurement of the 1S0–3P0 clock transition in 87Sr using a Cs fountain clock as a transfer oscillator,” J. Phys. Soc. Jpn. 84, 115002 (2015).
[Crossref]

D. Akamatsu, H. Inaba, K. Hosaka, M. Yasuda, A. Onae, T. Suzuyama, M. Amemiya, and F.-L. Hong, “Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb,” Appl. Phys. Express 7, 012401 (2014).
[Crossref]

Al-Masoudi, A.

A. Al-Masoudi, S. Dörscher, S. Häfner, U. Sterr, and C. Lisdat, “Noise and instability of an optical lattice clock,” Phys. Rev. A 92, 063814 (2015).
[Crossref]

S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
[Crossref]

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Amemiya, M.

D. Akamatsu, H. Inaba, K. Hosaka, M. Yasuda, A. Onae, T. Suzuyama, M. Amemiya, and F.-L. Hong, “Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb,” Appl. Phys. Express 7, 012401 (2014).
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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, 692–694 (2009).
[Crossref]

Amy-Klein, A.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Arias, F.

G. Petit, F. Arias, A. Harmegnies, G. Panfilo, and L. Tisserand, “UTCr: a rapid realization of UTC,” Metrologia 51, 33–39 (2014).
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Baillard, X.

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

Barrett, M. D.

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
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Barwood, G. P.

G. P. Barwood, P. Gill, G. Huang, and H. A. Klein, “Automatic laser control for a 88Sr+ optical frequency standard,” Meas. Sci. Technol. 23, 055201 (2012).
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Bauch, A.

A. Bauch, S. Weyers, D. Piester, E. Staliuniene, and W. Yang, “Generation of UTC(PTB) as a fountain-clock based time scale,” Metrologia 49, 180–188 (2012).
[Crossref]

S. Weyers, U. Hübner, R. Schröder, C. Tamm, and A. Bauch, “Uncertainty evaluation of the atomic caesium fountain CSF1 of the PTB,” Metrologia 38, 343–352 (2001).
[Crossref]

S. Weyers, A. Bauch, R. Schröder, and C. Tamm, “The atomic caesium fountain CSF1 of PTB,” in Frequency Standards and Metrology, Proceedings of the Sixth Symposium, P. Gill, ed. (World Scientific, 2002), pp. 64–71.

Beloy, K.

N. Hinkley, J. A. Sherman, N. B. Phillips, M. Schioppo, N. D. Lemke, K. Beloy, M. Pizzocaro, C. W. Oates, and A. D. Ludlow, “An atomic clock with 10−18 instability,” Science 341, 1215–1218 (2013).
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Benkler, E.

E. Benkler, C. Lisdat, and U. Sterr, “On the relation between uncertainties of weighted frequency averages and the various types of Allan deviations,” Metrologia 52, 565–574 (2015).
[Crossref]

Bernard, J. E.

P. Dubé, A. A. Madej, Z. Zhou, and J. E. Bernard, “Evaluation of systematic shifts of the 88Sr+ single-ion optical frequency standard at the 10−17 level,” Phys. Rev. A 87, 023806 (2013).
[Crossref]

Bilicki, S.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Bishof, M.

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
[Crossref]

Bize, S.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

Blatt, S.

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,” Metrologia 45, 539–548 (2008).
[Crossref]

M. M. Boyd, A. D. Ludlow, S. Blatt, S. M. Foreman, T. Ido, T. Zelevinsky, and J. Ye, “87Sr lattice clock with inaccuracy below 10−15,” Phys. Rev. Lett. 98, 083002 (2007).
[Crossref]

Bloom, B. J.

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
[Crossref]

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
[Crossref]

Bongs, K.

R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M. Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K. Bongs, and P. Gill, “Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time-variation of fundamental constants,” Phys. Rev. Lett. 113, 210801 (2014).
[Crossref]

Bookjans, E.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Boulanger, J.-S.

R. J. Douglas and J.-S. Boulanger, “Standard uncertainty for average frequency traceability,” in Proceedings of the 11th European Frequency and Time Forum (EFTF), Neuchâtel, Switzerland, March4–7, 1997, pp. 345–349.

Boyd, M. M.

A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, and P. O. Schmidt, “Optical atomic clocks,” Rev. Mod. Phys. 87, 637–701 (2015).
[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,” Metrologia 45, 539–548 (2008).
[Crossref]

M. M. Boyd, A. D. Ludlow, S. Blatt, S. M. Foreman, T. Ido, T. Zelevinsky, and J. Ye, “87Sr lattice clock with inaccuracy below 10−15,” Phys. Rev. Lett. 98, 083002 (2007).
[Crossref]

Braxmaier, C.

Bromley, S. L.

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
[Crossref]

Buczek, L.

Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
[Crossref]

Cacciapuoti, L.

L. Cacciapuoti and C. Salomon, “Space clocks and fundamental tests: the ACES experiment,” Eur. Phys. J. Spec. Top. 172, 57–68 (2009).
[Crossref]

Camisard, E.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Campbell, G. K.

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,” Metrologia 45, 539–548 (2008).
[Crossref]

Campbell, S. L.

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
[Crossref]

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
[Crossref]

Chapelet, F.

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

Chardonnet, C.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Chen, L.

T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
[Crossref]

Chiodo, N.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Chou, C. W.

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, 070802 (2010).
[Crossref]

Clairon, A.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

Czubla, A.

Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
[Crossref]

Das, M.

I. Ushijima, M. Takamoto, M. Das, T. Ohkubo, and H. Katori, “Cryogenic optical lattice clocks,” Nat. Photonics 9, 185–189 (2015).
[Crossref]

de Miranda, M. H. G.

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,” Metrologia 45, 539–548 (2008).
[Crossref]

Denker, H.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Diddams, S. A.

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,” Metrologia 45, 539–548 (2008).
[Crossref]

Dörscher, S.

A. Al-Masoudi, S. Dörscher, S. Häfner, U. Sterr, and C. Lisdat, “Noise and instability of an optical lattice clock,” Phys. Rev. A 92, 063814 (2015).
[Crossref]

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Douglas, R. J.

R. J. Douglas and J.-S. Boulanger, “Standard uncertainty for average frequency traceability,” in Proceedings of the 11th European Frequency and Time Forum (EFTF), Neuchâtel, Switzerland, March4–7, 1997, pp. 345–349.

Dubé, P.

P. Dubé, A. A. Madej, Z. Zhou, and J. E. Bernard, “Evaluation of systematic shifts of the 88Sr+ single-ion optical frequency standard at the 10−17 level,” Phys. Rev. A 87, 023806 (2013).
[Crossref]

Falke, S.

Fang, F.

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Fang, Z.-J.

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J. Leute, N. Huntemann, B. Lipphardt, C. Tamm, P. B. R. Nisbet-Jones, S. A. King, R. M. Godun, J. M. Jones, H. S. Margolis, P. B. Whibberley, A. Wallin, M. Merimaa, P. Gill, and E. Peik, “Frequency comparison of 171Yb+ ion optical clocks at PTB and NPL via GPS PPP,” arXiv:1507.04754v1 (2015).

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Katori, H.

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S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
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T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
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R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M. Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K. Bongs, and P. Gill, “Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time-variation of fundamental constants,” Phys. Rev. Lett. 113, 210801 (2014).
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J. Leute, N. Huntemann, B. Lipphardt, C. Tamm, P. B. R. Nisbet-Jones, S. A. King, R. M. Godun, J. M. Jones, H. S. Margolis, P. B. Whibberley, A. Wallin, M. Merimaa, P. Gill, and E. Peik, “Frequency comparison of 171Yb+ ion optical clocks at PTB and NPL via GPS PPP,” arXiv:1507.04754v1 (2015).

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M. J. Thorpe, L. Rippe, T. M. Fortier, M. S. Kirchner, and T. Rosenband, “Frequency-stabilization to 6 × 10−16 via spectral-hole burning,” Nat. Photonics 5, 688–693 (2011).
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G. P. Barwood, P. Gill, G. Huang, and H. A. Klein, “Automatic laser control for a 88Sr+ optical frequency standard,” Meas. Sci. Technol. 23, 055201 (2012).
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T. Tanabe, D. Akamatsu, T. Kobayashi, A. Takamizawa, S. Yanagimachi, T. Ikegami, T. Suzuyama, H. Inaba, S. Okubo, M. Yasuda, F.-L. Hong, A. Onae, and K. Hosaka, “Improved frequency measurement of the 1S0–3P0 clock transition in 87Sr using a Cs fountain clock as a transfer oscillator,” J. Phys. Soc. Jpn. 84, 115002 (2015).
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S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
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C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

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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, 070802 (2010).
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C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Krehlik, P.

Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
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Kuhl, A.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Kumagai, M.

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. Express 5, 022701 (2012).
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Laurent, P.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
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Le Coq, Y.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Le Targat, R.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Lea, S. N.

R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M. Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K. Bongs, and P. Gill, “Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time-variation of fundamental constants,” Phys. Rev. Lett. 113, 210801 (2014).
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Lecallier, A.

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
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Legero, T.

S. Häfner, S. Falke, C. Grebing, S. Vogt, T. Legero, M. Merimaa, C. Lisdat, and U. Sterr, “8 × 10−17 fractional laser frequency instability with a long room-temperature cavity,” Opt. Lett. 40, 2112–2115 (2015).
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C. Hagemann, C. Grebing, C. Lisdat, S. Falke, T. Legero, U. Sterr, F. Riehle, M. J. Martin, and J. Ye, “Ultra-stable laser with average fractional frequency drift rate below 5 × 10−19/s,” Opt. Lett. 39, 5102–5105 (2014).
[Crossref]

S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
[Crossref]

T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
[Crossref]

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Lemke, N.

S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
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N. Hinkley, J. A. Sherman, N. B. Phillips, M. Schioppo, N. D. Lemke, K. Beloy, M. Pizzocaro, C. W. Oates, and A. D. Ludlow, “An atomic clock with 10−18 instability,” Science 341, 1215–1218 (2013).
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Lemonde, P.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
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Leute, J.

J. Leute, N. Huntemann, B. Lipphardt, C. Tamm, P. B. R. Nisbet-Jones, S. A. King, R. M. Godun, J. M. Jones, H. S. Margolis, P. B. Whibberley, A. Wallin, M. Merimaa, P. Gill, and E. Peik, “Frequency comparison of 171Yb+ ion optical clocks at PTB and NPL via GPS PPP,” arXiv:1507.04754v1 (2015).

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S. Weyers, V. Gerginov, N. Nemitz, R. Li, and K. Gibble, “Distributed cavity phase frequency shifts of the caesium fountain PTB-CSF2,” Metrologia 49, 82–87 (2012).
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Li, T.-C.

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

Li, Y.

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

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. Express 5, 022701 (2012).
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K. Matsubara, H. Hachisu, Y. Li, S. Nagano, C. Locke, A. Nohgami, M. Kajita, K. Hayasaka, T. Ido, and M. Hosokawa, “Direct comparison of a Ca+ single-ion clock against a Sr lattice clock to verify the absolute frequency measurement,” Opt. Express 20, 22034–22041 (2012).
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Lin, B.-K.

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

Lin, Y.-G.

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

Lipinski, M.

Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
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Lipphardt, B.

N. Huntemann, C. Sanner, B. Lipphardt, C. Tamm, and E. Peik, “Single-ion atomic clock with 3 × 10−18 systematic uncertainty,” Phys. Rev. Lett. 116, 063001 (2016).
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H. Hachisu, M. Fujieda, S. Nagano, T. Gotoh, A. Nogami, T. Ido, S. Falke, N. Huntemann, C. Grebing, B. Lipphardt, C. Lisdat, and D. Piester, “Direct comparison of optical lattice clocks with an intercontinental baseline of 9000  km,” Opt. Lett. 39, 4072–4075 (2014).
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S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
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N. Huntemann, B. Lipphardt, C. Tamm, V. Gerginov, S. Weyers, and E. Peik, “Improved limit on a temporal variation of mp/me from comparisons of Yb+ and Cs atomic clocks,” Phys. Rev. Lett. 113, 210802 (2014).
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S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
[Crossref]

S. Falke, H. Schnatz, J. S. R. Vellore 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,” Metrologia 48, 399–407 (2011).
[Crossref]

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
[Crossref]

J. Leute, N. Huntemann, B. Lipphardt, C. Tamm, P. B. R. Nisbet-Jones, S. A. King, R. M. Godun, J. M. Jones, H. S. Margolis, P. B. Whibberley, A. Wallin, M. Merimaa, P. Gill, and E. Peik, “Frequency comparison of 171Yb+ ion optical clocks at PTB and NPL via GPS PPP,” arXiv:1507.04754v1 (2015).

Lisdat, C.

E. Benkler, C. Lisdat, and U. Sterr, “On the relation between uncertainties of weighted frequency averages and the various types of Allan deviations,” Metrologia 52, 565–574 (2015).
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A. Al-Masoudi, S. Dörscher, S. Häfner, U. Sterr, and C. Lisdat, “Noise and instability of an optical lattice clock,” Phys. Rev. A 92, 063814 (2015).
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S. Häfner, S. Falke, C. Grebing, S. Vogt, T. Legero, M. Merimaa, C. Lisdat, and U. Sterr, “8 × 10−17 fractional laser frequency instability with a long room-temperature cavity,” Opt. Lett. 40, 2112–2115 (2015).
[Crossref]

H. Hachisu, M. Fujieda, S. Nagano, T. Gotoh, A. Nogami, T. Ido, S. Falke, N. Huntemann, C. Grebing, B. Lipphardt, C. Lisdat, and D. Piester, “Direct comparison of optical lattice clocks with an intercontinental baseline of 9000  km,” Opt. Lett. 39, 4072–4075 (2014).
[Crossref]

C. Hagemann, C. Grebing, C. Lisdat, S. Falke, T. Legero, U. Sterr, F. Riehle, M. J. Martin, and J. Ye, “Ultra-stable laser with average fractional frequency drift rate below 5 × 10−19/s,” Opt. Lett. 39, 5102–5105 (2014).
[Crossref]

S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
[Crossref]

S. Falke, H. Schnatz, J. S. R. Vellore 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,” Metrologia 48, 399–407 (2011).
[Crossref]

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Locke, C.

Lodewyck, J.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Lopez, O.

C. Lisdat, G. Grosche, N. Quintin, C. Shi, S. M. F. Raupach, C. Grebing, D. Nicolodi, F. Stefani, A. Al-Masoudi, S. Dörscher, S. Häfner, J. L. Robyr, N. Chiodo, S. Bilicki, E. Bookjans, A. Koczwara, S. Koke, A. Kuhl, F. Wiotte, F. Meynadier, E. Camisard, M. Abgrall, M. Lours, T. Legero, H. Schnatz, U. Sterr, H. Denker, C. Chardonnet, Y. Le Coq, G. Santarelli, A. Amy-Klein, R. Le Targat, J. Lodewyck, O. Lopez, and P.-E. Pottie, “A clock network for geodesy and fundamental science,” arXiv:1511.07735v1 (2015).

Lorini, L.

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Yang, W.

A. Bauch, S. Weyers, D. Piester, E. Staliuniene, and W. Yang, “Generation of UTC(PTB) as a fountain-clock based time scale,” Metrologia 49, 180–188 (2012).
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Yasuda, M.

T. Tanabe, D. Akamatsu, T. Kobayashi, A. Takamizawa, S. Yanagimachi, T. Ikegami, T. Suzuyama, H. Inaba, S. Okubo, M. Yasuda, F.-L. Hong, A. Onae, and K. Hosaka, “Improved frequency measurement of the 1S0–3P0 clock transition in 87Sr using a Cs fountain clock as a transfer oscillator,” J. Phys. Soc. Jpn. 84, 115002 (2015).
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D. Akamatsu, H. Inaba, K. Hosaka, M. Yasuda, A. Onae, T. Suzuyama, M. Amemiya, and F.-L. Hong, “Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb,” Appl. Phys. Express 7, 012401 (2014).
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Ye, J.

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
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A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik, and P. O. Schmidt, “Optical atomic clocks,” Rev. Mod. Phys. 87, 637–701 (2015).
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B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
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C. Hagemann, C. Grebing, C. Lisdat, S. Falke, T. Legero, U. Sterr, F. Riehle, M. J. Martin, and J. Ye, “Ultra-stable laser with average fractional frequency drift rate below 5 × 10−19/s,” Opt. Lett. 39, 5102–5105 (2014).
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T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
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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,” Metrologia 45, 539–548 (2008).
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M. M. Boyd, A. D. Ludlow, S. Blatt, S. M. Foreman, T. Ido, T. Zelevinsky, and J. Ye, “87Sr lattice clock with inaccuracy below 10−15,” Phys. Rev. Lett. 98, 083002 (2007).
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Yu, D.-H.

D.-H. Yu, M. Weiss, and T. E. Parker, “Uncertainty of a frequency comparison with distributed dead time and measurement interval offset,” Metrologia 44, 91–96 (2007).
[Crossref]

Zang, E.-J.

Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

Zawada, M.

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

Zelevinsky, T.

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,” Metrologia 45, 539–548 (2008).
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M. M. Boyd, A. D. Ludlow, S. Blatt, S. M. Foreman, T. Ido, T. Zelevinsky, and J. Ye, “87Sr lattice clock with inaccuracy below 10−15,” Phys. Rev. Lett. 98, 083002 (2007).
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Zhang, W.

T. L. Nicholson, S. L. Campbell, R. B. Hutson, G. E. Marti, B. J. Bloom, R. L. McNally, W. Zhang, M. D. Barrett, M. S. Safronova, G. F. Strouse, W. L. Tew, and J. Ye, “Systematic evaluation of an atomic clock at 2 × 10−18 total uncertainty,” Nat. Commun. 6, 6896 (2015).
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B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
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Zhang, X.

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
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Zhou, Z.

P. Dubé, A. A. Madej, Z. Zhou, and J. E. Bernard, “Evaluation of systematic shifts of the 88Sr+ single-ion optical frequency standard at the 10−17 level,” Phys. Rev. A 87, 023806 (2013).
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Appl. Phys. B (1)

S. M. F. Raupach, T. Legero, C. Grebing, C. Hagemann, T. Kessler, A. Koczwara, B. Lipphardt, M. Misera, H. Schnatz, G. Grosche, and U. Sterr, “Subhertz-linewidth infrared frequency source with a long-term instability below 5 × 10−15,” Appl. Phys. B 110, 465–470 (2013).
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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. Express 5, 022701 (2012).
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D. Akamatsu, H. Inaba, K. Hosaka, M. Yasuda, A. Onae, T. Suzuyama, M. Amemiya, and F.-L. Hong, “Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb,” Appl. Phys. Express 7, 012401 (2014).
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Appl. Phys. Lett. (1)

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Y.-G. Lin, Q. Wang, Y. Li, F. Meng, B.-K. Lin, E.-J. Zang, Z. Sun, F. Fang, T.-C. Li, and Z.-J. Fang, “First evaluation and frequency measurement of the strontium optical lattice clock at NIM,” Chin. Phys. Lett. 32, 090601 (2015).

Eur. Phys. J. D (1)

X. Baillard, M. Fouché, R. L. 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. D 48, 11–17 (2008).
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H. Hachisu and T. Ido, “Intermittent optical frequency measurements to reduce the dead time uncertainty of frequency link,” Jpn. J. Appl. Phys. 54, 112401 (2015).
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G. Panfilo and T. E. Parker, “A theoretical and experimental analysis of frequency transfer uncertainty, including frequency transfer into TAI,” Metrologia 47, 552–560 (2010).
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S. Falke, H. Schnatz, J. S. R. Vellore 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,” Metrologia 48, 399–407 (2011).
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D.-H. Yu, M. Weiss, and T. E. Parker, “Uncertainty of a frequency comparison with distributed dead time and measurement interval offset,” Metrologia 44, 91–96 (2007).
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V. Gerginov, N. Nemitz, S. Weyers, R. Schröder, R. D. Griebsch, and R. Wynands, “Uncertainty evaluation of the caesium fountain clock PTB-CSF2,” Metrologia 47, 65–79 (2010).
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S. Weyers, V. Gerginov, N. Nemitz, R. Li, and K. Gibble, “Distributed cavity phase frequency shifts of the caesium fountain PTB-CSF2,” Metrologia 49, 82–87 (2012).
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E. Benkler, C. Lisdat, and U. Sterr, “On the relation between uncertainties of weighted frequency averages and the various types of Allan deviations,” Metrologia 52, 565–574 (2015).
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G. Petit, F. Arias, A. Harmegnies, G. Panfilo, and L. Tisserand, “UTCr: a rapid realization of UTC,” Metrologia 51, 33–39 (2014).
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A. Bauch, S. Weyers, D. Piester, E. Staliuniene, and W. Yang, “Generation of UTC(PTB) as a fountain-clock based time scale,” Metrologia 49, 180–188 (2012).
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Ł. Śliwczyński, P. Krehlik, A. Czubla, Ł. Buczek, and M. Lipiński, “Dissemination of time and RF frequency via a stabilized fibre optic link over a distance of 420  km,” Metrologia 50, 133–145 (2013).
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Nat. Commun. (2)

R. Le Targat, L. Lorini, Y. Le Coq, M. Zawada, J. Guéna, M. Abgrall, M. Gurov, P. Rosenbusch, D. G. Rovera, B. Nagórny, R. Gartman, P. G. Westergaard, M. E. Tobar, M. Lours, G. Santarelli, A. Clairon, S. Bize, P. Laurent, P. Lemonde, and J. Lodewyck, “Experimental realization of an optical second with strontium lattice clocks,” Nat. Commun. 4, 2109 (2013).

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[Crossref]

Nat. Photonics (3)

T. Kessler, C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M. J. Martin, L. Chen, and J. Ye, “A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity,” Nat. Photonics 6, 687–692 (2012).
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M. J. Thorpe, L. Rippe, T. M. Fortier, M. S. Kirchner, and T. Rosenband, “Frequency-stabilization to 6 × 10−16 via spectral-hole burning,” Nat. Photonics 5, 688–693 (2011).
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Nat. Phys. (1)

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Nature (1)

B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014).
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New J. Phys. (1)

S. Falke, N. Lemke, C. Grebing, B. Lipphardt, S. Weyers, V. Gerginov, N. Huntemann, C. Hagemann, A. Al-Masoudi, S. Häfner, S. Vogt, U. Sterr, and C. Lisdat, “A strontium lattice clock with 3 × 10−17 inaccuracy and its frequency,” New J. Phys. 16, 073023 (2014).
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Supplementary Material (1)

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Figures (5)

Fig. 1.
Fig. 1. Realization of a timescale TS from a microwave and an optical clock: The Cs clock transition frequency is compared against the maser flywheel frequency. The acquired offset y Cs y H is used to correct the classical timescale TS(Cs) generated from the maser utilizing a phase stepper ( Δ φ ). An equivalent scheme is applicable when referencing the timescale TS(Sr) to an optical frequency standard. For that purpose, the clock laser light is down-converted to the microwave regime using a femtosecond frequency comb (FC) before comparing against the flywheel. Moreover, both maser offsets can be analyzed to yield the Sr clock frequency in SI units.
Fig. 2.
Fig. 2. Stability as represented by the Allan deviation σ of the relevant oscillators. Solid black line, fountain clock CSF2; dashed black line, Sr lattice clock; data points, measured maser stability (triangles: versus lattice clock; diamonds: maser comparisons; empty dots: against fountain clocks; filled dots: ditto without linear drift); solid (dashed) green line, noise model for the maser with (without) linear drift removed. The inset again shows the stabilities of the maser (green) and fountain clock (black). The red curve shows the additional uncertainty u ext when the maser is used as a flywheel and data is extrapolated from an interval T Sr = 267 000    s to T ext . Dashed vertical lines indicate the direct and the optimum extrapolation measurement times.
Fig. 3.
Fig. 3. Results achieved during measurement campaign 2014. (a) Frequency deviation between the nominal 100 MHz maser output and the Sr lattice clock averaged over 10 s assuming the Sr clock transition frequency is equal to the recommendation of the CIPM for the secondary representation of the “second” [26], in total 267 000 s. (b) Weighting function used to derive the calibration uncertainty of the hydrogen maser’s frequency with respect to the Sr lattice clock for an interval of 10 6    s . (c) Estimated 1- σ time uncertainty range of TS(Sr) (red shaded area) and TS(Cs) (gray shaded area) including statistical and systematic contributions. The red solid line depicts the time error of a simulated timescale realization TS(Sr) with respect to an ideal reference; it is shown starting with the first corrected interval [ t = 0 corresponds to the Modified Julian Date (MJD) t 56934.6 ].
Fig. 4.
Fig. 4. (a) Comparison of measured absolute frequencies of the 5 s 2 S 0 1 5 s 5 p P 0 3 transition in Sr 87 . The values have been obtained from various references [3,14,2738]. PTB 14 and PTB 15 have been obtained in this work. The vertical line indicates the frequency recommended by the CIPM in 2013 for the secondary representation of the second by Sr lattice clocks [26]; the dashed lines show the assigned uncertainty. (b) Listing of the systematic uncertainty u B of the best Sr clocks worldwide [9,3640]. The gray line indicates the Cs systematic uncertainty of the absolute frequency measurement with the smallest overall uncertainty so far [3].
Fig. 5.
Fig. 5. Results achieved during measurement campaign 2015. (a) Frequency deviation between maser and the Sr clock. (b) 1- σ time uncertainty range of TS(Sr) (red) and TS(Cs) (gray), including statistical and systematic contributions.

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