[1] J. A. Scheer, J. L. Kurtz. Coherent Radar Performance Estimation(1993).

[2] T. M. Fortier, M. S. Kirchner, F. Quinlan, J. Taylor, J. C. Bergquist, T. Rosenband, N. Lemke, A. Ludlow, Y. Jiang, C. W. Oates, S. A. Diddams. Generation of ultrastable microwaves via optical frequency division. Nat. Photonics, 5, 425-429(2011).

[3] T. Nakamura, J. Davila-Rodriguez, H. Leopardi, J. A. Sherman, T. M. Fortier, X. Xie, J. C. Campbell, W. F. McGrew, X. Zhang, Y. S. Hassan, D. Nicolodi, K. Beloy, A. D. Ludlow, S. A. Diddams, F. Quinlan. Coherent optical clock down-conversion for microwave frequencies with 10^–18 instability. Science, 368, 889-892(2020).

[4] B. C. Young, F. C. Cruz, W. M. Itano, J. C. Bergquist. Visible lasers with subhertz linewidths. Phys. Rev. Lett., 82, 3799-3802(1999).

[5] Y. Y. Jiang, A. D. Ludlow, N. D. Lemke, R. W. Fox, J. A. Sherman, L.-S. Ma, C. W. Oates. Making optical atomic clocks more stable with 10^–16-level laser stabilization. Nat. Photonics, 5, 158-161(2011).

[6] T. L. Nicholson, M. J. Martin, J. R. Williams, B. J. Bloom, M. Bishof, M. D. Swallows, S. L. Campbell, J. Ye. Comparison of two dependent Sr optical clocks with 1 × 10^–17 stability at 10³  s. Phys. Rev. Lett., 109, 230801(2012).

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

[8] L. Jin, Y. Jiang, Y. Yao, H. Fu, Z. Bi, L. Ma. Laser frequency instability of 2 × 10^–16 by stabilizing to 30-cm-long Fabry-Perot cavities at 578  nm. Opt. Express, 26, 18699-18707(2018).

[9] D. G. Matei, T. Legero, S. Häfner, C. Grebing, R. Weyrich, W. Zhang, L. Sonderhouse, J. M. Robinson, J. Ye, F. Riehle, U. Sterr. 1.5  μm lasers with sub-10  mHz linewidth. Phys. Rev. Lett., 118, 263202(2017).

[10] J. M. Robinson, E. Oelker, W. R. Milner, W. Zhang, T. Legero, D. G. Matei, F. Riehle, U. Sterr, J. Ye. Crystalline optical cavity at 4  K with thermal-noise-limited instability and ultralow drift. Optica, 6, 240-243(2019).

[11] T. Bothwell, D. Kedar, E. Oelker, J. M. Robinson, S. L. Bromley, W. L. Tew, J. Ye, C. J. Kennedy. JILA SrI optical lattice clock with uncertainty of 2.0 × 10^–18. Metrologia, 56, 065004(2019).

[12] W. F. McGrew, X. Zhang, R. J. Fasano, S. A. Schäffer, K. Beloy, D. Nicolodi, R. C. Brown, N. Hinkley, G. Milani, M. Schioppo, T. H. Yoon, A. D. Ludlow. Atomic clock performance enabling geodesy below the centimeter level. Nature, 564, 87-90(2018).

[13] E. Oelker, R. B. Hutson, C. J. Kennedy, L. Sonderhouse, T. Bothwell, A. Goban, D. Kedar, C. Sanner, J. M. Robinson, G. E. Marti, D. G. Matei, T. Legero, M. Giunta, R. Holzwarth, E. Riehle, U. Sterr, J. Ye. Demonstration of 4.8 × 10^–17 stability at 1  s for two independent optical clocks. Nat. Photonics, 13, 714-719(2019).

[14] S. M. Brewer, J.-S. Chen, A. M. Hankin, E. R. Clements, C. W. Chou, D. J. Wineland, D. B. Hume, D. R. Leibrandt. ²⁷Al⁺ quantum-logic clock with a systematic uncertainty below 10^–18. Phys. Rev. Lett., 123, 033201(2019).

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

[16] M. S. Safronova, D. Budker, D. MeMille, D. F. J. Kimball, A. Derevianko, C. W. Clark. Search for new physics with atoms and molecules. Rev. Mod. Phys., 90, 025008(2018).

[17] C. W. Chou, D. B. Hume, T. Rosenband, D. J. Wineland. Optical clocks and relativity. Science, 329, 1630-1633(2010).

[18] M. Takamoto, I. Ushijima, N. Ohmae, T. Yahagi, K. Kokado, H. Shinkai, H. Katori. Test of general relativity by a pair of transportable optical lattice clocks. Nat. Photonics, 14, 411-414(2020).

[19] C. Sanner, N. Huntemann, R. Lange, C. Tamm, E. Peik, M. S. Safronova, S. G. Porsev. Optical clock comparison test of Lorentz symmetry. Nature, 567, 204-208(2019).

[20] S. Kolkowitz, I. Pikovski, N. Langellier, M. D. Lukin, R. L. Walsworth, J. Ye. Gravitational wave detection with optical lattice atomic clocks. Phys. Rev. D, 94, 124043(2016).

[21] R. M. Roberts, G. Blewitt, C. Dailey, M. Murphy, M. Pospelov, A. Rollings, J. Sherman, W. Williams, A. Derevianko. Search for domain wall dark matter with atomic clocks on board global positioning system satellites. Nat. Commun., 8, 1195(2017).

[22] J. D. Jost, J. L. Hall, J. Ye. Continuously tunable, precise, single frequency optical signal generator. Opt. Express, 10, 515-520(2002).

[23] Y. Yao, Y. Jiang, L. Wu, H. Yu, Z. Bi, L. Ma. A low noise optical frequency synthesizer at 700–990  nm. Appl. Phys. Lett., 109, 131102(2016).

[24] D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, S. B. Papp. An optical-frequency synthesizer using integrated photonics. Nature, 557, 81-85(2018).

[25] J. L. Hall. Nobel lecture: defining and measuring optical frequencies. Rev. Mod. Phys., 78, 1279-1295(2006).

[26] T. W. Hänsch. Nobel lecture: passion for precision. Rev. Mod. Phys., 78, 1297-1309(2006).

[27] L.-S. Ma, Z. Bi, A. Bartels, L. Robertsson, M. Zucco, R. S. Windeler, G. Wilpers, C. Oates, L. Hollberg, S. A. Diddams. Optical frequency synthesis and comparison with uncertainty at the 10^–19 level. Science, 303, 1843-1845(2004).

[28] D. Nicolodi, B. Argence, W. Zhang, R. Le Targat, G. Santarelli, Y. Le Coq. Spectral purity transfer between optical wavelengths at the 10^–18 level. Nat. Photonics, 8, 219-223(2014).

[29] Y. Yao, Y. Jiang, H. Yu, Z. Bi, L. Ma. Optical frequency divider with division uncertainty at the 10^–21 level. Natl. Sci. Rev., 3, 463-469(2016).

[30] H. Leopardi, J. Davila-Rodriguez, F. Quinlan, J. Olson, J. A. Sherman, S. A. Diddams, T. M. Fortier. Single-branch Er:fiber frequency comb for precision optical metrology with 10^–18 fractional instability. Optica, 4, 879-885(2017).

[31] A. Rolland, P. Li, N. Kuse, J. Jiang, M. Cassinerio, C. Langrock, M. E. Fermann. Ultra-broadband dual-branch optical frequency comb with 10^–18 instability. Optica, 5, 1070-1077(2018).

[32] E. Benkler, B. Lipphardt, T. Puppe, R. Wilk, F. Rohde, U. Sterr. End-to-end topology for fiber comb based optical frequency transfer at the 10^–21 level. Opt. Express, 27, 36886-36902(2019).

[33] M. Giunta, W. Hänsel, M. Fischer, M. Lezius, T. Udem, R. Holzwarth. Real-time phase tracking for wide-band optical frequency measurements at the 20th decimal place. Nat. Photonics, 14, 44-49(2020).

[34] K. Kashiwagi, Y. Nakajima, M. Wada, S. Okubo, H. Inaba. Multi-branch fiber comb with relative frequency uncertainty at 10^–20 using fiber noise difference cancellation. Opt. Express, 26, 8831-8840(2018).

[35] H. R. Telle, B. Lipphardt, J. Stenger. Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements. Appl. Phys. B, 74, 1-6(2002).

[36] N. Scharnhorst, J. B. Wübbena, S. Hannig, K. Jakobsen, J. Kramer, I. D. Leroux, P. O. Schmidt. High-bandwidth transfer of phase stability through a fiber frequency comb. Opt. Express, 23, 19771-19777(2015).

[37] C. Hagemann, C. Grebing, T. Kessler, S. Falke, N. Lemke, C. Lisdat, H. Schnatz, F. Riehle, U. Sterr. Providing 10^–16 short-term stability of a 1.5-μm laser to optical clocks. IEEE Trans. Instrum. Meas., 62, 1556-1562(2013).

[38] L. A. M. Johnson, P. Gill, H. S. Margolis. Evaluating the performance of the NPL femtosecond frequency combs: agreement at the 10^–21 level. Metrologia, 52, 62-71(2015).

[39] X. Chen, Y. Jiang, B. Li, H. Yu, H. Jiang, T. Wang, Y. Yao, L. Ma. Laser frequency instability of 6 × 10^–16 using 10-cm-long cavities on a cubic spacer. Chin. Opt. Lett., 18, 030201(2020).

[40] Y. Jiang, Z. Bi, L. Robertsson, L.-S. Ma. A collinear self-referencing set-up for control of the carrier-envelope offset frequency in Ti:sapphire femtosecond laser frequency combs. Metrologia, 42, 304-307(2005).

[41] D. Yeaton-Massey, R. X. Adhikari. A new bound on excess frequency noise in second harmonic generation in PPKTP at the 10^–19 level. Opt. Express, 20, 21019-21024(2012).

[42] L.-S. Ma, P. Jungner, J. Ye, J. L. Hall. Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by optical fiber or other time-varying path. Opt. Lett., 19, 1777-1779(1994).

[43] Y. Sun, Y. Yao, Y. Hao, H. Yu, Y. Jiang, L. Ma. Laser stabilizing to ytterbium clock transition with Rabi and Ramsey spectroscopy. Chin. Opt. Lett., 18, 070201(2020).

[44] M. Pizzocaro, P. Thoumany, B. Rauf, F. Bregolin, G. Milani, C. Clivati, G. A. Costanzo, F. Levi, D. Calonico. Absolute frequency measurement of the ¹S₀–³P₀ transition of ¹⁷¹Yb. Metrologia, 54, 102-112(2017).

[45] C. Y. Park, D.-H. Yu, W.-K. Lee, S. E. Park, E. B. Kim, S. K. Lee, J. W. Cho, T. H. Yoon, J. Mun, S. J. Park, T. Y. Kwon, S.-B. Lee. Absolute frequency measurement of ¹S₀(F = 1/2)–³P₀(F = 1/2) transition of ¹⁷¹Yb atoms in a one-dimensional optical lattice at KRISS. Metrologia, 50, 119-128(2013).

[46] 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, J. Ye. Systematic evaluation of an atomic clock at 2 × 10^–18 total uncertainty. Nat. Commun., 6, 6896(2015).

[47] G. J. Dick. Local oscillator induced instabilities in trapped ion frequency standards. Proceedings of the 19th Annual Precise Time and Time Interval Systems and Applications Meeting, 133-147(1987).