Zhang W., M.J. Martin, C. Benko, J.L. Hall, J. Ye, C. Hagemann, T. Legero, U. Sterr, F. Riehle, G.D. Cole, and M. Aspelmeyer, Optics Letters 39, 1980 (2014).
Kessler T., C. Hagemann, C. Grebing, T. Legero, U. Sterr, F. Riehle, M.J. Martin, L. Chen, and J. Ye, Nature Photonics 6, 687-692 (2012).
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