Author Affiliations

D. A. Long,^*,a G.-W. Truong,^a,b K. O. Douglass,^a S. E. Maxwell,^a R .D. van Zee,^a D. F. Plusquellic,^a and J. T. Hodges^a

^aNational Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg Maryland 20899 USA

^bFrequency Standards and Metrology Research Group, School of Physics, The University of Western Australia, Perth, Western Australia 6009 Australia

^*Corresponding author: david.long@nist.gov