In a 2010 experiment based on an older version of this clock[0], NIST succeeding in measuring the gravitational time dilation across a 33 cm vertical separation

In a 2010 experiment based on an older version of this clock[0], NIST succeeding in measuring the gravitational time dilation across a 33 cm vertical separation—a frequency difference of 4.1×10^{-17}—with 140,000 seconds of integration time (<2 days). I don't really understand how that worked. [0] https://sci-hub.se/https://doi.org/10.1126/science.1192720 https://sci-hub.se/https://doi.org/10.1126/science.1192720 ("Optical Clocks and Relativity" (2010))