Laboratory demonstration of geopotential measurement using transportable optical clocks

Abstract

We report an experimental demonstration of geopotential difference measurement using a pair of transportable 40Ca+ optical clocks (TOC-729-1 and TOC-729-3) in the laboratory, each of them with an uncertainty of 1.3 × 10-17 and an instability of 4.8 × 10-15/ √ τ, respectively. Referenced to a stationary clock of TOC-729-1, geopotential difference measurements are realized by moving TOC-729-3 to three different locations and the relevant altitude differences are measured with uncertainties at the level of 20 cm. After correcting the systematic shifts (including gravitational red shift), these two-clock frequency difference is measured to be -0.7(2.2) × 10-17, considering both the statistic (1.0 × 10-17) and the systematic (1.9 × 10-17) uncertainty. The frequency difference between these two clocks is within their respective uncertainties, verifying the reliability of transportable 40Ca+ optical clocks at the low level of 10-17.