Improved Limits for Violations of Local Position Invariance from Atomic Clock Comparisons

 We present data from precision frequency comparisons between two primary Cs fountain clocks and two optical clocks based on both an electric quadrupole (E2) and an electric octupole (E3) transition of single trapped Ytterbium ions at PTB. We measured the E3 transition frequency at 642 THz with 80 mHz uncertainty, the most accurate determination of an optical transition frequency to date. Furthermore, the frequency ratio of E2 and E3 transition has been determined with 3×10^-17 fractional uncertainty, improving upon previous measurements by an order of magnitude.

Repeated measurements of both the E3 transition frequency and the E3/E2 frequency ratio over several years are used for a test of Einstein's equivalence principle by analyzing the data for potential violations of local position invariance, that would manifest themselves in a variation of fundamental constants, measurable as changes of the atomic transition frequencies. From the observed long-term stability of the clocks, we improve by factors of about 20 and 2 the limits for fractional temporal variations of the fine structure constant to 1.0(1.1)×10^-18/yr and of the proton-to-electron mass ratio to -8(36)×10^-18/yr. Using the annual variation of the Sun's gravitational potential at Earth, we also improve limits for a potential coupling of both constants to gravity.