Atomic clocks measure time by counting the oscillations of laser light tuned to an atomic transition of known frequency. Optical clocks, the most sophisticated variety of atomic clock, probe optical transitions in ultracold atoms and can measure time with an uncertainty approaching 1 part in 1018. (Olympic athletes can be timed to the millionth of a second.)
Among their many applications, optical clocks can be used to measure Earth’s gravitational potential: According to general relativity, clocks raised to higher potentials have their atomic transition frequency ν increased by Δν = ν ΔU/c2, where ΔU is the change in potential and c the speed of light. Until recently the best optical clocks had always operated within the friendly confines of a controlled laboratory environment. Now an international group led by Christian Lisdat of the National Metrology Institute of Germany has developed an advanced, portable, strontium-based optical clock. With that device, the researchers were able to measure frequencies with a precision of 2 parts in 1015 and determine the potential difference between the National Institute of Metrological Research (INRIM) in Turin, Italy, and the Modane Underground Laboratory (LSM), located 90 km away in the middle of the Fréjus Road Tunnel that runs through the Alps to connect France and Italy.
Turin’s INRIM has its own atomic clocks based on cesium and ytterbium. For the first round of measurements, Lisdat and company brought their strontium clock to LSM. An optical fiber connecting the two labs enabled the Cs clock to measure the frequency of the Sr transition from afar. (Anticipated Yb-clock measurements were not possible.) Then the Sr clock was transported to INRIM, and the transition frequency was measured by both Cs and Yb clocks.
The 48 ± 1 Hz change in frequency determined by Lisdat’s team indicates that LSM and INRIM differ in altitude by about 1000 m. That value is in good agreement with, but not nearly as precise as, those obtained with conventional ground-based and satellite methods, which can reach decimeter precision. To match that, an optical clock would need to be exact to 1 part in 1017, a goal the Lisdat team expects to meet after their next round of improvements. (J. Grotti et al., Nat. Phys, in press.)
