Exploring Gravity with Optical Clocks

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Testing the foundations of gravitational physics and mapping Earth′s gravitational potential


Recent news: ESA selects "STE-QUEST" as one of four medium-size candidate missions for a flight in 2022+

In the near future it will become possible to measure several aspects of the physics of gravity with a significantly higher precision than so far. The key tool for these measurements are optical clocks, a type of atomic clock of unprecedented accuracy and stability. Optical clocks employ ultra-cold atoms whose electronic level energies can be used to define the frequencies of electromagnetic waves, for example those emitted by lasers. In the future, optical clocks could also employ molecules and use their vibrational energies.

At present, the accuracies of the best optical clocks has already reached the level of 1 part in 1017 (NIST, Boulder, USA). These are single-ion clocks. Optical lattice clocks, a type of clock using a relatively large number (tens of thousands) of neutral atoms trapped in a lattice of laser light, have currently reached accuracies of 1-3 parts in 1016. It is expected that in the near future, optical clocks with stability at a level on the order of 1 part in 1018 will eventually be achieved. Using clocks of this stability on board of a satellite, together with an appropriate frequency link for comparison with Earth clocks, will allow to

  I. improve tests of the principle of Local Position Invariance, by a factor of up to 105
  II. improve measurements of the gravitational frequency shift by a factor of up to 105
  III. measure the gravitational potential of the Earth at any particular location by measuring the frequency of a high-accuracy clock at that location
  IV. distribute high-accuracy time and frequency signals over the Earth and to space

Figure: Concept of a satellite mission with optical clocks to test fundamental principles of space-time structure. A satellite carrying an ensemble of dissimilar optical clocks orbits Earth on a highly elliptic orbit. The clocks, are intercompared with each other and with clocks that reside in a differing gravitational potential U, on the ground or on another satellite. The result of the intercomparisons is analyzed to search for possible deviations from the predictions of General Relativity and its underlying principles, in particular Local Position Invariance.