As it was mentioned atoms of 88Sr have an electronic level structure particularly suited for precise measurements of forces. Indeed 88Sr is an atomic species remarkably robust against decoherence processes due to a small atom-atom interaction and a zero orbital, spin, and nuclear angular momentum, which makes it insensitive to stray electric and magnetic fields.

In the experiment a sample of cold 88Sr atoms was loaded in a vertical optical lattice formed by green (532 nm) retro-reflected laser beam. The combination of the periodic optical potential and a linear gravitation potential gives rise to Bloch oscillations at frequency ?B=mg?L/2h, where m is the atomic mass, g is the gravity acceleration, ?L is wavelength of the lattice light, and h is the Planck constant. Since m, ?L and h are well known the force along the lattice axis can be determined by measuring the Bloch frequency ?B. We have observed persistent Bloch oscillations with a damping time longer than 10 seconds (G. Ferrari et al., Phys. Rev. Lett. 97, 060402 (2006)). We monitored ~4000 Bloch oscillations in a time of 7 s. This allows us to measure the local gravity with a sensitivity of 5 10-6 g.

Fig. 3: Bloch oscillations. Picture are taken after 10 ms of TOF

The high precision of this method is combined with intristically small size of the probe i.e. an atomic sample. Therefore this technique is promising for probing surface forces, such as Casimir-Polder forces or testing possible deviations form the Newtonian gravity. Indeed Casimir-Polder forces fall as ~1/z4 , where z is the distance between atom and surface. Thus a compact atomic sample is highly desirable to probe region where Casimir-Polder forces are relately strong. The same argument is valid for a study of non Newtonian gravity, which is ussualy writen in form of a Yukawa potential.

The strategy to probe non Newtonian gravity is following. We use an optical elevator to transfer atoms close to the surface, then we turn off the ascending beam, atoms are trapped in lattice formed by descending beeam and its reflection from the surface, than we drag atoms along the surface by mechanical movement of the descending beam. Finally we probe the force on top of materials with different density alluminum and gold. Both materials are shielded by a thin layer of gold to make Casimire-Polder potential uniform, so only a gravity contribution will be visible. Up to now we succeed in transfering about 10 103 atoms, to the trap formed by descending beam and its reflection at distance about 200 ?m from the surface. Then about 5 103 atoms were draged ~ 1 mm distance along the surface.

Fig. 4: Transferring atomic sample to the surface