Bloch oscillations measure gravity

In 1929 Felix Bloch predicted that electrons in a periodic crystal potential subjected to a weak electric field would not be linearly accelerated; rather, they would oscillate around their individual positions. Although never seen in actual crystals because of lattice defects, Bloch oscillations are observable with cold atoms in an optical lattice (see Physics Today, July 1997, page 30 , and August 2004, page 25 ). Experimenters at the University of Florence (Italy) have set up such a system and measured gravity on the micron scale. To do that, they cooled bosonic strontium-88 atoms—which not only have no angular momentum in the ground state but also enjoy extremely weak atom–atom interactions—to a mere 400 nK and loaded them into a vertically oriented optical lattice. Because the ⁸⁸Sr atoms formed a nearly ideal gas, the system remained coherent and stable for a remarkably long time, about 7 seconds. For each atom during that time, the physicists observed roughly 4000 oscillations and 8000 photon momenta coherently transferred. In the end, they measured the gravitational acceleration to be 9.80012(5) m/s². According to team leader Guglielmo Tino, unlike gravity-measuring experiments that use torsional balances or cantilevers, the Florence approach measures gravity directly and over shorter distances. In addition, the experiment can take place within microns of a surface, enabling future explorations of the Casimir force and deviations from Newtonian gravity.(G. Ferrari et al., Phys. Rev. Lett. 97 , 060402, 2006 http://dx.doi.org/10.1103/PhysRevLett.97.060402 ).