Atom interferometry with fermionic and bosonic isotopes of strontium for precision gravity measurements and test of the Equivalence Principle

Atom interferometry is rapidly growing as a new tool for high precision gravity measurements, finding important applications in applied and fundamental physics, especially in quantum-level tests of general relativity. In this context, testing the Einstein Equivalence Principle with quantum systems is motivated by the aim of improving the limits reached by classical tests with macroscopic bodies, but mostly by the possibility to perform qualitatively new tests with "test masses'' having well defined properties, in terms of spin, bosonic or fermionic nature, and proton-to-neutron ratio. In this thesis, two experiments based on ultra-cold strontium atom interferometry are presented. In the first one, we performed an experimental comparison of the gravitational acceleration for two different strontium isotopes: one which has zero total spin, the boson 88Sr, and one which has a half-integer spin, the fermion 87Sr. Gravity acceleration was measured by means of a genuine quantum effect, namely, the coherent delocalization of matter waves in an optical lattice. The results set an upper limit of 10^-7 for the Weak Equivalence Principle violation and for the existence of a possible spin-gravity coupling. In a second experiment a new interferometric scheme with the 88Sr isotope was developed with the purpose of setting the basis for a new generation of high precision gravimeters. We realized the first vertical Mach-Zehnder interferometer with 88Sr atoms based on large-momentum-transfer Bragg pulses. This isotope has specific favorable characteristics: it has no nuclear spin so that in the ground state it is a scalar particle which is virtually insensitive to stray magnetic fields, and its small scattering length results in reduced decoherence due to cold collisions. These unique properties make this isotope of superior interest for the highest precision gravimetric devices. We demonstrated atomic diffraction by a laser standing wave of up to eight photon recoils and the realization of a gravimeter with a sensitivity dg/g = 4x10^-8.