  We propose an interferometric scheme based on an untrapped nano-object
subjected to gravity. The motion of the center of mass (c.m.) of the free
object is coupled to its internal spin system magnetically, and a free flight
scheme is developed based on coherent spin control. The wavepacket of the test
object, under a spin-dependent force, may then be delocalized to a macroscopic
scale. A gravity induced dynamical phase (accrued solely on the spin state, and
measured through a Ramsey scheme) is used to reveal the above spatially
delocalised superposition of the spin-nano-object composite system that arises
during our scheme. We find a remarkable immunity to the motional noise in the
c.m. (initially in a thermal state with moderate cooling), and also a dynamical
decoupling nature of the scheme itself. Together they secure a high visibility
of the resulting Ramsey fringes. The mass independence of our scheme makes it
viable for a nano-object selected from an ensemble with a high mass
variability. Given these advantages, a quantum superposition with $100$ nm
spatial separation for a massive object of $10^9$ amu is achievable
experimentally, providing a route to test postulated modifications of quantum
theory such as continuous spontaneous localisation.

Quantum Physics