  Gravity differs from all other known fundamental forces since it is best
described as a curvature of spacetime. For that reason it remains resistant to
unifications with quantum theory. Gravitational interaction is fundamentally
weak and becomes prominent only at macroscopic scales. This means, we do not
know what happens to gravity in the microscopic regime where quantum effects
dominate, and whether quantum coherent effects of gravity become apparent.
Levitated mechanical systems of mesoscopic size offer a probe of gravity, while
still allowing quantum control over their motional state. This regime opens the
possibility of table-top testing of quantum superposition and entanglement in
gravitating systems. Here we show gravitational coupling between a levitated
sub-millimeter scale magnetic particle inside a type-I superconducting trap and
kg source masses, placed approximately half a meter away. Our results extend
gravity measurements to low gravitational forces of attonewton and underline
the importance of levitated mechanical sensors. Specifically, at a frequency of
26.7 Hz, a mass of 0.4 mg and showing Q-factors in excess of 10$^7$, we
obtained a force noise of 0.5 $fN\sqrt{Hz}$ . We simultaneously detect the
other 5 rotational and translational degrees of freedom.
