  Quantum mechanics dictates that a continuous measurement of the position of
an object imposes a random back action perturbation on its momentum. This
randomness translates with time into position uncertainty, thus leading to the
well known uncertainty on the measurement of motion. Here we demonstrate that
the quantum back action on a macroscopic mechanical oscillator measured in the
reference frame of an atomic spin oscillator can be evaded. The collective
quantum measurement on this novel hybrid system of two distant and disparate
oscillators is performed with light. The mechanical oscillator is a drum mode
of a millimeter size dielectric membrane and the spin oscillator is an atomic
ensemble in a magnetic field. The spin oriented along the field corresponds to
an energetically inverted spin population and realizes an effective negative
mass oscillator, while the opposite orientation corresponds to a positive mass
oscillator. The quantum back action is evaded in the negative mass setting and
is enhanced in the positive mass case. The hybrid quantum system presented here
paves the road to entanglement generation and distant quantum communication
between mechanical and spin systems and to sensing of force, motion and gravity
beyond the standard quantum limit.
