Quantum mechanics dictates that a continuous measurement of the position of an object imposes a random quantum back action (QBA) perturbation on its momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. The same happens when a measurement of a spin oscillating in magnetic field is performed. As a consequence, and in accordance with the Heisenberg uncertainty principle, the QBA puts a limitation—the so-called standard quantum limit—on the precision of sensing of position, force and fields. In this talk I will present the ideas  and experimental results  for measurement of motion of a mechanical oscillator with the precision not restricted by the QBA. This is achieved by measuring the motion in a special reference frame linked to an atomic spin system with an effective negative mass. The measurement is performed by light propagating through the atomic and mechanical oscillators. Applications to force sensing , clock synchronization  and gravitational wave detection  will be discussed.
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