The standard quantum limit constrains the precision of an oscillator position measurement. It arises from a balance between the imprecision and the quantum back-action of the measurement. However, a measurement of only a single quadrature of the oscillator can evade the back-action and be made with arbitrary precision. Here we demonstrate quantum back-action evading measurements of a collective quadrature of two mechanical oscillators, both coupled to a common microwave cavity. The work allows for quantum state tomography of two mechanical oscillators, and provides a foundation for macroscopic mechanical entanglement and force sensing beyond conventional quantum limits.
@article{arxiv.1608.06152,
title = {Quantum back-action evading measurement of collective mechanical modes},
author = {C. F. Ockeloen-Korppi and E. Damskägg and J. -M. Pirkkalainen and A. A. Clerk and M. J. Woolley and M. A. Sillanpää},
journal= {arXiv preprint arXiv:1608.06152},
year = {2016}
}