Quantum back-action evading measurement of collective mechanical modes
  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. 
C. F. Ockeloen-Korppi, E. Damsk\"agg, J.-M. Pirkkalainen, A. A. Clerk,   M. J. Woolley, M. A. Sillanp\"a\"a
cond-mat.mes-hall quant-ph