  We propose that the dispersion management of coherent atomic matter waves can
be exploited to overcome quantum back-action in condensate-based optomechanical
sensors. The effective mass of an atomic Bose-Einstein condensate modulated by
an optical lattice can become negative, resulting in a negative-frequency
optomechanical oscillator, a negative environment temperature, and
optomechanical properties opposite to those of a positive-mass system. This
enables a quantum-mechanics-free subsystem insulated from quantum back-action.
