  We propose and analyse a feasible experimental scheme for a quantum force
sensor based on the elimination of back-action noise through coherent quantum
noise cancellation (CQNC) in a hybrid atom-cavity optomechanical setup assisted
with squeezed vacuum injection. The force detector, which allows for a
continuous, broad-band detection of weak forces well below the standard quantum
limit (SQL), is formed by a single optical cavity simultaneously coupled to a
mechanical oscillator and to an ensemble of ultracold atoms. The latter acts as
a negative-mass oscillator so that atomic noise cancels exactly the back-action
noise from the mechanical oscillator due to destructive quantum interference.
Squeezed vacuum injection enforces this cancellation and allows to reach
sub-SQL sensitivity in a very wide frequency band, and at much lower input
laser powers.
