Quantum backaction evasion with reservoir engineering

We propose a backaction evading scheme for a free mass that combines reservoir engineering with velocity measurement. The underlying principle follows the double-pass-type speed meter, which measures the mirror's velocity using a nonreciprocal interaction. In our method, the nonreciprocal coupling is realized through reservoir engineering, following the recipe proposed by Metelmann and Clerk [Nonreciprocal Photon Transmission and Amplification via Reservoir EngineeringPhys. Rev. X 5, 021025 (2015)]. We show that reservoir engineering can reproduce the double-pass speed meter with optimal feedforward, using only reciprocal interactions. The resulting force sensitivity surpasses the standard quantum limit, providing an alternative route to quantum backaction evasion in cavity optomechanical systems.