Classical Pendulum Feels Quantum Back-Action

Abstract

Quantum mechanics, which has agreed with every experimental test, predicts superposition of position states even for macroscopic objects. Because of the massiveness, such states might have a key to investigate both quantum measurement problem and quantum gravity. Recently, the use of quasi-freely suspended mirror combined with a laser field was proposed to prepare such states. One of the key milestones for the quantum-optomechanical effects such as the generation of the entanglement state and a squeezed state of light is the observation of quantum back-action, which identifies the connection between the objects and quantumness of the light. Until now, this effect has been observed below the meso-scopic mass scale. However, it has not been observed yet in the macroscopic scale beyond Planck mass. This is partially due to a technical limitation-the radiation pressure of light will expose a free mass to instability such as anti-torsional spring effect. Also, there is the fundamental compromise between tolerance for the instability and sensitivity; sufficient tolerance with firm suspension makes the mass differ from free mass, and this results in increase of a thermal fluctuating force. Here we describe using a triangular optical cavity to overcome these limitations and observation of quantum back-action imposed on a suspended 5-mg mirror. The origin of quantum back-action is the momentum transfered to the mirror by light on its reflection. For the coherent light, the photon number fluctuates according to a Poisson distribution and this fluctuation gives the radiation pressure fluctuation. The pendulum mode excited by this force fluctuation was observed. Our result paves the way to the generation of macroscopic entangled states and also has possibility to test a role of gravity for solving the quantum measurement problem.