Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence^1,2,3. Quantum control of mechanical motion has been achieved by engineering the radiation–pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator^4,5,6,7. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states⁸. In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects^9,10, since their trapping potential is fully controllable. Here we optically levitate a femtogram (10⁻¹⁵ grams) dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its centre-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 0.43. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales¹¹.

宏观尺度上的量子力学测试需要对机械运动及其退相干^1,2,3的极端控制。^{机械运动的量子控制已经通过设计微机械振荡器和谐振器4,5,6,7}中的电磁场之间的辐射压力耦合来实现。此外，依赖于腔增强检测方案的基于测量的反馈控制已被用于将微机械振荡器冷却至其量子基态⁸。与机械系留系统相比，光悬浮纳米粒子特别有希望用于大质量物体的物质波实验^9,10，因为它们的诱捕潜力是完全可控的。在这里，我们光学悬浮飞克 (10 ^-15 克）低温自由空间中的介电粒子，它充分抑制了热效应，使测量反作用成为主要的退相干机制。通过有效的量子测量，我们可以对粒子的动力学进行量子控制。我们通过基于测量的反馈将其质心运动冷却到 0.65 运动量子的平均占有率，对应于 0.43 的状态纯度。没有光学谐振器及其带宽限制有望将可用于电磁场的完整量子控制转移到机械系统。再加上光俘获势是高度可控的，我们的实验平台提供了一条在宏观尺度¹¹研究量子力学的途径。