Feedback Cooling of All External Degrees of Freedom of a Levitated Nanoparticle for Exploring Macroscopic Quantum Mechanics

Controlling the motion of macroscopic mechanical oscillators with extreme precision in the quantum regime is a central challenge in modern physics. In this context, optically levitated nanoparticles offer a promising platform because they are well isolated from environmental noise and can be released into free space, enabling free expansion of their wave function. Recent experiments have achieved ground-state cooling of a single translational mode of a levitated nanoparticle. However, achieving ground-state cooling while simultaneously cooling all other modes has remained challenging. Furthermore, little is known about the dynamics of such particles during free flight near the ground state. This book demonstrates feedback cooling of all external degrees of freedom—three translational and three rotational motions—of a levitated nanoparticle. In particular, one of the translational modes is cooled to its quantum ground state. Moreover, it presents the first direct measurement of a nanoparticle’s velocity distribution in its motional ground state by temporarily releasing the particle from the optical trap. These achievements mark a significant step toward the observation of macroscopic quantum interference and pave the way for applications to ultra-high precision sensing, including accelerometry and torque detection. This book is intended for graduate students and researchers in atomic, molecular, and optical physics and quantum optics.