Simultaneous ground-state cooling of two mechanical modes of a levitated nanoparticle

The quantum ground state of a massive mechanical system is a steppingstone for investigating macroscopic quantum states and building high fidelity sensors. With the recent achievement of ground-state cooling of a single motional mode, levitated nanoparticles have entered the quantum domain. To overcome detrimental cross-coupling and decoherence effects, quantum control needs to be expanded to more system dimensions, but the effect of a decoupled dark mode has thus far hindered cavity-based ground state cooling of multiple mechanical modes. Here, we demonstrate two-dimensional (2D) ground-state cooling of an optically levitated nanoparticle. Utilising coherent scattering into an optical cavity mode, we reduce the occupation numbers of two separate centre-of-mass modes to 0.83 and 0.81, respectively. By controlling the frequency separation and the cavity coupling strengths of the nanoparticle's mechanical modes, we show the transition from 1D to 2D ground-state cooling while avoiding the effect of dark modes. Our results lay the foundations for generating quantum-limited high orbital angular momentum states with applications in rotation sensing. The demonstrated 2D control, combined with already shown capabilities of ground-state cooling along the third motional axis, opens the door for full 3D ground-state cooling of a massive object.