Phys. Rev. Lett. (in the press); preprint at

In quantum mechanics textbooks, particles trapped in a harmonic potential often serves as a paradigmatic system to study. They possess eigenstates labelled by sets of quantum numbers and their corresponding equally spaced eigenenergies have a non-zero minimum value — the zero-point energy. Although this textbook example has long been realized in the lab, transitioning a mesoscopic system from the classical realm to this quantum regime is still challenging. Now Felix Tebbenjohanns and co-workers have demonstrated this with a very simple setup.

The system consisted of a 136-nm-diameter silica particle trapped in a single-beam optical dipole trap. A measurement-based linear-feedback cooling scheme brought the system — which initially behaved entirely classically — directly to the quantum regime. As the particle approached its lowest energy state, light–particle scattering, which involves a further reduction of the energy, was supressed. The particle’s quantum ground-state motion could then be inferred by measuring the asymmetry of the spectrum of the scattered light.