Quantum collective motion of macroscopic mechanical oscillators

Collective phenomena in physics arise from interactions among numerous components in complex systems, leading to behaviors distinct from those of individual parts. Observing such phenomena in optomechanical systems has been challenging due to the stringent requirement of making arrays of nearly identical mechanical oscillators across various platforms, such as superconducting circuits, bulk-acoustic wave resonators, and levitated nanoparticles. Here, we overcome this challenge by demonstrating the collective behavior of multiple, nearly degenerate solid-state mechanical oscillators in a superconducting circuit platform. This is achieved through exceptionally low disorder in the mechanical frequencies, reaching levels as low as 0.1%. We show that by increasing the optomechanical coupling rates, the system undergoes a transition from individual to collective dynamics. By leveraging slight non-degeneracies in mechanical frequencies, we directly measure both the amplitude and relative phase of the collective modes. The coupling of the collective mode to the cavity via radiation pressure is enhanced with the number of oscillators involved, resembling super-radiance in atomic ensembles. High mechanical quality factors allow us to achieve ground-state cooling of the collective mode and observe its quantum sideband asymmetry. Finally, by utilizing a dissipative squeezing scheme, we squeeze the collective motion of the mechanical oscillators. Combining squeezing with reservoir engineering, we aim to entangle the motion of all oscillators.