Ultrastrong coupling between electron tunneling and mechanical motion.

The ultrastrong coupling of single-electron tunneling and nanomechanical motion opens exciting opportunities to explore fundamental questions and develop new platforms for quantum technologies. We have measured and modelled this electromechanical coupling in a fully-suspended carbon nanotube device and report a ratio of $g_m/\omega_m = 1.3$, where $g_m/2\pi = 420\pm20$~MHz is the coupling strength and $\omega_m/2\pi=324$~MHz is the mechanical resonance frequency. This is well within the ultrastrong coupling regime and the highest among current electromechanical platforms. Even higher ratios could be achieved with improvement on device design.