Ultrastrong light-matter coupling at 300 GHz with few (<80) electrons

[We demonstrate ultrastrong light-matter coupling between less than 80 electrons located in the last occupied Landau level of an high mobility 2 dimensional electron gas and a LC resonator with an extremely small effective mode volume V eff/? 3= 4× 10-10. The unperturbed frequency is 300 GHz and the measured normalized coupling ratio is ?/?= 0.35.object Object]