“…This has led to the demonstration of light-matter coupling with a number of quantum systems including phonons, 12 intersubband transitions (ISBTs) in quantum wells (QWs), 13,14 vibrational modes of molecules, 15 and cyclotron resonances. 16,17 The coupling of MMs to ISBTs in QWs is especially appealing for applications due to the tunability of the transition frequency, the large dipole moment, and the possibility of optical and/or electrical modulation. 18 However, the experimental demonstration of ultrastrong light-matter coupling in such a system is still open.…”