Nonlinear quantum control of Landau systems beyond Kohn's theorem

Controlling superpositions of electronic quantum states through nonlinear interactions is central to quantum computing. At THz frequencies, solid-state systems exhibit numerous elementary excitations suitable to this purpose, yet efficient dephasing mechanisms render their exploitation challenging. The cyclotron resonance, protected from Coulomb interactions by Kohn's theorem, is a unique exception. Here, single-cycle THz transients of an amplitude of up to 8.7 kVcm
^-1
facilitate coherent quantum control of a Landau system, leading to anharmonic Landau ladder climbing up to the 6
^th
rung, and population inversion. Strong coherent nonlinearities including four- and six-wave mixing revealed through two-dimensional THz spectroscopy unveil dynamic Coulomb interactions in a setting of nonperturbative light-matter interaction, beyond Kohn's theorem.