Coherent cyclotron motion beyond Kohn's theorem

TL;DR

This paper demonstrates that intense terahertz pulses can drive a 2D electron gas beyond Kohn's theorem, revealing nonlinear Coulomb interactions and enabling new ultrafast quantum control of Landau electrons.

Contribution

It experimentally shows nonlinear effects in cyclotron motion beyond Kohn's theorem using terahertz pulses, linking these effects to Coulomb interactions.

Findings

Observation of anharmonic Landau ladder climbing

Detection of terahertz four- and six-wave mixing signatures

Identification of Coulomb effects between electrons and ion background

Abstract

In solids, the high density of charged particles makes many-body interactions a pervasive principle governing optics and electronics[1-12]. However, Walter Kohn found in 1961 that the cyclotron resonance of Landau-quantized electrons is independent of the seemingly inescapable Coulomb interaction between electrons[2]. While this surprising theorem has been exploited in sophisticated quantum phenomena[13-15] such as ultrastrong light-matter coupling[16], superradiance[17], and coherent control[18], the complete absence of nonlinearities excludes many intriguing possibilities, such as quantum-logic protocols[19]. Here, we use intense terahertz pulses to drive the cyclotron response of a two-dimensional electron gas beyond the protective limits of Kohn's theorem. Anharmonic Landau ladder climbing and distinct terahertz four- and six-wave mixing signatures occur, which our theory links to dynamic Coulomb effects between electrons and the positively charged ion background. This new context for Kohn's theorem unveils previously inaccessible internal degrees of freedom of Landau electrons, opening up new realms of ultrafast quantum control for electrons.