Terahertz Coherent Control of Cyclotron Resonance in the Quantum Hall Regime

We report on the creation and coherent control of a superposition of many-electron quantum states (or a qubit) in a Landau-quantized GaAs two-dimensional electron gas (2DEG) using a sequence of coherent terahertz (THz) pulses. The first pulse excites electrons from the highest-filled Landau level (LL) to the lowest-unfilled LL, creating a superposition of the two LLs which re-emits a coherent THz wave. We found that the second THz pulse incident within the decoherence time stops or enhances the THz re-emission depending on its arrival phase. These results show that an arbitrary coherent control of the LL qubit is possible using THz pulses. We also performed a simulation within the framework of single-particle optical Bloch equations, which reproduced the experimental results surprisingly well. This agreement shows that the 2DEG behaves in the same way as a single-electron two-level system despite the fact that it contains a large density of interacting electrons. This finding extends the Kohn's theorem to a more general level of coherent dynamics.