Quantum control of a Landau-quantized two-dimensional electron gas in a GaAs quantum well using coherent terahertz pulses

We demonstrate coherent control of cyclotron resonance in a two-dimensional electron gas (2DEG). We use a sequence of terahertz (THz) pulses to control the amplitude of coherent cyclotron resonance oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the THz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using single-particle optical Bloch equations, showing no signatures of electron-electron interactions, which verifies the validity of Kohn's theorem for cyclotron resonance in the coherent regime.

Figure 1

(a) Schematic of the experimental setup. WGP: wire-grid polarizer. The incident THz pulses are linearly ($x$) polarized. (b) Transmitted THz pulse at 0 and 2 T and the reemitted THz wave, $E_{\mathrm{em}}$ ($x$ component, left and $y$ component, right). Traces are vertically offset for clarity. Inset: amplitude ratio $\left|t\right|$ in the frequency domain showing cyclotron resonance as a dip. (c) Parametric plot showing the cyclotron resonance active mode absorption at $\pm$2 T. (d) Bloch sphere for coherent cyclotron resonance.

Figure 2

Coherent control of cyclotron resonance. (a) Incident (upper) and reemitted field (lower) at several magnetic fields. The second pulse was created through the internal reflection inside a 430-$\mu$m-thick sapphire plate [Fig. 1a]. (b) Bloch vector dynamics and calculated density matrix elements.

Figure 3

(a) The interaction of the 2DEG with its self-reemitted wave. Thick gray arrows: reemitted THz wave from the 2DEG; thin red arrows: reflected and transmitted incident THz pulse. (b) The incident electric field (upper) and reemitted field (lower). The second incident THz pulse is due to the internal reflection inside the sample substrate. The solid black curve is an extended fitting curve to the initial oscillation from 5 to 17 ps with a sine wave.