Ultrafast spectroscopy of the quantum Hall ferromagnet

We report ultrafast spectroscopy experiments at the ν=1 quantum Hall state of a high-mobility two-dimensional electron system in a GaAs single quantum well. By spectrally resolved four-wave-mixing measurements we investigate the coherent dynamics of photo-excited electron–hole pairs and find extraordinarily long decoherence times for electrons in the upper spin level around ν=1. We interpret the observed slower decrease of the dephasing time for ν>1, as compared to ν<1, as a fingerprint of Skyrmion formation dynamics.