Dephasing dynamics accessed by high harmonic generation: Direct determination of quantum decoherence

Abstract We reveal the critical effect of ultrafast quantum dephasing on the polarization of high harmonic generation from massless Dirac fermions in graphene. Under the elliptically polarized pump pulse, the elliptically polarized high harmonic generation is produced and exhibits a characteristic polarimetry of the polarization ellipse, which is found to depend on the decoherence time T 2 for the dephasing in the multiphoton regime. T 2 could then be determined to be a few femtoseconds directly from the experimentally observed polarimetry of high harmonics. This shows a sharp contrast with the semimetal regime of higher pump intensity, where the polarimetry is irrelevant to T 2 . An access to the dephasing dynamics would extend the prospect of high harmonic generation into the metrology of femtosecond dynamical process in the coherent quantum control.