  Antiferromagnets exhibit distinctive characteristics such as ultrafast
dynamics and robustness against perturbative fields, thereby attracting
considerable interest in fundamental physics and technological applications.
Recently, it was revealed that the N\'eel vector can be switched by a
current-induced staggered (N\'eel) spin-orbit torque in antiferromagnets with
the parity-time symmetry, and furthermore, a nonsymmorphic symmetry enables the
control of Dirac fermions. However, the real-time dynamics of the magnetic and
electronic structures remain largely unexplored. Here, we propose a theory of
the ultrafast dynamics in antiferromagnetic Dirac semimetals and show that the
N\'eel vector is rotated in the picosecond timescale by the
terahertz-pulse-induced N\'eel spin-orbit torque and other torques originating
from magnetic anisotropies. This reorientation accompanies the modulation of
the mass of Dirac fermions and can be observed in real time by the
magneto-optical effects. Our results provide a theoretical basis for emerging
ultrafast antiferromagnetic spintronics combined with the topological aspects
of materials.
