  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 quasiparticles. However, the real-time dynamics of the
magnetic and electronic structures remain largely unexplored. Here, we propose
a theoretical framework for 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 due to magnetic anisotropies, which accompanies the modulation of the
mass of the Dirac quasiparticles. This reorientation can be measured by the
time-resolved magneto-optical Kerr effect. Our results provide a theoretical
basis for emerging ultrafast antiferromagnetic spintronics combined with the
topological aspects of materials.
