  A central prospect of antiferromagnetic spintronics is to exploit magnetic
properties that are unavailable with ferromagnets. However, this poses the
challenge of accessing such properties for readout and control. To this end,
light-induced manipulation of the transient ground state, e.g. by changing the
magnetic anisotropy potential, opens promising pathways towards ultrafast
deterministic control of antiferromagnetism. Here, we use this approach to
trigger a $\it{coherent}$ rotation of the entire long-range antiferromagnetic
spin arrangement about a crystalline axis in $GdRh_2Si_2$ and demonstrate
$\it{deterministic}$ control of this rotation upon ultrafast optical
excitation. Our observations can be explained by a displacive excitation of the
Gd spins$'$ local anisotropy potential by the optical excitation, allowing for
a full description of this transient magnetic anisotropy potential.
