  Undoing a unitary operation, $i.e$. reversing its action, is the task of
canceling the effects of a unitary evolution on a quantum system, and it may be
easily achieved when the unitary is known. Given a unitary operation without
any specific description, however, it is a hard and challenging task to realize
the inverse operation. Recently, a universal quantum circuit has been proposed
[Phys.Rev.Lett. 123, 210502 (2019)] to undo an arbitrary unknown
$d$-dimensional unitary $U$ by implementing its inverse with a certain
probability. In this letter, we report the experimental reversing of three
single-qubit unitaries $(d = 2)$ by linear optical elements. The experimental
results prove the feasibility of the reversing scheme, showing that the average
fidelity of inverse unitaries is $F=0.9767\pm0.0048$, in close agreement with
the theoretical prediction.
