Experimentally undoing an unknown single-qubit unitary

Understand

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.