  We propose a quantum noise eater for a single qubit and experimentally verify
its performance for recovery of a superposition carried by a dual-rail photonic
qubit. We consider a case when only one of the rails (e.g., one of
interferometric arms) is vulnerable to noise. A coherent but randomly arriving
photon penetrating into this single rail causes a change of its state, which
results in an error in a subsequent quantum information processing. We
theoretically prove and experimentally demonstrate a conditional full recovery
of the superposition by this quantum noise eater.
