  Long-distance optical quantum channels are necessarily lossy, leading to
errors in transmitted quantum information, entanglement degradation and,
ultimately, poor protocol performance. Quantum states carrying information in
the channel can be probabilistically amplified to compensate for loss, but are
destroyed when amplification fails. Quantum correction of the channel itself is
therefore required, but break-even performance -- where arbitrary states can be
better transmitted through a corrected channel than an uncorrected one -- has
so far remained out of reach. Here we perform distillation by heralded
amplification to improve a noisy entanglement channel. We subsequently employ
entanglement swapping to demonstrate that arbitrary quantum information
transmission is unconditionally improved -- i.e. without relying on
postselection or post-processing of data -- compared to the uncorrected
channel. In this way, it represents realisation of a genuine quantum relay. Our
channel correction for single-mode quantum states will find use in quantum
repeater, communication and metrology applications.
