  The phenomenon of quantum entanglement marks one of the furthest departures
from classical physics and is indispensable for quantum information processing.
Despite its fundamental importance, the distribution of entanglement over long
distances trough photons is unfortunately hindered by unavoidable decoherence
effects. Entanglement distillation is a means of restoring the quality of such
diluted entanglement by concentrating it into a pair of qubits. Conventionally,
this would be done by distributing multiple photon pairs and distilling the
entanglement into a single pair. Here, we turn around this paradigm by
utilising pairs of single photons entangled in multiple degrees of freedom.
Specifically, we make use of the polarisation and the energy-time domain of
photons, both of which are extensively field-tested. We experimentally chart
the domain of distillable states and achieve relative fidelity gains up to 13.8
%. Compared to the two-copy scheme, the distillation rate of our single-copy
scheme is several orders of magnitude higher, paving the way towards
high-capacity and noise-resilient quantum networks.
