  Entanglement [1, 2] enables powerful new quantum technologies [3-8], but in
real-world implementations, entangled states are often subject to decoherence
and preparation errors. Entanglement distillation [9, 10] can often counteract
these effects by converting imperfectly entangled states into a smaller number
of maximally entangled states. States that are entangled but cannot be
distilled are called bound entangled [11]. Bound entanglement is central to
many exciting theoretical results in quantum information processing [12-14],
but has thus far not been experimentally realized. A recent claim for
experimental bound entanglement is not supported by their data [15]. Here, we
consider a family of four-qubit Smolin states [16], focusing on a regime where
the bound entanglement is experimentally robust. We encode the state into the
polarization of four photons and show that our state exhibits both entanglement
and undistillability, the two defining properties of bound entanglement. We
then use our state to implement entanglement unlocking, a key feature of Smolin
states [16].
