  Einstein-Podolsky-Rosen steering is a quantum phenomenon wherein one party
influences, or steers, the state of a distant party's particle beyond what
could be achieved with a separable state, by making measurements on one half of
an entangled state. This type of quantum nonlocality stands out through its
asymmetric setting, and even allows for cases where one party can steer the
other, but where the reverse is not true. A series of experiments have
demonstrated one-way steering in the past, but all were based on significant
limiting assumptions. These consisted either of restrictions on the type of
allowed measurements, or of assumptions about the quantum state at hand, by
mapping to a specific family of states and analysing the ideal target state
rather than the real experimental state. Here, we present the first
experimental demonstration of one-way steering free of such assumptions. We
achieve this using a new sufficient condition for non-steerability, and,
although not required by our analysis, using a novel source of extremely
high-quality photonic Werner states.
