  We investigate the witnessing of two-qubit entangled states by sequential and
independent pairs of observers, with both observers of each pair acting
independently on their part of the shared state from spatially separated
laboratories, and subsequently passing their qubits to the next pair in the
sequence. It has previously been conjectured that not more than one pair of
observers can detect Clauser-Horne-Shimony-Holt "Bell-nonlocal" correlations in
a similar set-up. This is intriguing since it is possible to have an
arbitrarily long sequence of Bell-nonlocal correlations when only a single
observer is allowed to share a bipartite state with multiple observers at the
other end. It is therefore interesting to ask whether such restrictions are
also present when entangled correlations are considered in the scenario of
multiple pairs of observers. We find that a two-qubit entangled state can be
used to witness entanglement arbitrarily many times, by pairs of observers,
acting sequentially and independently. We prove the statement to be true when
the initial pair of observers in the sequence share any pure entangled state or
when they share a state from a class of mixed entangled states. We demonstrate
that the phenomenon can also be observed for a certain class of entangled
states in which an arbitrarily long sequence of observer pairs witnessing
entanglement is reached in the limit of the initial entanglement content
tending to a vanishing amount.
