  Heisenberg's uncertainty principle provides a fundamental limitation on an
observer's ability to simultaneously predict the outcome when one of two
measurements is performed on a quantum system. However, if the observer has
access to a particle (stored in a quantum memory) which is entangled with the
system, his uncertainty is generally reduced. This effect has recently been
quantified by Berta et al. [Nature Physics 6, 659 (2010)] in a new, more
general uncertainty relation, formulated in terms of entropies. Using entangled
photon pairs, an optical delay line serving as a quantum memory and fast,
active feed-forward we experimentally probe the validity of this new relation.
The behaviour we find agrees with the predictions of quantum theory and
satisfies the new uncertainty relation. In particular, we find lower
uncertainties about the measurement outcomes than would be possible without the
entangled particle. This shows not only that the reduction in uncertainty
enabled by entanglement can be significant in practice, but also demonstrates
the use of the inequality to witness entanglement.
