  In this paper, we present a new entanglement monotone for bipartite quantum
states. Its definition is inspired by the so-called intrinsic information of
classical cryptography and is given by the halved minimum quantum conditional
mutual information over all tripartite state extensions. We derive certain
properties of the new measure which we call "squashed entanglement": it is a
lower bound on entanglement of formation and an upper bound on distillable
entanglement. Furthermore, it is convex, additive on tensor products, and
superadditive in general.
  Continuity in the state is the only property of our entanglement measure
which we cannot provide a proof for. We present some evidence, however, that
our quantity has this property, the strongest indication being a conjectured
Fannes type inequality for the conditional von Neumann entropy. This inequality
is proved in the classical case.
