  Entanglement is one of the most fascinating aspects distinguishing quantum
from classical physics. It is the backbone of quantum information processing
which relies on engineered quantum systems. It also exists in natural systems
such as atoms and molecules, showcased in many experimental instances mostly in
the form of entangled photon pairs and a few examples of entanglement between
massive particles. Nevertheless, the control of entanglement in natural systems
has never been demonstrated. In artificially prepared quantum systems, on the
other hand, the creation and manipulation of entanglement lies at the heart of
quantum computing currently implemented in a wide array of two-level systems
(e.g. trapped ions, superconducting and semiconductor systems). These processes
are, however, relatively slow: the time scale of the entanglement generation
and control ranges from a couple of {\mu}s in case of trapped-ion quantum
systems down to tens of ns in superconducting systems. In this letter, we show
ultrafast optical control of entanglement between massive fundamental particles
in a natural system on a time scale faster than that available to engineered
systems. We demonstrate the sub-femtosecond control of electronic entangled
states in a single hydrogen molecule by applying few-photon interactions with
adjustable relative delays. This molecular entanglement is revealed in the
asymmetric electron emission with respect to the ejected proton in the
photodissociation of H2. We anticipate that these results open the way to
entanglement-based operations at THz speed.
