  The reversible transfer of quantum states of light in and out of matter
constitutes an important building block for future applications of quantum
communication: it allows synchronizing quantum information, and enables one to
build quantum repeaters and quantum networks. Much effort has been devoted
worldwide over the past years to develop memories suitable for the storage of
quantum states. Of central importance to this task is the preservation of
entanglement, a quantum mechanical phenomenon whose counter intuitive
properties have occupied philosophers, physicists and computer scientists since
the early days of quantum physics. Here we report, for the first time, the
reversible transfer of photon-photon entanglement into entanglement between a
photon and collective atomic excitation in a solid-state device. Towards this
end, we employ a thulium-doped lithium niobate waveguide in conjunction with a
photon-echo quantum memory protocol, and increase the spectral acceptance from
the current maximum of 100 MHz to 5 GHz. The entanglement-preserving nature of
our storage device is assessed by comparing the amount of entanglement
contained in the detected photon pairs before and after the reversible
transfer, showing, within statistical error, a perfect mapping process. Our
integrated, broadband quantum memory complements the family of robust,
integrated lithium niobate devices. It renders frequency matching of light with
matter interfaces in advanced applications of quantum communication trivial and
institutes several key properties in the quest to unleash the full potential of
quantum communication.
