  A quantum internet is the holy grail of quantum information processing,
enabling the deployment of a broad range of quantum technologies and protocols
on a global scale. However, numerous challenges exist before the quantum
internet can become a reality. Perhaps the most crucial of these is the
realization of a quantum repeater, an essential component in the long-distance
transmission of quantum information. As the analog of a classical repeater,
extender, or booster, the quantum repeater works to overcome loss and noise in
the quantum channels comprising a quantum network. Here, we review the
conceptual frameworks and architectures for quantum repeaters, as well as the
experimental progress towards their realization. We also discuss the various
near-term proposals to overcome the limits to the communication rates set by
point-to-point quantum communication. Finally, we overview how quantum
repeaters fit within the broader challenge of designing and implementing a
quantum internet.
