(Physics.aps.org) As researchers worldwide work toward a potential quantum internet, a major roadblock remains: How to build a device called a quantum repeater.
In the last decade or so, researchers around the world have taken big steps toward building quantum networks. While many groups have started testing small networks tens of miles in size, major obstacles, including the need to develop a key piece of hardware, lie in the way of larger quantum networks.
the same properties that make quantum networks useful present significant challenges. Ground-based networks, whether classical or quantum, often use optical fibers to direct information from place to place in the form of photons. As photons travel through a network, some will be lost over time as a result of impurities in the fibers, weakening the signal. In classical networks, devices called “repeaters” intermittently detect the signal, amplify it, and send it off again. But for information carried by photons in superpositions of states, or qubits, “it’s not possible to read the signal without perturbing it.
The key to long-distance quantum communication, researchers say, is to figure out how to build a “quantum repeater” equivalent to the existing classical one. Without a quantum repeater, a qubit would typically only be able to travel through a few miles or up to about 100 miles of fiber—far too little range for widespread networks.

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