Quantum communication without entanglement could perform faster than previously thought possible

In order to build a quantum internet – a network that is faster and more secure than the current internet – the key is the ability to transmit quantum information between remote quantum computers (i.e., nodes). The most familiar approach involves entangling the links between nodes and then using quantum repeaters at intermediate locations to provide entanglement swapping, extending the range of entanglement across km-long networks.



In such a system, the performance is inherently limited by the time it takes to establish entanglement between nodes. This time is at best the classical signaling time between the nodes, but with many schemes it is even longer, and increases as network size increases. Since the qubits that store the quantum information are unstable and quickly decohere, quantum memories are required to store quantum information for milliseconds or longer while they wait for entanglement. The result is a theoretical limitation on speed due to the system's design and the need for additional components – quantum memories – to enable a functioning network.





In a new study published in Nature Photonics, scientists from Japan and the UK have presented an alternative design for a quantum communication network that requires neither entanglement between nodes nor quantum memories. Instead, the scheme transmits quantum information in encoded form directly across the network, using devices that act as quantum repeaters (without entanglement) to transmit and receive information between each other. Because this approach doesn't use entanglement, the communications rate is not limited by the conventional restraints that limit the rates of entanglement-based approaches.