While the current internet relies on digital bits, or 0s and 1s, transmitted through electrical or optical signals, the quantum internet operates on a whole new level. It harnesses the power of quantum bits, or qubits, which are based on the intricate quantum property of entanglement.

This phenomenon links particles in such a way that measuring one instantly affects the state of another, regardless of the distance between them. Transmitting entangled qubits over vast distances necessitates a quantum repeater, a device capable of storing the entangled state in memory and replicating it for further transmission. These repeaters must be strategically placed along a long-distance network to maintain signal integrity from start to finish.

Although quantum repeaters are not a reality, two research groups have demonstrated enduring entanglement memory within quantum networks spanning tens of kilometres, a crucial feature for such devices.

Can Knaut and his team at Harvard University establish a quantum network with two nodes connected by a 35-kilometre optical fibre loop in Boston. Each node comprises a communication qubit for information transfer and a memory qubit for storing the quantum state for up to a second.

"Our experiment has brought us tantalisingly close to a quantum repeater demonstration," stated Knaut.

The team entangled a photon sent from the first node containing a specific type of diamond with the second node containing a similar diamond. Upon arrival, the photon entangled both nodes, and the diamonds were capable of preserving this state for a second.

Knaut anticipates that a fully operational quantum repeater using this technology could be showcased within the next few years, potentially connecting cities or countries through quantum networks.

Meanwhile, Xiao-Hui Bao and his colleagues at the University of Science and Technology of China linked three nodes, each around 10 kilometres apart in Hefei.

Their nodes employed supercooled rubidium atom clouds to produce entangled photons, which were then transmitted across the nodes. The central node coordinates the photons to connect the atom clouds as a memory form.

A significant breakthrough for Bao's network was synchronising the photon frequencies at the central node, a critical step for quantum repeaters linking different nodes. Although their storage time was shorter than Knaut's team, at 100 microseconds, it was sufficient for performing meaningful operations on the transmitted data.