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After creating the first multi-node quantum network, Dutch physicists at the QuTech Research Institute showed for the first time that quantum information can reliably teleport between network nodes that are not directly connected to each other.
Future quantum internet applications will draw their power from the ability to exchange quantum information (i.e., qubits) over a network. This will enable all sorts of applications, such as the secure exchange of information, the linking of multiple quantum computers to increase their computing power, or the use of high-precision coupled quantum sensors. The nodes of such a quantum network consist of small quantum processors.
They can be connected with ordinary optical fibers, but the loss of photons in the fibers (especially over long distances) limits the quality or accuracy of the connection. However, when a photon is lost, the quantum information it carries is also lost. Quantum teleportation offers the best way to reliably transfer quantum information between distant nodes even in the presence of network connections with high losses. But so far, no one has been able to exchange quantum information between two nodes that were not directly connected to each other within the network.
Teleportation in three steps
As you might imagine, in quantum teleportation, the qubit disappears from the transmitter side to reappear from the receiver side without crossing the space in between – so there is no chance of it being lost. Teleportation, however, requires a quantum entangled link between sender and receiver, a reliable method for reading quantum processors, and a capacity for temporary storage of qubits.
In 2021, the same group of researchers succeeded in creating the world’s first three-node quantum network (a network connecting three quantum processors) for the first time in the world. These nodes were some distance away, in the same building; one of them (named Bob) had a physical connection (optical fiber) to the other two (Alice and Charlie), which allowed him to establish intricate connections with each of them (each node had a communication qubit). Bob was given an extra qubit as a memory – he could store the previously generated quantum bond while a new bond was being established.
After the establishment of the Alice-Bob and Bob-Charlie quantum links, a set of quantum operations made it possible to create the Alice-Charlie quantum link. QuTech researchers established entanglement between three nodes and demonstrated that it is possible to teleport qubits between two neighboring nodes. This time, for the first time, they achieved teleportation between non-adjacent nodes: they teleported qubits from Charlie’s node to Alice’s node thanks to the intermediary node Bob.
This took three steps. First of all, it was necessary to establish the quantum entanglement between Alice and Charlie, as explained above. Next, we needed to create a qubit for teleportation. Finally, Charlie teleports to Alice: the researchers jointly measured Charlie’s qubit (the sender) and half of it in an entangled state (Alice has the other half). This measurement caused the teleportation of the quantum state of the qubit, i.e. the disappearance of information from Charlie’s side and its instantaneous appearance from Alice’s (receiver) side.
High precision procedure
Note that the quantum state appeared encrypted at the Alice level: the encryption key is determined by the result of Charlie’s measurement. So Charlie sends the result of the measurement to Alice, after which Alice can decode the qubit. After that, quantum information can be used. The researchers report that Alice was able to restore the quantum state with an accuracy of 71%. The experience is summarized in this video provided by QuTech:
To achieve this teleportation, the researchers improved their experimental device in several ways, starting with the detection system. Previously, signals indicating entanglement came from the same photodetectors that detected the photons used for this entanglement, which could lead to false signals. This time around, the team has implemented an additional detection path to rule out these false announcement signals. They also improved the qubit reading procedure and implemented active protection of memory qubits during entanglement generation.
The team now plans to increase the number of memory qubits to run more complex protocols, according to Physics World. It is also planned to operate the system outside the laboratory, in particular, through the optical fibers of a real network.
In the meantime, further research will be directed towards undoing steps 1 and 2 of the teleportation protocol, meaning that the team will attempt to create a teleportation qubit first before preparing the teleporter to perform the teleportation. This proves to be particularly difficult, since the quantum information to be teleported must be preserved during entanglement. This approach still has a considerable advantage, because in this case, teleportation can be carried out exclusively “on demand”, the researchers explain.