Sending quantum information from Alice to Charlie

Researchers supported by the EU-funded QIA and QNETWORK projects have demonstrated how quantum information can be teleported between two nodes that have no direct link to each other. Published in the journal ‘Nature’, their work brings us yet another step closer to a quantum internet with faster and more secure communication.

Just like in our current computer networks, quantum computers will need to be linked to transfer quantum bits, or qubits, of information between them. However, sending this information from one location, or node, to another can be problematic: If ordinary optical fibres are used, the loss of photons within the fibre results in information being lost. This problem can be overcome using a phenomenon called quantum entanglement, which makes it possible to connect two nodes separated by vast distances and in this way teleport information between nodes.

To be able to teleport qubits from one node to another, changes need to be made to the sender’s qubit by performing a so-called Bell state measurement (BSM) on it. This causes the qubit’s quantum state to disappear from the sender’s node and reappear at the receiver’s node in encrypted form. Finally, the BSM outcome is sent to the receiver via another channel (e.g. optical fibre), explaining what operation needs to be performed so that the quantum state – and hence the teleported information – can be decrypted.This had been done before with two adjacent nodes named Alice and Bob. Now, the researchers were able to demonstrate the teleportation of qubits between Alice and a third node, Charlie, by creating an entanglement between them through Bob.

For the teleportation experiment, the research team used the three-node quantum network they had created in 2021 at Dutch research institute QuTech (founded by QIA and QNETWORK project coordinator Delft University of Technology (TU Delft) together with independent research organisation TNO). However, to achieve high-fidelity teleportation, they first performed several upgrades. They solved the problem of false heralding signals caused by an unwanted second photon, addressed the issue of spectral diffusion, and improved memory qubit protection and the qubit readout procedure.

These improvements made it possible to teleport qubits between the non-neighbouring nodes of Alice and Charlie. The team first entangled Alice’s qubit with Charlie’s via Bob’s. The BSM was then applied to Charlie’s qubit, which resulted in its quantum state being teleported to Alice. The research team then sent the BSM outcome to Alice and retrieved the encrypted information with a fidelity of around 71 %.

According to a news item posted on ‘Physics World’, the next step envisioned by study senior author Prof. Ronald Hanson of TU Delft is to increase the number of memory qubits and to test the technology in a real network. “We are also cooperating with computer scientists to develop the quantum network control stack – a similar stack of control layers that currently run the Internet we all use today,” remarks Prof. Hanson, who is also one of the founders of QuTech.

Realised with support from QIA (Quantum Internet Alliance) and QNETWORK (Quantum networks wired by multi-spin entanglement), this work represents an important building block for the quantum networks of the future. It also paves the way for research into teleportation-based multi-node protocols and applications.

For more information, please see:

QIA project website

QNETWORK project