Researchers at Qutech are the first to demonstrate the transfer of information between non-neighboring nodes in a mini quantum network.
In 1997, researchers succeeded for the first time in transferring the polarization state of one photon to another. This successful demonstration of quantum teleportation was hailed as the first step toward building a quantum network, which has potential applications in secure communications, quantum computing and the development of a next-generation internet.
It took a quarter of a century, however, to expand ‘point-to-point’ teleportation to anything resembling a network. Due to the fiddly nature of quantum systems, it was only possible to teleport quantum information between nodes that are directly connected. Now, Qutech researchers have overcome that hurdle. In an article published in Nature, they demonstrate teleportation between two non-neighboring nodes in a rudimentary three-node network.
Teleportation may conjure up associations with science fiction movies, in which people and objects are transferred over large distances by taking them apart at the molecular level and putting them back together at the destination. That’s not quite how quantum teleportation works, however, since this involves the transfer of information only. All particles stay put, it’s just their quantum states that get transported.
The immobility of the particles is, in fact, considered to be a major advantage for quantum communication. After all, having delicate quantum states on the move is asking for trouble. A photon traveling through an optic fiber cable, for example, will encounter all kinds of disturbances and could even get lost altogether. Quantum teleportation should make the connection much more reliable.
The Qutech researchers worked with qubits based on defects in the crystal lattice of diamonds. Their network consists of three quantum nodes, nicknamed Alice, Bob and Charlie. Alice and Charlie have no direct physical connection, but they’re both directly linked to Bob. Acting as an intermediary, Bob sets up the connection between Alice and Charlie by interacting with both. That’s why only Bob is equipped with a memory bit as well as a communication bit: Bob needs to relay what was ‘said.’
Once Alice’s and Charlie’s qubits are entangled via Bob, a so-called Bell-state measurement causes the information to disappear on Charlie’s side and appear immediately on Alice’s side. This doesn’t mean that teleportation enables communication faster than the speed of light because additional information is needed to ‘decrypt’ the transferred qubit. This key, which is actually the outcome of the Bell-state measurement, needs to be sent classically to Alice.
Some way off
The authors’ main achievement was to improve the fidelity of the steps involved, allowing the addition of an extra node (which obviously involves more steps). For example, they protected the link between Alice and Bob in a shielded quantum memory qubit while the link between Bob and Charlie was being formed. This doubled the formation rate of sufficient-quality entanglement links in comparison with previous efforts. The researchers also implemented a novel memory readout protocol that efficiently discarded readouts with a high probability of being unreliable.
“This achievement isn’t only a win for fundamental science but also represents an advance in the real-world problem-solving required to move this fascinating quantum application to the next step,” comment Oliver Slattery at the US National Institute of Standards and Technology, and Yong-Su Kim, senior researcher at the Korea Institute of Science and Technology, in an accompanying Nature article. However, the experts caution that practical applications are still some way off. “This work makes clear the massive challenge ahead for the true realization of the quantum internet – but the Qutech researchers offer a potential path forward.”