Physicists at TU Delft have sent information from one side to the other in a simple quantum network. The information is ‘teleported’ along an intermediate node.
A quantum network is a network in which information is sent using so-called quantum entanglement. Particles that are entangled with each other have coupled properties. This means that if such a property changes in one particle, the other particle immediately changes with it – even if the particles are not close to each other.
Creating such a network is an important step towards a quantum internet. Such an internet is in principle a lot more difficult to hack than the current internet systems.
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The old-timers
You can build a quantum network with interconnected qubits – the quantum variant of traditional computer bits. These must then be entangled with qubits elsewhere in the network, rather than with the neighboring qubits to which they are already directly connected. Until now, network qubit entanglement has only been demonstrated for directly connected qubits.
science fiction
The team of physicist Ronald Hanson of TU Delft now has a simple network built from diamond-made qubits† They were arranged in three nodes, which, as always, were called Alice, Bob and Charlie. There was no direct connection between Alice and Charlie, only an indirect connection through Bob. Still, Alice and Charlie were entwined.
When two qubits are entangled, it is impossible to measure the state of one particle without changing the state of the other particle. So when Charlie’s quantum state was changed, Alice’s state also changed. This means that information ‘teleported’ past Bob without being modified.
“It’s really teleportation the way you see it in science fiction movies,” Hanson says. The condition, or information, disappears on one side and reappears on the other. Because the data doesn’t travel through the intervening space, it can’t be lost either.’
It was theoretically shown decades ago that you can exploit entanglement in this way. But only in this experiment has it been successfully proven in practice. This was possible because some of the qubits on the nodes were ‘memory qubits’. They can hold quantum states longer than standard qubits.
Eavesdropping safe
In a quantum internet, two nodes in the network change at exactly the same time. However, it is not necessarily faster than a conventional system. This is because users have to share information about network changes through traditional forms of communication.
A quantum web does have the advantage that it offers truly private functions, such as secure communication or data servers that can never discover the source of the data they crack. ‘In addition, there are probably many applications that we have yet to discover,’ says Hanson.
photons
Hanson and his team have thus been the first to build a quantum network in which non-neighboring nodes are entangled. In addition, other teams have experimented in recent years with various forms of quantum communication, including entangled photons.
“Testing these kinds of experiments on different platforms is very important,” says physicist Charles Adams from Durham University in the United Kingdom. ‘We don’t know yet which technology will succeed – maybe it will be some kind of mixture of different technologies.’