Delft Researchers Realize The First Quantum Network Based On Entanglement

A team of researchers from QuTech in Delft has built the first quantum network that connects three quantum processors. In addition, in a proof-of-principle demonstration, they performed two essential quantum network protocols. Their results are an important step towards the future quantum internet and have now been published in Science.

The quantum internet
Thanks to the Internet, computers from anywhere on earth can be connected to each other. Many applications have emerged that were completely unimaginable at the time of the creation of the Internet a few decades ago. Researchers around the world are currently working on the first implementations of the quantum internet - a network that connects quantum devices, such as quantum computers and sensors. Where the information on the current Internet is sent in bits (which can have the value 0 or 1), the future quantum Internet will use quantum bits that can be 0 and 1 at the same time. 'A quantum internet makes a range of new applications possible, from uncrackable communication and cloud computing with complete privacy of users to extremely accurate time registration, 'says Matteo Pompili, PhD candidate and member of the research team. 'And there are probably countless applications that we do not yet foresee, just like with the Internet 40 years ago.'

To a global network
In the past decade, the first steps towards a quantum internet have been taken by connecting two quantum devices through a direct physical link. However, in order to achieve a scalable quantum network, it is essential to be able to transmit quantum information via intermediate nodes - analogous to how routers do this in the classical Internet. In addition, the most promising quantum internet applications require that entangled quantum bits can be distributed across multiple nodes. Entanglement is a phenomenon that occurs on the quantum scale, with which particles are connected in a fundamental way, over small and large distances. It gives quantum computers their unprecedented computing power and is the fundamental component for sharing quantum information about the future quantum internet. By realizing their quantum network in the lab, the QuTech team - a collaboration between Delft University of Technology and TNO - was the first to succeed in connecting two quantum processors via an intermediate node, as well as creating shared entanglement. across multiple independent quantum processors.

How does the quantum network work?
The rudimentary quantum network consists of three nodes, some distance from each other in the same building. The researchers have designed a new architecture that makes it possible to scale up to a real quantum network, consisting of multiple connections. The middle node (named Bob) has a physical connection with the two outer nodes (named Alice and Charlie) so that an entangled connection can be established with each of these nodes. Bob also has an extra quantum bit that serves as memory for storing a quantum connection that has already been established while a subsequent connection is being established. After both quantum connections (Alice-Bob and Bob-Charlie) have been established, Bob converts them into a quantum connection Alice-Charlie with a series of quantum operations.

Ready for further use
An important feature of the network is that it announces with a “flag” signal when these - naturally probabilistic - protocols have been successfully implemented. Such an announcement is crucial because such protocols will have to be implemented in succession in the future scalable quantum internet. “Once established, we were able to keep the entangled connection and protect it from noise,” says Sophie Hermans, another member of the research team. "This means that we can basically use this quantum state to distribute cryptographic keys, perform a quantum calculation, or run any other quantum protocol."

Quantum Internet Demonstrator
This first entangled quantum network provides the researchers with a unique testing facility for developing and testing hardware, software and protocols for the quantum internet. "The future quantum internet will consist of countless quantum devices and intermediate nodes," says Ronald Hanson, leader of the research team. 'Colleagues at QuTech are already studying future compatibility with the existing data infrastructure.' The current proof-of-principle implementation will in due course be tested outside the laboratory, on existing fiber optics for telecommunications - namely on QuTech's Quantum Internet Demonstrator. The first metropolitan connection of this is scheduled to be completed in 2022.

Higher layers
In the lab, the researchers are now focusing on adding more quantum bits to their three-node network and on adding higher software and hardware layers. Pompili: 'Once these control and interface layers have been developed, anyone can develop and implement an application for the quantum network without having to understand how lasers or cryostats work. That is our ultimate goal. '

Financing
The research is supported by the EU Flagship on Quantum Technologies through the Quantum Internet Alliance project (EU Horizon 2020, grant agreement no. 820445); the Netherlands Organization for Scientific Research (NWO) via a VICI grant (project no. 680-47-624) and the Gravitation program Quantum Software Consortium (project no. 024.003.037 / 3368); the European Research Council (ERC) through an ERC Starting Grant (SW); and from a Consolidator Grant (grant agreement no. 772627 to RH) under the Horizon 2020 Research and Innovation Program of the European Union; Marie Skłodowska- Curie Actions - Nanoscale solid-state spin systems in emerging quantum technologies - Spin-NANO, grant agreement no. 676108; and an Erwin-Schrödinger fellowship (QuantNet, no. J 4229-N27) from the Austrian National Science Foundation (FWF).