Researchers establish an entanglement-based quantum network

Wednesday, 28 April, 2021

Researchers establish an entanglement-based quantum network

Researchers from Dutch research institute QuTech — a collaboration between Delft University of Technology (TU Delft) and the Netherlands Organisation for Applied Scientific Research (TNO) — have realised what is claimed to be the first multinode quantum network, connecting three quantum processors. In addition, the researchers achieved a proof-of-principle demonstration of key quantum network protocols.

Published in the journal Science, their findings mark an important milestone in the journey towards the quantum internet.

The power of the internet is that it allows any two computers on Earth to be connected with each other, enabling applications undreamt of at the time of its creation decades ago. Today, researchers around the world are working towards first versions of a quantum internet — a network that can connect any two quantum devices, such as quantum computers or sensors, over large distances. Whereas today’s internet distributes information in bits (that can be either 0 or 1), a future quantum internet will make use of quantum bits that can be 0 and 1 at the same time.

“A quantum internet will open up a range of novel applications, from unhackable communication and cloud computing with complete user privacy to high-precision time-keeping,” said Matteo Pompili, a PhD student at TU Delft. “And like with the internet 40 years ago, there are probably many applications we cannot foresee right now.”

The first steps towards a quantum internet were taken in the past decade by linking two quantum devices that shared a direct physical link. However, being able to pass on quantum information through intermediate nodes (analogous to routers in the classical internet) is essential for creating a scalable quantum network. In addition, many promising quantum internet applications rely on entangled quantum bits, to be distributed between multiple nodes.

Entanglement is a phenomenon observed at the quantum scale, fundamentally connecting particles at small and even at large distances. It provides quantum computers their enormous computational power and it is the fundamental resource for sharing quantum information over the future quantum internet. By realising their quantum network in the lab, QuTech researchers successfully connected two quantum processors through an intermediate node and established shared entanglement between multiple standalone quantum processors.

The rudimentary quantum network consists of three quantum nodes, at some distance within the same building. To make these nodes operate as a true network, the researchers had to invent a novel architecture that enables scaling beyond a single link. The middle node (called Bob) has a physical connection to both outer nodes (called Alice and Charlie), allowing entanglement links with each of these nodes to be established. Bob is equipped with an additional quantum bit that can be used as memory, allowing a previously generated quantum link to be stored while a new link is being established. After establishing the quantum links Alice–Bob and Bob–Charlie, a set of quantum operations at Bob converts these links into a quantum link Alice–Charlie. Alternatively, by performing a different set of quantum operations at Bob, entanglement between all three nodes is established.

Researchers work on one of the quantum network nodes, where mirrors and filters guide the laser beams to the diamond chip. Image credit: Marieke de Lorijn for QuTech.

An important feature of the network is that it announces the successful completion of these (intrinsically probabilistic) protocols with a ‘flag’ signal. Such heralding is crucial for scalability, as in a future quantum internet many of such protocols will need to be concatenated.

“Once established, we were able to preserve the resulting entangled states, protecting them from noise,” said team member Sophie Hermans, also a PhD student at TU Delft. “It means that, in principle, we can use these states for quantum key distribution, a quantum computation or any other subsequent quantum protocol.”

The entanglement-based quantum network provides the researchers with a unique testbed for developing and testing quantum internet hardware, software and protocols. As noted by team leader Ronald Hanson, “The future quantum internet will consist of countless quantum devices and intermediate nodes. “Colleagues at QuTech are already looking into future compatibility with existing data infrastructures.”

An animation explaining the experiment. Video credit: Slimplot for QuTech.

In due time, the current proof-of-principle approach will be tested outside the lab on existing telecom fibre — on QuTech’s Quantum Internet Demonstrator, of which the first metropolitan link is scheduled to be completed in 2022. Meanwhile, inside the lab, the researchers will focus on adding more quantum bits to their three-node network and on adding higher level software and hardware layers.

“Once all the high-level control and interface layers for running the network have been developed, anybody will be able to write and run a network application without needing to understand how lasers and cryostats work,” Pompili said. “That is the end goal.”

Top image caption: Co-authors Matteo Pompili and Sophie Hermans, both PhD students in the group of Ronald Hanson, at one of the quantum network nodes. Image credit: Marieke de Lorijn for QuTech.

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