Researchers Demo Multi-Node Quantum Network

Article By : John Walko

Researchers have demonstrated key quantum network protocols that could form the basis of a future quantum Internet.

Researchers at the Dutch QuTech research center in Delft have demonstrated a multi-node quantum network capable of connecting three quantum processors, or ‘qubits’.

They used the system to show a proof-of-principle demonstration of key quantum network protocols that they suggest could form the basis of a future quantum Internet.

Matteo Pompili (left) and Sophie Hermans (right), both PhD student in the group of Ronald Hanson, at one of the quantum network nodes.

The work is claimed to be the first time that more than two qubits — the quantum bits that do the calculations in quantum computing — have been linked together to form network endpoints.

It should be noted that scientists around the world have already shown or are working on similar networks, but to date have only reported on successfully linking two quantum processors, generally deploying fiber optics.

The Dutch researchers say their quantum network will open up a range of novel applications, from unhackable communications and cloud computing with complete user privacy to high-precision time-keeping. “And like with the Internet 40 years ago. There are probably many applications we cannot foresee right now,” commented Matteo Pompili, a member of the QuTech team.

The research is a collaboration between scientists at Delft University of Technology and TNO.

Arguably the biggest challenge in linking qubits together into a working quantum network is in establishing and maintaining a process called entanglement, which Albert  Einstein dubbed “spooky action at a distance.” The phenomenon, observed at the quantum scale, occurs when two quantum particles are coupled such that they become fundamentally connected, irrespective of how far they are from each other.

At its simplest, entanglement allows scientists to alter the state of a particle by changing the state of its distant entangled partner and in this way transmitting information across large gaps. However, maintaining a state of entanglement is very difficult, not least because the entangled system is prone to interact with the outside world and thus being destroyed by a phenomenon called decoherence.

Being able to pass on quantum information through intermediate nodes (analogous to routers in the classic Internet) is vital to building a working quantum network, say the scientists.

To solve the problem, the researchers devised a quantum network that comprises three quantum nodes, at some distance within the same building. The researchers had to invent a novel architecture that enables scaling beyond a single link so that they can operate as a true network.

In such a set-up, the photons would essentially ‘pass’ the entanglement from a qubit at one of the outer nodes to one at the middle one. This qubit has two; one to acquire an entangled state and another to store it. Once the entanglement between one outer node and middle node is stored, the middle on entangles the other outer node and the middle node is stored.

Finally, the middle node entangles its two qubits, causing those of the outer nodes to become entangled.

The researchers outlined the scheme in a more literal way in a recent report of the work .

Researchers work on one of the quantum network nodes, where mirrors and filters guide the laser beams to the diamond chip.

The middle node, dubbed Bob, has physical connections to both outer nodes (called Alice and Charlie), allowing entanglement links with both 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.

Having established the quantum links Alice-Bob and Bob-Charlie, a set of quantum operations at Bob converts there links into a further quantum link, Alice-Charlie.

The researchers also describe an alternative scenario where, by performing a different set of quantum operations at Bob, entanglement between all three nodes is established.

Another important feature of the network is that it announces the successful completion of these protocols with what is termed a ‘flag’ signal. This, again, is said to be a first and will be crucial for scalability, as in a future quantum internet, many such protocols would have to be concatenated.

The network created at Delft will be used as a test-bed for both developing and testing quantum internet hardware, software and protocols. And according to Professor Ronald Henson of the Technical University of Delft, who leads the research team, the groups at QuTech are already looking into future compatibilities with existing  data infrastructures.

The next step after that is scheduled to involve testing the proof-of-concept outside of the lab on an existing telecom fibre — on QuTech’s Quantum Internet Demonstrator, and the first metropolitan   links are expected to be complete late next year.

For now, all the nodes are within a short 10 to 20 meters to each other.  The researchers are aware that for this to be of practical use, the links will need to me more like kilometres apart. The greater the distance of travel, the more the number of quantum devices and intermediary nodes, and thus the more powerful and useful such a quantum network could be.

Pompili said the lab work is currently focusing on adding more quantum bits to the three-node network, as well as 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. That is the end goal”, he stressed.

Of course the Dutch team are not alone in such research. There are numerous other teams working –notably in the US and China–on similar schemes, but, as noted, to date have only reported linking two quantum processors.

For instance, earlier this year, researchers at the Cleland Labs of the University of Chicago showed how it was possible to entangle two separate qubits by connecting them via a cable. And scientists in China claim to have achieved entanglement over a record-breaking 1,200km.

This article was originally published on EE Times.

John Walko is a technology writer and editor who has been covering the electronics industry since the early 1980s. He started tracking the sector while working on one of the UK’s oldest weekly technology titles, The Engineer, then moved to CMP’s flagship UK weekly, Electronics Times, in a variety of roles including news deputy and finally editor in chief. He then joined the online world when CMP started the EDTN Network, where he edited the daily electronics feed and was founding editor of (which, over the years, has become the Wireless and Networking Designline). He was editor of EE Times Europe at its launch and subsequently held various positions on EE Times, in the latter years, covering the growing wireless and mobile sectors.

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