World’s First Quantum Network Built with Two Atoms, One Photon

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Andreas Neuzner / Max Planck Institute of Quantum Optics

We’ve just heard about a working quantum computer tucked inside a diamond, and we’re already talking about quantum networks that could herald a quantum Internet? Yep, and in fact more than just talking: Scientists say they’ve managed to create the world’s first working quantum network, a functional analogue of the kind you probably use at home or work, only in the quantum version, individual atoms form the network nodes and information is shuttled back and forth by photons.

Those photons zip along a 60-meter fiberoptic cable, bounding between two single atom “nodes” capable of transmitting, receiving as well as storing information. It works just like your garden variety telecommunications network, only with quantum data, and it’s no longer science fiction thanks to a team of German scientists at the Max Planck Institute of Quantum Optics (MPQ).


Remember back in the 1990s when the futurism buzz phrase was “quantum teleportation”? Where we discovered that quantum information could be transported from one location to another — to date, over a distance of about 10 miles — without the quantum bit (or “qubit”) itself being transmitted? It wasn’t quite Star Trek‘s transporter room, since the particles weren’t actually popping through space-time, but it’s often cited as the catalyst for the notion of quantum networks. An actual quantum network would need to be capable of sending, receiving and storing information reliably. The MPQ team’s quantum network is the first ever to do so.

“We have realized the first prototype of a quantum network,” said Dr. Stephan Ritter, the project’s lead. “We achieve reversible exchange of quantum information between the nodes.”

In a classical network, bits are represented by ones or zeroes. But in a quantum network, a quantum bit can represent both the “0” and “1” at the same time, a brain-twister known as superposition. In computing terms, that means quantum bits can operate much faster than classical ones. How do those quantum bits communicate? By mapping information about their quantum states to carrier particles — in this case, photons. The photons then move along a fiber optic cable conveying qubit states to other qubits, which can then send information back in the same fashion. And, as importantly, this quantum network is scalable, meaning it could be eventually used for communication over very long distance.

“We were able to prove that the quantum states can be transferred much better than possible with any classical network. In fact, we demonstrate the feasibility of the theoretical approach developed by professor Cirac,” said Ritter.

Professor Ignacio Cirac, a director at MPQ, proposed the framework for the experiment. In his team’s quantum network, individual rubidium atoms were lodged between two highly reflective mirrors placed less than a millimeter apart — a setup referred to as an “optical cavity.” The team then fired a laser at one of the atoms, calibrated so as not to disturb it and instead cause it to emit a photon, which then traversed the 60-meter fiberoptic cable to be absorbed by the second atom, transferring the first atom’s quantum information. What’s more, since the team was able to entangle both atoms — that is, get them to interact, then separate — it follows that additional atoms could be involved in the process, illustrating the system’s scalability.

“Entanglement of two systems separated by a large distance is a fascinating phenomenon in itself,” said Ritter. “However, it could also serve as a resource for the teleportation of quantum information. One day, this might not only make it possible to communicate quantum information over very large distances, but might enable an entire quantum internet.”

The experiment’s results were just published in the science journal Nature.
