Quantum networks are often described as the future of the internet -- but instead of transmitting classical information in bits, they send quantum information carried by photons. These networks could enable ultra-secure communication, link together distant quantum computers into a single, vastly more powerful machine, and create precision sensing systems that can measure time or environmental conditions with unprecedented accuracy.
To make such a network possible, so-called quantum network nodes -- that can store quantum information and share it via light particles - are needed. In their latest work, the Innsbruck team led by Ben Lanyon at the Department of Experimental Physics of the University of Innsbruck demonstrated such a node using a string of ten calcium ions in a prototype quantum computer. By carefully adjusting electric fields, the ions were moved one by one into an optical cavity. There, a finely tuned laser pulse triggered the emission of a single photon whose polarization was entangled with the ion's state.
The process created a stream of photons; each tied to a different ion-qubit in the register. In future the photons could travel to distant nodes and be used to establish entanglement between separate quantum devices. The researchers achieved an average ion-photon entanglement fidelity of 92 percent, a level of precision that underscores the robustness of their method.
"One of the key strengths of this technique is its scalability," says Ben Lanyon. "While earlier experiments managed to link only two or three ion-qubits to individual photons, the Innsbruck setup can be extended to much larger registers, potentially containing hundreds of ions and more." This paves the way for connecting entire quantum processors across laboratories or even continents.
"Our method is a step towards building larger and more complex quantum networks," says Marco Canteri, the first author of the study. "It brings us closer to practical applications such as quantum-secure communication, distributed quantum computing and large-scale distributed quantum sensing."
Beyond networking, the technology could also advance optical atomic clocks, which keep time so precisely that they would lose less than a second over the age of the universe. Such clocks could be linked via quantum networks to form a worldwide timekeeping system of unmatched accuracy.
The work, now published in Physical Review Letters, was financially supported by the Austrian Science Fund FWF and the European Union, among others, and demonstrates not only a technical milestone but also a key building block for the next generation of quantum technologies.
