A new register with thousands of entangled nuclei to scale quantum networks

Published in Nature Physics, the work demonstrates the introduction of a new type of optically connected qubits -- a critical advance in the development of quantum networks, where stable, scalable, and versatile quantum nodes are essential.

Quantum dots are nanoscale objects with unique optical and electronic properties that come from quantum mechanical effects. These systems are already used in technologies like display screens and medical imaging, and their adoption in quantum communication has been mostly due to their ability to operate as bright single-photon sources. However, effective quantum networks need more than just single-photon emission; they also require stable qubits that can interact with the photons and store quantum information locally. The new research builds on the inherent spins of the atoms forming the quantum dots as a functioning many-body quantum register to store information over extended periods.

A many-body system refers to a collection of interacting particles -- here, the nuclear spins inside the quantum dot -- whose collective behaviour gives rise to new, emergent properties that are not present in individual components. By using these collective states, the researchers created a robust and scalable quantum register.

The Cambridge team, in close collaboration with colleagues at the University of Linz, successfully prepared 13,000 nuclear spins into a collective, entangled state of spins known as a 'dark state.' This dark state reduces interaction with its environment, leading to better coherence and stability, and serves as the logical 'zero' state of the quantum register. They introduced a complementary 'one' state as a single nuclear magnon excitation -- a phenomenon representing a coherent wave-like excitation involving a single nuclear spin flip propagating through the nuclear ensemble. Together, these states enable quantum information to be written, stored, retrieved, and read out with high fidelity. The researchers demonstrated this with a complete operational cycle, achieving a storage fidelity of nearly 69% and a coherence time exceeding 130 microseconds. This is a major step forward for quantum dots as scalable quantum nodes.

"This breakthrough is a testament to the power many-body physics can have in transforming quantum devices," said Mete Atatüre, co-lead author of the study and Professor of Physics at the Cavendish Laboratory. "By overcoming long-standing limitations, we've shown how quantum dots can serve as multi-qubit nodes, paving the way for quantum networks with applications in communication and distributed computing. In the 2025 International Year of Quantum, this work also highlights the innovative strides being made at the Cavendish Laboratory toward realising the promise of quantum technologies."

The work represents a unique marriage of semiconductor physics, quantum optics, and quantum information theory. The researchers utilised advanced control techniques to polarise nuclear spins in gallium arsenide (GaAs) quantum dots, creating a low-noise environment for robust quantum operations.

"By applying quantum feedback techniques and leveraging the remarkable uniformity of GaAs quantum dots, we've overcome long-standing challenges caused by uncontrolled nuclear magnetic interactions," explained Dorian Gangloff, co-lead author of the project and Associate Professor of Quantum Technology. "This breakthrough not only establishes quantum dots as operational quantum nodes but also unlocks a powerful platform to explore new many-body physics and emergent quantum phenomena."

Looking ahead, the Cambridge team aims to extend the time their quantum register can store information to tens of milliseconds by improving their control techniques. These improvements would make quantum dots suitable as intermediate quantum memories in quantum repeaters -- critical components for connecting distant quantum computers. This ambitious goal is the focus of their new QuantERA grant, MEEDGARD, a collaboration with Linz and other European partners, to advance quantum memory technologies with quantum dots. Their current research was supported by EPSRC, the European Union, the US Office of Naval Research, and the Royal Society.