Physicists Harness 13,000 Entangled Spins To Unlock the Power of the “Dark State”

Scientists have harnessed many-body physics to transform quantum dots into scalable, stable quantum nodes.

By entangling nuclear spins into a ‘dark state,’ they created a quantum register capable of storing and retrieving quantum information with high fidelity. This leap forward brings quantum networks closer to reality, unlocking new possibilities for communication and computing.

A New Breakthrough in Quantum Networks

Published in Nature Physics, this research introduces a new type of optically connected qubits — an important step toward developing quantum networks that require stable, scalable, and adaptable quantum nodes.

Quantum dots are nanoscale structures with unique optical and electronic properties derived from quantum mechanics. Already used in technologies like display screens and medical imaging, they have gained attention in quantum communication due to their ability to emit single photons.

“This breakthrough is a testament to the power many-body physics can have in transforming quantum devices.”

Mete Atatüre

However, building effective quantum networks requires more than just photon emission. They also need stable qubits that can interact with photons and locally store quantum information. This study leverages the atomic spins within quantum dots, using them as a many-body quantum register capable of storing information for extended periods.

Harnessing Many-Body Physics for Quantum Storage

A many-body system refers to a collection of interacting particles—here, the nuclear spins inside the quantum dot—whose collective behavior 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.

Unlocking the Potential of Quantum Dots

“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 realizing the promise of quantum technologies.”

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

Overcoming Long-Standing Challenges

“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.”

The Future of Quantum Memory and Networks

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.

Reference: “A many-body quantum register for a spin qubit” by Martin Hayhurst Appel, Alexander Ghorbal, Noah Shofer, Leon Zaporski, Santanu Manna, Saimon Filipe Covre da Silva, Urs Haeusler, Claire Le Gall, Armando Rastelli, Dorian A. Gangloff and Mete Atatüre, 24 January 2025, Nature Physics.
DOI: 10.1038/s41567-024-02746-z

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