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  • Open Access

Quantum-Network Nodes with Real-Time Noise Mitigation Using Spectator Qubits

S.J.H. Loenen1,2,*, Y. Wang1,3,*, N. Demetriou1,2, C.E. Bradley1, and T.H. Taminiau1,2,†

  • *These authors contributed equally to this work.
  • †Contact author: T.H.Taminiau@TUDelft.nl
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PRX Quantum 7, 020306 – Published 10 April, 2026

DOI: https://doi.org/10.1103/zgm8-6b8l

Abstract

Quantum networks might enable quantum communication and distributed quantum computation. Solid-state defects are promising platforms for such networks, because they provide an optical interface for remote entanglement distribution and a nuclear-spin register to store and process quantum information. A key challenge toward larger networks is to improve the storage of previously generated entangled states during new entanglement generation. Here, we introduce a method that uses “spectator” qubits combined with real-time decision making and feedforward to mitigate dephasing of stored quantum states during remote entanglement sequences. We implement the protocol using a single nitrogen-vacancy (NV) center in diamond and demonstrate improved memory fidelity. Our results show that spectator qubits can improve quantum network memory using minimal overhead and naturally present resources, making them a promising addition for near-term testbeds for quantum networks.

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Popular Summary

Color centers in solids provide a promising platform for developing quantum networks. In such networks, the color-center electron spin couples to photons and acts as a communication qubit for distributing entanglement between nodes, while magnetically coupled nuclear spins serve as data qubits to store and process quantum states. To efficiently build large networks, entanglement must be distributed repeatedly, while the data qubits preserve previously generated quantum states. However, the entanglement process itself introduces noise that causes decoherence on the data qubits, limiting performance. In this work, the researchers introduce additional spectator qubits that monitor this noise and counteract it in real time through feedforward operations.

The noise arises from random evolution of the electron spin during the control sequences to distribute entanglement, which imprints unknown phases on the data qubits. Spectator qubits that are coupled to the same electron spin record the noise, enabling partial reversal of the decoherence on the data qubits. The researchers develop a mathematical framework for this concept, analyze strategies for implementation and optimization, and experimentally demonstrate improved protection of data qubits in a nitrogen-vacancy color center in diamond.

These results demonstrate a low-cost method for error reduction that leverages spin qubits already present in the system. This approach can be combined with other error-correction techniques and could help enable more complex protocols in near-term quantum networks.

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Outline

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PRX Quantum 7, 020306– Published 10 April, 2026
  • Received 20 May 2025
  • Revised 12 November 2025
  • Accepted 13 January 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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