Engineering Quantum Resources for Quantum Networking Using Single Rare-Earth Ions Inside Crystals

Citation

Wu, Chun-Ju (2026) Engineering Quantum Resources for Quantum Networking Using Single Rare-Earth Ions Inside Crystals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kgjp-xe35. https://resolver.caltech.edu/CaltechTHESIS:10262025-132142523

Abstract

Quantum networks are foundational components of quantum technology, enabling transformative applications in secure communication, distributed quantum computation, and enhanced sensing. Rare-earth ions in solid-state hosts represent a leading platform for building such networks due to their exceptional optical and spin coherence properties. This thesis details the experimental realization of a quantum network node using single ¹⁷¹Yb³⁺ ions in YVO₄ coupled to nanophotonic crystal cavities. We demonstrate the fundamental building blocks of quantum networks and develop multiple advanced capabilities, including multiplexing, protected nuclear spin storage, and high-dimensional qudit control, to expand the platform's power and versatility.

Using this platform, we demonstrate heralded remote entanglement between two physically separate devices. A key innovation is a novel entanglement distribution protocol that employs real-time feedforward to cancel spectral diffusion on timescales slower than a single experiment by rephasing the optical transition based on photon arrival time. We also apply real-time phase compensations to entangle ¹⁷¹Yb ions with different optical frequencies. By combining this novel protocol with multiple spectrally distinguishable ions, we demonstrate heralding of a three-ion W state and implement multiplexed remote entanglement. This multiplexing approach increases the entanglement rate by nearly a factor of two, showcasing a scalable pathway to mitigate network overhead.

Beyond establishing remote entanglement, we explore the local nuclear spin environment of ¹⁷¹Yb as an integrated quantum resource. We harness the four symmetrically located ⁵¹V nuclear spins to generate multi-qubit Greenberger–Horne–Zeilinger states. Critically, we identify and experimentally verify a decoherence-protected subspace within these states that exhibits insensitivity to common-mode magnetic field noise. By developing a sequence to transfer quantum information into this protected subspace, we establish the ⁵¹VV nuclear ensemble as an integrated, noise-resilient quantum memory.

To further expand the platform's capabilities, we demonstrate coherent control over the four-level ground state of the ¹⁷¹Yb ion, operating it as a qudit. Through development of a new device architecture that enables microwave driving of all transitions and comprehensive characterization of their coherence properties, this work establishes the foundation for higher-dimensional quantum communication protocols that offer significant advantages in network capacity and efficiency.

Collectively, these results establish the ¹⁷¹Yb:YVO₄ system as a uniquely versatile platform and demonstrate the feasibility of building scalable quantum networks using single rare-earth ions in crystals.

Thesis Files