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Quantum 2.0 Conference and Exhibition

Toward Qudit Quantum Networking with Rare-earth Ions

Will Pajak, Chun-Ju Wu, Emanuel Green, Andrei Faraon

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We demonstrate coherent control of a qudit system formed by four hyperfine levels of 171Yb in YVO4. This is achieved using a photonic crystal resonator leveraging the birefringence of the crystal.
DOI
10.1364/quantum.2025.qm3a.4

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I present quantum nano-photonic devices based on nanophotonic resonators coupled to rare-earth-ions in crystals. The rare-earth ions exhibit long coherence times on optical transitions, which makes them suitable for optical quantum memories. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic rare-earth ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Besides ensemble memories, single rare-earth-ions coupled to nano-resonators can be used as single optically addressable quantum bits where the quantum state is mapped on their Zeeman or hyperfine levels with long coherence time. Our solid-state nano-photonic quantum light-matter interfaces can be integrated with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at the nodes of quantum networks.
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.