Control and single-shot readout of an ion embedded in a nanophotonic cavity

Distributing entanglement over long distances using optical networks is an intriguing macroscopic quantum phenomenon with applications in quantum systems for advanced computing and secure communication. Building quantum networks requires scalable quantum light–matter interfaces based on atoms, ions or other optically addressable qubits. Solid-state emitters5, such as quantum dots and defects in diamond or silicon carbide , have emerged as promising candidates for such interfaces. So far, it has not been possible to scale up these systems, motivating the development of alternative platforms. A central challenge is identifying emitters that exhibit coherent optical and spin transitions while coupled to photonic cavities that enhance the light–matter interaction and channel emission into optical fibres. Rare-earth ions in crystals are known to have highly coherent 4f–4f optical and spin transitions suited to quantum storage and transduction, but only recently have single rare-earth ions been isolated and coupled to nanocavities. The crucial next steps towards using single rare-earth ions for quantum networks are realizing long spin coherence and single-shot readout in photonic resonators. Here we demonstrate spin initialization, coherent optical and spin manipulation, and high-fidelity single-shot optical readout of the hyperfine spin state of single ¹⁷¹Yb³⁺ ions coupled to a nanophotonic cavity fabricated in an yttrium orthovanadate host crystal. These ions have optical and spin transitions that are first-order insensitive to magnetic field fluctuations, enabling optical linewidths of less than one megahertz and spin coherence times exceeding thirty milliseconds for cavity-coupled ions, even at temperatures greater than one kelvin. The cavity-enhanced optical emission rate facilitates efficient spin initialization and single-shot readout with conditional fidelity greater than 95 per cent. These results showcase a solid-state platform based on single coherent rare-earth ions for the future quantum internet

Coherent Control of Rare-Earth Ions in On-Chip Devices for Microwave-to-Optical Transduction

Entangling microwave and optical photons is essential to harness disparate technologies for building larger scale quantum networks. We demonstrate coherent microwave-to-optical transduction using a nanobeam waveguide containing rare-earth ions in a dilution refrigerator

Toward Coherent Control of Single Yb^(3+) Ions in a Nanophotonic Cavity

We report on detection and coherent optical driving of single Yb^(3+) ions coupled to a nanophotonic resonator fabricated in the YVO_4 host crystal and outline a path toward control of single ^(171)Yb^(3+) spins

Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation

Coherent Control of Rare-Earth Ions in On-Chip Devices for Microwave-to-Optical Transduction

Entangling microwave and optical photons is essential to harness disparate technologies for building larger scale quantum networks. We demonstrate coherent microwave-to-optical transduction using a nanobeam waveguide containing rare-earth ions in a dilution refrigerator

Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation

Single Rare-Earth Ions in Solid-State Hosts: A Platform for Quantum Networks

Solid-state defects have emerged as leading candidates for quantum network nodes due to their compatibility with scalable device engineering and local nuclear spins for quantum processing. Rare-earth ions in crystalline hosts are particularly attractive due to their long optical and spin coherence times at cryogenic temperatures. However, until recently, detection and utilization of single rare-earth ions in quantum technologies has been hindered by their inherently weak optical transitions. In this thesis I present progress towards realizing a novel quantum network node architecture using single ¹7¹Yb³⁺ ions in YVO₄, coupled to a nanophotonic cavity. First, we demonstrate coherent operation of single ¹7¹Yb³⁺ ions as optically addressed qubits. To do this, we leverage first order insensitivity of optical and spin transitions to electric and magnetic fields, thereby protecting the qubits from environmental noise. We demonstrate initialization, high fidelity control and readout of a hyperfine spin qubit with long quantum storage times. We also characterize the optical transitions and find a lifetime-limited echo coherence, thereby enabling a coherent spin-photon interface. Next, we focus on realizing an auxiliary quantum register. The high-fidelity spin control of our ¹7¹Yb³⁺ qubit is leveraged to access local nuclear spins. These spins comprise a dense ensemble which serves as a deterministic quantum resource. We utilize Hamiltonian engineering to generate tailored interactions, enabling polarization, coherent control and preparation of many-body nuclear spin states. Finally, we implement a spin-wave based memory protocol and demonstrate storage and retrieval of quantum states. Moving beyond a single quantum node, in the final section of this thesis we will realize a small-scale quantum network using this platform. As a first step we demonstrate time-resolved quantum interference between photons emitted by ions in two separate devices. Then, we demonstrate a novel heralded entanglement protocol which incorporates optical dynamical decoupling and frequency erasure via precise photon detection. This protocol counteracts both static and dynamic inhomogeneity in the ions’ optical transition frequencies, thereby enabling entanglement generation between any pair of qubits in a scalable fashion. These results showcase single rare-earth ions as a promising platform for the future quantum internet.</p