In this paper, we investigate three schemes for implementing Controlled-Z (CZ) gates between individual ytterbium (Yb) rare-earth ions doped into yttrium orthovanadate (YVO 4 or YVO). Specifically, we investigate the CZ gates based on magnetic dipolar interactions between Yb ions, photon scattering off a cavity, and a photon interference-based protocol, with and without an optical cavity. We introduce a theoretical framework for precise computations of state and gate infidelities, accounting for noise effects. We then compute the state fidelity for each scheme to evaluate the feasibility of their experimental implementation. Based on these results, we compare the performance of the two-qubit gate schemes and discuss their respective advantages and disadvantages. We conclude that the probabilistic photon interference-based scheme offers the best fidelity scaling with cooperativity and is superior with the current technology of Yb values, while photon scattering is nearly deterministic but slower with less favourable fidelity scaling as a function of cooperativity. The cavityless magnetic dipolar scheme provides a fast, deterministic gate with decent fidelities if close ion localization can be realized. While focusing on 171 Yb 3 + :YVO system as a case study, the theoretical tools and approaches developed in this work are broadly applicable to other spin qubit systems.
In this paper, we investigate three schemes for implementing Controlled-Z (CZ) gates between individual ytterbium (Yb) rare-earth ions doped into yttrium orthovanadate (YVO 4 or YVO). Specifically, we investigate the CZ gates based on magnetic dipolar interactions between Yb ions, photon scattering off a cavity, and a photon interference-based protocol, with and without an optical cavity. We introduce a theoretical framework for precise computations of state and gate infidelities, accounting for noise effects. We then compute the state fidelity for each scheme to evaluate the feasibility of their experimental implementation. Based on these results, we compare the performance of the two-qubit gate schemes and discuss their respective advantages and disadvantages. We conclude that the probabilistic photon interference-based scheme offers the best fidelity scaling with cooperativity and is superior with the current technology of Yb values, while photon scattering is nearly deterministic but slower with less favourable fidelity scaling as a function of cooperativity. The cavityless magnetic dipolar scheme provides a fast, deterministic gate with decent fidelities if close ion localization can be realized. While focusing on 171 Yb 3 + :YVO system as a case study, the theoretical tools and approaches developed in this work are broadly applicable to other spin qubit systems.
Quantum computing offers powerful advantages for simulating quantum many-body systems and for accelerating specific computational tasks beyond classical capabilities, motivating major advances in hardware, control, and error-correction toward scalable and fault-tolerant platforms. Realizing a utility-scale quantum computer requires high-fidelity operations, and hence, the study of hardware errors is of paramount importance. In this thesis, we develop techniques that advance the existing tool sets for analyzing hardware imperfections in quantum operations, thereby enabling a more accurate understanding of how dif-ferent quantum platforms can reach the required fault-tolerance thresholds. In Chapter 3, we study the implementation of two-qubit Controlled-Z (CZ) gates in the solid-state rare-earth platform 171Yb3+:YVO. The chapter compares three different physical routes to en-tangling individual ytterbium ions: direct magnetic dipole–dipole coupling between closely spaced ions, photon scattering mediated by an optical cavity, and heralded gates based on two-photon in-terference, with both cavity-assisted and cavity-free variants. To evaluate these proposals on equal footing, we develop and use theoretical tools for estimating state and gate fidelities in the presence of realistic imperfections. In particular, we apply a perturbative treatment to the magnetic-dipolar gate, while the photon-scattering and photon-interference schemes are analyzed using a combina-tion of existing analytic methods and numerical simulations. This provides a quantitative basis for assessing the feasibility, strengths, and limitations of each mechanism. The comparison shows that the photon-interference-based protocol has the most favorable fidelity scaling with cavity co-operativity, making it the strongest candidate for currently accessible 171Yb3+:YVO parameters. Photon-scattering gates offer the advantage of near-deterministic operation, but this comes at the cost of slower gate times and less favorable cooperativity scaling. The magnetic-dipolar approach, by contrast, gives a fast and deterministic cavity-free gate, but requires ions to be positioned suf-ficiently close together. Although the analysis is carried out for 171Yb3+:YVO as a concrete case study, the techniques used to model, benchmark, and compare these gates are applicable more generally to other solid-state spin-qubit platforms. Motivated by the need for a more systematic treatment of noise in photon-heralded opera-tions, Chapter 4 develops an analytic perturbative method for probabilistic quantum gates between non-interacting emitters. Based on the Zero-Photon-Generation formalism, this method describes light–matter dynamics conditioned on time-integrated photon-detection outcomes and decomposes the resulting conditional map into its ideal contribution and leading noise-dependent corrections. In this way, it produces closed-form expressions for process matrices and effective Pauli error weights, giving a direct link between microscopic hardware imperfections and the noise models used in circuit-level and error-correction descriptions. The method is flexible enough to treat errors from different parts of the experimental architecture, including imperfections in photon emission and optical-control components. We benchmark it on a repeat-until-success CZ gate, where the perturbative results match numerical simulations in the low-error regime, and then use it to study phase-shifter calibration errors as an example of coherent optical-control noise. These results give a physics-informed route for optimizing gate parameters, comparing different imperfections, and constructing effective error models for fault-tolerant quantum computing with hybrid light–matter platforms. Together, these results strengthen the connection between microscopic hardware imperfections and effective error models, providing tools for assessing and optimizing quantum operations on the path toward fault-tolerant quantum computation.
Quantum light-matter interfaces that can reversibly map quantum information between photons and atoms are essential for building future quantum networks. Crystals doped with rare-earth ions (REIs) are an attractive solid-state platform for such light-matter interfaces due to their exceptional optical and spin coherence properties at cryogenic temperatures. Building scalable REI-based technology has proven to be challenging due to the inherently weak coupling of REIs with light. This thesis explores the integration of REIs with nanophotonic resonators to overcome this weak light-matter interaction and enable efficient, scalable quantum light-matter interfaces. Specifically, this work focuses on the development of quantum nanophotonics with ytterbium in yttrium orthovanadate. This thesis begins with an introduction to a nanophotonic platform based on photonic crystal cavities fabricated directly in rare-earth host materials and highlights the initial successes of this platform with neodymium-doped materials. This motivates an examination of the optical and spin coherence properties of 171Yb:YVO4, a REI material that was previously unexplored for quantum technology applications. This material is found to have strong optical transitions compared to other REI-doped materials, a simple energy level structure, and long optical and spin coherence lifetimes. The focus then turns to the detection and coherent manipulation of single ytterbium ions coupled to nanophotonic cavities. The Purcell-enhancement in these cavities enables efficient optical detection and spin initialization of individual ytterbium ions. We identify ions corresponding to different isotopes of ytterbium and show that the coupling of electron and nuclear spin in ytterbium-171 at zero-field gives rise to strong electron-spin-like transitions that are first-order insensitive to magnetic field fluctuations. This allows for coherent microwave control and the observation of long spin coherence lifetimes at temperatures up to 1 K. We then make use of the optical selection rules and energy structure of 171Yb:YVO4 to demonstrate high-fidelity single-shot optical readout of the spin state. These results establish nanophotonic devices in 171Yb:YVO4 as a promising platform for solid-state quantum light-matter interfaces.
T center defects in silicon provide an attractive platform for quantum technologies due to their unique spin properties and compatibility with mature silicon technologies. We investigate several gate protocols between single T centers, including two probabilistic photon interference-based schemes, a near-deterministic photon scattering gate, and a deterministic magnetic dipole-based scheme. In particular, we study a photon interference-based scheme with feedback which can achieve success probabilities above 50%, and use the photon-count decomposition method to perform the first analytical calculations of its entanglement fidelity and efficiency while accounting for imperfections. We also calculate the fidelity and efficiency of the other schemes. Finally, we compare the performance of all the schemes, considering current and near-future experimental capabilities. In particular, we find that the photon interference-based scheme with feedback has the potential to achieve competitive efficiency and fidelity, making it interesting to explore experimentally.
Rare-earth ion crystals are a promising platform for implementing solid-state quantum interfaces, yet there are few combinations of rare-earth and host crystals that simultaneously deliver large optical absorption, narrow inhomogeneous linewidths, and long coherence times. This work investigates the optical and spin properties of 171Yb3+ ions in YVO4. The observed properties and simple energy-level structure of the 171Yb3+ ion show that this material has significant potential for creating efficient quantum interfaces, such as ensemble-based memories, microwave-to-optical transducers, and optically addressable single rare-earth ion spin qubits.
We demonstrate optical probing of spectrally resolved single Nd^{3+} rare-earth ions in yttrium orthovanadate. The ions are coupled to a photonic crystal resonator and show strong enhancement of the optical emission rate via the Purcell effect, resulting in near radiatively limited single photon emission. The measured high coupling cooperativity between a single photon and the ion allows for the observation of coherent optical Rabi oscillations. This could enable optically controlled spin qubits, quantum logic gates, and spin-photon interfaces for future quantum networks.
171Yb3+ ions in yttrium orthovanadate are promising candidates for building quantum networks, based on their excellent optical and spin properties and access to a secondary nuclear-spin quantum register. In this study the authors transfer GaAs photonic crystal nanobeam resonators to the surface of the host YVO4 to enhance the optical transitions of the Yb3+ ions and efficiently collect the emitted photons. This hybrid quantum photonic interface facilitates addressing of single Yb3+ ions and paves the way for the implementation of a scalable photonic architecture in the near infrared.