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.
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.
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.
Rare-earth ions in solids are a promising platform for quantum technologies because they combine atom-like optical and spin transitions with the practical advantages of a solid-state host compatible with optical and microwave resonators. This thesis studies cavity-coupled 171Yb3+ ensembles in oxide crystals as a platform for collective cavity-QED and many-body physics, motivated by the goal of using rare-earth ions as hybrid quantum interconnects for quantum computation and networking. The central theme is that coupling many rare-earth ions to a shared resonator mode creates a rich setting for studying fundamental collective and many-body physics, while also providing a practical route to microwave-to-optical conversion, protected spin storage, and interfaces between superconducting circuits and optical photons. In 171Yb3+:YVO4, a nanophotonic cavity coupled to an inhomogeneously broadened ion ensemble reveals collective cavity QED in a solid. This work led to the discovery of collectively induced transparency, a cavity-QED phenomenon arising from collective interference in a driven, disordered ensemble. The same system exhibits optical superradiance and subradiance, and supports an interacting microwave spin system in which dipolar exchange competes with disorder, allowing studies of quantum thermalization. With Floquet control, these spin dynamics can be modified to reveal discrete time-crystal signatures. In 171Yb3+:CaWO4, related experiments demonstrate microwave superradiance, one-axis twisting, and many-body gap protection in a solid-state spin ensemble. These physics results are developed alongside quantum-technology applications. The 171Yb3+:YVO4 platform further enables low-noise microwave-to-optical transduction with percent-level on-chip efficiency and added noise near the single-photon level, establishing rare-earth ensembles as a promising approach to optical interconnects for superconducting quantum systems. In 171Yb3+:CaWO4, cavity-mediated gap protection extends Ramsey coherence and supports the development of a spin-based microwave quantum memory, including a classical-regime demonstration of storage and optical readout. The final part develops the superconducting qubit architecture needed to drive a rare-earth transducer with single microwave excitations, including qubit readout, tunable-coupler SWAP control, and cable-mode characterization. Together, these results establish cavity-coupled rare-earth ensembles as a versatile platform for studying fundamental cavity QED and many-body physics and for developing quantum technologies.
Quantum interconnects allow disparate quantum systems to be entangled, leading to more powerful integrated quantum technology and increases in scalability. The foundation for such technology, including photonic quantum memories and coherent microwave-to-optical (M2O) transducers, have already been developed in rare-earth ion (REI) crystals. Here we demonstrate improved REI quantum device functionality in an on-chip platform that dramatically strengthens the ions’ interactions with optical fields and integrates with planar microwave technology. Using a photonic crystal nanobeam fabricated in a Nd-doped yttrium vanadate (YVO) crystal, we harness the enhanced ion-photon interactions that create single photon Rabi frequencies as large as 60 MHz. In particular, the large AC Stark shift is used to control an ensemble of approximately 4000 ions for photonic quantum memory applications. We demonstrate AC Stark shift control of the storage time in the atomic frequency comb protocol as well as the possibility of memories based on an all-optical variation of the hybrid photon echo rephasing protocol. The spin state of the REIs can also be addressed directly through the integration of microwave striplines and coplanar waveguide cavities. The achievement of optically detected magnetic resonance in on-chip waveguides and nanophotonic cavities in Nd:YVO will be presented along with the initial progress of achieving coherent M2O conversion using Raman heterodyne spectroscopy. With photonic quantum memories and sources, single ion qubits, and quantum M2O all feasible in the one integrated platform, REI technology is a promising platform for enabling large scale integration of diverse quantum resources.
Quantum interconnects allow disparate quantum systems to be entangled, leading to more powerful integrated quantum technology and increases in scalability. The foundation for such technology, including photonic quantum memories and coherent microwave-to-optical (M2O) transducers, have already been developed in rare-earth ion (REI) crystals. Here we demonstrate improved REI quantum device functionality in an on-chip platform that dramatically strengthens the ions’ interactions with optical fields and integrates with planar microwave technology. Using a photonic crystal nanobeam fabricated in a Nd-doped yttrium vanadate (YVO) crystal, we harness the enhanced ion-photon interactions that create single photon Rabi frequencies as large as 60 MHz. In particular, the large AC Stark shift is used to control an ensemble of approximately 4000 ions for photonic quantum memory applications. We demonstrate AC Stark shift control of the storage time in the atomic frequency comb protocol as well as the possibility of memories based on an all-optical variation of the hybrid photon echo rephasing protocol. The spin state of the REIs can also be addressed directly through the integration of microwave striplines and coplanar waveguide cavities. The achievement of optically detected magnetic resonance in on-chip waveguides and nanophotonic cavities in Nd:YVO will be presented along with the initial progress of achieving coherent M2O conversion using Raman heterodyne spectroscopy. With photonic quantum memories and sources, single ion qubits, and quantum M2O all feasible in the one integrated platform, REI technology is a promising platform for enabling large scale integration of diverse quantum resources.
This thesis describes a number of coherent processes, such as quantum information processing, superradiance and electromagnetically induced transparency, which have been experimentally implemented using rare-earth-ion-doped crystals. The rare-earths are a class of elements that have in common, an atomic structure that allows for very long lifetimes as well as coherence times, both on optical transitions and on spin transitions. These ions can be naturally trapped inside host crystals, and with the phonon vibrations removed by cooling down to cryogenic temperatures (< 4 K), they can be used for a number of different quantum information processes. Most experiments were carried out using PrYSO, but also other crystal types were investigated, such as Nd:YVO4, and La2(WO4)3. The characterization of the ions were done with a variety of methods, including among others, photon echo techniques and electromagnetically induced transparency. Quantum computing is a rapidly growing field and there are still many potential candidates for its implementation. In our work we have utilized the spin states of the Pr rare-earth ion as a qubit, and demonstrated arbitrary single qubit gates, which are important pieces towards a quantum computing realization in these systems. Most of the experimental work done for the thesis was carried out using an ensemble approach, that has the advantage of giving a strong readout signal, but for future scaling to multiple qubits, single instances are more promising, which is discussed. Quantum memories are crucial components in applications such as quantum networks and long distance quantum communication. In this thesis, work has been done to investigate how quantum memories can be implemented in rare-earth crystals. In particular, there are two protocols, CRIB and AFC, which were suggested with rare-earth crystals directly in mind, and both of these require high optical depth, for maximum recall efficiency. In this thesis, implications of being in the high optical depth regime, with phenomena such as superradiance, a collective effect that could cause the stored light to be immediately reemitted, and slow light effects that come from performing the storage inside spectrally structured materials, were investigated. In order to carry out the phase-sensitive experiments, a laser system with a very narrow linewidth of ~1 kHz at 606 nm was constructed, by locking the laser to a semi-persistent spectral hole. In addition, many experiments required advanced pulse shapes, such as complex sechyp pulses or pulses obtained from optimal control theory. In order to be able to accurately create such shapes, an elaborate system using an arbitrary waveform generator and two well calibrated AOMs, controlled from a computer, was also built.
The integration of rare-earth ions in an on-chip photonic platform would enable quantum repeaters and scalable quantum networks. While ensemble-based quantum memories have been routinely realized, implementing single rare-earth ion qubit remains an outstanding challenge due to its weak photoluminescence. Here we demonstrate a nanophotonic platform consisting of yttrium vanadate (YVO) photonic crystal nanobeam resonators coupled to a spectrally dilute ensemble of Nd ions. The cavity acts as a memory when prepared with spectral hole burning, meanwhile it permits addressing of single ions when high-resolution spectroscopy is employed. For quantum memory, atomic frequency comb (AFC) protocol was implemented in a 50 ppm Nd:YVO nanocavity cooled to 480 mk. The high-fidelity quantum storage of time-bin qubits is demonstrated with a 80% efficient WSi superconducting nanowire single photon detector (SNSPD). The small mode volume of the cavity results in a peak atomic spectral density of <10 ions per homogeneous linewidth, suitable for probing single ions when detuned from the center of the inhomogeneous distribution. The high-cooperativity coupling of a single ion yields a strong signature (20%) in the cavity reection spectrum, which could be detected by our efficient SNSPD. We estimate a signal-to-noise ratio exceeding 10 for addressing a single Nd ion with its 879.7nm transition. This, combines with the AFC memory, constitutes a promising platform for preparation, storage and detection of rare-earth qubits on the same ship.