Long-range interactions between emitters give rise to collective phenomena, including superradiance, spin squeezing, and coherence protection, that are important to both fundamental physics and quantum technologies. Despite progress in cold atoms, coherent cavity-mediated all-to-all interactions have not yet been realized in a solid-state ensemble. Here we demonstrate such interactions in a ^171Yb^3+:CaWO_4 crystal coupled to a microwave resonator, observing superradiant emission on resonance and unitary one-axis twisting dynamics in the dispersive regime. The same interaction also opens a many-body energy gap that suppresses inhomogeneous dephasing, extending the ensemble Ramsey coherence time from tens of microseconds to milliseconds without decoupling pulses. These results establish a solid-state platform for collective many-body physics with direct implications for quantum technologies. Specifically, the observed one-axis twisting dynamics opens a path towards spin squeezing for entanglement-enhanced quantum metrology, and the extended coherence due to gap-protection is relevant for both microwave photon storage and precision measurement.

Controlling light photon-by-photon is central to quantum optics. At a fundamental level, photon interactions are mediated by their coupling to atoms, and ultimate control requires deterministic light-matter interfacing of single photons to single atoms. Extending this paradigm to radiatively couple multiple individual atoms in a deterministic and scalable manner opens the arena of many-body quantum optics. Here, we realize such a setting by coherently coupling solid-state artificial atoms to a nanophotonic waveguide and demonstrate higher-order photon correlations that are controlled by the number of quantum emitters. We study the scaling of nonlinear photonic transport induced by emitter-photon scattering and demonstrate that adding a quantum emitter generates higher-order photon correlations. Specifically, we experimentally observe genuine three-photon correlations from a pair of collectively coupled emitters, while contributions from lower photon numbers are suppressed. In addition, we scale to three resonant quantum emitters coupled to the waveguide. These advancements demonstrate the onset of many-body quantum optics in waveguide quantum electrodynamics, enabling new photonic quantum simulators, the creation of many-body entangled states, and the exploration of novel quantum phase transitions.

Understanding and modeling energy transfer mechanisms in rare-earth-doped nanomaterials are essential for advancing luminescent technologies used in bioimaging, optical thermometry, and solid-state lasers. In this work, we investigate the photoluminescence dynamics of Yb3+ and Er3+ ions in Y2O3 nanoparticles over a wide concentration range (0.5-17%), using both direct and up-conversion excitation. Luminescence decays of green, red, and near-infrared transitions were measured and analyzed using a single rate equation model incorporating radiative and non-radiative processes, energy transfer mechanisms, and defect-related quenching. Using specific measurements to determine each model parameter in a reliable way, we successfully reproduce experimental trends across most concentrations and excitation paths. This unified approach thus provides a sound and predictive framework for modeling energy transfer in rare-earth-doped materials and offers valuable insights for optimizing photoluminescent properties in nanostructured systems.

Radiative coupling between quantum emitters leads to a range of spectacular emission phenomena. Dicke studied the foundations of collectively enhanced and suppressed decay, commonly referred to as super- and subradiance. Collective effects can further result in directionality of the emission, thus offering a complimentary implementation of chiral quantum optics. Waveguide quantum electrodynamics (QED) allows coupling between spatially separated emitters, enabling selective driving. In this work, we control the emission direction for a pair of quantum dots embedded in a bidirectional photonic crystal waveguide offering independent electrical tuning. Notably the emitters are 13 m apart, which corresponds to 26 effective wavelengths, but are nevertheless radiatively coupled. The directionality arises from a dispersive dipole-dipole interaction, which shifts the energy of the collective states, so that the emitter pair effectively forms an artificial molecule. We show that the emission direction can be switched from left- to rightwards by manipulating the relative driving phase while collectively exciting the emitters. In addition, we observe directional photon statistics under continuous driving, with, for example, single photons detected on one output port, and photon pairs on the other. With pulsed excitation, both emitters are fully inverted and correlated photon pairs are observed in time-resolved intensity correlation measurements. This work demonstrates a novel implementation of chiral quantum optics using quantum dots coupled via a non-chiral waveguide, and reports key steps for scaling up as a multi-emitter waveguide QED platform.

Optically addressable solid-state spins have been extensively studied for quantum technologies, offering unique advantages for quantum computing, communication, and sensing. Advancing these applications is generally limited by finding materials that simultaneously provide lifetime-limited optical and long spin coherences. Here, we introduce 171Yb3+ ions doped into a CaWO4 crystal. We perform high-resolution spectroscopy of the excited state, and demonstrate all-optical coherent control of the electron-nuclear spin ensemble. We find narrow inhomogeneous broadening of the optical transitions of 185 MHz and radiative-lifetime-limited coherence time up to 0.75 ms. Next to this, we measure a spin-transition ensemble line width of 5 kHz and electron-nuclear spin coherence time reaching 0.15 seconds at zero magnetic field between 50 mK and 1 K temperatures. These results demonstrate the potential of 171Yb3+:CaWO4 as a low-noise platform for building quantum technologies with ensemble-based memories, microwave-to-optical transducers, and optically addressable single-ion spin qubits.

Optically addressable solid-state spins have been extensively studied for quantum technologies, offering unique advantages for quantum computing, communication, and sensing. Advancing these applications is generally limited by finding materials that simultaneously provide lifetime-limited optical and long spin coherences. Here, we introduce ^171Yb^3+ ions doped into a CaWO_4 crystal. We perform high-resolution spectroscopy of the excited state, and demonstrate all-optical coherent control of the electron-nuclear spin ensemble. We find narrow inhomogeneous broadening of the optical transitions of 185 MHz and radiative-lifetime-limited coherence time up to 0.75 ms. Next to this, we measure a spin-transition ensemble line width of 5 kHz and electron-nuclear spin coherence time reaching 0.15 seconds at zero magnetic field between 50 mK and 1 K temperatures. These results demonstrate the potential of ^171Yb^3+:CaWO_4 as a low-noise platform for building quantum technologies with ensemble-based memories, microwave-to-optical transducers, and optically addressable single-ion spin qubits.

Dense and shallow ensembles of negatively charged nitrogen-vacancy center (NV -) with good optical and spin properties play a key role in the performance enhancement of diamond-based quantum sensors. Ion implantation enables precise control of NV(- )depth and density. However, at high ion fluence, this method is limited by low NV- creation yields and sample amorphization. Additionally, shallow NV- spin properties deteriorate due to surface proximity. In this paper, we study N-2(+) ion implantation at energies between 10 and 15 keV with fluences as high as 1 x 10(15) ions/cm(2) at temperatures of 20, 400, and 800 degrees C to investigate the influence of implantation temperature on lattice damage, NV- creation yield, and NV- spin properties. Our results show that diamond maintains structural integrity at 800 degrees C with fluences up to 1 x 10(15) ions/cm(2) without amorphization. Furthermore, high-temperature implantation improves NV- creation yields up to five times without compromising T-2(& lowast;), T-2, and T-1, making it a promising approach to enhance the magnetic field sensitivity of NV(- )ensembles.

Optically addressable solid-state spins have been extensively studied for quantum technologies, offering unique advantages for quantum computing, communication, and sensing. Advancing these applications is generally limited by finding materials that simultaneously provide lifetime-limited optical and long spin coherences. Here, we introduce $^{171}$Yb$^{3+}$ ions doped into a CaWO$_4$ crystal. We perform high-resolution spectroscopy of the excited state, and demonstrate all-optical coherent control of the electron-nuclear spin ensemble. We find narrow inhomogeneous broadening of the optical transitions of 185 MHz and radiative-lifetime-limited coherence time up to 0.75 ms. Next to this, we measure a spin-transition ensemble line width of 5 kHz and electron-nuclear spin coherence time reaching 0.15 seconds at zero magnetic field between 50 mK and 1 K temperatures. These results demonstrate the potential of $^{171}$Yb$^{3+}$:CaWO$_4$ as a low-noise platform for building quantum technologies with ensemble-based memories, microwave-to-optical transducers, and optically addressable single-ion spin qubits.

We present a comprehensive study of spin relaxation dynamics at cryogenic temperatures in a rare-earth-doped crystal used for quantum memory applications: ^171Yb:Y_2SiO_5. Spin relaxation is indeed a major limiting factor for both the efficiency and storage time of quantum memory protocols based on atomic frequency combs in rare-earth materials. The relaxation dynamics among the four ground-state hyperfine levels were simultaneously investigated by optically perturbing the spin population distribution and monitoring its return to thermal equilibrium through optical absorption spectroscopy. By applying different types of perturbations, we were also able to distinguish between two types of relaxation processes, induced by spin-phonon and spin-spin interactions. Below 1 K, we observed that the re-thermalization of the Yb^3+ ion population takes several hours, driven solely by direct phonon absorption or emission. However, the effective lifetime of individual spin states is much shorter - on the order of several seconds in low-doped (2 ppm) samples and of milliseconds in 10 ppm samples - due to spin-spin interactions. These findings provide valuable guidelines for optimizing doping levels and operating temperatures in rare-earth-doped crystals for quantum applications. Notably, they suggest that atomic frequency combs with lifetimes of several hours could be realized using ^171Yb:Y_2SiO_5 crystals with slightly less than 2 ppm doping and operating near 1 K.

Quantum state lifetimes T_{2}, or equivalently homogeneous linewidths Γ_{h}=1/πT_{2}, are a key parameter for understanding decoherence processes in quantum systems and assessing their potential for applications in quantum technologies. The most common tool for measuring narrow optical homogeneous linewidths, i.e., long T_{2}, is the measurement of coherent photon echo emissions, which however gives very weak signal when the number of emitters is small. This strongly hampers the development of nanomaterials, such as those based on rare earth ions, for quantum communication and processing. In this work, we propose, and demonstrate in an erbium doped crystal, a measurement of photon echoes based on incoherent fluorescence detection and its variance analysis. It gives access to T_{2} through a much larger signal than direct photon echo detection, and, importantly, with a laser which is incoherent over the measurement timescale, on the order of a few T_{2}. Our results thus open the way to efficiently assess the properties of a broad range of emitters and materials for applications in quantum nanophotonics.

Thin films provide nanoscale confinement together with compatibility with photonic and microwave architectures, making them ideal candidates for chip-scale quantum devices. In this work, we propose a thin film fabrication approach yielding the epitaxial growth of Eu3+ doped Y2O3 on silicon. We combine two of the most prominent thin film deposition techniques: chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). We report sub-megahertz optical homogeneous linewidths up to 8 K for the Eu3+ dopants in the film, and lowest value of 270 kHz. This result constitutes a ten-fold improvement with respect to previous reports on the same material, opening promising perspectives for the development of scalable and compact quantum devices containing rare-earth ions.

Waveguide quantum electrodynamics (QED) has opened a new frontier in quantum optics, which enables the radiative coupling of distantly located emitters via the spatially extended waveguide mode. This coupling leads to modified emission dynamics and previous work has reported the observation of increased intensity correlations (an antidip) when probing the resonance response of multiple emitters. However, the interference between independent emitters has been shown to lead to a similar response. Here, we directly observe resonant energy transfer between two distant quantum emitters by recording an antidip in the intensity correlations, g^{(2)}(τ), while driving only one of the emitters. Under the condition that only a single emitter is driven, the antidip in photon coincidences is a distinctive signature of emitter-emitter coupling, which enables the transfer of energy from the driven to the undriven emitter. Interestingly, the observed mechanism is a long-range and waveguide-engineered version of resonant Förster transfer, which is responsible for the transport of energy between chlorophylls in the photosynthesis. Building on the established coupling, we demonstrate collective driving of the coupled emitter pair. Specifically, we control the relative driving phase and amplitude of the emitters and apply this collective excitation scheme to selectively populate the long-lived subradiant state. This results in suppressed emission, i.e., the peculiar situation where driving two emitters as opposed to one effectively reduces the probability of photon emission. Our work presents novel emission regimes and excitation schemes for a multiemitter waveguide QED system. These can be exploited to deterministically generate emitter-emitter entanglement and advanced photonic states providing robustness against losses for photonic quantum computation and quantum communication.

Rare earth doped crystals show, at low temperatures, extremely narrow optical homogeneous linewidths, as well as long spin coherence lifetimes, a unique combination in the solid state. This makes these materials attractive for optical quantum technologies like quantum communication and processing. Most of the results in this field have so far used bulk crystals because of their exceptional spectroscopic properties. Crystalline thin films can combine these properties with the possibilities offered by integration in photonic circuits in terms of compactness, stability, energy efficiency, and scalability. In this review, recent results on different platforms containing rare earth ions and targeting quantum technologies, including lithium niobate and silicon films, and oxide films deposited on Si, are summarized. Current approaches for obtaining thin films and devices are described, together with RE spectroscopic properties and applications to quantum technologies. The opportunities and challenges offered or faced by the different platforms are also discussed.

We report the optical study of europium nitrate hexahydrate crystals Eu(NO3)3. 6H2O and their deuterated version Eu(NO3)3. 6D2O conducted at 50 mK. The crystals show sub-GHz inhomogeneous linewidths for the 7F0 <-> 5 D0 transition and optical coherence times up to 136 mu s for the non-deuterated crystal and 750 mu s for the deuterated one. In addition, using the spectral hole burning technique, the hyperfine splittings in the absence of an external field have been estimated for the ground and optical levels. These results suggest that rare-earth nitrate crystals could represent a promising class of materials for quantum information applications as well as for fundamental physics experiments requiring narrow optical transitions and good coherence properties.

We perform optical spectroscopy on 171 Yb 3+ rare-earth ions doped in CaWO 4 . We characterize the temperature dependent lifetime and the magnetic field dependence of the transitions.

Quantum emitters embedded in photonic circuits are fundamental for for quantum information applications. Here, evanescent coupling between Yb:Y 2 O 3 thin films and GaInP photonic crystals is explored to achieve high-quality factors and efficient photon collection.

Realizing a sensitive photon-number-dependent phase shift on a light beam is required both in classical and quantum photonics. It may lead to new applications for classical and quantum photonics machine learning or pave the way for realizing photon-photon gate operations. Non-linear phase-shifts require efficient light-matter interaction, and recently quantum dots coupled to nanophotonic devices have enabled near-deterministic single-photon coupling. We experimentally realize an optical phase shift of $0.19 \pi \pm 0.03$ radians ($\approx 34$ degrees) using a weak coherent state interacting with a single quantum dot in a planar nanophotonic waveguide. The phase shift is probed by interferometric measurements of the light scattered from the quantum dot in the waveguide. The nonlinear process is sensitive at the single-photon level and can be made compatible with scalable photonic integrated circuitry. The work may open new prospects for realizing high-efficiency optical switching or be applied for proof-of-concept quantum machine learning or quantum simulation demonstrations.

Deterministic photon sources allow long-term advancements in quantum optics. A single quantum emitter embedded in a photonic resonator or waveguide may be triggered to emit one photon at a time into a desired optical mode. By coherently controlling a single spin in the emitter, multi-photon entanglement can be realized. We demonstrate a deterministic source of three-qubit entanglement based on a single electron spin trapped in a quantum dot embedded in a planar nanophotonic waveguide. We implement nuclear spin narrowing to increase the spin dephasing time to $T_2^* \simeq 33$ ns, which enables high-fidelity coherent optical spin rotations, and realize a spin-echo pulse sequence for sequential generation of high-fidelity spin-photon and spin-photon-photon entanglement. The emitted photons are highly indistinguishable, which is a key requirement for subsequent photon fusions to realize larger entangled states. This work presents a scalable deterministic source of multi-photon entanglement with a clear pathway for further improvements, offering promising applications in photonic quantum computing or quantum networks.

Understanding the dephasing dynamics of quantum materials with a low number of emitters is crucial for optimizing them for quantum nanophotonics. Here, we investigate the recovery of photon echoes from fluorescence emissions in rare-earth doped systems.

Creating dense and shallow nitrogen vacancy (NV) ensembles with good spin properties, is a prerequisite for developing diamond-based quantum sensors exhibiting better performance. Ion implantation is a key enabling tool for precisely controlling spatial localisation and density of NV colour centres in diamond. However, it suffers from a low creation yield, while higher ion fluences significantly damage the crystal lattice. In this work, we realize N2 ion implantation in the 30 to 40 keV range at high temperatures. At 800 C, NV ensemble photoluminescence emission is three to four times higher than room temperature implanted films, while narrow electron spin resonance linewidths of 1.5 MHz, comparable to well established implantation techniques are obtained. In addition, we found that ion fluences above 2E14 ions per cm2 can be used without graphitization of the diamond film, in contrast to room temperature implantation. This study opens promising perspectives in optimizing diamond films with implanted NV ensembles that could be integrated into quantum sensing devices.