Alban Ferrier

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.

Among rare-earth ion-doped crystals that are of high interest for quantum technologies, yttrium orthosilicate [Y2SiO5 (YSO)] crystal has demonstrated the longest coherence times. Etching optical guiding structures enables the creation of complex structures offering the advantage of technological scalability. We report on the dry etching of YSO crystal wafers performed by inductive coupled plasma (ICP) reactive ion etching. The material etching rate, the etch rate selectivity versus the material mask, and the etched side wall’s shape have been measured for various ICP process parameters and gas combinations including Ar-alone, Cl2:Ar, and Cl2:N2 plasma chemistry. These gas-combination choices have enabled identifying the etching mechanisms involved. Etching results have evidenced that Ar ions are the primary contributors for YSO etching, with Cl ions being also efficient, while Cl neutrals play a marginal role. ICP YSO etching is predominantly a physical process carried out by ions. Several ICP powers and RF powers have been investigated. The highest 80 nm/min etch rate is obtained under the Ar-alone plasma, but at the expense of trenching, while chlorine-based etching provides no trenching and smooth side walls. The choice of the dielectric material mask is also discussed.

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 Eu ³⁺ doped Y ₂ O ₃ 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 Eu ³⁺ 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.

Rare earth (RE) doped oxides have demonstrated great potential for photonic applications and have also appeared as promising candidates for quantum memory devices and microwave to optical transducers. Here, we investigate the potential of Chemical Vapor Deposited (CVD) europium-doped Y₂O₃ thin films on silicon as a new platform for integrated quantum devices. We aim at improving the optical properties of such thin films by carefully controlling the RE ion's environment. In particular, we study the effect of annealing post treatments and demonstrate that a significant source of broadening of the optical transition arises from interfacial reactions with the silicon substrate. We thus propose to encapsulate RE ions between undoped high-quality thick layers in order to limit the impact of interfacial reactions on their properties during thermal annealing. Using this approach, we succeeded in measuring a narrow inhomogeneous linewidth of 18 GHz and an ultra-narrow homogeneous linewidth of 5 MHz inferred from spectral hole width. These results are promising towards the use of these engineered RE doped thin films for the development of a scalable nanostructured spin-photon interface. In addition, our strategy could be applied to a large variety of oxide films for a broad range of applications.

The interaction of single quantum emitters with an optical cavity enables the realization of efficient spin-photon interfaces, an essential resource for quantum networks. The dynamical control of the spontaneous emission rate of quantum emitters in cavities has important implications in quantum technologies, e.g., for shaping the emitted photons’ waveform or for driving coherently the optical transition while preventing photon emission. Here we demonstrate the dynamical control of the Purcell enhanced emission of a small ensemble of erbium ions doped into a nanoparticle. By embedding the nanoparticles into a fully tunable high finesse fiber based optical microcavity, we demonstrate a median Purcell factor of 15 for the ensemble of ions. We also show that we can dynamically control the Purcell enhanced emission by tuning the cavity on and out of resonance, by controlling its length with sub-nanometer precision on a time scale more than two orders of magnitude faster than the natural lifetime of the erbium ions. This capability opens prospects for the realization of efficient nanoscale quantum interfaces between solid-state spins and single telecom photons with controllable waveform, for non-destructive detection of photonic qubits, and for the realization of quantum gates between rare-earth ion qubits coupled to an optical cavity.

Quantum memories for light are essential components in quantum technologies like long-distance quantum communication and distributed quantum computing. Recent studies have shown that long optical and spin coherence lifetimes can be observed in rare earth doped nanoparticles, opening exciting possibilities over bulk materials, e.g., for enhancing coupling to light and other quantum systems, and material design. Here, we report on coherent light storage in Eu³⁺:Y₂O₃ nanoparticles using the Stark echo modulation memory (SEMM) quantum protocol. We first measure a nearly constant Stark coefficient of 50 kHz/(V/cm) across a bandwidth of 15 GHz, which is promising for broadband operation. Storage of light is then demonstrated with an effective coherence lifetime of 5 μs. Pulses with two different frequencies are also stored, confirming frequency-multiplexing capability, and are used to demonstrate the memory high phase fidelity. These results open the way to nanoscale optical quantum memories with increased efficiency, bandwidth, and processing capabilities.