Defect Engineering for Quantum Grade Rare-Earth Nanocrystals

Abstract

Nanostructured systems that combine optical and spin transitions offer new functionalities for quantum technologies by providing efficient quantum light-matter interfaces. Rare earth (RE) ions doped nanoparticles are promising in this field as they show long-lived optical and spin quantum states, a unique feature among nanomaterials. However, further development of their use in highly demanding applications such as scalable single ion based quantum processors, requires controlling defects that currently limit coherence lifetimes. In this work, we demonstrate that a novel posttreatment process that includes multi-step high-temperature annealing followed by high-power microwave oxygen plasma processing advantageously improves key properties for quantum technologies. We obtain single crystalline nanoparticles (NPs) of 100 nm diameter, presenting bulk-like inhomogeneous linewidths (Γ inh) and population lifetimes (T 1). Furthermore, a significant coherence lifetime (T 2) extension, up to a factor of 5, is successfully achieved by modifying the oxygen-related point defects in the NPs by the oxygen plasma treatment. These promising results confirm the potential of these engineered RE NPs to integrate devices such as cavity-based single photon sources, quantum memories and processors. In addition, our strategy could be applied to a large variety of oxides to obtain outstanding crystalline quality NPs for a broad range of applications.

Controlling the interfacial reactions and environment of rare-earth ions in thin oxide films towards wafer-scalable quantum technologies

“…Recently we have assessed the potential of both Atomic Layer Deposited (ALD) 30 and Chemically Vapor Deposited (CVD) 31 Eu-doped Y 2 O 3 films and managed to successfully burn spectral holes with a 22 MHz linewidth in the CVD films. This is at the state-of-the-art for thin films but still an order of magnitude larger than in optimized nanoparticles 17,32 . This suggests that additional dephasing mechanisms intrinsic to thin films exist due to interactions with various defects, grain boundaries, residual disorder or strain 33 .…”

“…This broadening is attributed to the presence of point or defects especially located close to the substrate's interface. 17 Annealing at 1100 °C for 2 h in air more than doubles the linewidth to 96 GHz with a blue shift of the central frequency. This blue shift already reported for ALD films 23 , is possibly related to the generation of thermal stress due to the thermal expansion coefficient mismatch between silicon and yttria 49,50 .…”

“…In order to keep the curative aspect of annealing while limiting disorder 17,26,30,52 , the Eu ions of interest in the active layer were protected with undoped Y 2 O 3 buffer and a cap layers of similar thicknesses of 80 nm (Fig. 5-a).…”

“…In fact, processing them by a top-down approach into nanoscale systems in which single ion emitters are precisely spatially localized and addressable represents an important challenge in those macrosystems [11][12][13] . As an alternative, specially designed and optimized Eu:Y 2 O 3 nanoparticles have been recently developed leading to record optical and spin coherence times [14][15][16][17] for a nanoscale material and have opened up interesting integration perspectives to cavities 18 .…”

“…One way to reduce native defects in as-grown films is to use a high-temperature annealing post treatment 17 . The temperature and duration of this post treatment are key parameters since thermallyactivated diffusion of impurities or reaction with the substrate are known to take place.…”