PCSI2027 Session PCSI-MoM2: Point Defects & Quantum Emitters

Diamond is the leading solid-state platform for quantum networking and room-temperature deployable quantum sensing technologies. The continued evolution of quantum technologies in diamond requires heterogenous material platforms for sophisticated functionalities, device integration and packaging, and improved qubit performance. At UChicago and Argonne National Laboratory, we are creating pristine single-crystal diamond thin-films that host coherent color center qubits and utilizing direct-bonding processes to integrate them with a wide range of materials including fused silica, sapphire, thermal oxide, lithium niobate, tantalum, silicon and YIG (figure 1).¹ High-resolution imaging of a diamond membrane/sapphire interface showcases crystalline reconstruction, signifying bonding between the diamond and host substrate, and hosted color centers maintain excellent coherence throughout processing (figure 2).

In this talk, I will present our diamond thin-film synthesis and integration methods, and present in-depth characterization of the thin-films following integration, and update on recent results from the Advanced Photon Source at Argonne. I will highlight performance of the thin-films in quantum device architectures for networking and sensing, including record-setting qubit and photonic performance. I will demonstrate how material heterostructures composed of thin-film diamond bonded to thin-film lithium niobate unlock new functionalities for quantum networking nodes. For instance, we show that by utilizing the piezoelectric effect in lithium niobate, we can dynamically control strain within the diamond layer and tune critical qubit properties. I will also demonstrate how integration of thin-film diamond with glass substrates can be utilized to generate significant tensile strain in diamond, unlocking axial-free qubits within the diamond.

Atomic defects in solid-state materials have attracted much recent attention due to their potential to realize photonic quantum information processing in various material platforms. Quantum light sources from atomic defects in two-dimensional (2D) materials have been particularly promising for developing quantum technologies in van der Waals (vdW) materials and heterostructures. However, studies on room-temperature (RT) quantum emitters in 2D materials have been limited so far, urging the identification of novel 2D materials that can host RT quantum emitters to enable a wider range of fundamental studies and quantum applications. In this talk, we present the observation of RT quantum emission from α-MoO₃, a vdW semiconductor which has a bandgap of 3 eV [1]. By annealing exfoliated ultrathin α-MoO₃flakes, we found localized sharp emitter peaks around the energy of 2 eV. With the photon antibunching curve in the second-order correlation function measurements, single-photon nature of the emitted light is demonstrated. The density functional theory calculation further supports that the oxygen vacancy center is a highly promising candidate of the observed emitters. Our work provides α-MoO₃as a novel platform to study atomic defects and quantum light sources in vdW systems for realizing RT quantum technologies.

[1] J. Lee, H. Wang, K.-Y. Park, S. Huh, D. Kim, M. Yu, C. Kim, K. S. Thygesen, J. Lee, Nano Lett. 25, 1142 (2025).

Cooperative light emission arises when multiple indistinguishable quantum emitters interact coherently through a shared electromagnetic field, dramatically changing emission characteristics and offering opportunities for generating entangled quantum states for quantum computing, quantum information processing, and secure communications. A key example is superradiance, in which coupled quantum light emitters radiate collectively and emit in phase, producing an enhanced, high-intensity burst of light.¹ However, superradiance remains challenging to realize in solid-state materials because it requires interacting emitters to be identical, while quantum emitters in these systems are inevitably subject to variations in their local environment, including strain, carrier fluctuations, and surrounding defects. Two-dimensional semiconductors, particularly monolayer tungsten diselenide (WSe₂), offer a promising platform due to their atomically thin structure, strong excitonic interactions, and sensitivity to local strain, enabling spatial and spectral control of quantum emitters which provides new opportunities for realizing scalable cooperative quantum light sources.

In this work, we demonstrate superradiant emission and the transition from single-photon to superradiant light emission in strained monolayer WSe₂ suspended over dense nanocavity arrays, with uniform localized strain controlled through electrostatic interactions.² Computational studies have predicted a critical inter-emitter spacing for achieving superradiance in two-dimensional systems, where is the emission wavelength.³ By comparing devices with inter-emitter spacings of and , we show that emitters remain isolated single-photon sources when , whereas reducing the spacing below this threshold produces pronounced photon bunching, fast radiative lifetime, and enhanced peak intensity—the characteristic signatures of superradiance. Furthermore, electrostatic back-gating enhances both emitter intensity and superradiant emission, consistent with increased exciton populations funneling toward deeply bound defects under electrostatic gating. These results demonstrate a powerful new platform for highly controlled, large-scale quantum light emission in two-dimensional materials, and establish inter-emitter spacing and electrostatic gating as effective controls for engineering cooperative quantum light emission in two-dimensional materials.

N-polar InGaN/GaN heterostructures are attractive for optoelectronic and energy-conversion devices because the reversed polarization field can facilitate the separation and extraction of photogenerated carriers. However, point defects may limit their optical and electronic properties through nonradiative recombination. The photothermal deflection spectroscopy signal intensity in the bandgap was higher for N-polar GaN than for Ga-polar GaN, suggesting a larger contribution from defect-related states. In this context, experimental characterization of nitrogen vacancies (V_N) remains a crucial issue. We investigated the valence-band states of Ga- and N-polar GaN by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The measured spectra were compared with calculated densities of states (DOS) for pristine GaN and GaN containing V_N. The pristine GaN DOS reproduced the Ga-polar spectrum well, whereas the N-polar sample showed a tail-like feature near the valence-band maximum that was not reproduced by the pristine calculation. Calculations including V_N produced additional states in the corresponding energy region. To evaluate V_N, the difference DOS, DOS_VN - DOS_pristine, was compared with the residual XPS spectrum obtained by subtracting the scaled defect-free DFT spectrum from the experimental XPS spectrum. The broadened difference DOS was scaled to reproduce the experimental residual spectrum. From the scaling factor, the near-surface V_N concentration in the N-polar sample was estimated to be approximately 6 × 10¹⁹ cm^-3. These results suggest that valence-band XPS combined with first-principles calculations can provide an approach for evaluating V_N-related electronic states in GaN.