Tongcang Li

Optical refrigeration of solids with anti-Stokes fluorescence has been widely explored as a vibration-free cryogenic cooling technology. A minimum temperature of 87 K has been demonstrated with rare-earth-ion-doped crystals using optical refrigeration. However, the depletion of the upper-lying energy levels in the ground-state manifold hinders further cooling to below the liquid nitrogen ( LN 2 ) temperatures, restricting its applications. In this work, we introduce a Purcell-enhanced optical refrigeration method to circumvent this limitation. This approach enhances the emission of high-energy photons by coupling the emitters to an optical cavity, blueshifting the mean emission wavelength. Such Purcell-enhanced emission facilitates cooling starting from a lower-energy level in the ground-state manifold, which exhibits a higher occupation below the LN 2 temperatures. Using experimentally measured optical coefficients, our theoretical analysis predicts a minimum achievable internal temperature of about 38 K for a Yb 3 + : YLiF 4 nanocrystal near a cavity under realistic conditions. The proposed method is applicable to other rare-earth-ion-doped materials and semiconductors and will have applications in creating superconducting and other quantum devices through solid-state cooling.

Despite seamless integration of hexagonal boron nitride (hBN) with on‐chip devices, the intrinsically low optical quantum yield of spin‐active boron vacancy () defects remains a significant limitation to the sensitivity of hBN‐based quantum sensors. Here, we demonstrate an hBN quantum sensor with enhanced quantum yield and high DC magnetic field sensitivity (η _DC ), achieved by coupling defects in hBN with a nanostructured plasmon‐strain microwave waveguide architecture.This platform is realized by fabricating arrays of alumina‐coated gold nanopillars, or plasmonic nanoresonators (PNRs), onto the constricted region of a microwave‐efficient, single‐port gold coplanar waveguide. The alumina coating acts as a dielectric barrier that suppresses photoluminescence (PL) quenching, while gold nanopillars enhance local electromagnetic fields and induce strain‐driven perturbations of the defect energy levels, causing accelerated photo‐emission. This synergistic effect results in a ∼tenfold enhancement in PL and improves optically detected magnetic resonance to −17% for on‐PNR regions, exceeding comparable prior works by over an order of magnitude. Consequently, we achieve an η _DC of 9.4 µT/√Hz, approaching the highest reported values for defects. This research establishes a strategy for designing and fabricating highly sensitive quantum sensors that operate at room temperature without requiring extensive optimization of laser or microwave fields.

The U.S. Center for Quantum Technologies (CQT) is a multi-university consortium established under the National Science Foundation's (NSF) Industry-University Cooperative Research Centers (IUCRC) program. Led jointly by Purdue University, Indiana University (both Bloomington and Indianapolis campuses), and the University of Notre Dame, CQT integrates academic research with industrial and governmental collaboration to accelerate quantum innovation. This perspective outlines the consortium's strategic mission, interdisciplinary research agenda, and its role in shaping the future of quantum-enabled technologies through collaborative development, translational impact, and workforce cultivation.

Two-dimensional (2D) materials have emerged as a versatile and powerful platform for quantum technologies, offering atomic-scale control, strong quantum confinement, and seamless integration into heterogeneous device architectures. Their reduced dimensionality enables unique quantum phenomena, including optically addressable spin defects, tunable single-photon emitters, low-dimensional magnetism, gate-controlled superconductivity, and correlated states in Moiré superlattices. This Roadmap provides a comprehensive overview of recent progress and future directions in exploiting 2D materials for quantum sensing, computation, communication, and simulation. We survey advances spanning spin defects and quantum sensing, quantum emitters and nonlinear photonics, computational theory and data-driven discovery of quantum defects, spintronic and magnonic devices, cavity-engineered quantum materials, superconducting and hybrid quantum circuits, quantum dots, Moiré quantum simulators, and quantum communication platforms. Across these themes, we identify common challenges in defect control, coherence preservation, interfacial engineering, and scalable integration, alongside emerging opportunities driven by machine$-$learning$-$assisted design and integrated experiment$-$theory feedback loops. By connecting microscopic quantum states to mesoscopic excitations and macroscopic device architectures, this Roadmap outlines a materials-centric framework for integrating coherent quantum functionalities and positions 2D materials as foundational building blocks for next-generation quantum technologies.

The U.S. Center for Quantum Technologies (CQT) is a multi-university consortium established under the National Science Foundation's (NSF) Industry-University Cooperative Research Centers (IUCRC) program. Led jointly by Purdue University, Indiana University (both Bloomington and Indianapolis campuses), and the University of Notre Dame, CQT integrates academic research with industrial and governmental collaboration to accelerate quantum innovation. This perspective outlines the consortium's strategic mission, interdisciplinary research agenda, and its role in shaping the future of quantum-enabled technologies through collaborative development, translational impact, and workforce cultivation.

Entanglement is central to quantum science and technology. Atomic defects in two-dimensional (2D) van der Waals (vdW) materials offer exciting prospects for quantum sensing, with spatial resolution reaching 1 nm demonstrated using scanning probe techniques. However, entangling qubits in vdW materials remains elusive. Here we report room-temperature quantum entanglement between an optically addressable electron spin and a strongly coupled $^{13}$C nuclear spin in hexagonal boron nitride (hBN). We extend the electron spin coherence to 38 $μ$s with dynamical decoupling, and create maximally entangled Bell states with a fidelity up to 0.89. We further use the nuclear spin as a long-lived quantum memory to enhance AC magnetic field sensing via correlation spectroscopy. These results establish entangled spin qubits in hBN as a robust platform for advanced quantum technologies based on 2D materials.

Optically active spin defects in van der Waals (vdW) materials have emerged as versatile quantum sensors, enabling applications for a wide range of quantum phenomena in condensed matter systems. Their ease of exfoliation and compatibility with device integration make them promising candidates for future quantum technologies. Here we report the observation and room-temperature coherent control of ensemble spin defects in the high-temperature crystalline phase of germanium disulfide (β-GeS₂), a two-dimensional (2D) semiconductor with low nuclear spin density. The defects exhibit spin-1/2 behavior, and their dynamics can be explained by a weakly coupled spin-pair model. We implement dynamical decoupling techniques to extend the coherence time (T₂) by a factor of 20. Finally, we use density functional theory (DFT) calculations to estimate the structures and spin densities of two possible spin defect candidates. This work will help to expand the field of quantum sensing with spin defects in 2D materials.