Jared H. Cole

The negatively charged nitrogen‐vacancy (NV) center in diamond is a leading solid‐state quantum emitter, offering spin‐photon interfaces over a wide temperature range with applications from electromagnetic sensing to bioimaging. While NV centers in bulk diamond are well understood, embedding them in nanodiamond (ND) introduces complexities from size, NV location, and NV polarizations. NVs in ND show altered fluorescence properties, including longer lifetimes, lower quantum efficiency, and higher sensitivity to dielectric surroundings, which arise from radiative suppression, surface‐induced non‐radiative decay, and escape inefficiency at the diamond‐background interface. Prior models typically addressed isolated aspects, such as dielectric contrast or surface quenching, without integrating full quantum‐optical NV behavior with classical electrodynamics. We present a hybrid framework coupling rigorous electromagnetic simulations with a quantum‐optical NV model including phonon sideband dynamics. NV emission is found to depend strongly on ND size, NV position, and surrounding refractive index. Our results explain observations such as shallow NVs in water‐coated ND appearing brighter than deeper ones in air. This integrated model provides a unified framework for realistic NV in ND emission scenarios and informs the design of efficient NV‐based sensors and quantum devices, advancing understanding of quantum emitter photophysics in nanoscale crystals.

Superconducting quantum computers leverage superconducting qubits, systems with well-defined energy levels, the spacing of which is determined by the properties of embedded Josephson junctions. Indeterminacy of Josephson junction manufacturing limits large-scale superconducting circuits; however, junction-by-junction tuning affords the possibility of an order of magnitude improvement of determinacy—although the origin of this tuning effect is uncertain. Understanding how tuning techniques may physically modify the tunnel barrier of a Josephson junction is important and will enable these techniques to be optimized. We develop a model of junction tuning based on depinning theory to interpret a phase diagram of the tuning rate. We extract the dependence on temperature, time-varying voltages, and oscillation frequency. Comparing the effect of junction tuning on room-temperature I − V ’s, electrical breakdown, and frequency of higher-energy levels of transmon qubits indicates that the tuning process leaves the barrier thickness unchanged but modifies its composition.

When a quantum emitter (QE) is placed in close proximity to a plasmonic metal nanoparticle within an external optical field, a dipole-dipole coupling arises, resulting in a highly tunable hybrid nanosystem that surpasses the optical manipulation capabilities of the individual components. These hybrid systems enable the exploration and manipulation of optical fields at the intersection between classical and quantum phenomena. Theoretical models of this interaction have typically been limited to the extreme orientations, where the QE and plasmonic dipoles are polarized either along or perpendicular to the interparticle axis, for analytical tractability. In this work, we generalize the semiclassical optical dipole-dipole interaction model for a two-level quantum emitter in a plasmonic field for arbitrary polarization angles. We show that the total field experienced by the quantum emitter at intermediate angles does not necessarily align with the external input field and discuss the implications of varying the polarization angle of the external input field. Published by the American Physical Society 2025

Photoluminescent defects in diamond, such as the nitrogen-vacancy (NV) color center, are at the forefront of emerging optical quantum technologies. Most emit in the visible and near-infrared spectral region below 1000 nm (NIR-I), limiting their applications in photonics, fiber communications, and biology. Here, we show that the nitrogen-vacancy-nitrogen (N₂V) center, which emits in the visible and near-infrared-II (NIR-II, 1000-1700 nm), is ubiquitous in as-synthesized and processed nitrogen-doped diamond, ranging from bulk samples to nanoparticles. We demonstrate that N₂V is also present in commercially available state-of-the-art NV diamond sensing chips made via chemical vapor deposition (CVD). In high-pressure high-temperature (HPHT) diamonds, the photoluminescence (PL) intensity of both N₂V charge states, N₂V⁰ in the visible and N₂V^- in the NIR-II, increases with increasing substitutional nitrogen concentration. We determine the PL lifetime of N₂V^- to be 0.3 ns and compare a quantum optical and density functional theory model of the N₂V^- with experimental PL spectra. Finally, we show that detonation nanodiamonds (DND) exhibit stable PL in the NIR-II, which we attribute to the N₂V color center, and use this NIR-II PL to image DNDs inside skin cells. Our results contribute to the scientific and technological exploration and development of the N₂V color center and help elucidate interactions with other color centers in diamond.

Localized surface plasmons (LSPs) are collective oscillations of free electrons in metal nanoparticles that confine electromagnetic waves into subwavelength regions, making them an ideal platform for light-matter coupling. To design and understand plasmonic structures, numerical computations of Maxwell's equations are commonly used. However, obtaining physical insight from these numerical solutions can be challenging, especially for complex-shaped nanoparticles. To circumvent this, we introduce mode decomposition strategies within the boundary element method (BEM). By employing singular value decomposition (SVD) and quasi-normal mode (QNM) decomposition, we break down optical responses into elementary modes. QNMs offer deeper insights into frequency and damping, while SVD modes allow for more accurate spectral reconstruction with fast computation. These techniques provide a deeper understanding of LSPs and facilitates the design of metal nanoparticles for efficient light-matter interaction.

The photophysical processes of singlet fission and triplet fusion have numerous emerging applications. They involve the separation of a photo-generated singlet exciton into two dark triplet excitons and the fusion of two dark triplet excitons into an emissive singlet exciton, respectively. The role of the excimer state and the nature of the triplet-pair state in these processes have been a matter of contention. Here we analyse the room temperature time-resolved emission of a neat liquid singlet fission chromophore and show that it exhibits three spectral components: two that correspond to the bright singlet and excimer states and a third component that becomes more prominent during triplet fusion. This spectrum is enhanced by magnetic fields, confirming its origins in the recombination of weakly coupled triplet pairs. It is thus attributed to a strongly coupled triplet pair state. These observations unite the view that there is an emissive intermediate in singlet fission and triplet fusion, distinct from the broad, unstructured excimer emission.