Quantum Nanophotonic Materials, Devices, and Systems 2024

In this research, we explored the electronic characteristics of a biconical quantum dot (QD) constructed from GaAs, employing the finite element method. Our investigation commenced with the computation of wave functions and energies for the ground state and the initial four excited states. This enabled us to discern the influence of the quantum dot's geometry on its electronic configuration. Utilizing the derived wave functions and energies for a single electron, we determined the oscillator strengths for various quantum transitions. We observed significant absorption phenomena during transitions from the ground state to the next four excited states.

The interaction of light with matter can generate different types of scattering that can be applied to quantum optics, photonics and integrated optics appropriately. The generation of certain pulses in certain waveguides can produce certain nonlinear optical pulses with important properties such as low energy and information loss applied, for example, to communication systems and quantum information. The method we describe is based on the generation and propagation of nonlinear optical pulses in multidimensional material structures: waveguides in 1 dimension, planar systems in 2 dimensions and crystalline systems in 3 dimensions. These structures can be appropriately designed to generate nonlinear and quantum optical pulses depending on the crystal structure and electrical susceptibility of the material. These optical pulses can be appropriately characterized whose respective scattering signals can be identified and processed providing an application basis for emerging technologies for transmission and processing systems based on quantum optics and quantum computing.

The rise of quantum technologies, especially the development of quantum networks, has made it important to develop reliable and robust quantum nodes which can communicate with each other through exchange of qubits. The biggest challenge in the way of developing such networks is to find suitable qubits which do not easily succumb to decoherence. With a scheme that relies on the coupling between single photons and atom-like transitions among spin states in a diamond colour center, it is possible to exploit both the strong coherence properties of photons and the easy control of spin states within the colour centers. In this work, we aim to couple single photons, generated by a germanium vacancy (GeV) center located within a nanodiamond, to other GeV centers in other nanodiamonds. The advantage of using nanodiamond-based GeV centers is in the ease of photon extraction along with the ability to control the colour center more easily through external fields without sacrificing spectral purity.

Single organic molecules make excellent single photon sources [1], emitting photons with high efficiency and at favourable wavelengths for coupling to other quantum systems, including alkali atoms. I will discuss techniques to create organic crystals containing single photon emitting molecules, and our recent results applying strain to these crystals to tune their emission wavelength [2]. I will show that subsequent photons emitted by a single molecule can undergo quantum interference at a beam splitter, a vital tool in optical quantum computing and communication, and will discuss how discuss how to assess photon indistinguishability using both continuous and pulsed lasers [3]. While molecular excitation and radiative emission is efficient, generated photons can be difficult to collect without the use of photonic structures. I will discuss our recent efforts in coupling single molecules to waveguides [4] and nanophotonic cavities. [1] C. Toninelli et al., Nature Materials 20, 1615 (2021). [2] A. Fasoulakis et al., Nanoscale 15, 177 (2023). [3] R. C. Schofield et al., Phys. Rev. Research 4, 013037 (2022). [4] S. Boissier et al., Nature Commun. 12, 706 (2021).

Magnetometry plays a crucial role in various remote sensing applications, aiding Earth-based navigation and geological surveying, as well as contributing to planetary and Earth sciences in space. While typically, magnetometers are placed on spacecraft booms to reduce interference, there is growing interest in deploying multiple compact sensors on a space craft. One promising approach uses optically detected magnetic resonance (ODMR) of defects in diamond, silicon carbide (SiC), or hexagonal boron nitride. While NV centers in diamond offer high sensitivity, they usually require a magnetic bias field. SiC's silicon vacancy, with a single orientation, enables zero-field magnetometry, making it appealing for spacecraft application. This research focuses on defect engineering to enhance SiC-based magnetometer performance, exploring the impact of radiation fluences on key properties across different SiC polytypes. This work advances the development of optically pumped quantum magnetometers using silicon carbide, making ODMR-based sensors more attractive for space applications.

Future quantum networks will enable unprecedented applications, ranging from secure communication to quantum-sensor networks. Qubits based on single rare-earth ions are promising candidates to act as nodes in such networks. We make use of single Yb3+ ions in YVO4, embedded in photonic crystal cavities. The vanadium (V) nuclear spins provide a dense nuclear spin bath, which can be used to store quantum states. Due to the Yb ions' orientation relative to neighboring V spins, control has been limited to next-nearest-neighbor spins. We work towards extending the resources for quantum state storage by the development of the nearest-neighbor spin control, enabled by an off-axis off-chip radiofrequency drive. Furthermore, we investigate what limits the qubit lifetime and coherence, using a crystal with a lowered concentration of Yb and paramagnetic defects. Additionally, we study the effect of an interacting spin bath using a cluster correlation expansion model. This work will enable the further exploration of sensing protocols in complex spin environments, the development of coherence preserving pulse sequences, and storage of quantum states in high-dimensional Hilbert spaces.

Atom-sized quantum sensors constructed from a spin-based qubit system can provide unparalleled sensitivity along with atomic resolution. Nitrogen vacancy (NV) centers in diamond have emerged as one of the leading solid-state quantum sensing systems, but the central challenge remains in the suboptimal optical readout. Resonant nanophotonic strategies to improve spin-photon interactions and opportunities to reach near-unity absorption-based spin readout fidelity will be discussed. The studied quantum diamond-spin metasurface paves the way for a new type of quantum micro(nano)scopy.

The study explores hexagonal boron nitride (hBN) as a host for optically addressable spin defects. It investigates the interaction between boron vacancy defects (V_B^-) and carbon-related visible emitters in hBN crystals, demonstrating coherent control and optical readout at room temperature. The research also assesses the spin properties of these emitters under various optical excitation and readout wavelengths conditions and examines the effects of electron irradiation and annealing on hBN samples of various forms – powders, bulk crystals and films grown by vapour epitaxy. This contributes to understanding the spin properties of color centers in hBN, advancing the use of quantum sensors in two-dimensional materials.

Recently, fluorescent point defects in silicon have been explored as promising candidates for single photon sources, which may pave the way towards the integration of quantum photonic devices with existing silicon-based electronic platforms. However, the current processes for creating such defects are complex, and commonly require one or two implantation steps. In this work, we have demonstrated implantation-free methods for obtaining G and W-centers in commercial silicon-on-insulator substrates using femtosecond laser annealing. We also demonstrate an enhancement of the color centers’ optical properties by coupling them with photonic structures. For example, we have shown an improvement in emission directivity for G centers by embedding them into silicon Mie resonators fabricated by dewetting, achieving an extraction efficiency exceeding 60% with standard numerical apertures. We will also address the control of emission polarization by embedding color centers in photonic crystals.

Cu2O, also known as cuprous oxide, has garnered attention as a solid-state material that holds promise for hosting excitonic Rydberg states. These states are characterized by large principal quantum numbers, resulting in significantly expanded wavefunctions. Consequently, Cu2O exhibits strong dipole-dipole interactions, making it an appealing platform for solid-state quantum technologies. Of particular interest are thin-film Cu2O samples, which can be fabricated with careful control to minimize defects and enable the observation of extreme single-photon nonlinearities through the Rydberg blockade.

In this work, we demonstrate an unprecedentedly bright heralded single-photon source with ideal single photon purity. The source is based on spontaneous four wave mixing in silicon spiral waveguide excited by a pump with a repetition rate of 2.5~GHz. We proposed an explicit theoretical limit of purity of heralded single-photon source, depending on Possoinian and thermal-like light pump, respectively. The measured coincidence counting rate exceeding 1.5~MHz, and the value of second-order correlation function reaches the limit with a lowest value of 0.000945, which has never achieved by on-chip SFWM sources.

Directionally unbiased multi-ports are novel photonic components where each port could equally serve as an input and an output point for light. This new concept of linear-optical devices enables the design of next-generation classical and quantum photonic devices for applications in sensing, metrology, and information processing. Though some unbiased multi-ports have been realized as collections of free space optics, their implementation in a graph network is impractical due to their sensitivity to misalignment and the strict coherence requirements of their fundamental interference phenomena. Therefore, developing chip-integrated embodiments of interconnected, unbiased multi-ports will provide an experimental platform for novel quantum photonics devices. This include enhanced-sensitivity interferometers for navigation, low-power optical modulators, quantum entanglement routing, and discrete-time Hamiltonian simulation. This talk will discuss recent advances in the design, fabrication, and implementation of nanoscale, directionally unbiased photonic integrated circuits (PICs) while introducing the new linear optical devices they enable.

Integrated quantum photonic system has become a promising candidate for next-generation quantum optical system. Thanks to the scalability and strong light-matter interactions, such nanophotonic systems can realize compactness, large-scale integration, stability, and novel optical functionalities simultaneously. In this talk, I will introduce our experimental works in the fields of the integrated lithium niobate photonics. First, we demonstrate a set of nonlinear photon-pair generations for the nonlinear quantum photonic sources. Next, I will show our works on lithium-niobate integrated quantum photonic circuits for the application in quantum communications and variational quantum simulations.

We report the designs and the fabrication of optical intensity masks which enable trapping of two-dimensional arrays of cooled atoms of two atomic species, using single laser. Compared to previous realizations using active optical components, e.g., spatial light modulators, these passive optical masks reduce the complexity of neutral-atom experiments. The optical intensity masks are easily scalable to enable the trapping of large arrays of single atoms, enabling future applications in quantum sensing, networking, and computing.

In this presentation, we delve into hBN’s potential as a host for photon emitters. In our recent publication, we introduced a method for creating defects in hBN with tailored spectral properties and spatial distributions using ion beam irradiation. We demonstrate that gallium ions efficiently produce emitters, with Raman spectroscopy identifying defect vibrational signatures. Spectral tuning over 200 nm is achieved through thermal annealing, regardless of ion species, energy, or density. This process is confirmed by Raman spectroscopy, indicating changes in defects' configurations. Coupling a focused ion beam system with annealing, we achieve precise control over emitters' spectral and spatial properties, advancing quantum technologies by enabling customization of emitter properties in hBN.

We introduce a pioneering platform combining open cavity design with confocal microscopy and spectroscopy, tailored for quantum technology and science studies. This innovative setup excels in investigating quantum phenomena such as the Purcell effect, Rabi splitting, and light-matter interactions in 2D materials, alongside the development of quantum light sources. The open cavity enhances the interaction between light and matter, facilitating the observation of quantum effects at unprecedented detail. Coupled with the spatial resolution of confocal microscopy, this platform allows for precise, localized studies of quantum behaviors in nanoscale systems. It represents a significant leap forward in quantum technology research, providing a versatile tool for exploring the frontiers of quantum science.

We hereby present our recent work related to the interaction of light with vapor and 2-D materials, enhanced by metasurfaces. Both free space and guided wave configurations will be discussed. We will show how Rubidium vapor and other species can modulate the phase and amplitude of the local field, benefiting from the strong enhancement of the light-matter interactions.

Quantum yield (QY) is a crucial performance factor of the semiconductor to be considered in light-emitting photonic applications. In two-dimensional (2D) transition metal dichalcogenides (TMDs), high density of lattice defects and strong Coulomb interaction are mainly responsible for their low QY compared to that of bulk semiconductor materials. In this presentation, I will discuss the methods to enhance the QY of 2D TMDs, by the use of heterogeneous interfacing with metallic substrate, hBN layers or wide-band gap quantum dots. [1-4] [1] Y. Lee, et al, Nat. Commun. 12, 7095 (2021) [2] Y. Lee and T. T. Tran, et al, ACS Photon. 9, 873 (2022) [3] T. T. Tran et al., ACS Nano 18, 220 (2024) [4] A. S. Sharbirin, manuscript in preparation

In this work, i will discuss the emerging field of hBN quantum photonics. I will show how to engineer scalable photonic devices from hBN, and present new spectroscopic evidence for spectral hole burning and evidence for coherent emission.

Coupling optical transitions to a single mode of an optical cavity can to enable generation of indistinguishable single photons, nonlinear-optical applications, quantum transduction, and control of chemical pathways. For all of these applications, coupling strengths need to be large compared to decoherence rates of the emitter. I will discuss progress towards this goal for various quantum emitters, including semiconductor nanocrystals (quantum dots), defects in silicon, and organic molecules. I will emphasize in particular the use of plasmonic nanocavities, which can have mode volumes well below the diffraction limit, and thus can provide coupling strengths than enable quantum photonics at room temperature.