In recent years, elementary quantum optical structures, called photonic molecules (PMs), have been carefully studied both experimentally and theoretically [1, 2]. There structures are formed from high quality factor solid-state microresonators (MR). These devices may be integrated with singlephoton sources that generate and guide photon flows in a system and high-sensitivity detectors that fix the arrival of a photon and, preferably, its polarization [3]. As for the element base for quantum computation, the main effort of scientists is now focused on the search for the optimal geometry of a solidstate photonic chip [4]. Here, we propose the design of three-unit PM optimized to obtain good transport and dissipation properties. To design PM supporting optical-band frequencies, one uses photon cells with geometric dimensions on the order of a few microns. MRs supporting whispering gallery modes (e.g. microrings) can form quasi-one-dimensional optical structures. We optimize diamond microring parameters calculating the eigenfrequencies and the electrical field distributions of the single microring in a broad range of inner and outer radii as well as thicknesses. Analytical consideration of the PM-system composed of three MRs is given within the formalism of tight-binding phenomenological Hamiltonian:
In recent years, elementary quantum optical structures, called photonic molecules (PMs), have been carefully studied both experimentally and theoretically [1, 2]. There structures are formed from high quality factor solid-state microresonators (MR). These devices may be integrated with singlephoton sources that generate and guide photon flows in a system and high-sensitivity detectors that fix the arrival of a photon and, preferably, its polarization [3]. As for the element base for quantum computation, the main effort of scientists is now focused on the search for the optimal geometry of a solidstate photonic chip [4]. Here, we propose the design of three-unit PM optimized to obtain good transport and dissipation properties. To design PM supporting optical-band frequencies, one uses photon cells with geometric dimensions on the order of a few microns. MRs supporting whispering gallery modes (e.g. microrings) can form quasi-one-dimensional optical structures. We optimize diamond microring parameters calculating the eigenfrequencies and the electrical field distributions of the single microring in a broad range of inner and outer radii as well as thicknesses. Analytical consideration of the PM-system composed of three MRs is given within the formalism of tight-binding phenomenological Hamiltonian:
The optical properties of a photonic molecule consisting of three diamond microring cavities are theoretically investigated. The probability of single-photon excitation (optical response) of a photonic molecule by a weak laser field in the steady-state mode with regard to the dissipative effects is calculated using the model analogous to the tight binding approximation. It is shown that the spectrum can be fine tuned by depositing additional layers onto the photonic-molecule surface. The dependences of the wavelength of the mode corresponding to the zero-phonon optical transition in the NV center on the thickness of these layers and refractive index of their material are established. The NV center localized in the electromagnetic field antinode effectively interacts with the photonic molecule eigenmode, which can be observed as anticrossing points in the dependences of the optical response of the system on exciting laser and transition frequencies.
Photonic molecules (PMs) are artificial nanoscale photonic structures that play important roles in the fundamental optics field. PM quantum optics has recently become a promising research field, because it provides novel quantum optical phenomena including Rabi oscillation, the Stark effect, the Purcell effect, the photon blockade effect, bound states in the continuum, electromagnetically induced transparency, and Autler–Townes splitting. With the constant improvements in theoretical PM quantum optics research, many newly integrated photonic devices have been proposed and experimentally demonstrated, showing major potential for fabrication of next-generation, high-performance integrated photonic chips. This review provides a universal overview of the rapidly developing PM quantum optics field, including fundamental mechanisms, realization frameworks, novel quantum optical phenomena, and applications in newly developed photonic devices while also giving a general summary of the remaining challenges and proposing possible development directions for PM quantum optics.
Luminescent lattice defects (so called “color centers”) in diamond provide a robust, solid-state architecture for exploring quantum optics, with many facilitating direct access to highly coherent electron and nuclear spins. The past two decades have seen tremendous advances in our ability to prepare and control the quantum states of these atom-like systems. For instance, demonstrations that leverage the diamond nitrogen vacancy (NV–) center as a spin-photon interface have included the generation of indistinguishable photons, a quantum bit memory exceeding 1 second, entanglement of two distance solid-state qubits, and deterministic long distance quantum teleportation. While diamond color centers are an attractive platform for scalable quantum information processing, the diamond solid-state matrix, by its very nature, inhibits efficient photon collection into either free-space or fiber-optical systems. Due to its large refractive index, emitted photons are inevitably trapped within the diamond crystal by total internal reflection, limiting count rates and overall scalability of information processing via quantum optical protocols. To this end, there has been significant progress towards integrating individual diamond color centers within well-defined radiation channels: namely, those provided by monolithic diamond nanophotonic networks. Currently, on-chip low loss (~ few db/cm) diamond waveguide networks are available, as well as key passive photonic elements like micron scale ring and disk resonators and photonic crystal cavities, with high optical quality (Q) factors. Specifically, we recently demonstrated a scalable ‘angled-etching’ nanofabrication method for realizing nanophotonic networks starting from bulk single-crystal diamond substrates (MJ Burek et al., Nano Letters 2012, MJ Burek et al., Nat Comm. 2014). Angled-etching employs anisotropic oxygen-based plasma etching at an oblique angle to the substrate surface, resulting in suspended optical structures with triangular cross-sections. Using this approach, we demonstrated high Q-factor (> 10^5) optical nanocavities fabricated in bulk single-crystal diamond, operating over a wide wavelength range (visible to telecom). Beyond isolated photonic devices, we have further developed free-standing angled-etched waveguides which efficiently route photons between diamond optical nanocavities, while maintaining physical support through attachment to the bulk substrate. Although on-chip diamond nanophotonics realized by angled-etching continue to mature, efficient off-chip optical coupling schemes which provide near seamless transition of photons on-chip into commercial optical fibers remain an outstanding challenge. This is especially pertinent in applications involving single photons, such as quantum optics with diamond color centers. Herein, we demonstrate a high efficiency fiber-optical interface with aforementioned on-chip diamond nanophotonic networks, achieving > 90% power coupling at both telecom and visible wavelengths. The coupling scheme utilizes a single mode optical fiber, with one end fabricated into a conical taper, to adiabatically transition guided light between on-chip tapered diamond waveguides and off-chip optical fiber networks. Additionally, we develop techniques to robustly pigtail single-mode fibers with diamond waveguide couplers, yielding a packaged technology that may potentially enable deployment in harsh environments, including vacuum, sub-Kelvin cryogenics, or even liquid/biological environments. With these newly developed optical integration schemes, single-crystal diamond is now a viable integrated nanophotonics platform, servicing a range of applications, from non-linear optics and chemical sensing, to quantum science and cavity optomechanics.
With their ability to process and transfer quantum information, large-scale entanglement-based quantum networks could be at the heart of a new age of quantum information, enabling fundamentally new applications such as distributed quantum computation, quantum communication, and quantum enhanced sensing. Due to their long spin coherence, controllable local qubit registers and optically active spins, color centers in diamond are prime candidates for nodes of such a network, and have enabled some of the most advanced quantum network demonstrations to date. These demonstrations include the distribution of a 3-node GHZ state across a quantum network, entanglement swapping, entanglement distillation, and memory-enhanced quantum communication. To move beyond current proof-of-principle networks, a further increase of entanglement generation rates is crucial. This thesis presents theoretical and experimental work on enhancing the spin-photon interface of color centers in diamond to achieve this goal, making use of the Purcell effect. The discussed work follows two main directions: embedding of color centers in open, tuneable Fabry-Perot micro-cavities, and in all-diamond photonic crystal cavities. First, we describe theoretical and experimental progress towards cavity-enhanced quantum networks based on nitrogen-vacancy (NV) centers in diamond. Due to their first order sensitivity to electric fields, we choose to embed NV centers in microns-thin diamond membranes that can be integrated into open fiber-based micro-cavities. We develop analytical methods to optimize the design of such open cavity systems for maximum Purcell enhancement of embedded color centers. We demonstrate a method to fabricate optically coherent NV centers in microns-thin diamond membranes, and use such structures to demonstrate the resonant excitation and detection of coherent, Purcell enhanced NV emission. A theoretical model in excellent agreement with our results suggests our system can improve entanglement rates between distant NV centers by two orders of magnitude with near-term improvements to the setup. Second, we describe progress towards an efficient spin-photon interface of group-IV color centers in diamond by coupling them to photonic nanostructures, allowing for large-scale integration. Due to their first-order insensitivity to electric fields, group-IV color centers can be brought in close proximity to surfaces (~ 100 nm), allowing for sub-wavelength mode volumes and thus very high Purcell factors. We numerically optimize photonic crystal cavity designs to maximize the Purcell enhancement of embedded emitters, test the robustness of our designs to real-world fabrication imperfections, and devise a method to efficiently interface nanophotonic structures. We then proceed to fabricate all-diamond photonic crystal cavities, making use of a dry etching technique that is selective to different crystallographic directions, and characterize the resulting optical quality factors. Finally, we marry the developed fabrication methods to fabricate microns-sized diamond platelets that can be transferred from a holding frame in a controlled fashion. This capability could be crucial for the realization of hybrid photonic circuits that we expect to be at the heart of future large-scale quantum networks.
Abstract We propose an efficient microwave-photonic modulator as a resource for stationary entangled microwave-optical fields and develop the theory for deterministic entanglement generation and quantum state transfer in multi-resonant electro-optic systems. The device is based on a single crystal whispering gallery mode resonator integrated into a 3D-microwave cavity. The specific design relies on a new combination of thin-film technology and conventional machining that is optimized for the lowest dissipation rates in the microwave, optical, and mechanical domains. We extract important device properties from finite-element simulations and predict continuous variable entanglement generation rates on the order of a Mebit/s for optical pump powers of only a few tens of microwatts. We compare the quantum state transfer fidelities of coherent, squeezed, and non-Gaussian cat states for both teleportation and direct conversion protocols under realistic conditions. Combining the unique capabilities of circuit quantum electrodynamics with the resilience of fiber optic communication could facilitate long-distance solid-state qubit networks, new methods for quantum signal synthesis, quantum key distribution, and quantum enhanced detection, as well as more power-efficient classical sensing and modulation.
In recent years, there has been a spurring research interest growing on photonics integrated circuits as the way to scale up quantum communications. In particular, building a reliable integrated source of entangled photons is one of the main challenges remaining for commercial exploitation in quantum sensing and metrology. Therefore, advances in design of integrated sources of entangled photons is a necessary and crucial task to be performed for miniaturization and cost-effectiveness. Microring resonators are one of the most promising candidates to generate entangled photons via nonlinear interactions, especially Spontaneous Four-Wave Mixing (SFWM). But fabrication limitations in reproducibility of high-quality resonators mandate the use of active devices. Here we present the results of preparing the excitation pulse with different spectral, intensity and dispersion parameters to optimize the quantum efficiency in passive devices after fabrication. Different numerical methods such as Finite Difference Time Domain (FDTD) and Split-Step Fourier Method (SSFM) are used to reproduce the nonlinear propagation, interplay with dispersion and losses. Exploiting the degree of freedom of the excitation pulse can bring more energy efficient and passive devices as quantum entangled pair sources in an integrated chip. Considerations on different degrees of entanglement are also discussed for applications in quantum sensing and quantum metrology.