Engineering photon-photon interactions is fundamentally challenging because photons in vacuum do not interact with each other. While their interactions can be mediated using optical nonlinearities, these effects are negligible for individual photons. This thesis explores two topics related to optical nonlinearities in waveguides. In the first part, we perform a numerical simulation study of hollow core antiresonant reflecting optical waveguides (ARROWs) fabricated using standard lithographic techniques in the context of their suitability as a platform for on-chip photonic quantum information processing. We investigate the effects of the core size, the number of pairs of antiresonant layers surrounding the hollow core, and the refractive index contrast between cladding materials on propagation losses in the waveguide. Additionally, we explore the feasibility of integrating these waveguides with Bragg gratings and dielectric metasurface mirrors to form on-chip cavities, that when loaded with atomic ensembles could act as nonlinear optical devices controllable with single photons. \n \nThe second part of this thesis studies the application of a 3 level quantum emitter coupled to a directional optical waveguide to deterministically subtract a single photon from a propagating optical pulse. Subtracting a single photon from a light state is one of the most fundamental operations with important applications in quantum information processing. However, current methods to subtract a photon such as using a low reflectivity beam splitter suffer from inherently low success probabilities as well as a strong dependence on the number of photons in the input. We explore implementing this single photon subtraction operation in our proposed system when the optical input is a continuous wave coherent state, coherent pulsed state containing a finite number of photons, or a Fock state.
Engineering photon-photon interactions is fundamentally challenging because photons in vacuum do not interact with each other. While their interactions can be mediated using optical nonlinearities, these effects are negligible for individual photons. This thesis explores two topics related to optical nonlinearities in waveguides. In the first part, we perform a numerical simulation study of hollow core antiresonant reflecting optical waveguides (ARROWs) fabricated using standard lithographic techniques in the context of their suitability as a platform for on-chip photonic quantum information processing. We investigate the effects of the core size, the number of pairs of antiresonant layers surrounding the hollow core, and the refractive index contrast between cladding materials on propagation losses in the waveguide. Additionally, we explore the feasibility of integrating these waveguides with Bragg gratings and dielectric metasurface mirrors to form on-chip cavities, that when loaded with atomic ensembles could act as nonlinear optical devices controllable with single photons. \n \nThe second part of this thesis studies the application of a 3 level quantum emitter coupled to a directional optical waveguide to deterministically subtract a single photon from a propagating optical pulse. Subtracting a single photon from a light state is one of the most fundamental operations with important applications in quantum information processing. However, current methods to subtract a photon such as using a low reflectivity beam splitter suffer from inherently low success probabilities as well as a strong dependence on the number of photons in the input. We explore implementing this single photon subtraction operation in our proposed system when the optical input is a continuous wave coherent state, coherent pulsed state containing a finite number of photons, or a Fock state.
The ability to control light at the single photon level is essential for fully harnessing the power of quantum information theory, and precision measurements. In this thesis, we study two phenomena which can help us to create, and shape light at the smallest scale. In the first part of the thesis, we explore deterministic single photon subtraction based upon single photon Raman interaction of a single three-level Λ-type quantum emitter in a bi-modal cavity or coupled with a chiral waveguide. We study the effect of photon subtraction from different types of optical inputs –- continuous-wave or pulsed coherent states and Fock states, and dependence of the fidelity of subtraction upon key system parameters. We also discuss the suitability and prospects of using different emitter-waveguide systems reported in literature to successfully extract a single photon from an optical input. We find that a quantum dot coupled with a photonic crystal waveguide with high group index could subtract photons with fidelity close to unity. Moreover, we explore how such a SPRINT-based single photon subtraction process can be used to create non-Gaussian states with negative values of Wigner distribution, and negative conditional entropies. Such states can be used as quantum resources in various fields of quantum information theory. Finally, as an interesting example, we also discuss how this mechanism can also be used to create non-classical Fock states of arbitrary photon number. \nIn the second part of this thesis, we study how to shape the emission from a single solid-state quantum emitter, in particular an NV⁻ center in diamond, using two dimensional patterns with sub-wavelength features. We propose a pattern etched on the surface of a diamond sample that enhances the NV's emission in a particular direction, and maximizes coupling with a waveguide substantially far away from the diamond-air interface. Our proposed structure was designed using the adjoint optimization technique, which significantly reduces the amount of computational resources compared to brute-force methods to design an optical element with desired properties. Our structure exhibits a higher directionality of emission compared to other nanophotonic structures reported in the literature -- solid immersion lenses, nanopillars, and bull's eye structures. Finally, we also discuss in details the steps pertaining to setting up a confocal microscope in our laboratory for imaging NV centers, and characterizing our proposed device.
This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.
Realising a platform for efficiently guiding and manipulating light at the singlephoton level lies at the heart of optical quantum information processing. In this thesis we aid in the pursuit of this goal by analysing few-photon transport properties in photonic devices while keeping in mind possible applications and experimental challenges. We especially focus on single and two-photon transport properties in one-dimensional waveguides coupled to (artificial) atoms modelled as two and three-level emitters. We begin by introducing key concepts relevant for this thesis and then explain how to model such waveguide geometries. We show how directional-dependent coupling between a waveguide and a localized quantum system can be modelled from a chain of harmonic oscillators by allowing the system to couple to more than one site. Using the input-output formalism and scattering matrix theory we determine a general form for the single and two-photon S-matrices for a waveguide coupled to a two-level system. We also explain how to engineer a quantum optical Fanoresonance waveguide geometry and calculate how this modifies the S-matrices, the few-photon scattering probabilities, and the scattered output spectra. We show that the nonlinear effects in such a system generates highly correlated directional scattering probabilities. We device a protocol that can generate maximal entanglement between two spectrally distinct solid-state emitters by the use of multi-photon scattering. The results also reveal a rich structure for multi-photon states interacting with non-identical emitters. As a final investigation, we quantify how spectral diffusion in quantum dots affects finite-width single and two-photon scattering. Single-photon scattering probabilities are described by a simple convolution but this is not the case for a two-photon state scattering off a two-level emitter. We show that the effect of spectral wandering depletes the nonlinear scattering properties, but that it affects co and counter-propagating output probabilities differently.
Waveguide quantum electrodynamics (waveguide QED) describes the interaction between an electromagnetic field confined to a one-dimensional waveguide with atom-like quantum emitters close by. The characteristics of these kinds of systems are the possibility for strong and even ultrastrong interactions between the photonic and atom-like systems, and practically infinite range interactions between the emitters. These properties are valuable for a wide range of quantum optical applications, like quantum communication, quantum networks and quantum metrology. In this thesis we focus on two challenges in quantum optics, namely the generation and scattering of multiphoton states. In the first part of this thesis we demonstrate how waveguide QED systems can be exploited for the generation of multiphoton states, in particular of single-mode Fock states and superpositions thereof as well as multi-mode photonic states with metrological applications. The basic setup for this goal is an ensemble of quantum emitters coupled to a waveguide. In the so-called atomic mirror configuration symmetric Dicke states (or a superposition thereof) decay superradiantly to the ground state and emit the desired multiphoton state, which can be efficiently collected at the ends of the waveguide. We propose various protocols for the preparation of these Dicke states in waveguide QED systems and characterize the emitted photonic state. It turns out that in the low excitation regime, that is, if the number of photons is much lower than the ensemble size, the photonic state is emitted into a single mode. This single-mode structure is fundamental to current proposals for applications in quantum metrology with optical interferometers. Outside the low excitation regime, a multi-mode photonic state is generated, for which the metrological capabilities were unknown. We were able to show that these states still lead to quantum-enhanced optical interferometry. In the second part of this thesis we demonstrate how the scattering of multiphoton states on a single quantum emitter coupled to the waveguide can be used for testing the limits of the light-matter interaction strength. In particular, we investigate systems in the so-called ultrastrong coupling regime, where the coupling strength between the photonic and atom-like system is of the order of the emitter's transition energy. In this regime, many methods and approximations used in quantum optics, especially the Rotating Wave Approximation, break down and one needs to develop new analytical and numerical methods to study these systems. One approach is the so-called polaron Transformation. We show how this transformation can be applied for predicting the scattering amplitude of few photons scattering on the emitter. The comparison of these results with numerical simulations using matrix product states shows a good agreement for moderate coupling strengths. Both parts together show that waveguide QED systems are promising candidates for the generation of multiphoton states as well as for the investigation of fundamental limits probed through the scattering of multiphoton states. The latter also finds application in the implementation of photon-photon nonlinearities induced by the interaction with the quantum emitters. Apart from these and many other theoretical predictions, waveguide QED systems are also undergoing rapid progress in experiments. Therefore, we expect these kinds of systems to bring forth several advances in the field of quantum optics.
This Thesis is focused on the study of quantum electrodynamics in photonic crystal waveguides. We investigate the interplay between a single quantum dot and the fundamental mode of the photonic crystal waveguide. We demonstrate experimental coupling eciencies for the spontaneous emission into the mode exceeding 98% for emitters spectrally close to the band-edge of the waveguide mode. In addition we illustrate the broadband nature of the underlying eects, by obtaining coupling eciencies above 90% for quantum dots detuned from the band edge by as far as 20nm. These values are in good agreement with numerical simulations. Such a high coupling eciency implies that the system can be considered an articial 1D-atom, and we theoretically show that this system can generate strong photon-photon interaction, which is an essential functionality for deterministic optical quantum information processing. We present the progress towards the experimental demonstration of such a strong photon-photon nonlinearity. We present the rst signatures of resonant transmission in photonic crystal waveguides and discuss some of the encountered diculties and future directions of the experiments. In the nal part of this Thesis we discuss a novel type of photonic crystal waveguide and show its applications for on-chip quantum information processing. This structure was designed for the ecient mapping of two orthogonal circular dipole transitions to dierent propagation paths of the emitted photon, i.e. exhibits chiral quantum-dot-waveguide coupling. Such a structure is ideally suited for a number of applications in quantum information processing and among others we propose an on-chip spin-photon interface, a single photon transistor, and a deterministic cNOT gate.
Controlling light photon-by-photon is central to quantum optics. At a fundamental level, photon interactions are mediated by their coupling to atoms, and ultimate control requires deterministic light-matter interfacing of single photons to single atoms. Extending this paradigm to radiatively couple multiple individual atoms in a deterministic and scalable manner opens the arena of many-body quantum optics. Here, we realize such a setting by coherently coupling solid-state artificial atoms to a nanophotonic waveguide and demonstrate higher-order photon correlations that are controlled by the number of quantum emitters. We study the scaling of nonlinear photonic transport induced by emitter-photon scattering and demonstrate that adding a quantum emitter generates higher-order photon correlations. Specifically, we experimentally observe genuine three-photon correlations from a pair of collectively coupled emitters, while contributions from lower photon numbers are suppressed. In addition, we scale to three resonant quantum emitters coupled to the waveguide. These advancements demonstrate the onset of many-body quantum optics in waveguide quantum electrodynamics, enabling new photonic quantum simulators, the creation of many-body entangled states, and the exploration of novel quantum phase transitions.