The main goal of this thesis is to study nonlinear photonic devices from the perspective of generation of non-classical states of light through the process of parametric down-conversion, also considering practical aspects. Periodically-poled waveguides as well as relatively new Bragg reflection waveguides are studied as a promising bright sources for integrated platforms. Possible limitations by imperfections of fabrication process are considered as well. More common bulk crystals are also used to generate more complex entangled states and fields, whose correlations properties are studied not only at single-photon level, but also in a mesoscopic domain.
The main goal of this thesis is to study nonlinear photonic devices from the perspective of generation of non-classical states of light through the process of parametric down-conversion, also considering practical aspects. Periodically-poled waveguides as well as relatively new Bragg reflection waveguides are studied as a promising bright sources for integrated platforms. Possible limitations by imperfections of fabrication process are considered as well. More common bulk crystals are also used to generate more complex entangled states and fields, whose correlations properties are studied not only at single-photon level, but also in a mesoscopic domain.
In this thesis we study the generation of non-classical states of light using nonlinear integrated photonics. We focus first on third-order parametric down-conversion (TOPDC), a process in which pump photons can be down-converted into photon triplets, and which could serve as a source of non-Gaussian states and tripartite discrete-variable entanglement. We investigate the possibility of implementing TOPDC in conventional integrated structures. By comparing the estimated triplet generation rates in a waveguide and a microring resonator, we establish that non-resonant structures appear to be a more promising platform for a near-term demonstration of TOPDC, due to the high generation bandwidth enhancing the rate of triplet generation. We estimate generation rates of ~10 triplets per second for a 1-cm-long waveguide driven by a 100 mW CW pump, a rate that is comparable to or higher than those quoted for other proposals. We then estimate the non-Gaussianity that might be expected in a supermode of the field generated by our sample structures. The predicted non-Gaussianity is negligible in both structures, for different reasons: The generation rate in the ring is too low, while the field generated by the waveguide is highly multimode. We conclude that although a simple SiN waveguide may be a suitable source for a first demonstration of optical TOPDC, the design will need to be improved significantly for observations of non-Gaussianity or tripartite entanglement in the generated light. Indeed, state generation protocols based on conventional spontaneous four-wave mixing sources and postselection appear to be more promising in the near term. We adopt this approach to design integrated sources of postselected multipartite states in a frequency bin encoding scheme. We introduce sources of W states and GHZ states, with kHz generation rates expected for picojoule pump pulses. This platform appears to be promising for the generation of more arbitrary states, including high-dimensional ones consisting of entangled qudits. We review a graph-based strategy that may be useful for designing other devices, and for understanding the limitations of this platform.
In this thesis, nonlinear periodic structures and their applications in both classical and quantum regime are investigated. New theoretical models are developed, and novel applications of nonlinear periodic structures are proposed and demonstrated. The theoretical studies, both design and simulation, are based on but not limited to InGaAsP material. A new method, namely the time-domain transfer-matrix (TDTM), is presented to simulate optical pulse propagation in layered media with resonant nonlinearity. As there were no satisfactory methods in the literature to model this problem, in order to validate and compare with the TDTM method, the standard FDTD method is generalized to include the rate equation in the analysis of semi-conductors. Also in this work, optical manipulation of absorption in periodic structures is studied for the first time. Thanks to the large accessible nonlinearity that results from the absorption saturation and frequency selectivity of periodic structures, a sensitive and compact optical limiter is designed. The novel design and modeling work developed in this thesis has provided new insights and tools to the utilization of resonant nonlinearities in compact all-optical devices.The experimental studies are based on quasi-phase matched AlGaAs superlattice waveguides. These devices have been previously designed and used for classical optical wavelength conversion such as second harmonic generation and difference frequency generation. In this work, these devices are exploited for spontaneous down conversion, which is a quantum effect, for the first time, through this process, entangled photon pairs are generated. Unprecedented performance, in terms of brightness and purity, of III-V semiconductor-based entangled photon sources has been demonstrated here. Moreover, the quantum properties of these entangled photons are characterized. The experimental studies presented in this thesis open up new application areas for III-V nonlinear optical devices as quantum sources and convincingly demonstrate the promising role of such devices will play in future quantum technologies.
Integrated optical quantum circuits based on photonic waveguiding structures are increasingly gaining attention as a possible solution for scalable quantum technologies with important applications to quantum simulations. Quantum communication provides secure information transmission, but the distance over which quantum states of light can be distributed without significant disturbance is limited due to inescapable losses and noise in optical elements. Loss is the greatest challenge facing the implementation of integrated photonic technologies, and it is inescapable in experimental reality. In recent years there is a rise of interest in structures with spatially inhomogeneous losses. Light propagation in waveguiding structures with spatially distributed sections of loss can be used for implementation of quantum plasmonic circuits, which are able to strongly confine light to sub-wavelength dimensions, as well as for parity-time (PT) symmetric structures, with phase transition associated with PT-symmetry breaking, which opens new possibilities for light manipulation. The PhD thesis contains research on the controllable classical and quantum dynamics of optical frequency conversion processes in quadratically nonlinear photonic integrated circuites in the presence of losses. Namely, I discuss spontaneous parametric down-conversion (SPDC), sum-frequency generation (SFG) and optical parametric amplification (OPA) in nonlinear structures governed by non-Hermition Hamiltonians. I explore the fundamental features of multi-photon generation in integrated nonlinear waveguides. I have been shown that arrays of coupled nonlinear waveguides can serve as a robust integrated platform for the generation of entangled photon states with nonclassical spatial correlations through spontaneous parametric down-conversion (SPDC), and that the operation of such quantum circuit is tolerant even to relatively high losses. Furthermore, I have studied the bi-photon multimode quantum emission tomography in waveguide structures with spatially inhomegeneous losses. The PhD thesis also covers the research on the effect of these losses in waveguide couplers possessing parity-time (PT) symmetry. I have identified an anti-PT spectral symmetry of a parametric amplifier based on those couplers. Finally, I describe the single-photon conversion to a photon pair, and identify opportunities for the efficient enhancement of this process.
Nonlinear photonics studies situations in which light passing through a material can be used to create, reshape, or convert other light in ways that are not possible in ordinary optical systems. In recent years, this has become especially important for quantum photonics, where carefully prepared states of light are used as resources for emerging technologies such as secure communication, ultra-precise measurement, and new forms of information processing. A major challenge, however, is that these effects are delicate, difficult to control, and highly sensitive to the imperfections that arise when trying to build practical devices on a chip. This thesis examines how such nonlinear quantum-optical processes can be both engineered more deliberately and modelled more realistically, with the broader goal of helping move quantum photonic technology from proof-of-principle laboratory demonstrations toward robust and scalable implementation. To do this, the thesis first places nonlinear quantum photonics in a broader technological context and explains the main physical ideas needed to understand how quantum states of light are generated, transformed, and measured. It then develops this picture through a set of concrete examples. These include a method for generating quantum light on a chip while avoiding one of the field’s standard fabrication steps, a proposed chip-scale device that could help connect otherwise incompatible quantum systems by changing the colour and spectral width of light, and a modelling framework that links quantum theory directly to realistic fabricated waveguides, including common imperfections such as loss, non-uniformity, and writing errors. The thesis also reviews a wider class of design methods in which the nonlinear properties of a material are deliberately patterned in order to tailor the light produced. Taken together, the central message is that progress in quantum photonics will depend not only on discovering new effects, but on learning how to design, fabricate, and model these devices in ways that make them practical outside idealized laboratory conditions.
Integrated photonics is playing a key role in the development of quantum technologies covering applications ranging from quantum communication to computing and from simulations to sensing. In this context, material platforms enabling the chip-scale integration of quantum components are a key issue on the way towards real-world and large-scale technological implementations. In this paper, we review our last achievements on AlGaAs Bragg reflection waveguides emitting nonclassical states of light by spontaneous parametric down conversion. The choice of this platform combines the advantages of a mature fabrication technology, room temperature operation, emission in the telecom C-Band and compliance with electrical injection. We demonstrate the ability of these devices to generate a large variety of quantum states spanning from biphoton frequency combs to broadband polarization entanglement enabling promising applications in quantum processing as well as flexible quantum networks.
The aim of this thesis is to explore nonlinear nanostructures – photonic systems with one or more dimension with nanoscale feature size – as new platforms for parametric photon-pair generation. This is motivated by the need for photon-pair sources in many technological applications of quantum optics, such as quantum communication, imaging and computing. The initial hypothesis is that nonlinear nanostructures are a promising platform to complement established photon-pair source schemes based on bulk crystals or photonic integrated circuits in terms of quantum state-engineering, footprint, functional density etc. However, nonlinear pair generation, e.g. via spontaneous parametric down-conversion (SPDC), is a new subfield in nanophotonics with a very limited number of studies existing. This constitutes the knowledge-gap addressed in this thesis. Therefore, the objective is to provide first experimental demonstrations of SPDC in subwavelength-scale and nonlinear nanostructures. We investigate the ability of such systems to control the properties of two-photon quantum states such as frequency and angular spectrum or polarization entanglement. To this end we analyze nanostructures with different degrees of complexity, starting with an unstructured thin crystal with one nanoscale dimension, leading up to a 3D nano-scale resonator with subwavelength-sized features in all dimensions. Furthermore, we test different types of nonlinear materials such as lithium niobate, group III-V semiconductors and emerging, highly nonlinear materials such as molybdenum disulfide. Additionally, we also advance the theoretical understanding by developing a comprehensive model of the quantum process of nonlinear parametric pair-generation in photonic nanostructures.