The PhD thesis work here presented was carried out within the SiQuro project at the Nanoscience laboratory of the University of Trento. The project started in September 2013 and lasted four years. It was funded by the Provincia Autonoma di Trento (PAT). SiQuro’s goal was to bring the quantum world into integrated photonics by using the silicon platform and, therefore, permitting the integration of quantum photonics with electronics. The vision was to have low cost and mass manufacturable integrated quantum photonic circuits for a variety of different applications in quantum computing and secure communications. It must be said that SiQuro was a challenging and ambitious project, nevertheless important achievements in the quantum photonics arena were reached. My thesis is concentrated on the generation, manipulation and detection of quantum states of light. On one side, this was carried on in strained silicon waveguides, with the final goal to generate MIR entangled photon pairs via SPDC. Alongside, the generation and manipulation of correlated photon pair sources by means of spontaneous FWM in traditional silicon waveguides and microring resonators at telecom wavelength was also investigated. For the detection of MIR photon pairs, a suitable detection unit was developed as well. Moreover, even though the long-term goal of the project was the realization of a silicon quantum photonic circuit, I also implemented free-space quantum optical experiments. For this, I exploited a bulk nonlinear crystal, namely lithium niobate (LiNbO3), which has a well-known sizeable χ(2) nonlinearity.
The PhD thesis work here presented was carried out within the SiQuro project at the Nanoscience laboratory of the University of Trento. The project started in September 2013 and lasted four years. It was funded by the Provincia Autonoma di Trento (PAT). SiQuro’s goal was to bring the quantum world into integrated photonics by using the silicon platform and, therefore, permitting the integration of quantum photonics with electronics. The vision was to have low cost and mass manufacturable integrated quantum photonic circuits for a variety of different applications in quantum computing and secure communications. It must be said that SiQuro was a challenging and ambitious project, nevertheless important achievements in the quantum photonics arena were reached. My thesis is concentrated on the generation, manipulation and detection of quantum states of light. On one side, this was carried on in strained silicon waveguides, with the final goal to generate MIR entangled photon pairs via SPDC. Alongside, the generation and manipulation of correlated photon pair sources by means of spontaneous FWM in traditional silicon waveguides and microring resonators at telecom wavelength was also investigated. For the detection of MIR photon pairs, a suitable detection unit was developed as well. Moreover, even though the long-term goal of the project was the realization of a silicon quantum photonic circuit, I also implemented free-space quantum optical experiments. For this, I exploited a bulk nonlinear crystal, namely lithium niobate (LiNbO3), which has a well-known sizeable χ(2) nonlinearity.
The PhD thesis work here presented was carried out within the SiQuro project at the Nanoscience laboratory of the University of Trento. The project started in September 2013 and lasted four years. It was funded by the Provincia Autonoma di Trento (PAT). \nSiQuro’s goal was to bring the quantum world into integrated photonics by using the silicon platform and, therefore, permitting the integration of quantum photonics with electronics. The vision was to have low cost and mass manufacturable integrated quantum photonic circuits for a variety of different applications in quantum computing and secure communications. It must be said that SiQuro was a challenging and ambitious project, nevertheless important achievements in the quantum photonics arena were reached. My thesis is concentrated on the generation, manipulation and detection of quantum states of light. On one side, this was carried on in strained silicon waveguides, with the final goal to generate MIR entangled photon pairs via SPDC. Alongside, the generation and manipulation of correlated photon pair sources by means of spontaneous FWM in traditional silicon waveguides and microring resonators at telecom wavelength was also investigated. For the detection of MIR photon pairs, a suitable detection unit was developed as well. Moreover, even though the long-term goal of the project was the realization of a silicon quantum photonic circuit, I also implemented free-space quantum optical experiments. For this, I exploited a bulk nonlinear crystal, namely lithium niobate (LiNbO3), which has a well-known sizeable χ(2) nonlinearity.
This thesis work covers both classical and quantum aspects of nonlinear propagation of photons in nanophotonic Silicon waveguides. The work has been carried out within the framework of the project SIQURO, which aims to bring the quantum world into integrated photonics by using the Silicon platform and, therefore, permitting in a natural way the integration of quantum photonics with electronics. The research towards on chip bright quantum sources of photon pairs has been done by investigating Multi Modal Four Wave Mixing in micrometer-size waveguides, thus exploiting the large third order nonlinearity of Silicon. The possibility to induce second order nonlinearities by straining its unit cell has been also analyzed through the study of the electro-optic effect. This has been done with the aim to promote Silicon as a platform for the integration of quantum sources of entangled photons based on Spontaneous Parametric Down Conversion. New quantum interference effects have been reported in a free space unbalanced Mach Zehnder interferometer asymmetrically excited by colour entangled photon pairs. Innovative designs of integrated quantum circuits have been proposed, which extend the capabilities of the quantum circuits demonstrated so far and provide additional functionalities. This work represents a step forward to the realization of self subsistent integrated devices for quantum enhanced measurement, quantum computation and quantum crypthography.
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.
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.
Entangled photons have acted as a workhorse in many quantum technologies, including cryptography, imaging, sensing, and computation. With the growing demands of quantum technology, the entangled photon pair sources are required to be compact and integrable. For the last few years, the field of quantum optics in the nanoscale has demonstrated photon pair generation in ’flat’ optics such as thin layers, Van der Waals crystals, micro-, nano-antennas, and metasurfaces. The word ’flat’ is used because such optical platforms have sub-wavelength thicknesses. Due to the short travel length inside the sources, they can possess multiple functions that their bulky counterparts cannot have. This doctoral thesis addresses photon pair generation in ’flat’ optics via second- and third-order nonlinear processes, which are spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM). The main aims of the thesis are to understand how the photon pair generation is realized via SPDC and SFWM on subwavelength-scale platforms, and to make a bridge between photon pair generation via two nonlinear processes under the presence of optical resonances. The thesis covers two experimental parts related to SPDC and SFWM, and one future expectation of the SFWM in metamaterials. The first part of the thesis addresses one of the multi-functionalities that has recently been reported: the emission direction control of the photon pairs. A 150 nm-thick gallium phosphide (GaP) metasurface is used as a source. The metasurface supports a quasi-bound state in the continuum (quasi-BIC) resonance, enhancing the electric field inside the material. I generate the entangled photons through the SPDC process. The generated photons are in two different wavelengths under the given pump wavelength: signal photon at the resonance, and idler photon out of the resonance. The photon pair generation rate is enhanced by 66 times compared to the unpatterned GaP slab since the vacuum field in the signal mode is enhanced by the resonance. The generated photons obey the transmission spectrum of the metasurface. As a result, the signal photons at resonance are mostly reflected, and the idler photons are transmitted after generation. This result is the first work of photon demultiplexing by a metasurface. The second part of the thesis covers the photon pair generation through SFWM in thin nonlinear films. Silicon-rich silicon nitride (SiN) layers are selected as the sources. The layers have an intriguing characteristic that the third-order nonlinear susceptibility (chi-3) differs with the nitrogen doping rate. I generate the photon pairs through chi-3 variant SiN layers. The thicknesses of the samples are all sub-wavelength level, having a few hundred nanometers. Time-energy correlation measurements are conducted for all samples, and the second-order correlation functions (g(2)(0)) are calculated. The maximum g(2)(0) of the samples exceeds 2. Interestingly, I observed two-photon interference effects between photon pairs generated in the layers and the fused-silica substrate, which allows me to estimate the chi-3 values of the samples by knowing the reference: chi-3 value of the substrate. The last part of the thesis introduces designs of metasurfaces and possible experimental schemes for photon pair generation through SFWM. Yet, due to its weak light-matter interaction in third-order nonlinear processes, the photon pair generation through SFWM has not been observed in metasurfaces. However, it is fairly possible to enhance the vacuum fields at the output photon modes and generate photon pairs because preliminary work in unpatterned slabs has already been observed in this thesis. Finally, I introduce a fabricated metasurface. It needs improvements, but it is quite promising for the ongoing project.
Quantum information processing offers new protocols and functionalities in the field of communications by providing an absolute security guaranteed by the quantum physics laws as well as in the field of quantum computing, with the perspective to drastically speed up massively parallel computations. The second quantum revolution is now about to take place with theoretical concepts being brought into concrete applications thanks to integrated technological solutions. Photons are an excellent physical implementation of quantum bits for these two applications. Photonic quantum information processing requires three key functionalities: a source of single photons, a reconfigurable circuits for their manipulation and a single photon detector. The objective of this PhD work is to design, fabricate and characterize superconducting single photon detectors integrated on silicon photonic circuits, with very high efficiency above 95%, a large spectral operating range at telecom wavelength and the ability to resolve the number of received photons. This PhD will take place at CEA, in collaboration between INAC and LETI institutes to benefit from their complementary expertise. This PhD work will be conducted in parallel with developments of single photon sources and will bring an essential building block for the integration of complex circuits able to generate, manipulate and detect tens of photonic qubits for secured communications and quantum computing.