Photon manipulation in silicon nanophotonic circuits

Quantum-based communication systems can potentially achieve the ultimate security from eavesdropping and greatly reduce the operating powers on chip. Light-speed transmission, noise immunity, and low noise properties make photons indispensable for quantum communication to transfer a quantum state through a transmission line. Furthermore, the field of silicon nanophotonics is fast growing field which is driven by the attractive and promising improvements it has to offer in high speed communication systems and on chip optical interconnects. Consequently, there is a high demand to develop the building blocks for photon manipulation in silicon nanophotonic circuits. The goal of the work is to enable high performance optoelectronic computing and communication systems that overcome the barriers of electronics and dramatically enhance the performance of circuits and systems. We will focus our attention on solving some of the issues with the current systems regarding photon storage, routing, isolation, switching, and energy conversion. We realize a continuously tunable optical memory which breaks the time-bandwidth limit by more than thirty times. This enabled the storage of ultra-short pulses of light for hundreds of picoseconds. Also, we investigate on-chip photon scattering when transmitted through micro-scale optical cavities. In addition, we develop novel dynamic quantum mechanical models that predict quantum-like behavior of single and multi-photon wavepackets. Furthermore, we report for the first time that efficient red shifts in silicon are achievable with free carrier injection which generally produces blue wavelength shifts. We realize adiabatic wavelength conversion and discrete photonic transitions of single photons in silicon cavities. Moreover, we demonstrate a basic quantum network on chip with an on-chip photon source. We present a novel design for CMOS compatible optical isolator on silicon chip using a system of active cavities. And finally, we analyze a novel ultra-fast broadband modulator in silicon based on free-carrier absorption effect in SOI waveguides integrated with Schottky diodes

Optics in Our Time

Optics, Lasers, Photonics, Optical Devices; Quantum Optics; Popular Science in Physics; History and Philosophical Foundations of Physic

Pair creation and pair annihilation in Bose-Einstein condensates

This thesis covers three applications of Bose-Einstein condensates and related phenomena, in the theme of pair creation and pair annihilation. First, Bose-Einstein condensates (BEC) are viewed as one of the candidates to implement a sonic black hole. This can lead to the observation of analog Hawking radiation, resulting from a phonon pair creation at a black-hole horizon (BH). Such implementation has been achieved in a resent experiment by J. Steinhauer, in which a black-hole/white-hole pair has been produced. He also reported the observations of self-amplifying Hawking radiation, via a lasing mechanism operating between the black and white-hole horizons. Through our simulations, we find that the observations should be attributed not to the black hole laser effect, but rather to a growing zero-frequency bow wave, generated at the white-hole horizon. The relative motion of the two horizons produces a Doppler shift of the bow wave at the BH, where it stimulates a monochromatic Hawking radiation. We also find that shot-to-shot atom number variations and quantum fluctuations give density-density correlations consistent with those reported in the experiments. In particular, atom number variations can produce a spurious correlation signal. Secondly, a sonic black hole/white hole pair and phonon pair creation can also be realized using a ring-shaped condensate. Here we focus on the phonon spectroscopy of a ring condensate. We probe the phonon excitation spectrum by applying a harmonically driven barrier to a 23Na Bose-Einstein condensate in a ring-shaped trap. When excited resonantly, these wavepackets display a regular periodic structure. The resonant frequencies depend upon the particular configuration of the barrier, but are commensurate with the orbital frequency of a sound wave traveling around the ring. Energy transfer to the condensate over many cycles of the periodic wavepacket motion causes enhanced atom loss from the trap at resonant frequencies. Solutions of the Gross-Pitaevskii (GP) equation exhibit quantitative agreement with the experimental data. Thirdly, positronium (Ps) BECs have been of experimental and theoretical interest due to their potential application as the gain medium of a gamma-ray laser. Ps BECs are intrinsically spinor due to the presence of ortho- (o-Ps) and para-positronium (p-Ps), whose annihilation lifetimes differ by three orders of magnitude. We study the spinor dynamics and annihilation processes in the p-Ps/o-Ps system using both solutions of the GP equations and a rate-equation approach. For an initially unpolarized condensate, there is a threshold density at which spin mixing between o-Ps and p-Ps occurs. Beyond this threshold, there are unstable spatial modes accompanied by spin mixing. To ensure a high production yield above the critical density, a careful choice of external field must be made to avoid the spin mixing instability

Analogue Cosmology Experiments with Sodium Bose-Einstein Condensates

Due to their high degree of controllability and precise measurement capabilities, ultracold ensembles of neutral atoms are a leading platform for performing quantum simulations. In this thesis, I will describe the design and construction of an analog quantum simulator based on $^{23}$Na Bose-Einstein Condensates (BEC). Our system can produce and trap BECs in arbitrary-shaped quasi two-dimensional optical dipole traps, which can be dynamically altered during an experimental sequence. Such controlled variation of the BEC's spatial mode enables exploration of open questions in superfluidity, atomtronics, and analogue cosmology. I will describe the implementation of our system to study the inflationary dynamics of the early universe and report our recent results on the simulation of cosmological Hubble friction. We expand and contract a toroidally shaped BEC and analyze the time evolution of its collective phonon modes. These excitations are analogous to fluctuating scalar fields in an expanding universe. The changing metric of the expanding or contracting background BEC results in dilation of the phonon field through a term dependent on the expansion speed, similar to Hubble friction in inflationary models of the universe. We conclusively demonstrate the analogy by experimentally measuring Hubble attenuation and amplification. Our measured strength of Hubble friction disagrees with recent theoretical work [J. M. Gomez Llorente and J. Plata, {\it Phys. Rev. A} {\bf 100} 043613 (2019) and S. Eckel and T. Jacobson, {\it SciPost Phys.} {\bf 10} 64 (2021)], suggesting inadequacies in the current model

Nano-film functionalized exposed core fibers enabling resonance-driven dispersive wave tailoring

Light sources with specific optical properties are the backbone of optical technologies such as spectroscopy or hyperspectral imaging. Yet, the creation of broadband, stable, and spectrally flat light sources, especially at low pump energies, remains a particular challenge. Supercontinuum generation (SCG) is a well-established method for broadband light generation in optical fibers. For tailorable SCG spectra, it is essential to accurately design and precisely control the dispersion of fibers with new methods. This thesis aims to explore nonlinear frequency conversion in resonance-enhanced fibers to create tunable broadband light sources with tailored properties at low pump energies. By depositing high refractive index nano-films with different thicknesses on the surface of the exposed fiber core, the dispersion of the fibers and thus the output spectrum of SCG can be tuned. Different nano-film geometries are investigated, featuring TiO2 nano-films with a uniform thickness, Ta2O5 nano-films with a gradually increasing thickness along the fiber length, and periodically structured Ta2O5 nano-films. Experiments and simulations reveal the advantages of a longitudinally varying dispersion over uniformly coated fibers concerning an enhanced spectral flatness and an enlarged bandwidth. Furthermore, periodically structured nano-films lead to multi-color tailorable higher-order dispersive waves via quasi phase-matching, which are outside of the wavelength range of classical soliton-based SCG. Resonance-based modifications of the fiber dispersion by using nano-films are a powerful new tool to efficiently shape nonlinear frequency conversion in SCG even at low pump energies. It has high technological potential for the realization of novel, ultrafast, broadband, and stable nonlinear light sources for biophotonics, environmental, life sciences, medical diagnostics, and metrology

Photons micro-ondes, mesure et informatique quantique

Les circuits supraconducteurs constituent aujourd’hui une architecture prometteuse vers la réalisation d’un ordinateur quantique universel. Cependant, avant qu’un tel ordinateur voie le jour, plusieurs défis doivent être surmontés autant au niveau matériel que logiciel. Les circuits supraconducteurs forment aussi une architecture intéressante pour la réalisation d’expériences fondamentales en optique quantique grâce aux jonctions Josephson qui permettent aux photons micro-ondes d’interagir directement entre eux. Dans cette thèse, j’ai abordé certains des grands défis du domaine des circuits supraconducteurs dans le contexte de l’informatique quantique et de l’optique quantique micro-onde. Je me suis tout d’abord intéressé à la détection de photons micro-ondes uniques (chapitre 3). Avec mes collaborateurs, nous avons ainsi proposé deux méthodes pour réaliser un détecteur de photons uniques à grande efficacité quantique. Une de ces méthodes se base sur un ensemble de qubits supraconducteurs (section 3.2), tandis que l’autre promet une large bande de détection en se basant sur un métamatériau unidimensionnel (section 3.3). La réalisation expérimentale de ces propositions permettrait d’ajouter une pièce importante dans la boîte à outils de l’optique quantique micro-onde. Je me suis ensuite penché sur la correction d’erreurs dans les qubits supraconducteurs (section 4). Plus précisément, j’ai développé un circuit expérimentalement compact permettant de mesurer les propriétés d’un ensemble de qubits, une opération essentielle pour les techniques de correction d’erreurs quantiques. Finalement, j’ai consacré une partie de mon doctorat à modéliser des expériences effectuées dans le groupe d’Andreas Wallraff à l’ETH Zürich (chapitre 5). Nous avons ainsi réalisé trois expériences basées sur un couplage variable de type Raman entre un qubit supraconducteur et un résonateur micro-onde. Dans la première expérience, nous avons réalisé le premier transfert d’état quantique déterministe entre deux qubits supraconducteurs distants (section 5.2). Nous avons ensuite amélioré cette expérience en utilisant un code de détection d’erreur pour transférer l’information quantique de manière plus robuste entre les deux qubits (section 5.3). Ces expériences représentent une étape importante vers la réalisation d’une architecture modulaire de l’ordinateur quantique. Finalement, nous avons développé une méthode pour rapidement initialiser des qubits supraconducteurs dans leur état fondamental (section 5.4), une opération essentielle de l’ordinateur quantique

Bulk continuum generation: the ultimate tool for laser applications and spectroscopy

This thesis investigates bulk continuum generation. A full study of all relevant parameter is given. In addition, its application in ultrafast and widly tunable amplifiers and spectrometers is shown