Examensarbeten för masterexamen
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- PostOptimization of coupling interfaces between nanophotonics waveguides and optical fibers(2025) Haidar Hussein, Ali; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Torres Company, Victor; Gao, Yan; Shekhawat, VijayThe integration of photonic components into compact and efficient circuits necessitates effective coupling mechanisms between optical fibers and on-chip waveguides. This thesis focuses on optimizing the coupling efficiency between silicon nitride (Si3N4) waveguides and optical fibers using linear adiabatic edge tapers. Addressing the significant challenge of minimizing coupling losses, the study explores the interplay between taper geometry, cladding thickness, taper and physical gaps for both standard single-mode fibers (SMFs) and lensed fibers. Utilizing both advanced simulation tools—Finite-Difference Eigenmode (FDE), Eigenmode Expansion (EME), and Finite-Difference Time-Domain (FDTD) methods—and experimental validation, the research systematically investigates the parameters influencing coupling efficiency. FDE simulations provide initial insights into mode mismatch losses due to mode field diameter (MFD) disparities. EME simulations are employed to optimize taper lengths, ensuring adiabatic mode transformation with minimal loss. FDTD simulations offer a comprehensive 3D analysis, capturing detailed electromagnetic interactions at the coupling interface. Experimental prototypes were fabricated and tested to validate the simulation results and assess practical fabrication considerations. The results demonstrate that optimal coupling for SMFs is achieved with a taper length of approximately 1mm, a facet width between 100nm and 150nm (with 125nm yielding the lowest loss), and a cladding thickness of 6µm, resulting in coupling losses around 0.75dB. For lensed fibers, both simulations and experiments indicate that a taper length of 50µm, a facet width between 230nm and 290nm (with 290nm yielding the losses in the simulations around 0.37 dB per facet and an experimental loss of around 1.1dB per facet which is possible to reduce even further), and a cladding thickness of 3µm yield optimal coupling efficiency. The slight discrepancies between simulated and experimental results are attributed to fabrication imperfections, including sidewall roughness and taper dimension deviations. The study underscores the critical impact of physical gaps and beam divergence on coupling efficiency, emphasizing the necessity of precise alignment and minimal separation in practical applications. Experimental results confirm that minimizing the taper and physical gaps is essential for lensed fiber coupling, with coupling losses yielding a constant value for gaps that are larger than 0.5µm. The thesis identifies limitations related to fabrication imperfections and suggests avenues for future research, including broadband performance analysis and the incorporation of fabrication-induced variations into simulations. The findings offer valuable design considerations for enhancing fiber-to-chip coupling in Si3N4 phov tonic integrated circuits, contributing to the advancement of efficient and scalable optical communication technologies.
- PostVirtual radar simulations for interior sensing(2025) Mallipudi Venkata Satya, Surya Naga Maruthi Ramya; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Vassilev, Vessen; Johansson, FilipThis master’s thesis investigates the validation of radar simulation data for in-cabin sensing applications, utilizing AVxcelerate by Ansys to replicate and test real radar scenarios virtually. The research primarily focuses on comparing virtual radar readings from the simulation environment with real radar measurements gathered in a lab setting, aiming to ensure accuracy and consistency in radar data for future in-cabin safety applications. In this study, a controlled environment was established to minimize discrepancies between simulated and real-world conditions, with experiments conducted on a single target in two scenarios: stationary and controlled motion. Data from both settings were analyzed to assess the reliability of the simulation for representing real-world radar behavior. The findings contribute foundational insights for advancing radar-based occupant monitoring systems, supporting features like driver drowsiness detection and child presence alerts in automotive interiors.
- PostTensile-strained crystalline aluminium nitride nanomechanical resonators(2024) Nindito, Laurentius Radit; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Wieczorek, Witlef; Ciers, Anastasiia; Wieczorek, WitlefHigh-Q_m nanomechanical resonators have proven to be a promising platform for advancing quantum technology. Resonators with Q_m×f_m products exceeding 6.2×10^12 Hz can sustain at least one coherent oscillation at room temperature, enabling their use in emerging quantum applications such as engineering long-lived quantum states and quantum sensing. Silicon nitride has become the favored material in this regard due to its great mechanical properties. However, it is an amorphous material that lacks additional functionalization capabilities beyond its admirable mechanical characteristics. We therefore explore crystalline aluminum nitride (AlN) as a promising alternative platform for high-Q_m nanomechanical resonators. Like other crystalline nitride materials, we expect AlN to possess robust mechanical properties. Moreover, the lack of centrosymmetry in its crystal structure gives rise to its piezoelectricity, making it a particularly versatile material for electromechanical applications. In this thesis, we studied four tensile-strained crystalline aluminum nitride samples with thickness ranging from 90nm to 295 nm. We extracted their elastic properties, including Young’s modulus, residual stress, and intrinsic quality factor. We then designed and realized phononically-shielded high-Q_m nanomechanical resonators out of them. Our bestperforming device achieved a quality factor of 8.6 × 10^6 and a Q_m × f_m product as high as 1.5 × 10^13, sufficient to provide a coherent oscillation at room temperature.
- PostTowards fault-tolerant quantum error correction with the surface-GKP code(2024) Jaeken, Thomas; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Ferrini, Giulia; Hillmann, Timo; Sorée, BartQuantum computers have been predicted to be of great importance in the future. However, realization of this technology comes with many challenges. The fragile nature of quantum phenomena necessitates the development of fault-tolerant computation. The need for robust error correction schemes is evident. One of the most promising efforts at this time is the surface code. Recently, it became apparent that the surface code can synergize with the Gottesman-Kitaev-Preskill (GKP) code. This thesis explores that concatenated code within a circuit-level noise model approximating reality as close as possible, through classical Monte Carlo simulations relying on the state-twirling approximation and relates. We reproduce the results of ref. [1, Noh and Chamberland, Phys. Rev. A 101, 012316 (2020)] and expand on them. We simulate the concatenated code in different experimental setups within the parameter space of the noise model and expose relations between the results. This leads to, among others, an analogy of error-flow with current in electrical circuits. This behavior is not directly recognized in analogous simulations of the discrete surface code and was not reported previously. It corroborates the recent theory by ref. [2, Conrad et al., Quantum 6, 648 (2022)] that the concatenation of GKP codes with stabilizer codes are a particular case of general multi-mode GKP codes. From individual simulations of the threshold for each noise source in the model, we learn that two-qubit gate noise is the most critical, while measurement noise is the most tolerable. Finally, we investigate the threshold of the concatenated code, σ*gkp as a function of the measurement efficiency and confirm the concern that this is a critical issue for practical realizations. The result of this work is a better understanding of the effect that different kinds of noise have on the logical error rate and can potentially support experimental implementations in the future.
- PostImplementation of qubit reset for fixed-frequency transmons in tunable-coupler architectures(2024) Yan, Zixian; Chalmers tekniska högskola / Institutionen för mikroteknologi och nanovetenskap (MC2); Chalmers University of Technology / Department of Microtechnology and Nanoscience (MC2); Bylander, Jonas; Chen, Liangyu; Tancredi, GiovannaUnconditional and fast qubit reset is a key element to decrease algorithm computation time as the lifetime of the physical qubits continuously grows. For example, in quantum error correction (QEC), fast qubit reset in ancilla qubits is highly desired to accelerate the surface code algorithm. This thesis reports a qubit reset protocol utilizing a tunable coupler to transfer excitation from the qubit to the dedicated readout resonator in an architecture consisting of fixed-frequency transmons pairwise coupled by tunable couplers. The reset pulse is designed and optimized based on the Roland-Cerf protocol for resetting the |e⟩-state adiabatically in a two-level system (TLS) with an adiabatic pulse, demonstrating an improvement in reset fidelity compared to linear pulse in simulation. By changing the pulse shape, the evolution follows the shortcut-to-adiabaticity (STA) path within some parameter regions, enabling faster and better qubit reset. For resetting |f⟩-state, the numerical results also give adiabatic and STA pulse shapes similar to that given by the Roland-Cerf protocol in a two-level system, thus enabling us to model the |f⟩-state reset model as an approximate TLS system. We verify our theoretical prediction by running the reset protocols on a 25-qubit chip. The experiment results show fast reset operations while keeping low reset errors, verifying the validity of the proposed pulses [1]. However, the presence of other qubits limits the reset fidelity, and therefore, frequency separation between coupled qubits should be a parameter to be carefully considered at the design stage.