Quantum Control and Feedback in Qubit Systems

Abstract

This thesis examines the behavior of a single-qubit quantum system controlled by feedback. Key parameters such as coupling strength, dissipation, and detuning are
explored to improve quantum coherence and stability. Based on the results, practical methods for controlling quantum systems are proposed to better understand quantum control techniques and then improve the performance and stability of quantum systems. These findings provide a foundation for future research into more complex systems, adaptive feedback approaches, and their applications in quantum computing. “The grammar of the text in this thesis was improved with chat gpt, but the actual contents and text were solely produced by the author”.

Popular Abstract

Computers like any technology keep on improving at ever increasing speeds. Sometimes however, it is not simple improvements that we witness but huge leaps forward. One such leap forward that gets the scientific community most excited about is the promise of quantum computers. Super quantum computers will allow us to solve problems that we cannot even approach today. They have the theoretical ability to solve in seconds computational problems that would take today's most advanced computer millions of years to figure out. Clean energy, agricultural optimization or drug discovery are only a few of many problems that quantum computers can be instrumental in overcoming. Traditional computers have bits that can only have one of two values, zero or one. On the other hand, quantum computers are based on qubits. Qubits have two super powers: superposition and entanglement. Superposition is qubits' ability to exist as both 0 and 1 simultaneously. Entanglement of two qubits is when these two qubits' values are bound to each other no matter how far apart they are. Superposition allows a single bit to process multiple possibilities at once, while entanglement connects multiple bits to process them simultaneously.
Indeed, it is superposition and entanglement of qubits that allow quantum computers the ability to process massive amounts of data in parallel. However, we are not there yet. We still have many unanswered questions and challenges to conquer. One of the biggest obstacle in our way to achieve quantum computers is "qubits decoherence". Decoherence stems from quantum systems' excessive sensitivity. Simple external noise, changes in temperature or in tiny electromagnetic fields can interfere with qubits' state. The slightest interference may cause qubits to lose the information they’re holding, leading to erroneous computations rendering the whole operation useless. For example, imagine trying to knit a sweater while riding a roller coaster. You would need to make precise adjustments to keep your stitches even and your yarn from getting tangled, even as the roller coaster twists and turns. Similarly, in quantum computing, we need to make precise adjustments to keep the qubits stable and functioning correctly. Controlling a qubit means being able to steer it precisely to the appropriate state for accurate calculations. Quick and reliable feedback is imperative to enable correcting any unwanted changes before they lead to errors.
This is where my research comes into the picture. Examining qubits interaction with their environment. Through exploring mathematical functions, I study some possible ways to reliably influence or even control qubits. As we work to master these techniques, we hope to pave the way toward more robust quantum systems, bringing us closer to practical, large-scale quantum computers. The ability to reliably
control qubits would mark a significant leap forward, not only for quantum computing but also for our understanding of the quantum world itself. It’s an exciting and intricate journey, and every small advancement brings us one step closer to a world where quantum technology could transform industries and scientific research on a global scale. “The grammar of this text was improved with chat gpt, but the actual contents and text were solely produced by the author”.

• author: Lihimdi, Meriem
• supervisor: Peter Samuelsson; Daniel Holst
• organization: Mathematical Physics; Department of Physics
• course: FYSK04 20242
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Qubit Systems, Quantum Control and Feedback, Qubit Dynamics, Continuous Measurements
• language: English
• id: 9185531
• date added to LUP: 2025-02-21 09:27:14
• date last changed: 2025-02-21 09:27:14