Name of the student: Ms. Khaire Siddhi Satish
Date/Time: 15th July 2025/ 11:00 AM
Venue: Lecture hall-1
Research Supervisor: Prof. Baladitya Suri
Affiliation: Department of Instrumentation and Applied Physics, Indian Institute of Science, Bengaluru.
Title: Towards microwave excitation-free broadband single photon source using LandauZener transitions in a flux qubit
Abstract: Single photon sources are essential resources for quantum computation, quantum communication, quantum information processing, quantum cryptography and other applications. Over the past two decades, superconducting circuits have emerged as the leading platform for quantum computation and information processing owing to their design flexibility, real-time tunability, compact footprint, scalability, and compatibility with standard CMOS fabrication techniques. Therefore, single photon sources operating within the frequency range of superconducting circuits have become a focus of research. Microwave photons have around three orders of magnitude lower energy as compared to their optical counterparts, making them susceptible to thermal photons even at lower temperatures. The design of such sources is extremely demanding, requiring ultra-low-noise components and precise timing of signals. Due all these complexities, microwave single photon sources remain difficult to achieve. Most state-of-the-art microwave single photon sources use a common principle of generation of single photons. A superconducting qubit which is approximated as a two-level system (TLS) is excited using a coherent microwave signal followed by spontaneous radiative decay of the TLS leading to emission of a single microwave photon. Microwave photons from the coherent drive inevitably leak into the single photon output, effectively “contaminating” the emitted photons. This can be mitigated through various techniques, but cannot be entirely eliminated.
In the first and main part of the thesis, I present a novel single photon source based on a fundamentally different way of exciting a qubit which involves Landau-Zener Transitions (LZTs). A non-adiabatic parameter sweep is used in the form of a DC pulse sequence which excites the qubit without the need for microwave excitations. This results in negligible leakage of microwave photons to the single photon output. This method ii of excitation is without precedent and allows for highly efficient, wide-band tunable and low-jitter, fulfilling majority of the criteria of a practical microwave single photon source. I then delve into the details of the design and fabrication process of such a single photon source followed by a description of the experimental setup involved in basic spectroscopy and time domain measurements of a superconducting qubit. I also talk about the methods of verification of single photon nature of the emitted field using single photon state tomography as well as correlation functions. Subsequently, I discuss at length the various iterations of single photon sources that I fabricated and measured. I explored two architectures based on a charge qubit or a Cooper pair box (CPB) and a flux qubit as they fulfilled the necessary criteria to support LZTs.
In the second part of the thesis, I investigate multiple loss mechanisms and their sources which affect the lifetimes of superconducting qubits. I do a systematic study of quality factors of superconducting resonators with varying substrates, superconducting metals and fabrication processes to investigate these loss mechanisms. This study showed that superconducting circuits fabricated on tantalum thin films suffer the least from dielectric noise which is the most dominant noise channel. I extend this study by designing and fabricating a tantalum transmon which is expected to have high lifetimes and coherence times. I finish by prescribing few additional measures to minimize loss arising from various channels of decoherence.
About the student: Ms. Siddhi did her BSc in Electronics from Fergusson College in Pune, followed by MSc in Electronics from University of Pune. She worked as a project fellow in IISER Pune for a year before joining the Quantum Technologies Lab at IISc as a PhD student in Jan 2019.