Towards microwave excitation-free broadband single photon source using Landau-Zener 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 of excitation is without precedent and allows for high efficiency, wide-band tunability and low jitter, fulfilling majority of the criteria of a practical microwave single photon source. In Chapter 3 and 4, I 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. In Chapter 5, 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 coherence times 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 decay channel. I extend this study by designing and fabricating a tantalum transmon which is expected to have higher lifetimes and coherence times. I finish by prescribing few additional measures to minimize loss arising from various decoherence channels.