Abstract : This is the final report for the ARO/LPS project at Yale University, on the RF control and measurements of superconducting qubits. During this project, we performed the proposed research work, which was revolving around four major axes: I. The improvement of qubit coherence by investigating the nature of the various sources of decoherence, II. The improvement of readout fidelity by establishing the parameters influencing the efficiency and speed, III. The novel manipulation and entanglement of multiple qubits by developing gate hardware, protocols and room temperature electronics extendable to systems comprising more than two qubits, and IV. The development of a scalable architecture combining superconducting linear resonators and superconducting qubit. The overall goal of the project was to assess the potential of superconducting qubits for the construction of a quantum computer based on superconducting Josephson circuits.
Abstract : This is the final report for the ARO/LPS project at Yale University, on the RF control and measurements of superconducting qubits. During this project, we performed the proposed research work, which was revolving around four major axes: I. The improvement of qubit coherence by investigating the nature of the various sources of decoherence, II. The improvement of readout fidelity by establishing the parameters influencing the efficiency and speed, III. The novel manipulation and entanglement of multiple qubits by developing gate hardware, protocols and room temperature electronics extendable to systems comprising more than two qubits, and IV. The development of a scalable architecture combining superconducting linear resonators and superconducting qubit. The overall goal of the project was to assess the potential of superconducting qubits for the construction of a quantum computer based on superconducting Josephson circuits.
Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms. Benefiting from the well-developed technology of microfabrication and microwave engineering, superconducting qubits have great advantages in design flexibility, controllability, and scalability. Over the past decade, there has been rapid progress in the field, which greatly improved our understanding of qubit decoherence and circuit optimization. The single-qubit coherence time has been steadily raised to the order of 10 to 100 μs, allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control. Here we review recent progress in the coherence and readout of superconducting qubits.
The thesis investigates three challenges in superconducting quantum computing: efficient quantum gate compilation, scalable microwave interfacing of quantum processors (QPUs), and electrical annealing of Josephson junctions.The first part addresses an inefficiency in contemporary two-qubit gate compilation. Universal quantum computation requires only a single entangling two-qubit gate alongside single-qubit rotations, but this minimal gate set requires up to three additional two-qubit gates for every unsupported gate. I show how access to multiple inequivalent two-qubit gate classes overcomes this inefficiency. I experimentally realised an unsupported SWAP operation using one parametric CZ and one parametric iSWAP gate on a circuit-QED architecture. A Kraus-Cirac analysis shows that this combination belongs to the SWAP equivalency class, the fundamental reason for the reduction in two-qubit gates. The experiment demonstrates a rare case of simultaneous CZ and iSWAP support in the same quantum circuit, illustrating how access to multiple inequivalent gate classes can reduce compilation overhead on real hardware.The second part explores direct digital synthesis of microwave signals for interfacing with quantum processors. Conventional control uses heterodyne upconversion with IQ mixers, requiring compensation of LO leakage and IQ skew. I investigated a mixerless architecture based on high-speed DAC/ADC systems to generate and analyse the required microwave signals. CZ and iSWAP gate experiments subsequently verify that the platform is equivalent to conventional QPU setups.The third part investigates post-fabrication electrical tuning of Josephson junctions, in which voltage pulses modify junction resistance to correct fabrication-induced transmon-frequency variations. The speed of resistance change was found to depend exponentially on pulse amplitude. A novel stepwise protocol achieved a 270% resistance increase in an 11-month-old 11.5 kΩ junction, corresponding to a ~2 GHz reduction in transmon frequency, the largest reported selective resistance modification to date. Simulations show that resistance, charging-energy, and superconducting-gap uncertainties contribute equally to the uncertainty in the final qubit frequency. The results support a picture where electrical manipulation modifies the energy landscape, accelerating natural junction ageing.The thesis demonstrates that investigating gate-level compilation strategies, fully digital QPU control, and post-fabrication tuning, helps advance the understanding of how to approach scalable superconducting quantum processors.
Abstract : This is the final report for the IARPA CSQ project at Yale University, on the coherence limits of superconducting charge qubits. During this project, we performed the proposed work investigating the various sources of decoherence and their origins in different materials, using the participation ratio paradigm that we developed and disseminated in the community. The original project goal was to increase by an order of magnitude the coherence time of the state-of-the-art qubits (transmons and fluxonium designs) from a few microseconds to greater than 20 microseconds. By project end, we realized qubit and cavity devices with times in excess of a millisecond, or an improvement by almost three orders of magnitude. These results, especially improved 3D designs, have been widely adopted and successfully reproduced by many groups across the superconducting qubit community. At program end, we have an existence proof that qubits satisfying the quantum error correction threshold are possible with existing materials and fabrication techniques, leading to a growing belief that large-scale quantum computation with solid-state devices will eventually be realized.
Quantum computers have the potential to solve problems which are classically intractable. Superconducting qubits present a promising path to building such a computer. Recent experiments with these qubits have demonstrated the principles of quantum error correction, quantum simulation, quantum annealing, and more. Current research with superconducting qubits is focused on two primary goals: creating a fully fault tolerant logical qubit out of many physical qubits using surface code error correction, and demonstrating an exponential speedup over any classical computer for a well-defined computational problem. To achieve either of these goals requires high precision control of three components: single qubit gates, two qubit gates, and qubit measurement. In this thesis, we use randomized benchmarking to characterize single qubit gates with 99.95\\% fidelity and two qubit gates wiht 99.5\\% fidelity in superconducting transmon qubits. In addition, we use standard decoherence measurements as well as newly developed extensions of randomized benchmarking to determine the limiting sources of error. Finally, we explore the surprisingly complicated dynamics of measuring the transmon state through a coupled resonator, and show that fully understanding this process requires breaking a few "standard" assumptions.
Superconducting qubits are a leading platform for quantum computation, yet decoherence and scalability challenges continue to limit progress toward fault-tolerant systems. This review synthesizes research on architectures, noise sources, and error correction to clarify performance trade-offs and guide development. The transition from Transmon to Fluxonium has dramatically improved qubit performance, achieving millisecond-scale coherence times. However, this success reveals a fundamental limit of coherence now being primarily restricted by the physical properties of the materials, specifically dielectric losses occurring at the Josephson junction leads and within the surrounding materials. As overcoming these material limits becomes increasingly difficult, system-level engineering determines whether high-coherence qubits can scale.Scaling quantum systems is difficult because current cooling and control systems create too much heat and this forces a shift to integrated digital superconducting circuits and cryogenic light-based links. While these hardware challenges restrict scaling, experiments in error correction show that logical qubits can already outperform physical ones. For example, a distance-7 surface code improved logical qubit lifetime by 2.4×. Achieving practical fault tolerance, however, requires moving beyond simple error models. Future systems must combine techniques like Dynamical Decoupling and Machine Learning-based Optimal Control with dynamic classical control capable of handling complex, correlated faults such as Multi-Bit Burst Errors. Progress in materials, cryogenic system integration for control, and adaptive error management must advance together to realize scalable, high-fidelity quantum hardware.
Superconducting qubits are a promising technology for building a scalable quantum computer. An important architecture employed in the field is called Circuit Quantum Electrodynamics (circuit QED), where such qubits are combined with high quality microwave cavities to study the interaction between artificial atoms and single microwave photons. The ultra-strong coupling achieved in these systems allows for control and readout of the quantum state of qubits to perform quantum information processing. The work on circuit QED performed in this thesis consisted of realizing an experimental setup for qubit experiments in a new laboratory, investigating the coherence and decay of higher energy levels of superconducting transmon qubits and finally demonstrating a novel coaxial form of circuit QED. Designing and building a 3D circuit QED setup involved the following main accomplishments: producing high quality 3D cavities; designing and installing the cryogenic microwave setup as well as the room temperature amplification and data acquisition circuitry; successfully developing a recipe for the fabrication of Josephson junctions; controlling and measuring superconducting 3D transmon qubits at 10mK. Several qubits were fully characterised and have shown coherence times of several microseconds and relaxation times up to 25μs. Superconducting qubits in fact possess higher energy levels that can provide significant computational advantages in quantum information applications. In experiments performed at MIT, preparation and control of the five lowest states of a transmon qubit was demonstrated, followed by an investigation of the phase coherence and decay dynamics of these higher energy levels. The decay was found to proceed mainly sequentially with relaxation times in excess of 20μs for all transitions. A direct measurement of the charge dispersion of these levels was performed to explore their characteristics of dephasing. This experiment was also reproduced on a 3D transmon fabricated and measured in Oxford, where due to a higher effective qubit temperature a multi-level decay model including thermal excitations was developed to explain the observed relaxation dynamics. Finally, a coaxial transmon, which we name the coaxmon, is presented and measured with a coaxial LC readout resonator and input/output coupling ports placed inline along the third dimension. This novel coaxial circuit QED architecture holds great promise for developing a scalable planar grid of qubits to build a quantum computer.