In order to build a useful quantum computer, qubits with long coherence times are needed. Longer coherence times mean that more qubit operations can be performed reliably, which is essential for executing complex quantum algorithms. Recently, there has been significant progress in this field and state-of-the-art superconducting qubits have reached relaxation and dephasing times near the millisecond range. However, it would still be highly beneficial to improve qubit coherence times even further to eventually move forward from the noisy intermediate-scale quantum era. One potential way to achieve this improvement is to utilize a superconducting three-dimensional (3D) cavity as part of a qubit design where nonlinearity is realized with some structure inside the cavity. These cavities can have extremely high quality factors which shows potential for long qubit relaxation times even in the range of tens of milliseconds. In this thesis, we present and experimentally test a novel unimon-type qubit design based on a vacuum-gap Josephson junction inside a superconducting 3D cavity resonator. The junction is located at the center of a high-purity aluminum vacuum cavity, between two pillars extending from the top and the bottom. It is formed with a 3D structure by breaking a thin neck into two parts leaving a vacuum gap between them which forms the insulating layer of the junction. In order to control the vacuum-gap distance, the bottom of the cavity can be moved using a screw and the top wall is controlled using a high-precision cryogenic actuator. We present design simulations, conduct preliminary experiments on the moving of the cavity walls and carry out room-temperature spectroscopy measurements to identify the resonant modes and their quality factors. The obtained experimental results are quite limited due to certain issues with the first prototypes of the design. However, identifying these issues provides pivotal information for improving the design in the future. Although the preliminary results show that further research is needed, this thesis provides a solid base for developing the device further towards a high-coherence superconducting qubit.
In order to build a useful quantum computer, qubits with long coherence times are needed. Longer coherence times mean that more qubit operations can be performed reliably, which is essential for executing complex quantum algorithms. Recently, there has been significant progress in this field and state-of-the-art superconducting qubits have reached relaxation and dephasing times near the millisecond range. However, it would still be highly beneficial to improve qubit coherence times even further to eventually move forward from the noisy intermediate-scale quantum era. One potential way to achieve this improvement is to utilize a superconducting three-dimensional (3D) cavity as part of a qubit design where nonlinearity is realized with some structure inside the cavity. These cavities can have extremely high quality factors which shows potential for long qubit relaxation times even in the range of tens of milliseconds. In this thesis, we present and experimentally test a novel unimon-type qubit design based on a vacuum-gap Josephson junction inside a superconducting 3D cavity resonator. The junction is located at the center of a high-purity aluminum vacuum cavity, between two pillars extending from the top and the bottom. It is formed with a 3D structure by breaking a thin neck into two parts leaving a vacuum gap between them which forms the insulating layer of the junction. In order to control the vacuum-gap distance, the bottom of the cavity can be moved using a screw and the top wall is controlled using a high-precision cryogenic actuator. We present design simulations, conduct preliminary experiments on the moving of the cavity walls and carry out room-temperature spectroscopy measurements to identify the resonant modes and their quality factors. The obtained experimental results are quite limited due to certain issues with the first prototypes of the design. However, identifying these issues provides pivotal information for improving the design in the future. Although the preliminary results show that further research is needed, this thesis provides a solid base for developing the device further towards a high-coherence superconducting qubit.
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.
Superconducting circuits provide a promising approach for constructing a large-scale quantum computer in the future. In theory, a fault-tolerant quantum computer is suitable for efficiently simulating quantum-mechanical systems, such as molecules, or factorizing large integers - to name a few examples. However, reaching a practical quantum advantage will probably require scaling up the number of qubits in current quantum-processing units by several orders of magnitude while retaining high controllability and high coherence of individual qubits. In this thesis, we first provide a review of the most common superconducting qubit types in order to shed light on the factors that affect the coherence, anharmonicity, and operation of single qubits. Using the insight gained from the review, we theoretically study a novel superconducting qubit circuit that consists of a grounded coplanar waveguide resonator incorporating an embedded Josephson junction in the center conductor of the resonator. The non-linear Josephson junction renders the normal modes of the resonator anharmonic, as a result of which one of the normal modes can be utilized to implement a qubit. The studied circuit, nicknamed as the coplanarium, may provide a high-coherence superconducting qubit since it contains no isolated superconducting islands rendering the qubit immune to dephasing arising from low-frequency charge noise. We derive the quantum Hamiltonian of the distributed circuit from first principles both with and without an external magnetic flux. Using numerical calculations based on our theoretical results, we show that the anharmonicity of the coplanarium qubit can exceed 500 MHz for a qubit frequency of 5 GHz at a flux-insensitive sweet spot if an appropriate external magnetic flux is applied. Importantly, the parameter values used in the numerical simulations are experimentally feasible, and the characteristic impedance of the resonator needs to be no greater than Z0 = 100 Omega. The relatively high anharmonicity can be attributed to a partial cancellation of the quadratic potential-energy terms associated with the Josephson junction and the linear inductance of the resonator. In conclusion, the promising coherence and anharmonicity properties may render the coplanarium qubit an interesting alternative for reducing leakage errors, and the ratio between coherence times and the duration of quantum logic gates in superconducting quantum processors.
Quantum computers utilize qubits to store and process quantum information. In superconducting quantum computers, qubits are implemented as quantum superconducting resonant circuits. The circuits are operated only at the two energy states, which form the computational basis for the qubit. To suppress leakage to uncomputational states, superconducting qubits are designed to be anharmonic oscillators, which is achieved using one or more Josephson junctions, a nonlinear superconducting element. One of the main challenges in developing quantum computers is minimizing the decoherence caused by environmental noise. Decoherence is characterized by two coherence times, T1 for depolarization processes and T2 for dephasing. This thesis reviews and investigates the decoherence properties of superconducting qubits. The main goal of the thesis is to analyze the tradeoff between anharmonicity and dephasing in a qubit unimon. Recently developed unimon incorporates a single Josephson junction shunted by a linear inductor and a capacitor. Unimon is tunable by external magnetic flux, and at the half flux quantum bias, the Josephson energy is partially canceled by the inductive energy, allowing unimon to have relatively high anharmonicity while remaining fully protected against low-frequency charge noise. In addition, at the sweet spot with respect to the magnetic flux, unimon becomes immune to first-order perturbations in the flux. The sweet spot, however, is relatively narrow, making unimon susceptible to dephasing through the quadratic coupling to the flux noise. In the first chapter of this thesis, we present a comprehensive look into the basic theory of superconducting qubits, starting with two-state quantum systems, followed by superconductivity and superconducting circuit elements, and finally combining these two by introducing circuit quantum electrodynamics (cQED), a framework for building superconducting qubits. We follow with a theoretical discussion of decoherence in two-state quantum systems, described by the Bloch-Redfield formalism. We continue the discussion by estimating decoherence using perturbation theory, with special care put into the dephasing due to the low-frequency 1/f noise. Finally, we review the theoretical model of unimon, which is used in the numerical analysis. As a main result of this thesis, we suggest a design parameter regime for unimon, which gives the best ratio between anharmonicity and T2.
As quantum computing advances towards practical implementation, preserving coherence in superconducting circuits remains a central challenge in developing scalable systems. Advances in materials science and fabrication techniques have significantly improved component quality over the past decade. In particular, treatments such as chemical etching and protective coatings have demonstrated reductions in two-level system (TLS) losses by reducing oxide-related dissipation. However, these methods are insufficient to prevent re-oxidation when devices are exposed to the atmosphere between fabrication and measurement, introducing decoherence channels and hindering reproducibility across cooldown cycles. This thesis addresses these issues by fabricating and evaluating a hermetically sealed three-dimensional aluminum cavity capable of preserving vacuum and preventing oxidation during cooldown cycling. We begin by fabricating the cavity, after which we test its vacuum integrity using a helium leak detector and its structural integrity under repeated thermal changes. After this, we characterize test samples and assess the cavity’s suitability for standardized quality factor and two-level system loss measurements. The results show that the cavity remains sealed across varying temperatures and supports reliable characterization measurements. These results demonstrate that hermetically sealed cavities present a promising strategy for reducing decoherence and improving the reproducibility of superconducting qubit experiments, although further research is warranted to evaluate the long-term viability of the device.
Superconducting microwave cavities with ultra-high Q-factors are revolutionizing the field of quantum computing, offering long coherence times exceeding 1 ms, which is critical for realizing scalable multi-qubit quantum systems with low error rates. In this work, we provide an in-depth analysis of recent advances in ultra-high Q-factor cavities, integration of Josephson junction-based qubits, and bosonic-encoded qubits in 3D cavities. We examine the sources of quantum state dephasing caused by damping and noise mechanisms in cavities and qubits, highlighting the critical challenges that need to be addressed to achieve even higher coherence times. We critically survey the latest progress made in implementing single 3D qubits using superconducting materials, normal metals, and multi-qubit and multi-state quantum systems. Our work sheds light on the promising future of this research area, including novel materials for cavities and qubits, modes with nontrivial topological properties, error correction techniques for bosonic qubits, and new light-matter interaction effects.
In a quantum computer, the information carriers, which are bits in ordinary computers, are implemented as devices that exhibit coherent superpositions of physical states and entanglement. Such components, known as quantum bits or qubits, can be realized with various different types of two-state quantum systems. Quantum computers will be built for computational speed, with hoped for applications especially in cryptography and in other tasks where classical computers remain inefficient. Circuit quantum electrodynamics (cQED) is a quantum-computer architecture which employs superconducting electronic components and microwave photon fields as building blocks. Compared to cavity quantum electrodynamics (CQED), where atoms are trapped in physical cavities, cQED is more attractive in that its qubits are tunable and conveniently integrable with the electronics already in use. This architecture has shown some of the most promising qubit designs, despite their coherence times reaching tens of microseconds, are still below the state of the art with spin qubits, which reach milliseconds. Coherence times are historically the most relevant parameters describing the fitness of a qubit, although these days not necessarily the limiting factor. This thesis presents a comprehensive set of theoretical and experimental methods for measuring the characteristic parameters of superconducting qubits. We especially study transmission-line-shunted plasma oscillation qubits, or transmons, and presents experimental results for a single sample. Transmon capacitively couples a superconducting quantum interference device (SQUID) with a coplanar waveguide (CPW) resonator, often with added frequency tunability utilizing an external magnet. The number of superconducting charge carriers tunnelled through a junction in the SQUID are used as qubit degrees of freedom. Readout of the qubit state is carried out by measuring transmission through the CPW. A cryogenic setup is employed with measurement and driving pulses delivered from microwave sources. Steady-state spectroscopy is employed to determine the resonance frequencies of the qubit and the resonator, qubit-resonator coupling constants, and the energy parameters of the qubit. Pulse-modulated measurements are employed to determine the coherence times of the qubit. The related analysis- and simulation programs and scripts are collected togithub.com/patomaki.