The photon storage time in a superconducting coplanar waveguide (CPW) resonator is contingent on the loaded quality factor, primarily dictated by the input and output capacitance of the resonator. The phase-slip based superconducting quantum interference device (PS-SQUID) comprises two phase-slip (PS) junctions connected in series with a superconducting island in between. The PS-SQUID can manifest nonlinear capacitance behavior, with the capacitance finetuned by the gate voltage to minimize the impact of magnetic field noise as much as possible. By substituting the coupling capacitance of the CPW resonator with the PS-SQUID, the loaded quality factor of the resonator can be changed by three orders, thus, we get a microwave photon switch in superconducting quantum computation architectures. Furthermore, by regulating the loaded quality factors, the coupling strength between the CPW and superconducting quantum circuits can be controlled, enabling the ability to manipulate stationary qubits and flying qubits.
The photon storage time in a superconducting coplanar waveguide (CPW) resonator is contingent on the loaded quality factor, primarily dictated by the input and output capacitance of the resonator. The phase-slip based superconducting quantum interference device (PS-SQUID) comprises two phase-slip (PS) junctions connected in series with a superconducting island in between. The PS-SQUID can manifest nonlinear capacitance behavior, with the capacitance finetuned by the gate voltage to minimize the impact of magnetic field noise as much as possible. By substituting the coupling capacitance of the CPW resonator with the PS-SQUID, the loaded quality factor of the resonator can be changed by three orders, thus, we get a microwave photon switch in superconducting quantum computation architectures. Furthermore, by regulating the loaded quality factors, the coupling strength between the CPW and superconducting quantum circuits can be controlled, enabling the ability to manipulate stationary qubits and flying qubits.
A phase-slip flux qubit, exactly dual to a charge qubit, is composed of a superconducting loop interrupted by a phase-slip junction. We propose a tunable phase-slip flux qubit by replacing the phase-slip junction with a charge-related superconducting quantum interference device (SQUID) consisting of two phase-slip junctions connected in series with a superconducting island. This charge-SQUID acts as an effective phase-slip junction controlled by the applied gate voltage and can be used to tune the energy-level splitting of the qubit. In addition, we show that a large inductance inserted in the loop can reduce the inductance energy and consequently suppress the dominating flux noise of the phase-slip flux qubit. This enhanced phase-slip flux qubit is exactly dual to a transmon qubit.
The key components in superconducting quantum computing architectures are qubits, couplers, microwave elements and low-noise amplifiers among others. In a variety of these subsystems, nonlinear circuit elements are an essential ingredient which enables their functionality. In this thesis, we study two different superconducting elements used to implement nonlinear capacitors. First we simulated a novel Josephson-junction-based tuneable nonlinear capacitor to couple flux qubits. Such a device implements a YY interaction, which has the potential to enhance the performance and extend the capabilities of current generation quantum annealers. We show that the interaction strength of the YY coupling is on the order of MHz and tuneable across zero. We discuss experimental realisations of this device to verify its function. Next, we present our study of a low temperature parametric amplifier utilizing the nonlinear capacitance of quantum phase-slip nanowires. We have fabricated a single-nanowire and two-nanowire variants which shunt a quarter-wavelength superconducting coplanar waveguide resonator. We characterised the nonlinear properties of the first variant such as the power-dependent resonance frequency, idler generation and signal gain, while in the second variant we developed a lock-in detection method to investigate the gate voltage response and demonstrate that the device acts as a gate-voltage-tuneable resonator. We also report on the progress towards a fabrication process for sputtered Al overlap junctions in the LCN cleanroom. Back-sputter and in-situ oxide growth techniques are developed as an alternative to shadow evaporation. These techniques are better suited for large-scale integration as they do not rely on angular deposition, but they require better control over the interfaces due to patterning the two electrodes separately. We characterised the processes using resistance measurements and characterised their low temperature properties. We show that our junctions exhibit overdamped dynamics and the critical current is suppressed with RF radiation.
The leading approach to attempt quantum computing is based on superconductors. In particular, Circuit Quantum Electrodynamic systems (circuit QED) are used to couple photons stored in a microwave resonator to a quantum bit (qubit), allowing the qubit to be controlled and measured. A lumped element parallel RLC circuit will be used to model a superconducting coplanar microwave resonator, in order to gain an intuitive understanding of how various parameters affect its electromagnetic properties, and how they can be optimised to provide desired specifications. Resonator quality factors will also be realised, which describe the effects of coupling the microwave resonator to external input/output transmission lines. The qubit is generated by a superconducting capacitively shunted Josephson Junction; the transmon, and its operation and non-linear behaviour will be discussed in this report. The transmon is integrated with a microwave resonator to generate the circuit QED system, and the derivation of the systems Hamiltonian will allow us to determine the equations describing the behaviour of the system, and therefore how it may be controlled and measured. The Bloch sphere will be introduced, which provides a useful geometric representation of the state space of a two-level quantum system. Mapping the derived Hamiltonian to the time-dependent Schrodinger equation facilitates realisation of control mechanisms of the qubit based on rotations about the Bloch sphere.
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.
Superconducting resonators are powerful tools widely used in circuit Quantum Electrodynamics [1]. High-quality factors, tunability, and several implemented layouts make them ideal circuital elements for various applications from quantum metrology [2], to quantum computation [3] and quantum information [4]. Here, we report on the simulation and design of superconducting Coplanar Waveguide (CPW) resonators whose resonance frequencies are in the range of 4–8 GHz. Typical properties of superconducting resonators employed in standard transmon qubits can be achieved by designing a λ/4 resonator made by Niobium (Nb) and modeling the capacitive coupling to provide a high-quality factor useful for its future employments in more advanced quantum circuits [5]. Nb gives the advantage of better stability of the superconducting properties of the material against the application of a magnetic field, compared those achievable by Aluminum (Al), thus guaranteeing an additional degree of freedom in the implementation of complex quantum architectures [5]–[7].
A quantum computer is a promising addition to a classical computer due to increase in performance on certain computational problems. A classical computer computes by manipulating bits, which assume a value either 0 or 1, using logical gates. Similarly, a quantum computation is carried out by manipulating quantum bits, so-called qubits, using quantum gates. The main advantage of qubits is that they can be not only in the states representing 0 and 1, but also in any superposition of these two states. Many different physical realizations for qubits have been proposed. One of the most promising candidates for the hardware of quantum computing are superconducting circuits. Here, the qubit can be represented in number of ways. For example, the two different states can be the direction of current circulating in a superconducting loop, or presence and absence of a photon in a transmission line. In this thesis, we study a tunable phase gate for microwave photons. The gate is implemented by a transmission line interrupted by three superconducting quantum interference devices (SQUIDs), which we model as inductors. We theoretically show that this system fulfills the requirements of a phase gate and that the tunability of the phase shift is frequency dependent. In addition, we consider a non-ideal system by including the effects of the capacitance associated with the SQUIDs. We find that the capacitance has no adverse effects, and in the best case, it may even increase the range of tunability. We also measure the phase shift at frequency of 6.3 GHz and find that the system is well described by the theory. To our knowledge, similar phase gate has not been experimentally studied before.