Majorana zero modes have been proposed as building blocks of intrinsically fault-tolerant quantum computers. Currently, externally applied magnetic fields are necessary to induce Majorana zero modes in carefully engineered systems. Topological qubit state readout is realized by parity sensing of Majorana islands. However, fast and high fidelity conventional cQED parity readout is incompatible with the magnetic fields needed for Majorana physics. In this thesis, magnetic field insensitive microwave CPW resonators with artificial Abrikosov vortex pinning sites have tested and implemented in a graphene based transmon qubit. It has been shown that these artificial pinning sites reliably trap vortices and are able to retain their zero field Qi ~ 105 up to perpendicular fields of 35 mT. By application of these resonators, we have described the successful continuous wave qubit spectroscopy of a graphene transmon qubit at B|| = 1 T with a minimal linewidth of 166 MHz and demonstrated manipulation of the qubit frequency between 3.2-7 GHz with electric field. This is the first ever measured superconducting qubit that shows these properties at a magnetic field of 1 T.
Majorana zero modes have been proposed as building blocks of intrinsically fault-tolerant quantum computers. Currently, externally applied magnetic fields are necessary to induce Majorana zero modes in carefully engineered systems. Topological qubit state readout is realized by parity sensing of Majorana islands. However, fast and high fidelity conventional cQED parity readout is incompatible with the magnetic fields needed for Majorana physics. In this thesis, magnetic field insensitive microwave CPW resonators with artificial Abrikosov vortex pinning sites have tested and implemented in a graphene based transmon qubit. It has been shown that these artificial pinning sites reliably trap vortices and are able to retain their zero field Qi ~ 105 up to perpendicular fields of 35 mT. By application of these resonators, we have described the successful continuous wave qubit spectroscopy of a graphene transmon qubit at B|| = 1 T with a minimal linewidth of 166 MHz and demonstrated manipulation of the qubit frequency between 3.2-7 GHz with electric field. This is the first ever measured superconducting qubit that shows these properties at a magnetic field of 1 T.
The technology of quantum computing is believed to solve certain computational problems significantly faster than classical computers, enabling classically inaccessible problems. However, the technology is still in its infancy and it is still too early to conclude which physical system(s) will form the basis of tomorrow's quantum computer. Small-scale quantum processors, built on the superconducting transmon qubit, demonstrated already the anticipated quantum speed-up. Despite this tremendous milestone, scaling up to a full-fledged quantum computer is far from trivial: with these qubits, the flux-control causes crosstalk between qubits and heating, the room-temperature microwave control is hardly scalable and comes with high energetic radiation, and most importantly, the loss of quantum information in time leads to computational errors. Recent advances in various hybrid semiconductor materials enabled novel voltage-controlled transmons, gatemons, which are less susceptible to heating and crosstalk. Even more exciting, gatemons can be designed to host Majorana zero modes in a way that renders the qubit inherently protected against decoherence. In order to induce Majorana zero modes in such nanowire systems strong magnetic fields are required. Problematically, magnetic fields destroy the superconductivity that these microwave circuits rely on. In this thesis we integrate three types of hybrid Josephson junctions in magnetic field compatible microwave devices. Chapter 4 demonstrates the first graphene-based transmon. Due to the mono-atomic thickness of graphene in combination with magnetic field resilient coplanar waveguide resonators, we are able to operate the transmon circuit at an in-plane magnetic field of 1 Tesla. Chapter 5 embeds an InAs-Al semiconducting nanowire Josephson junction in a high quality factor superconducting transmission line resonator, demonstrating on-chip microwave generation. The gate-controllable semiconducting platform lends itself for fast pulse control, providing a perspective for the coherent on-chip manipulation of artificial two-level systems, in particular superconducting qubits such as transmons. Chapter 6 continues the development of InAs-Al nanowire transmons. The offset-charge-sensitive regime, additional plunger gates and magnetic field compatibility prepares the platform for the detection of coherent coupling between Majorana zero modes, a phenomena which unfortunately still remains unobserved. Additionally, we realise the first InSb-Al gatemon. The higher spin-orbit coupling makes InSb to be a preferred material in the search for Majorana signatures. Chapter 7 reports on the dynamics of quasiparticle tunneling events in real-time across the InAs-Al nanowire junction in a transmon architecture. The magnetic field compatibility of our device up to 1 Tesla together with in-situ voltage-controlled quasiparticle trap engineering, allows us to measure the survival of the charge-parity lifetime up to strong magnetic fields. A result which is extremely important in the research field of topological quantum computing, where the qubit space is defined in the charge-parity.
Majorana bound states (MBSs) are novel particles predicted to be created when superconductor/semiconductor hybrid structures with strong spin-orbit coupling are subjected to strong magnetic fields. Expected to exhibit non-Abelian exchange statistics, they could form the basis of a new kind of quantum computer that is inherently protected from environmental noise, a common problem that has frustrated other quantum computing platforms. The current techniques used to measure these particles are highly sensitive, having provided the best evidence yet for their existence, but they are intrinsically too slow to form the basis of a useful quantum computer. To remedy this, this thesis integrates exotic materials into high frequency superconducting circuits that have been engineered to be resilient to strong magnetic fields, creating hybrid devices that potentially allow for fast and precise measurement and control of MBSs and their properties. Several proposals to demonstrate the novel exchange statistics of MBSs use a specific type of superconducting qubit, the `transmon', for fast readout of the state of the MBSs. Problematically, the strong magnetic fields required to induce MBSs would destroy the superconductivity traditional transmons rely on, preventing them from operating as intended. To resolve this, the key constituent components of the transmon, the superconducting resonator and the Josephson junction have been engineered separately to become resilient to strong magnetic fields. Chapter 4 explores how nanofabrication techniques and careful consideration of the properties of thin superconducting films can be used to engineer superconducting co-planar waveguide resonators that remain operational in strong parallel magnetic fields of \\SI{6}{\\tesla} and perpendicular magnetic fields of \\SI{20}{\\milli \\tesla}, an order of magnitude greater than previously reported. Building on the results of Chapter 4, Chapter 5 utilises a graphene based Josephson junction, where the monoatomic thickness of the graphene provides an inherent protection against parallel magnetic fields, allowing us to demonstrate operation of a transmon circuit at a parallel magnetic field of \\SI{1}{\\tesla}. Advances in nanowire material growth intended to improve the signatures of MBS are used in Chapter 6 to create a low power, highly coherent on-chip microwave source. With broad potential applications in superconducting circuits, it demonstrates a platform well suited for the detection of unique radiation that MBSs are predicted to emit. The thesis is concluded by Chapter 7, which describes the engineering and development of a nanowire based transmon qubit capable of measuring key properties of MBSs in the qubit's energy spectrum.
Circuit quantum electrodynamics has proven to be a powerful tool to probe mesoscopic effects in hybrid systems and is used in several quantum computing (QC) proposals that require a transmon qubit able to operate in strong magnetic fields. To address this we integrate monolayer graphene Josephson junctions into microwave frequency superconducting circuits to create graphene based transmons. Using dispersive microwave spectroscopy we resolve graphene's characteristic band dispersion and observe coherent electronic interference effects confirming the ballistic nature of our graphene Josephson junctions. We show that the monoatomic thickness of graphene renders the device insensitive to an applied magnetic field, allowing us to perform energy level spectroscopy of the circuit in a parallel magnetic field of 1 T, an order of magnitude higher than previous studies. These results establish graphene based superconducting circuits as a promising platform for QC and the study of mesoscopic quantum effects that appear in strong magnetic fields.
A practical quantum computer demands a physical quantum bit (qubit) that is scalable, has a well-defined initial state, and brings together coherence with reliable control and read-out. A compelling idea is to put together a locally protected Majorana qubit with a superconducting qubit. The superconducting qubit is used to read out the Majorana states while the Majorana qubit may act as a quantum memory. This thesis presents a series of studies on quantum transport in semiconducting nanowires coupled to superconductors to investigate various platforms for realizing Majorana. These studies are followed by microwave measurements on superconducting circuits compatible with semiconducting-superconducting heterostructures. We first evaluate the potential of Ge/Si core/shell nanowires by achieving induced superconductivity as well as estimating spin-orbit coupling. Next we explore the transport mediated by Andreev bound states formed in InSb nanowire quantum. A subgap negative differential conductance is investigated together with the coalescing Andreev resonances at zero bias relevant for the correct interpretation of Majorana experiments done on the same structures. We conclude our studies of semiconducting nanowires by exploring tunnel junctions in Sn-InSb nanowires that are prepared by in-situ shadowing using nearby nanowires and flakes. Tin shells are found to induce a hard superconducting gap persisting up to high magnetic fields. We observe the two-electron charging effect from a small superconducting island of Sn-InSb. This effect is attributed to charge parity stability, which makes this nanowire system an intriguing candidate for superconducting and topological quantum circuits. These Sn-InSb junctions are then used as the nonlinear element in a transmon qubit design where we observe a dispersive coupling between this nanowire Josephson junction and a superconducting resonator. We also present our progress towards building a magnetic field resilient superconducting circuit that allows integration with semiconducting and topological structures. In the last chapter, InSb semiconducting nanowires are utilized as shadow masks prior to superconductor deposition on an InAs quantum well. We study Josephson current properties in Josephson junctions made from these nanowire shadows. Our results point to highly transparent junctions that can be developed further for hybrid superconductor-semiconductor qubit systems.
Quantum computation is a multifaceted field of research aiming for the physical realization of quantum systems and their manipulation. This thesis discusses the combination of two notable approaches in the pursuit of a fully operational quantum computer---circuit quantum electrodynamics and topological quantum computation based on Majorana quasiparticles. In circuit quantum electrodynamics quantum information is stored into small superconducting circuit elements whose interaction with electromagnetic radiation in the range of microwaves allows to process quantum information very efficiently. This approach has proven extremely useful for control and readout of superconducting qubits, i.e., small circuit elements that carry quantum information. Because of remarkably strong light-matter couplings that can be achieved for superconducting qubits in microwave resonators, the circuit quantum electrodynamics architecture is particularly useful to perform highly sensitive quantum measurements.Superconductivity by itself is an intriguing state of matter that shows a great variety of different phenomena. In particular, the discovery of topological phases in superconductors opened new horizons for quantum computation. One notable system that admits topological superconductivity is a semiconductor-superconductor nanowire with special zero modes occurring at its ends. These so-called Majorana zero modes are remarkably robust against decoherence and therefore well-suited for fault-tolerant quantum computation.The first part of this thesis examines the coupling of Majorana zero modes to electromagnetic radiation with microwave frequencies. The light-matter coupling mechanism that is considered here arises for Majorana zero modes located at a voltage-biased superconducting tunneling junction. The emission of microwave radiation in presence of Majorana zero modes gives rise to coherent radiation which is emitted at half of the usual Josephson frequency. On the basis of this fractional Josephson radiation, we propose a microwave readout scheme for Majorana qubits. As usual for typical measurements in circuit quantum electrodynamics, the proposed readout implements a quantum non-demolition measurement for Majorana qubit.In the last part of the thesis we propose a novel scheme for the implementation of measurement-induced entanglement between remote superconducting qubits as required for quantum communication. By detecting a single photon, that passes a Mach-Zehnder interferometric setup, deterministic entanglement with single-shot efficiency is achieved. This scheme essentially relies on the strong coupling between the qubits and the photon.
Motivated by recent experimental progress towards the measurement and manipulation of Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantum electrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting $\ensuremath{\lambda}/4$ resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet spots, this device is highly insensitive to environmental noise. It enables high-fidelity single-shot quantum nondemolition readout of the state of a pair of Majorana fermions encoding a topologically protected qubit. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet spot, this device can be used as a highly sensitive charge detector with a sensitivity better than ${10}^{\ensuremath{-}4}\text{e}/\sqrt{\text{Hz}}$ and bandwidth larger than 1 MHz.