Coupling single spins to superconducting circuits represents a powerful route toward scalable hybrid quantum technologies. At the intersection of quantum information processing, nanoscale sensing, and condensed matter physics, this emerging research direction aims to unite the best qualities of two very different quantum systems. Spins—whether associated with electrons, nuclei, or defects—are among the most coherent quantum systems known, with coherence times that can exceed seconds. However, their weak interaction with electromagnetic fields makes them notoriously difficult to control and read out individually. Superconducting circuits, on the other hand, offer exceptional control, scalability, and strong coupling to electromagnetic fields, but they tend to suffer from shorter coherence times. Bridging these two platforms can yield hybrid quantum systems that harness the best of both worlds: the long-lived quantum memory of spins and the fast, flexible control of superconducting devices. An additional key motivation for integrating single spins with superconducting circuits is the ability to perform spectroscopy at the nanoscale, enabling the investigation of quantum systems with unprecedented spatial and energy resolution. Spins, due to their sensitivity to magnetic and electric fields, serve as exquisite local probes of their environment. This synergy not only offers powerful tools for probing quantum phenomena at the nanoscale but also opens the possibility of hybrid quantum processors combining long-lived spin states with fast superconducting-circuit control.
Coupling single spins to superconducting circuits represents a powerful route toward scalable hybrid quantum technologies. At the intersection of quantum information processing, nanoscale sensing, and condensed matter physics, this emerging research direction aims to unite the best qualities of two very different quantum systems. Spins—whether associated with electrons, nuclei, or defects—are among the most coherent quantum systems known, with coherence times that can exceed seconds. However, their weak interaction with electromagnetic fields makes them notoriously difficult to control and read out individually. Superconducting circuits, on the other hand, offer exceptional control, scalability, and strong coupling to electromagnetic fields, but they tend to suffer from shorter coherence times. Bridging these two platforms can yield hybrid quantum systems that harness the best of both worlds: the long-lived quantum memory of spins and the fast, flexible control of superconducting devices. An additional key motivation for integrating single spins with superconducting circuits is the ability to perform spectroscopy at the nanoscale, enabling the investigation of quantum systems with unprecedented spatial and energy resolution. Spins, due to their sensitivity to magnetic and electric fields, serve as exquisite local probes of their environment. This synergy not only offers powerful tools for probing quantum phenomena at the nanoscale but also opens the possibility of hybrid quantum processors combining long-lived spin states with fast superconducting-circuit control.
One of the key elements in circuit quantum electrodynamics (cQED) is the superconducting microwave resonator, which can enable strong coupling to and between nanoscale quantum systems, and as a result has emerged as an essential tool for both quantum information and quantum sensing. In this thesis, I will discuss new applications of superconducting resonators in both of these fields, which illustrates how quantum information and quantum sensing can be strongly tied together. As the field of quantum information continues to grow, various platforms for realizing a quantum computer have been proposed. One attractive platform is to control and manipulate the spin of an electron to form spin qubits, which have demonstrated a long coherence time, weak coupling to the environment, and high fidelity single- and two-qubit gates. However, scaling beyond two qubits has proven challenging, as has incorporating long-range coupling, although both are crucial for making spin qubits a scalable quantum computing platform. The confinement of electromagnetic fields in superconducting resonators enables strong coupling to small systems, such as the single electrons of a spin qubit. In the first part of this thesis, I will describe how a singlet-triplet spin qubit coupled to a high-impedance superconducting resonator constitutes a promising architecture for mediating long-ranged interactions based on a longitudinal coupling between the resonator and qubit. I will discuss the technical developments needed to successfully implement this novel type of interaction in a hybrid spin qubit-resonator device. This type of interaction has several advantages over the more commonly studied transverse coupling scheme: it does not require careful tuning of the qubit frequency to match that of the resonator, which facilitates scaling to a large number of qubits, and also avoids the loss of coherence, which is inevitable in resonant coupling. This type of interaction therefore opens promising paths for generating high-fidelity long-ranged two-qubit coupling by employing superconducting microwave resonators. By adapting techniques used in quantum information, we have also developed a new technique in quantum sensing, where we employ superconducting resonators as a probe of quantum materials such as mesoscopic systems, two-dimensional materials, and heterostructures, which are otherwise challenging to experimentally access with conventional techniques. In the second part of my thesis, I will discuss how implementing cQED techniques from quantum information allows us to probe the symmetry of the order parameter in unconventional superconductors. Unconventional superconductors are not readily available, and we therefore study an engineered system that comprises of interfacing a ferromagnet and superconductor, in which unconventional pairing can be induced into the ferromagnet. This platform allows us to simultaneously excite spin waves, a collective excitation of spins, in the ferromagnet, which opens new directions for studying interactions between coherently coupled magnons (quanta of spin waves) and unconventional superconducting states. Finally, by applying the same technique to a two-dimensional (2D) superconductor, tungsten ditelluride, we leverage this platform in an attempt to study the order parameter of its intrinsic superconducting phase. The nature of the order parameter in this material remains an open question, as both topological and superconducting phases can exist, possibly simultaneously. This work thus demonstrates a unique use of superconducting resonators beyond quantum information applications.
Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins, and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers.
Hybrid semiconductor-superconductor qubits have recently emerged as a promising alternative to traditional platforms, combining material advantages with device-level tunability. A defining feature is their gate-tunable Josephson coupling, enabling superconducting qubit architectures with full electric-field control and offering a path toward scalable, low-crosstalk quantum processors. This approach seeks to merge benefits of superconducting and semiconductor qubits, for instance by encoding quantum information in the spin of a quasiparticle occupying an Andreev bound state, thus combining long coherence times with fast, flexible control. Progress has accelerated through bottom-up engineering of Andreev states in coupled quantum dot arrays, leading to architectures such as minimal Kitaev chains hosting Majorana zero modes. In parallel, Hamiltonian-protected designs aim to enhance resilience against local noise and decoherence by exploiting superconducting phase dynamics and discrete charge or flux degrees of freedom. This article reviews recent theoretical and experimental advances in hybrid qubits, providing an overview of physical mechanisms, device implementations, and emerging architectures, with emphasis on their potential for (topologically) protected quantum information processing. While many designs remain at proof-of-concept stage, rapid progress suggests practical demonstrations may soon be achievable.
Circuit quantum electrodynamics is a currently very active field of research. Since the discoveries that an artificial spin, the so-called qubit, can be implemented using a superconducting non-linear circuit and can coherently interact with the electromagnetic field at the single-photon level, it has gathered strong interest for its potential for quantum computing but also for its ability to create, manipulate and detect microwave states with an exquisite precision. This manuscript describes three experiments harnessing and building on the tools and concepts developed for circuit quantum electrodynamics to couple microwave fields to other fundamental entities, namely phonons and electron spins. In a first experiment, we show the coherent coupling of superconducting to surface acoustic wave phonons. It demonstrates that acoustic waves can be used to carry quantum information and distribute entanglement. We then explore the coupling of superconducting circuits to electronic spins, and its potential for sensing and quantum information storage. The second experiment concentrates at building a high-sensitivity spectrometer able to detect spins in picoliter sized samples, targeting samples from chemical or biological origins. The third experiment is about implementing a quantum memory using a spin ensemble possessing second-long coherence. We namely explore the potential of circuit-enhanced radiation cooling to polarize the spin ensemble and enhance the efficiency of the quantum memory storage.
Superconducting quantum devices provide excellent connectivity and controllability, while semiconductor spin qubits stand out with their long-lasting quantum coherence, fast control, and potential for miniaturization and scaling. In the last few years, remarkable progress has been made in combining superconducting circuits and semiconducting devices into hybrid quantum systems that benefit from the physical properties of both constituents. Superconducting cavities can mediate quantum-coherent coupling over long distances between electronic degrees of freedom such as the spin of individual electrons on a semiconductor chip and, thus, provide essential connectivity for a quantum device. Electron spins in semiconductor quantum dots have reached very long coherence times and allow for fast quantum gate operations with increasing fidelities. We summarize recent progress and theoretical models that describe superconducting–semiconducting hybrid quantum systems, explain the limitations of these systems, and describe different directions where future experiments and theory are headed.
We review progress on the use of electron spins to store and process quantum information, with particular focus on the ability of the electron spin to interact with multiple quantum degrees of freedom. We examine the benefits of hybrid quantum bits (qubits) in the solid state that are based on coupling electron spins to nuclear spins, electron charge, optical photons, and superconducting qubits. These benefits include the coherent storage of qubits for times exceeding seconds; fast qubit manipulation; single qubit measurement; and scalable methods for entangling spatially separated, matter-based qubits. In this way, the key strengths of different physical qubit implementations are brought together, laying the foundation for practical solid-state quantum technologies.