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.
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.
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 spin, 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.
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.
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.
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.
The controlled, coherent manipulation of quantum-mechanical systems is an important challenge in modern science and engineering, with significant applications in quantum information science. Solid-state quantum systems such as electronic spins, nuclear spins, and superconducting islands are among the most promising candidates for realization of quantum bits (qubits). However, in contrast to isolated atomic systems, these solid-state qubits couple to a complex environment which often results in rapid loss of coherence, and, in general, is difficult to understand. Additionally, the strong interactions which make solid-state quantum systems attractive can typically only occur between neighboring systems, leading to difficulties in coupling arbitrary pairs of quantum bits. This thesis presents experimental progress in understanding and controlling the complex environment of a solid-state quantum bit, and theoretical techniques for extending the distance over which certain quantum bits can interact coherently. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond is used to gain insight into its mesoscopic environment. Furthermore, techniques for exploiting coherent interactions between the electron spin and a subset of the environment are developed and demonstrated, leading to controlled
Quantum computing is an attractive and multidisciplinary field, which became a focus for experimental and theoretical research during the last decade. Among other systems, such as ions in traps and superconducting circuits, solid state based qubits are considered to be promising candidates for use in first experimental tests of quantum hardware. Here we report recent progress in quantum information processing with point defects in diamond. Qubits are defined as single spin states (electron or nuclear). This allows exploration of long coherence times (up to seconds for nuclear spins at cryogenic temperatures). In addition, the optical transition between ground and excited electronic states allows coupling of spin degrees of freedom to the state of the electromagnetic field. Such coupling gives access to spin state read-out via spin-selective scattering of photons. This also allows the use of spin states as robust memory for flying qubits (photons).