Abstract Some of the stable isotopes of silicon and carbon have zero nuclear spin, whereas many of the other elements that constitute semiconductors consist entirely of stable isotopes that have nuclear spins. Silicon and diamond crystals composed of nuclear-spin-free stable isotopes (28Si,30Si, or12C) are considered to be ideal host matrixes to place spin quantum bits (qubits) for quantum-computing and -sensing applications, because their coherent properties are not disrupted thanks to the absence of host nuclear spins. The present paper describes the state-of-the-art and future perspective of silicon and diamond isotope engineering for development of quantum information-processing devices.
Abstract Some of the stable isotopes of silicon and carbon have zero nuclear spin, whereas many of the other elements that constitute semiconductors consist entirely of stable isotopes that have nuclear spins. Silicon and diamond crystals composed of nuclear-spin-free stable isotopes (28Si,30Si, or12C) are considered to be ideal host matrixes to place spin quantum bits (qubits) for quantum-computing and -sensing applications, because their coherent properties are not disrupted thanks to the absence of host nuclear spins. The present paper describes the state-of-the-art and future perspective of silicon and diamond isotope engineering for development of quantum information-processing devices.
Spin is a quantum mechanical property that describes the magnetic orientation of elementary and composite particles such as electrons and nuclei. For spin-half particles placed in a magnetic field, two states with differing energies are defined. Proposals exist for developing a revolutionary kind of computer that encodes bits of information on the states of a simple quantum two-level system - like those of a spin in a magnetic field. These quantum computers exploit the fundamental principles of quantum mechanics in order to perform certain calculations exponentially faster than their classical counterparts. Key to the development of such a device is the identification of potential systems that are suitable for employment as a quantum bit (qubit). This thesis explores quantum bits constructed from the spin states of single atoms hosted in silicon semiconductor devices. Silicon is an exceptional platform for spin-based quantum computing - in addition to being the dominant material in the microelectronic industry, it is also predominantly composed of nuclear spin-zero isotopes, providing very little background magnetic disturbance. The donor-atom phosphorus (31P) is investigated here. It possesses a nucleus that binds an additional electron to its positive charge, providing two distinct spin qubits. Here, several world-first experimental milestones in spin-based quantum information processing are reported. First, the readout of the electron spin is demonstrated in single-shot, using a silicon device which combines implanted phosphorus donors with a sensitive nanoscale electrometer. Then, the electron spin is coherently operated as a qubit by delivering on-chip microwave excitations. Combining the electron spin detection and control capabilities enables a quantum non-demolition measurement of the single 31P nuclear spin. Experiments yield exceptional nuclear readout fidelities - the highest for any solid-state qubit. Through an application of nuclear magnetic resonance, high-fidelity quantum control of the nuclear spin qubit is demonstrated. Finally, a single spin-bearing silicon nucleus (29Si) coupled to the electron spin is detected in single-shot and operated as a qubit. These results, along with the clear pathways identified for improving device performance, illustrate that single-atom electron and nuclear spin qubits in silicon constitute an excellent basis on which to construct a quantum computer.
The understanding of quantum mechanics enabled the development of technology such as transistors and has been the foundation of today’s information age. Actively using quantum mechanics to build quantum technology may cause a second revolution in handling information. However, to execute meaningful algorithms, largescale quantum computers have to be built. Such systems are constructed from many qubits, the quantum version of the classical bit. While exciting progress is being made across a range of different qubit platforms, achieving the radical scalability that is necessary to build a largescale processor could be a roadblock. Huge challenges are put on reproducibility, inand output connectivity and material quality. Qubits based on the spins of electrons and holes confined in semiconductor quantum dots may have an important advantage in constructing quantum processors. This platform can profit from the advanced semiconductor industry that was responsible for the first computing revolution. Group IV semiconductors such as silicon and germanium have a high compatibility with industrial semiconductor manufacturing and contain stable isotopes with zero nuclear spin. The materials can be isotopically purified and serve as excellent hosts for spins with long quantum coherence. In Chapter 3 we present quantum dot arrays in silicon metaloxidesemiconductor (SiMOS), strained silicon (Si/SiGe) and strained germanium (Ge/SiGe). A nearly identical integration scheme based on an overlapping gate structure can be used to define quantum dots in each platform. Each platform has its own opportunities, which are carefully assessed. By employing charge sensing we confirm that all quantum dots can be depleted to the singleelectron regime. We compare capacitive crosstalk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. Using this crossplatform integration, we can study qubits in each platform with minimal overhead. Long coherence times, excellent singlequbit gate fidelities and twoqubit logic have been demonstrated with SiMOS spin qubits, making it one of the leading platforms for quantum information processing. However, due to the high disorder at the Si/SiO2 interface compared to Ge/SiGe and Si/SiGe interface, quantum dots defined in SiMOS are small and achieving sufficient control over single electrons has been a long standing challenge. In Chapter 5 we show experiments on a double quantum dot that can be isolated from its reservoir. We demonstrate a tunable tunnel coupling between single electrons up to 13 GHz and tunable tunnel rates down to below 1 Hz. These results mark an important step towards the required degree of control over the location of and coupling between quantum dots, necessary for the operation of a large array.
Abstract Quantum coherence underpins the performance of all quantum technologies, from error-resistant operations to long-lived quantum memories and ultrasensitive sensors. Despite steady advances, coherence remains fundamentally constrained by environmental noise, most notably, from nuclear spins embedded in the host lattice. Protocol-level strategies, such as dynamical decoupling or operation at clock transitions, mitigate this noise externally but impose control overhead. In contrast, materials-based approaches—especially isotopic purification—directly suppress decoherence at its source. Here, we introduce a unified periodic table of quantum coherence that quantifies coherence limits element by element, incorporating both natural isotope distributions and practical routes for isotopic enrichment. Importantly, this framework provides, for the first time, theoretical upper bounds ( T 2,max ) for all stable elements, offering a quantitative ceiling on what isotopic purification can achieve. The table reveals where purification yields decisive improvements, where alternative strategies must prevail, and where supply or regulatory barriers constrain feasibility. Beyond guiding the search for solid-state qubit hosts, it uncovers overlooked opportunities in functional oxides and related compounds. More broadly, this framework positions isotope engineering as an increasingly important tool for identifying and realizing future quantum-coherent materials. Impact statement Quantum coherence is the essential resource for all quantum technologies, yet it is fundamentally constrained by nuclear spins in the host lattice for solid-state qubits. While active controls can mitigate decoherence externally, materials-based approaches—most notably isotopic purification—suppress noise at its physical origin. In this work, we introduce a unified periodic table of quantum coherence that establishes, for the first time, theoretical upper bounds on spin coherence ( T 2,max ) across the entire set of stable elements. By systematically incorporating both natural isotope distributions and realistic pathways for isotopic enrichment, this framework reveals where purification can yield decisive improvements, where alternative strategies must dominate, and where supply or regulatory barriers impose practical limits. Beyond guiding the search for solid-state spin qubit hosts, the analysis uncovers overlooked opportunities in materials and highlights new design rules for exploiting isotope engineering. More broadly, this periodic table positions isotopic purification as a universal principle for identifying, evaluating, and realizing quantum-coherent materials. The results thus have implications not only for advancing quantum information science, but also for materials synthesis, isotopic supply chains, and the integration of quantum coherence considerations into the broader practice of materials design. Graphical abstract
Quantum sensors utilize quantized energy levels, quan- tum superposition states, and eventually quantum entan- glement, which are also key elements in quantum comput- ing. Electronic and nuclear spins in semiconductors may serve as quantum sensing elements or qubits. The phos- phorus donor in silicon is an ideal qubit in terms of coherence time because of highest chemical purity of host material with isotopic 28Si enrichment and a very low den- sity of structural defects. However, it requires a cryogenic temperature to avoid ionization of the donor. In contrast in diamond, due to the wide band gap, the nitrogen-vacancy (NV) center exhibits optical initialization and readout of its spin state and a long coherence time even above room temperature, promoting its research and development as quantum sensors or qubits. Nevertheless, the syn- thesized diamond crystals with NV centers are not so per- fect as silicon. Therefore, the host material characteriza- tion is essential to improve the bulk-ensemble sensi- tivity to the magnetic field. The present study shows how to measure the spin properties of the diamond as a sens- ing host via the NV centers themselves and how to utilize such information to improve the sensor performance.
A new method of quantum information processing is suggested based on precise placing of 31 P atoms in a quasi one-dimensional 28 Si nanowire using isotope engineering and neutron transmutation doping of the grown structures. The interqubit coupling is proposed to be based on the indirect interaction of 31 P nuclear spins with electrons localized in a nanowire, which allows control of the coupling between distant qubits and even between qubits separated by nonqubit nodes. Numerical estimations show the feasibility of the proposed device.