Diamond Flaws Maintain Electron Spin 11 Seconds

Scientists are continually refining the building blocks of quantum technology, and recent progress in solid-state spin defects is particularly noteworthy. Takashi Yamamoto and colleagues at Tohoku University report record electron-spin coherence times in nitrogen-vacancy (NV) centres within isotopically engineered diamond. The research achieves a sharp improvement in maintaining quantum information. This breakthrough surpasses the limitations of existing solid-state qubits and opens new possibilities for prolonged quantum information storage. Maintaining coherence for this duration was previously unattainable due to sensitivity to environmental noise and the challenge of simultaneously preserving both spin and optical coherence. The prolonged coherence is crucial because quantum information is fragile and easily lost to environmental interactions; extending this timeframe allows for more complex quantum operations to be performed before the information degrades. This is a significant step towards building fault-tolerant quantum computers.

Extended coherence times and narrow linewidths in isotopically engineered diamond nitrogen-vacancy

Defect-electron-spin coherence reached 11.2 seconds, a substantial increase from the previously reported maximum of 6.8 milliseconds for a Hahn echo. This breakthrough surpasses the limitations of existing solid-state qubits and opens new possibilities for prolonged quantum information storage. Maintaining coherence for this duration was previously unattainable due to sensitivity to environmental noise and the challenge of simultaneously preserving both spin and optical coherence. The prolonged coherence is crucial because quantum information is fragile and easily lost to environmental interactions; extending this timeframe allows for more complex quantum operations to be performed before the information degrades. This is a significant step towards building fault-tolerant quantum computers.

A near-lifetime-limited homogeneous linewidth of 16.9MHz was exhibited by coherent optical transitions, indicating high-quality crystal structure and minimal disturbance to the NV centre’s ability to emit and absorb light. Precise control of $^{13}\mathrm{C}$ concentration, coupled with mitigation of 50Hz electrical noise and tailored decoupling sequences, enabled this record performance; these techniques minimise disturbances to the delicate quantum states within the diamond. The linewidth is a measure of the spectral purity of the optical transition; a narrower linewidth indicates a more well-defined energy level and reduces the probability of unwanted transitions. This is vital for efficient and precise control of the NV centre using light. Dynamical decoupling sequences are specifically designed pulse sequences that effectively average out the effects of certain types of noise, extending the coherence time by preventing the accumulation of phase errors.

The diamond growth process yielded material with low-ppb (parts per billion) nitrogen concentrations, important for minimising defects that can disrupt quantum coherence. Achieving this level of purity alongside precise control over the $^{13}\mathrm{C}$ concentration represents a major advancement in materials engineering for quantum technologies. Nitrogen impurities act as paramagnetic centres, introducing additional noise and shortening coherence times. Minimising these impurities is therefore essential for achieving high-fidelity quantum control. Although these 11.2 second coherence times are a substantial leap forward, they currently rely on carefully controlled laboratory conditions and do not yet demonstrate sustained coherence within more complex, real-world quantum devices. Further work will focus on translating these results into functional quantum devices, including exploring methods to maintain coherence in less ideal environments.

Enhancing qubit coherence via carbon isotope purification in diamond

Isotopic engineering proved central to achieving these extended coherence times, a technique involving careful control of the different forms, or isotopes, of carbon atoms within the diamond itself. Disturbances to the delicate quantum state of the nitrogen-vacancy (NV) centres, tiny imperfections in the diamond behaving like artificial atoms, were minimised by reducing the prevalence of the $^{13}\mathrm{C}$ isotope. This precise control over atomic composition is akin to carefully tuning an instrument to eliminate unwanted vibrations, creating a more stable environment for maintaining quantum information. The $^{13}\mathrm{C}$ isotope possesses a nuclear spin, which interacts with the electron spin of the NV centre, causing decoherence. By reducing the concentration of $^{13}\mathrm{C}$, these interactions are weakened, leading to longer coherence times.

Large, high-quality diamond layers with consistent isotopic purity were enabled by this approach, essential for reliable qubit performance and scalability. Using MPCVD, diamond layers were grown with carefully controlled carbon isotope composition to minimise disturbances to quantum states within nitrogen-vacancy (NV) centres. The resulting diamond contained a $^{13}\mathrm{C}$ concentration ranging from 0.0139 to 1.0456 percent, alongside low levels of nitrogen, below 20 parts per billion, demonstrating the effectiveness of the growth process. MPCVD, or Microwave Plasma-Chemical Vapour Deposition, is a sophisticated technique allowing for precise control over the diamond’s composition during growth. The ability to consistently produce diamond with such low $^{13}\mathrm{C}$ concentrations is a significant achievement, paving the way for the fabrication of large-scale quantum devices.

Record coherence times do not yet translate to efficient quantum communication

Maintaining both extended electron-spin coherence and the ability to efficiently emit light from these nitrogen-vacancy (NV) centres remains a significant hurdle. Achieving practical spin-photon entanglement, essential for transmitting quantum information, requires optimising the efficiency of this light emission. Despite these advances in coherence, quantifying the efficiency of converting spin information into photons remains a critical challenge. The emitted photons act as carriers of the quantum information, and a low emission efficiency limits the range and fidelity of quantum communication.

Record-breaking electron-spin coherence times of up to 11.2 seconds were achieved using tailored decoupling sequences and a real-time noise reduction system, primarily focusing on the characteristics of the NV centre itself. Reliably creating and capturing single photons entangled with the electron spin is necessary for demonstrating practical quantum communication. Exceptionally stable diamond materials now benefit sustained quantum control, successfully combining extended electron-spin coherence, lasting over eleven seconds, with coherent light emission from nitrogen-vacancy centres, addressing a key obstacle in developing solid-state quantum technologies. These centres are atomic-scale defects within diamond acting as qubits, the fundamental units of quantum information. Achieving both properties simultaneously is particularly important for building quantum networks capable of transmitting information securely, paving the way for future advancements in quantum communication systems. The development of efficient and scalable methods for generating and detecting these entangled photons is now a primary focus of research in this field, with potential applications ranging from secure communication to distributed quantum computing.

The research successfully demonstrated record electron-spin coherence times of 11.2 seconds in nitrogen-vacancy centres within isotopically engineered diamond. This achievement is important because maintaining both long coherence and efficient light emission is crucial for creating stable qubits and transmitting quantum information. By combining tailored decoupling sequences with a real-time noise reduction system, researchers have improved control over these spin qubits. The authors suggest further investigation into impurity incorporation in diamond will be necessary to advance quantum technologies.