By tuning carbon-13, limiting nitrogen contamination, and suppressing mains-frequency magnetic noise, researchers tested how long a single NV-center spin could remain coherent without sacrificing its optical performance.
Paper: Ten-second electron-spin coherence in isotopically engineered diamond. AI-generated abstract conceptual image created using ChatGPT/OpenAI
In a recent paper in npj Quantum Information, researchers achieved an electron-spin coherence time of 11.2(8) seconds under CPMG dynamical decoupling in isotopically engineered synthetic diamond.
They produced high-purity, (111)-oriented diamond layers using controlled chemical vapor deposition (CVD) while controlling the carbon-13 (13C) concentration and limiting nitrogen contamination.
The resulting material provided a cleaner host for nitrogen-vacancy (NV) centers, reducing sources of material-related electric and magnetic noise and supporting long spin coherence times alongside near-lifetime-limited optical linewidths.
This work shows that controlling material impurities and external magnetic interference can extend electron-spin coherence in solid-state quantum systems while preserving optical coherence.
The foundation of quantum network functionality
Solid-state color centers, particularly NV centers embedded in synthetic diamond, offer a promising platform for quantum networks, distributed quantum computing, quantum sensing, and other applications.
An NV center can act as a solid-state quantum node, with its electron spin interacting with emitted photons and surrounding nuclear spins. Their performance depends heavily on the purity and structure of the diamond host.
NV centers are highly sensitive to electric-field noise from moving charges and magnetic-field noise from nuclear and electron spins. Longer coherence times can be achieved with lower concentrations of magnetic isotopes, such as 13C, and fewer charge-producing impurities.
Controlling the isotopic composition and impurity levels during diamond growth helps produce reliable solid-state quantum systems. The paper also treats 13C concentration as a trade-off, since its nuclear spins can serve as qubits.
Techniques for diamond growth and purity control
To prepare a high-purity diamond host, researchers used microwave-plasma chemical vapor deposition (MPCVD) to grow homoepitaxial diamond layers on low-dislocation, high-pressure high-temperature (HPHT) diamond substrates.
They selected the (111) orientation because it improves optical detection into single-mode fibers and preferentially directs CVD-grown NV centers along the surface-normal [111] direction, while making impurity control more difficult.
The study controlled 13C concentration by mixing natural-abundance methane with carbon-12 (12C) enriched methane. Secondary ion mass spectrometry (SIMS) measured isotope profiles across the diamond layers, while nitrogen was below its 20 ppb detection limit. Researchers inferred tighter nitrogen bounds from reactor leak rates and nitrogen-incorporation efficiency.
After diamond growth, dual-beam focused ion beam (FIB) milling was used on the Fukuoka sample to fabricate solid-immersion lenses directly into the diamond surface, improving photon-collection efficiency.
Direct laser lithography was used to pattern gold microwave striplines for electron-spin control. The NV centers were characterized using Hahn echo and Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling pulse sequences to measure electron spin coherence.
A real-time feedforward compensation system was also developed to reduce external 50 Hz magnetic noise from the electrical mains.
Coherence and noise control
This research reported what the authors describe as the longest coherence time for a single electron spin in a solid. Researchers inferred nitrogen concentrations at low parts-per-billion (ppb) levels rather than measuring them directly with SIMS.
The lowest-13C diamond layers reached a concentration of 0.0013%, while the Kagawa sample had a lower-bound nitrogen estimate of 0.1-0.5 ppb, with an upper bound below 3.2 ppb. The 11.2-second CPMG measurement was performed on an NV in the Fukuoka sample.
These low impurity levels reduced important sources of nuclear-spin and charge noise that can adversely affect NV-center coherence.
Initial Hahn echo measurements indicated electron spin coherence times of about 2 ms in the presence of 50 Hz magnetic noise. By synchronizing measurements with the mains frequency and applying real-time feedforward compensation, the Hahn-echo coherence time increased to 6.8(1) ms.
Separate CPMG measurements did not use that feedforward correction; researchers timed the sequences to avoid 50 Hz noise at lower pulse counts and used up to 24,000 microwave pulses, extending the electron-spin coherence time to 11.2(8) seconds. Above roughly 1,000 pulses, 50 Hz interference was no longer the limiting noise source.
The diamond also maintained narrow optical transitions, with measurements yielding a near-lifetime-limited homogeneous optical linewidth of 16.9(4) MHz after charge-resonance selection. Measurements incorporating 515 nm repumping yielded a broader, inhomogeneous linewidth of 222(3) MHz, reflecting charge-induced shifts in the optical transition frequency.
The analysis found that spectral diffusion, meaning changes in the optical transition frequency over time, was mainly laser-induced, while no single microscopic defect was identified.
The authors suggest vacancy- or hydrogen-related defects under 637 nm light, with substitutional nitrogen contributing under 515 nm illumination.
Implications for quantum networks and sensing
The engineered diamond material could support future technologies requiring long electron-spin coherence and coherent optical transitions. In distributed quantum computing and quantum communication systems, long-lived electron spin states could enable higher-fidelity qubit operations and quantum network protocols.
That result was obtained in a single-NV experiment under dynamical decoupling rather than in a full quantum-network implementation, so device-level testing will be necessary to assess performance in networked quantum systems.
The low concentrations of impurities are also relevant to quantum sensing. NV centers can detect magnetic and electric fields, with lower internal noise extending coherence and measurement times. These properties may improve magnetic and electric-field sensing applications.
Future directions in diamond-based quantum systems
The study demonstrated that homoepitaxial growth of isotopically engineered (111)-oriented diamonds can provide long electron-spin coherence alongside coherent optical transitions.
Feedforward compensation improved Hahn-echo performance, while the CPMG measurements used tailored pulse timing rather than the real-time feedforward correction.
The material maintained a near-lifetime-limited optical linewidth of 16.9(4) MHz, while spectral diffusion remained higher than that of a comparable NV center in a non-isotopically purified (100)-grown diamond.
Future work could focus on reducing charge-active defects and external magnetic interference, refining growth conditions, and testing the material in integrated quantum devices.
The preferred 13C concentration depends on the application, as a lower 13C concentration can increase coherence time while reducing the number of nuclear spins available as qubits.
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