Microscopic Study of Optically Stable Coherent Color Centers in Diamond Generated by High-Temperature Annealing

Single color centers in solid have emerged as promising physical platforms for quantum information science. Creating these centers with excellent quantum properties is a key foundation for further technological developments. In particular, the microscopic understanding of the spin-bath environments is the key to engineer color centers for quantum control. In this work, we propose and demonstrate a distinct high-temperature annealing (HTA) approach for creating high-quality nitrogen vacancy (N-V) centers in implantation-free diamonds. Simultaneously using the created N-V centers as probes for their local environment we verify that no damage is microscopically induced by the HTA. Nearly all single N-V centers created in ultralow-nitrogen-concentration membranes possess stable and Fourier-transform-limited optical spectra. Furthermore, HTA strongly reduces noise sources naturally grown in ensemble samples, and leads to more than threefold improvements of decoherence time and sensitivity. We also verify that the vacancy activation and defect reformation, especially H3 and P1 centers, can explain the reconfiguration between spin baths and color centers. This distinct approach will become a powerful tool in vacancy-based quantum technology.

Figure 1

The protocol of the HTA method and the N-V center creations in different samples. (a) The time sequence of HTA. The peak temperature of $1700{}^{\circ }\mathrm{C}$ is held for 30 min in a high-temperature furnace under a vacuum. (b) A confocal image ($y$-$x$ scan) of the ultralow [N] sample PPB-B after annealing, revealing noticeable single N-V center creations. (c) A confocal image ($y$-$x$ scan) of raw and annealed sample HPHT100 with fixed measurement parameters. Ensemble N-V centers are created in this sample. (d) A confocal image ($z$-$x$ scan) of raw and annealed sample CVD1 with fixed measurement parameters. The color scale denotes the PL counts, which provide a fair comparison between the same sample before and after annealing. (e) The increase in PL of ensemble samples CVD1 and HPHT100 after the annealing treatment. Normalized optical spectra of sample CVD1 under 520-nm excitation are shown in the inset.

Figure 2

Optical properties of created N-V centers in a 20-$\mu \mathrm{m}$-thick diamond membrane. (a) Confocal image of the cross section of the membrane. The $z$ axis marks the appearance depth (the correction fact due to the effective numerical aperture of the objective and refractive index of diamond is around 2.7). (b) A PLE spectrum showing the six sublevels of optical excited states. (c) An enlargement of the spectrum line, which has the Fourier-transform-limited linewidth of 24 MHz. (d) 40 rounds of repetitive PLE scan. This confirms the spectral stability of the sample.

Figure 3

The universal enhancement of electron spin properties in different types of samples. (a),(b) The FID and Hahn echo signals from a single N-V center created by this method in diamond sample PPB-B. (c) The $T_2$ measurements of sample HPHT100 before and after annealing in the same sample spot [spot IV in (d)]. (d) The summary of $T_2$ time of the ensemble samples CVD1 and HPHT100, providing a direct comparison of the raw and annealed data. Inset summarizes the enhancement of the $T_2$ time. The label I corresponds to a randomly chosen spot in sample CVD1. The labels (II, III, and IV) correspond to three randomly chosen spots in sample HPHT100. For sample PPB-B, the external magnetic field strength is $546\mathrm{G}$. For sample CVD1, the external magnetic field strengths are $80\mathrm{G}$ before the annealing and $79\mathrm{G}$ after annealing. For sample HPHT100, the external magnetic field strengths are $49\mathrm{G}$.

Figure 4

The change in the spin bath after HTA and the related mechanism. (a) P1 concentration of all samples. Inset shows a set of DEER spectra of sample HPHT100 before and after annealing. An explicit decrease in contrast is observed. (b) Optical emission spectra of sample CVD1 under 405-nm excitation. (c) Schematics of the combination of P1 centers and vacancy into N-V center and $NV{N}^0\mathrm{or}NV{N}^{-}$.