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Among the wealth of single fluorescent defects recently detected in silicon, the G center catches interest for its telecom single-photon emission that could be coupled to a metastable electron spin triplet. The G center is a unique defect where the standard Born-Oppenheimer approximation breaks down as one of its atoms can move between 6 lattice sites under optical excitation. The impact of this atomic reconfiguration on the photoluminescence properties of G centers is still largely unknown, especially in silicon-on-insulator (SOI) samples. Here, we investigate the displacement of the center-of-mass of the G center in silicon. We show that single G defects in SOI exhibit a multipolar emission and zero-phonon line fine structures with splittings up to $\sim1$ meV, both indicating a motion of the defect central atom over time. Combining polarization and spectral analysis at the single-photon level, we evidence that the reconfiguration dynamics are drastically different from the one of the unperturbed G center in bulk silicon. The SOI structure freezes the delocalization of the G defect center-of-mass and as a result, enables to isolate linearly polarized optical lines. Under above-bandgap optical excitation, the central atom of G centers in SOI behaves as if it were in a 6-slot roulette wheel, randomly alternating between localized crystal sites at each optical cycle. Comparative measurements in a bulk silicon sample and ab initio calculations highlight that strain is likely the dominant perturbation impacting the G center geometry. These results shed light on the importance of the atomic reconfiguration dynamics to understand and control the photoluminescence properties of the G center in silicon. More generally, these findings emphasize the impact of strain fluctuations inherent to SOI wafers for future quantum integrated photonics applications based on color centers in silicon.

Among the wide variety of single fluorescent defects investigated in silicon, numerous studies have focused on color centers with a zero-phonon line around $1.28 \mu$m and identified to a common carbon-complex in silicon, namely the G center. However, inconsistent estimates regarding their quantum efficiency cast doubt on the correct identification of these individual emitters. Through a comparative analysis of their single-photon emission properties, we demonstrate that these single color centers are split in two distinct families of point defects. A first family consists of the genuine single G centers with a well-identified microscopic structure and whose photoluminescence has been investigated on ensemble measurements since the 60's. The remaining defects belong to a new color center, which we will refer to as G$^{\star}$ center, whose atomic configuration has yet to be determined. These results provide a safeguard against future defect misidentifications, which is crucial for further development of quantum technologies relying on G or G$^{\star}$ center quantum properties.

Generating single photons on demand in silicon is a challenge to the scalability of silicon-on-insulator integrated quantum photonic chips. While several defects acting as artificial atoms have recently demonstrated an ability to generate antibunched single photons, practical applications require tailoring of their emission through quantum cavity effects. In this work, we perform cavity quantum electrodynamics experiments with ensembles of artificial atoms embedded in silicon-on-insulator microresonators. The emitters under study, known as W color centers, are silicon tri-interstitial defects created upon self-ion implantation and thermal annealing. The resonators consist of circular Bragg grating cavities, designed for moderate Purcell enhancement ($F_p=12.5$) and efficient luminescence extraction ($\eta_{coll}=40\%$ for a numerical aperture of 0.26) for W centers located at the mode antinode. When the resonant frequency mode of the cavity is tuned with the zero-phonon transition of the emitters at 1218 nm, we observe a 20-fold enhancement of the zero-phonon line intensity, together with a two-fold decrease of the total relaxation time in time-resolved photoluminescence experiments. Based on finite-difference time-domain simulations, we propose a detailed theoretical analysis of Purcell enhancement for an ensemble of W centers, considering the overlap between the emitters and the resonant cavity mode. We obtain a good agreement with our experimental results assuming a quantum efficiency of $65 \pm 10 \%$ for the emitters in bulk silicon. Therefore, W centers open promising perspectives for the development of on-demand sources of single photons, harnessing cavity quantum electrodynamics in silicon photonic chips.

We report the fabrication of G centers in silicon with an areal density compatible with single photon emission at optical telecommunication wavelengths. Our sample is made from a silicon-on-insulator wafer which is locally implanted with carbon ions and protons at various fluences. Decreasing the implantation fluences enables to gradually switch from large ensembles to isolated single defects, reaching areal densities of G centers down to $\sim$0.2 $\mu$m$^{-2}$. Single defect creation is demonstrated by photon antibunching in intensity-correlation experiments, thus establishing our approach as a reproducible procedure for generating single artificial atoms in silicon for quantum technologies.

Controlling the quantum properties of individual fluorescent defects in silicon is a key challenge towards advanced quantum photonic devices prone to scalability. Research efforts have so far focused on extrinsic defects based on impurities incorporated inside the silicon lattice. Here we demonstrate the detection of single intrinsic defects in silicon, which are linked to a tri-interstitial complex called W-center, with a zero-phonon line at 1.218${\mu}$m. Investigating their single-photon emission properties reveals new information about this common radiation damage center, such as its dipolar orientation and its photophysics. We also identify its microscopic structure and show that although this defect does not feature electronic states in the bandgap, Coulomb interactions lead to excitonic radiative recombination below the silicon bandgap. These results could set the stage for numerous quantum perspectives based on intrinsic luminescent defects in silicon, such as quantum integrated photonics, quantum communications and quantum sensing.