Single-photon emitters and spin-photon interfaces in silicon

Abstract

Single photons enable the distribution of quantum information over large distances and thus play a major role in quantum technologies such as communication and computing. Solid-state emitters are practical and efficient sources of single photons that can be manufactured in large numbers. When combined with a spin, the resulting spin-photon interfaces can store quantum states for extended periods and serve as the basis for quantum networks and repeaters. Among the many host materials explored over the past few decades, silicon stands out for its advanced nanofabrication, the maturity of its integrated photonics and microelectronics, and its high isotopic purity, which leads to exceptionally long spin coherence. These properties position silicon single-photon emitters and spin-photon interfaces among the most promising hardware platforms for implementing quantum networks and distributed quantum information processors. This review summarizes the current state of the art and open challenges towards coherent single-photon sources and scalable spin-photon interfaces based on color centers and erbium dopants in nanophotonic silicon structures.

Figures (15)

• Figure 1: Single-photon source and spin-photon interface. (a) A single-photon source is realized by embedding a single emitter (orange) in a solid-state host (grey box and zoom-in to the silicon lattice). After excitation, the system emits light (curly arrows) via optical decay to the ground state. In an ideal system, all emissions occur into a well-defined mode of the electromagnetic field, which is then coupled to a detection setup (bottom), e.g., via optical fibers (grey). The single-photon nature of the field is verified by autocorrelation measurements (inset) in a Hanbury-Brown and Twiss setup. However, real emitters in bulk crystals will emit at multiple frequencies (red and blue) and modes (light blue). This limitation can be overcome by embedding the emitters into nanostructures. (b) In a spin-photon interface, the emitters have several ground-state spin levels, e.g., $\ket{\uparrow}$ and $\ket{\downarrow}$, to facilitate long-term storage of quantum information. Ideally, following excitation, the emitters will only decay radiatively to a single ground state in a mode that is efficiently collected (dark blue wavy arrow). In real devices, some photons may be lost (light blue), and the excited state may decay non-radiatively (grey arrow). Furthermore, the emitter may optically decay to other energy levels than its ground state $\ket{g}$, including other orbital states $\ket{i}$ or states with excited phonon modes $\ket{q}$. This leads to the emission of photons at different frequencies (red curly arrows).
• Figure 2: Single-photon emitters embedded in nanophotonic silicon devices. (a) The most prominent color centers are named by letters (G, T, C and W centers). They are formed by specific arrangements of lattice impurities such as Carbon (C), Oxygen (O), and Hydrogen (H), as well as interstitial Silicon (Si) atoms. Their energy levels have a significant contribution from the orbitals of the surrounding lattice sites. In contrast, Erbium dopants, typically in their trivalent charge state ($\text{Er}^{3+}$), exhibit optical transitions between energy levels in their partially filled inner 4f-shell, making them less sensitive to lattice perturbations. (b) The emission wavelength of the emitters shown in (a) falls within the major telecommunication bands (O-, E-, S-, C-, and L-band; colors). Here, the transmission of light through low-loss optical fibers can still approach $10\%$ after 100km (grey data from petrovich_broadband_2025). (c) The optical stability of the energy levels, and thus the coherence of the emitted photons, is limited by various sources of decoherence. Charge traps at the surface and in bulk can lead to a fluctuating charge environment. In addition, fluctuating magnetic fields can arise from paramagnetic impurities and the nuclear spin bath. Finally, coupled local and extended phonon modes, as well as external vibrations, may also reduce the coherence.
• Figure 3: Level structure of erbium dopants in silicon. Erbium exhibits eight crystal field levels ($Z_1,...Z_8$) in the ground state manifold ($^{4}I_{15/2}$) and seven ($Y_1,...Y_7$) in the excited state manifold ($^{4}I_{13/2}$). The levels are further split under an external magnetic field, forming an effective spin-1/2 system in the lowest crystal-field levels of each manifold. In addition to the electronic spin, the isotope $^{167}\mathrm{Er}$ exhibits a nuclear spin of 7/2, whose levels are split on the order of GHz via the hyperfine interaction. All levels can also be further split by the superhyperfine interaction with the nuclear spins surrounding the Er dopants (not shown). The optical transitions are indicated with arrows; one often distinguishes electron-spin-preserving (blue/green solid) and spin-flip (blue dashed) transitions. The nuclear-spin-flip transitions are not shown for simplicity.
• Figure 4: Crystal field and magnetic properties of Er:Si in site A. (a) Crystal field spectroscopy of site A at low temperature (2K). (top) A narrow filter is set to the $Y_1\rightarrow Z_1$ transition, and the excitation laser is swept to measure the excited-state crystal-field levels. (bottom) The excitation laser is fixed at the $Z_1 \rightarrow Y_2$ transition, and the narrow filter is swept, revealing the emission to the lowest six ground-state levels. Adapted from gritsch_narrow_2022. (b) Spin properties. The sample is rotated in a magnetic field of 1.9T such that the field angle varies from $[001]$ to $[1\bar{1}0]$ and the spin-preserving and spin-flip transitions are measured. The large number of lines indicates low site symmetry, which is determined to be $C_{2v}$ from a fit (solid and dashed lines, shown only for one half of the symmetric spectrum) to the data. (c) Sketch of the silicon unit cell with exemplary positions of the Er dopants according to the extracted site symmetries of $C_{2v}$ (site A) and $C_{s}$ (site B). Thus, site A is located on the two-fold rotational axis (red dotted line) that passes through the unit cell center, and site B on the $\{110\}$ mirror plane (shaded area). Adapted from holzapfel_characterization_2025.
• Figure 5: Basic properties of the T center. (a) Energy levels of the ground- and the two BE excited states, (b) which split under a magnetic field, giving rise to four main optical transitions (A-D) associated with the electron and hole spins, and (c) hyperfine transitions associated with nuclear spins. (d) Atomic configuration, showing the arrangement of the two C atoms and the H atom forming the defect. (e) Excited state lifetime measurement of the TX$_0$ transition. Images adapted from bergeron_silicon-integrated_2020.