Two-dimensional materials have emerged as a promising platform for quantum photonics due to their unique electronic and optical properties. Among these properties, second-harmonic generation and spontaneous parametric down-conversion play a critical role in the development of quantum light sources. The atomic-scale thickness and strong light-matter interactions in 2D materials enable efficient nonlinear optical processes, making them suitable for integrated quantum photonic circuits.

Second-harmonic generation in 2D materials arises from the broken inversion symmetry in certain crystal structures, such as monolayer transition metal dichalcogenides. The nonlinear susceptibility in these materials is significantly enhanced compared to bulk semiconductors due to quantum confinement effects. Studies have shown that monolayer MoS2 exhibits a second-order nonlinear susceptibility approximately three orders of magnitude higher than traditional nonlinear crystals like lithium niobate. The layer-dependent nature of this effect is crucial, as odd-layer TMDCs retain non-centrosymmetricity, while even-layer structures exhibit suppressed SHG due to restored inversion symmetry.

Spontaneous parametric down-conversion in 2D materials provides a pathway for generating entangled photon pairs, a fundamental resource for quantum communication and computing. The efficiency of SPDC is influenced by the material’s nonlinear coefficients and phase-matching conditions. Unlike bulk nonlinear crystals, 2D materials do not require stringent phase matching due to their ultra-thin geometry, simplifying integration into photonic devices. Recent experiments have demonstrated SPDC in monolayer WS2, with photon pair generation rates competitive with conventional nonlinear media. The spectral tunability of SPDC in 2D materials further enhances their utility, as the bandgap can be engineered via strain, doping, or heterostructuring.

The thickness dependence of nonlinear optical processes in 2D materials is a critical factor for device design. While monolayer materials exhibit the strongest nonlinear response, multilayer stacks can be engineered to balance efficiency and light absorption. For instance, bilayer MoSe2 shows a reduction in SHG intensity by a factor of four compared to its monolayer counterpart, but stacking with alternating twist angles can recover some nonlinear enhancement through interlayer coupling. The trade-off between thickness and nonlinear efficiency must be carefully considered when designing quantum light sources for specific applications.

Integration of 2D materials with silicon photonics presents both opportunities and challenges. Silicon waveguides and resonators can enhance light-matter interaction in 2D materials, boosting nonlinear conversion efficiencies. Heterogeneous integration techniques, such as direct transfer or van der Waals bonding, have been employed to couple TMDCs with silicon nitride microring resonators. Experimental results indicate a tenfold increase in SHG intensity when monolayer WSe2 is coupled to a high-Q photonic cavity compared to free-space excitation. However, interfacial defects and strain effects can degrade performance, necessitating precise fabrication control.

For quantum applications, the purity and indistinguishability of generated photons are paramount. 2D materials offer advantages in this regard due to their narrow excitonic linewidths at cryogenic temperatures. Measurements on hBN-encapsulated MoTe2 reveal single-photon emission with a purity exceeding 99% and indistinguishability of over 90%, meeting the requirements for scalable quantum networks. The deterministic positioning of quantum emitters in 2D materials via strain engineering or defect creation further enhances their potential for integrated quantum photonics.

The scalability of 2D material-based quantum light sources remains an active area of research. Wafer-scale growth techniques, such as metal-organic chemical vapor deposition, have achieved uniform monolayer coverage with less than 5% thickness variation across 4-inch substrates. This progress in material synthesis is critical for transitioning from laboratory demonstrations to practical devices. Additionally, the compatibility of 2D materials with CMOS processing steps enables co-integration with classical electronic circuits, a necessary feature for hybrid quantum-classical systems.

Environmental stability is another consideration for practical deployment. While some 2D materials degrade under ambient conditions, encapsulation strategies using hBN or Al2O3 have demonstrated long-term stability with minimal degradation in nonlinear optical performance after six months in air. Passivation layers must be optimized to preserve quantum emitter properties while providing sufficient protection from oxidation and contamination.

The unique properties of 2D materials also enable novel device architectures for quantum light generation. For example, patterned strain engineering can create arrays of identical quantum emitters with controlled spacing, a capability not easily achieved in traditional semiconductor systems. Similarly, the gate-tunability of excitonic states in 2D materials allows for dynamic control of photon emission wavelengths, enabling reconfigurable quantum light sources without requiring external nonlinear optical components.

Future developments in this field will likely focus on improving the efficiency and scalability of 2D material-based quantum light sources. Advances in material quality, device integration, and quantum emitter control will determine their viability for large-scale quantum photonic circuits. The combination of strong nonlinearities, tunable optical properties, and compatibility with existing photonic platforms positions 2D materials as a key enabler for next-generation quantum technologies.

The exploration of second-harmonic generation and spontaneous parametric down-conversion in 2D materials has opened new avenues for compact, efficient quantum light sources. As research progresses, these materials may overcome current limitations in brightness, stability, and scalability, ultimately enabling practical quantum photonic devices for communication, computation, and sensing applications. The integration with silicon photonics provides a clear pathway toward chip-scale quantum systems, bridging the gap between fundamental research and technological implementation.