Single-photon sources are a critical component in quantum technologies, enabling applications such as quantum cryptography, quantum communication, and photonic quantum computing. Among the various candidates for single-photon emitters, quantum dots (QDs) stand out due to their high brightness, tunable emission wavelengths, and potential for on-chip integration. This article explores the use of quantum dots as single-photon sources, focusing on Purcell enhancement, photon indistinguishability, resonant excitation methods, and their integration with photonic cavities and waveguides.

Quantum dots are nanoscale semiconductor structures that confine charge carriers in all three spatial dimensions, leading to discrete energy levels analogous to those of atoms. This atomic-like behavior makes them excellent candidates for generating single photons on demand. When an electron-hole pair (exciton) recombines in a QD, it emits a single photon with high efficiency. However, achieving optimal performance requires careful engineering of the QD environment to enhance emission properties and ensure high photon purity.

Purcell enhancement is a key mechanism for improving the emission rate and directionality of QD-based single-photon sources. The Purcell effect arises when a QD is coupled to a photonic cavity, which modifies the local density of optical states and increases the spontaneous emission rate. The Purcell factor, given by F = (3/4π²)(λ³/n³)(Q/V), where λ is the emission wavelength, n is the refractive index, Q is the cavity quality factor, and V is the mode volume, quantifies this enhancement. High-Q cavities with small mode volumes can significantly boost the emission rate, reducing the radiative lifetime and improving photon extraction efficiency. For instance, QDs embedded in photonic crystal cavities have demonstrated Purcell factors exceeding 10, leading to enhanced brightness and reduced multiphoton emission probabilities.

Photon indistinguishability is another critical metric for quantum technologies, particularly in applications requiring quantum interference, such as linear optical quantum computing. Indistinguishable photons must exhibit identical spectral, temporal, and spatial properties. QDs can generate highly indistinguishable photons under resonant excitation, where the laser energy matches the QD transition energy. Resonant excitation avoids charge fluctuations and spectral diffusion caused by non-resonant pumping, which can degrade photon coherence. Studies have shown that resonantly excited QDs can achieve indistinguishability values above 95%, making them competitive with other single-photon sources like trapped ions or defect centers in diamond.

Resonant excitation methods for QDs include strict resonant pumping, two-photon excitation, and phonon-assisted excitation. Strict resonant pumping involves direct excitation of the QD ground state to the exciton state, but this requires careful filtering to separate the excitation laser from the emitted photons. Two-photon excitation leverages a virtual intermediate state to avoid laser scattering, while phonon-assisted excitation uses acoustic phonons to bridge the energy mismatch between the laser and QD transition. Each method has trade-offs in terms of setup complexity and photon purity, with strict resonant pumping generally offering the highest indistinguishability at the cost of increased experimental difficulty.

Integration of QDs with photonic cavities and waveguides is essential for scalable quantum photonic circuits. Photonic crystal cavities, micropillars, and ring resonators are commonly used to enhance QD emission and direct photons into desired optical modes. Waveguides coupled to QDs enable on-chip routing of single photons, which is crucial for integrated quantum photonics. For example, QDs embedded in GaAs waveguides have demonstrated efficient photon coupling with propagation losses below 1 dB/cm. Additionally, hybrid integration of QDs with silicon photonics platforms has been explored to leverage existing CMOS-compatible fabrication techniques.

Challenges remain in achieving deterministic positioning of QDs within photonic structures and maintaining high performance at elevated temperatures. Advances in nanofabrication, such as in-situ electron beam lithography and strain-tuning techniques, are addressing these issues. Furthermore, the development of electrically driven QD single-photon sources could simplify integration with existing optoelectronic systems.

In summary, quantum dots are a promising platform for single-photon generation in quantum technologies. Purcell enhancement via photonic cavities boosts emission rates, resonant excitation ensures high photon indistinguishability, and integration with waveguides enables scalable photonic circuits. Continued progress in materials engineering and nanophotonics will further solidify the role of QDs in advancing quantum communication and computation.