Semiconductor quantum dots have emerged as leading candidates for efficient, on-demand single-photon sources, a critical component in quantum communication, computing, and cryptography. Their atom-like discrete energy levels enable the generation of photons one at a time, exhibiting quantum mechanical properties essential for photonic quantum technologies. This article examines the fundamental characteristics that make quantum dots suitable for this role, focusing on antibunching behavior, emission purity, and photon indistinguishability, followed by their integration with photonic platforms.
A defining feature of quantum dots as single-photon emitters is their antibunching behavior, demonstrated through second-order correlation function measurements. When excited optically or electrically, a single quantum dot emits only one photon per recombination event due to its discrete energy structure. The g(2)(0) value, measured using a Hanbury Brown-Twiss interferometer, quantifies this property. For an ideal single-photon source, g(2)(0) approaches zero, indicating negligible probability of simultaneous photon emission. Experimental studies report g(2)(0) values below 0.01 for high-quality quantum dots at low temperatures, confirming their ability to suppress multi-photon events. This characteristic is crucial for quantum key distribution protocols, where multi-photon emissions could enable eavesdropping.
The purity of single-photon emission, characterized by the linewidth of the zero-phonon line, determines the temporal coherence of emitted photons. For quantum dots, inhomogeneous broadening caused by charge fluctuations in the surrounding matrix and homogeneous broadening due to electron-phonon interactions typically limit the linewidth. Cryogenic temperatures reduce phonon-induced decoherence, with some systems achieving linewidths approaching the transform limit. Spectral diffusion, caused by charge trapping in nearby defects, remains a challenge, though techniques such as resonant excitation and surface passivation have shown improvements. Studies demonstrate linewidths as narrow as 1-10 µeV for isolated quantum dots under optimized conditions, approaching the theoretical limit set by the radiative lifetime.
Photon indistinguishability is another critical metric for quantum interference applications such as linear optical quantum computing. Two photons are indistinguishable if their spectral, temporal, and polarization properties are identical. Quantum dots can generate indistinguishable photons when excited resonantly, avoiding relaxation through multiple energy levels that introduce timing jitter. The Hong-Ou-Mandel interference experiment quantifies indistinguishability through photon bunching probability. State-of-the-art quantum dots exhibit two-photon interference visibilities exceeding 90%, with some systems reaching 99% under pulsed resonant excitation. The remaining distinguishability often stems from fine structure splitting in the excitonic states, which can be mitigated through strain engineering or applied electric fields.
Integrating quantum dots with photonic structures enhances their performance as single-photon sources by improving collection efficiency and enabling additional functionality. Micropillar cavities with distributed Bragg reflectors increase the emission rate through the Purcell effect while directing light into a well-defined optical mode. Experiments show Purcell factors of 5-10 for quantum dots in such cavities, significantly boosting brightness. Photonic crystal waveguides offer another platform, providing directional emission and facilitating on-chip photon routing. Recent advances demonstrate coupling efficiencies exceeding 90% between quantum dots and waveguide modes. More complex photonic circuits incorporate quantum dots with beam splitters, phase shifters, and detectors to enable integrated quantum photonic processing.
Coupling quantum dots to optical resonators also improves photon indistinguishability by reducing the radiative lifetime and associated timing uncertainty. The Purcell enhancement shortens the emission time window, decreasing sensitivity to environmental fluctuations. However, maintaining spectral alignment between the quantum dot and cavity remains challenging due to fabrication imperfections and spectral diffusion. Active tuning methods using temperature, strain, or electric fields help maintain resonance, with some systems achieving stable operation over hours. Hybrid approaches combining photonic structures with electrical contacts enable deterministic triggering of single-photon emission, essential for scalable quantum networks.
Material systems for quantum dot single-photon sources include III-V semiconductors such as InAs/GaAs and II-VI compounds like CdSe/ZnS. Each offers distinct advantages in terms of emission wavelength, stability, and compatibility with photonic integration. InAs quantum dots grown by molecular beam epitaxy emit in the near-infrared, compatible with fiber optic networks, while CdSe dots provide visible emission for free-space applications. Emerging materials such as perovskite quantum dots show promise due to their high oscillator strengths and solution processability, though their stability under continuous operation requires further improvement.
Scalability remains a key challenge for deploying quantum dot single-photon sources in practical quantum technologies. While individual devices demonstrate excellent performance, producing large arrays of identical sources with uniform properties is difficult due to inherent variations in quantum dot size and composition. Advances in site-controlled growth techniques and post-fabrication tuning methods are addressing this issue. Another direction involves hybrid integration, where pre-selected quantum dots are transferred to optimized photonic circuits using pick-and-place techniques or optical trapping.
The operating temperature of quantum dot single-photon sources presents another practical consideration. While most high-performance devices require cryogenic cooling, recent progress in wide-bandgap materials and robust photonic designs has enabled operation at higher temperatures, with some systems functioning at liquid nitrogen temperatures or above. This reduces the complexity and cost of deployment in real-world applications.
Future developments will likely focus on improving the brightness, indistinguishability, and operating conditions of quantum dot single-photon sources while maintaining their excellent antibunching properties. Combining these sources with integrated photonic circuits and quantum memories could enable fully functional quantum networks. The unique combination of quantum optical properties and solid-state practicality positions quantum dots as a versatile platform for advancing quantum technologies.