Recent research conducted by scientists at the National University of Singapore (NUS) has revealed a groundbreaking discovery regarding the generation of entangled photon pairs. The study highlights the significant role that excitonic resonances play in enhancing the efficiency of generating these entangled photons, ultimately leading to the development of ultra-thin quantum light sources.
Quantum entanglement serves as a fundamental principle in various quantum technologies. It refers to a phenomenon where the properties of two quantum particles become interconnected, even when they are separated by vast distances. In the case of entangled photons, which are weightless particles of light, they are typically produced through a process called spontaneous parametric down-conversion (SPDC) by exposing specific types of crystals known as non-linear optical crystals to a light source referred to as the “pump” beam. However, it is worth noting that SPDC is inherently considered as a rather inefficient process.
The research team, led by Associate Professor Su Ying Quek from the Department of Physics at NUS, discovered that the efficiency of SPDC can be significantly improved by harnessing the interactions between excitons in non-linear optical crystals. Excitonic interactions arise from the interplay between negative and positive charges produced when light interacts with the crystal. These excitons, which consist of pairs of opposite charges, emerge as the fundamental excitations of the crystal, playing a crucial role in boosting SPDC efficiency.
In their study, the team utilized fully quantum mechanical calculations to analyze the non-linear optical response of crystals to incident light, taking into account excitonic effects. The lead author of the study, Dr. Fengyuan Xuan, explained that the increased proximity of the opposite charges within the crystal leads to a higher probability of transitions between fundamental excitations, ultimately enhancing SPDC efficiency. This effect was clearly demonstrated when comparing the team’s results with more traditional treatments that neglect the interaction between negative and positive charges.
One notable finding of the research is the potential use of ultrathin crystals to overcome technical challenges associated with SPDC, such as the phase matching problem. While ultrathin crystals were typically avoided for SPDC due to the perception of reduced efficiency with material volume, the team discovered that stronger excitonic interactions in these crystals can counteract this effect. This makes ultrathin crystals a promising candidate for producing entangled photons, offering a viable solution for next-generation quantum-photonic devices.
By applying their theoretical approach to NbOI2, a layered non-linear optical material, the team was able to study both SPDC and second harmonic generation (SHG), the reverse process of SPDC. They found that excitonic enhancement was particularly pronounced when the frequency of the “pump” beam closely matched an excitation frequency in the crystal. Additionally, the team demonstrated that SPDC efficiency could be further improved if one of the entangled photons shared a frequency matching another excitation frequency in the crystal. These discoveries open up new avenues for generating entangled photons using ultrathin materials, facilitating their integration into hybrid quantum-photonic platforms for the advancement of future devices.