Scientists demonstrate that excitonic interactions improve the efficiency of entangled photon generation

Scientists at the National University of Singapore (NUS) have shown that excitonic resonances and exciton transitions can significantly increase the efficiency of entangled photon pair generation. This could lead to the development of efficient ultrathin quantum light sources.

Quantum entanglement is the cornerstone of many quantum technologies. In simple terms, it describes a phenomenon in which the properties of two quantum particles are linked, even when they are very far apart.

Entangled photons, which are massless particles of light, are typically generated by a light beam (called a “pump beam”) onto certain types of crystals called nonlinear optical crystals, through a process called spontaneous parametric down-conversion (SPDC). However, SPDC is inherently a rather inefficient process.

The research team, led by Associate Professor Su Ying Quek from the NUS Department of Physics, showed that the efficiency of SPDC can be improved by exploiting the many-body excitonic interactions present in the nonlinear optical crystal.

These excitonic interactions occur between negative and positive charges that are created when light interacts with the crystal. Called excitons, these pairs of opposite charges come from the crystal’s fundamental excitations. The team showed that when these charges are closer together, the efficiency of the SPDC increases dramatically, depending on the energy or frequency of the light.

The research results were published in the journal Physical Exam Letters.

These predictions were made using fully quantum mechanical calculations to analyze the nonlinear optical response of crystals to incident light and to account for excitonic effects.

Dr Fengyuan Xuan, lead author of the work, explains: “SPDC is fundamentally a nonlinear optical process that involves transitions between fundamental excitations in the crystal. The probability of these transitions increases when the opposing charges due to the excitations in the crystal are located closer to each other.

“This effect became evident when our results were compared to a more conventional treatment that neglects the interaction between negative and positive charges.”

Professor Quek said: “Using ultrathin crystals can eliminate a technical challenge associated with SPDC, known as the phase-matching problem. Although ultrathin crystals are generally avoided for SPDC because their efficiency was thought to decrease with the volume of the material, the stronger excitonic interactions in these ultrathin crystals can mitigate this effect. This makes ultrathin crystals a viable source for producing entangled photons.”

The team applied the theoretical approach to the NbOI₂a layered nonlinear optical material, to study both SPDC and second harmonic generation (SHG), the inverse process of SPDC. They simulated SHG intensities as a function of the polarization angle of the incident light and found that these simulations agreed well with previously published experimental work.

They also found that excitonic amplification is particularly strong when the frequency of the “pumping” beam closely matches an excitation frequency in the crystal. Moreover, SPDC can be further enhanced if one of the entangled photons has a frequency that matches another excitation frequency in the crystal.

“These findings pave the way for the generation of entangled photons using ultrathin materials, which can be more easily integrated into hybrid quantum-photonic platforms for next-generation devices,” added Professor Quek.