Chiral Metasurface Boosts Entangled Photon Pair Generation and Circular Polarization

The creation of efficient sources of entangled photons remains a significant challenge in the advancement of quantum technologies, and current methods often struggle with low generation rates and limited control over photon properties. Sky Semone, Matthew Brandsema, and Christos Argyropoulos, from The Pennsylvania State University, address these limitations by demonstrating a new approach to generating entangled photon pairs. Their research introduces an ultrathin metasurface that harnesses a strong resonance known as a quasi-bound state in the continuum to dramatically increase the efficiency of the process. Importantly, this innovative design also enables the creation of circularly polarised, or chiral, entangled photons, a crucial feature missing from existing sources and one that unlocks potential applications in quantum communication and sensing.

Single-photon pairs are essential resources for numerous quantum technologies, yet current sources based on spontaneous parametric down-conversion often exhibit low efficiency and lack control over polarization. This work addresses these limitations by demonstrating a novel chiral plasmonic metasurface that leverages a strong quasi-bound state in the continuum resonance, significantly increasing single-photon pair generation efficiency and realising circular polarized photon emission.

Symmetry-Protected Bound States in Photonic Structures

Bound States in the Continuum (BICs) represent a fascinating area of research in photonics, offering unique ways to control light and matter interactions. These states are localized, existing despite being within the range of energies normally allowed for propagating waves, arising from carefully engineered symmetry or destructive interference. Real-world structures often exhibit imperfections, leading to quasi-BICs which still exhibit strong localization, though with some energy loss. Metasurfaces, artificial materials structured at the nanoscale, play a crucial role in creating the conditions necessary for BICs.

Symmetry protection and destructive interference are key mechanisms employed in their design, unlocking a wide range of potential applications. BICs offer exciting possibilities for lasing, sensing, and molecular imaging, enabling the creation of efficient lasers, highly sensitive sensors, and enhanced detection of molecules. They can also enhance nonlinear optical effects and create hyperspectral imaging systems, offering detailed spectral information. This rapidly growing field promises a new generation of photonic devices with enhanced performance and functionality.

Efficient Entangled Photon Generation via Metasurface Design

Researchers have developed a new metasurface capable of generating entangled single photons with significantly improved efficiency and unique circular polarization, representing a substantial advance in quantum photonics. This innovative device overcomes limitations of existing methods, which typically suffer from weak signal generation and lack polarization control. The newly designed metasurface utilizes specifically shaped nanoscale structures to dramatically enhance spontaneous parametric down-conversion, a technique used to create pairs of entangled photons. The key to this breakthrough lies in the metasurface’s asymmetric design, incorporating tilted silver nanorods and a carefully engineered nanocavity.

This configuration creates a quasi-bound state in the continuum resonance, effectively trapping and amplifying light within the structure, resulting in a substantial increase in photon pair generation efficiency compared to traditional materials. Importantly, the asymmetry of the nanorods introduces chirality, allowing precise control over the circular polarization of the emitted photons, a crucial feature for many quantum communication and sensing applications. The metasurface demonstrates exceptionally high circular dichroism, strongly favoring the generation of photons with a specific circular polarization. Simulations reveal that the electric field is significantly enhanced within the nanogap of the metasurface when excited with light of the preferred circular polarization, further boosting photon generation efficiency. This level of control, combined with the strong chiral response, positions this metasurface as a promising candidate for next-generation integrated quantum light sources.

Efficient Metasurface Generates Polarized Photon Pairs

This research demonstrates a new method for creating entangled single photons with enhanced efficiency and control over their polarization. The team developed a novel metasurface, an ultrathin material structured at the nanoscale, that significantly boosts the generation of photon pairs through spontaneous parametric down-conversion, enabling the creation of circularly polarized photons, a characteristic crucial for many quantum technologies. The demonstrated device achieves a record-breaking level of efficiency compared to existing methods, generating a substantial number of photon pairs per second. The metasurface leverages a unique resonance, known as a quasi-bound state in the continuum, to maximize photon pair creation and facilitate their emission into free space, offering a promising platform for developing advanced quantum communication and sensing technologies. While the current work focuses on demonstrating the principle and achieving high efficiency, the authors acknowledge that further research could explore optimizing the metasurface design for specific applications and broadening the range of achievable polarization states.