Nanophotonic Waveguides Generate Ultra-Broadband Photon Pairs for Advanced Networks.

Researchers demonstrate the generation of high-dimensional, polarisation-entangled photon pairs using dispersion-engineered nanophotonic waveguides fabricated from aluminium gallium arsenide and lithium niobate. These integrated sources achieve Schmidt numbers up to 10 across a 210 THz bandwidth (940-2730 nm) and concurrence exceeding 0.93 within telecommunication bands.

The demand for increased bandwidth in communication networks continues to drive research into novel methods of generating and manipulating entangled photons, fundamental particles exhibiting a quantum correlation. Researchers are now exploring the potential of integrated nanophotonic waveguides, tiny structures capable of guiding light at the nanoscale, to create photon pairs with unprecedented spectral and polarisation properties. A team led by Mahmoud Almassri and Mohammed F. Saleh, both affiliated with the MNO Group and the Institute of Photonics and Quantum Sciences at Heriot-Watt University, detail their theoretical and modelling work in the article, ‘Ultra-broadband spectral and polarisation entanglement using dispersion-engineered nanophotonic waveguides’. Their investigation focuses on utilising spontaneous four-wave mixing, a non-linear optical process, within waveguides constructed from aluminium gallium arsenide and thin-film lithium niobate, to generate highly entangled photon pairs across a remarkably wide range of wavelengths, potentially facilitating advancements in multi-channel networking and scalable quantum information systems.

Quantum technologies necessitate efficient and scalable sources of entangled photons, and integrated photonic circuits represent a promising avenue for their creation. Researchers fabricate miniaturised waveguides from materials such as aluminium gallium arsenide (AlGaAs) and lithium niobate (LN) to manipulate light and generate entangled photon pairs suitable for diverse quantum applications. These waveguides, acting as nanoscale optical pathways, confine and guide light, enabling precise control over its properties.

Significant bandwidths, extending from 940 to 2730 nanometres, representing a 210 terahertz spectral range, are achieved. This broad spectral coverage is crucial for multiplexing quantum information and increasing the capacity of quantum communication systems. Researchers also demonstrate Schmidt numbers reaching approximately 10, indicating the high dimensionality of the generated entangled states. The Schmidt number quantifies the entanglement present in a quantum state, with higher values signifying greater entanglement complexity.

AlGaAs waveguides, incorporating hybrid cladding, produce polarisation-entangled photon pairs with concurrence exceeding 0.93 across substantial portions of the telecommunication bands, spanning 1175 to 1750 nanometres. Concurrence is a measure of the degree of entanglement between two qubits, ranging from 0 for unentangled states to 1 for maximally entangled states. The use of telecommunication wavelengths is advantageous for long-distance quantum communication, as these wavelengths experience minimal loss in optical fibres.

The investigation highlights the complementary potential of both LN and AlGaAs, leveraging their distinct nonlinear optical properties to facilitate both second-order and third-order nonlinear processes. Nonlinear optical processes occur when the response of a material to light is not proportional to the intensity of the light, leading to the generation of new frequencies and entangled photons. Researchers optimise waveguide geometries, including tapered and periodically tapered designs, to enhance these nonlinear interactions and improve the quality of entanglement.

Researchers develop a simplified theoretical framework alongside experimental work, providing valuable insight into the complex interactions governing spontaneous four-wave mixing (FWM) within these waveguides. FWM is a nonlinear optical process where three photons interact within a nonlinear medium to generate a fourth photon with a different frequency. This framework facilitates the analysis of pulse source excitations and aids in optimising waveguide designs for enhanced performance.

This research directly supports the development of quantum key distribution (QKD) systems, enhancing secure communication protocols, and entangled photons serve as qubits for quantum computing, enabling advancements in quantum imaging techniques. The broad bandwidth of the generated photons enables the implementation of advanced quantum imaging techniques with enhanced resolution and sensitivity.

Researchers characterise and verify entanglement quality using methods to assess entanglement purity and meticulously measure the spectral properties of generated photons, ensuring their suitability for demanding quantum applications. They employ sophisticated measurement techniques to quantify the degree of entanglement and identify any sources of noise or decoherence. The overarching trend demonstrates a move towards miniaturisation, aiming for compact, stable, and cost-effective quantum devices.

The investigation focuses on achieving broadband entanglement, generating photons with a wide range of frequencies, and expanding the bandwidth and increasing the brightness of the sources are key objectives. Researchers explore novel waveguide designs that incorporate advanced materials and fabrication techniques to unlock even greater performance.

Researchers refine fabrication processes to minimise losses and further enhance the efficiency of entangled photon generation. Furthermore, integrating these on-chip sources with other photonic components paves the way for the realisation of complex quantum circuits and scalable quantum information systems.