Chip-scale Quantum Light Source Advances Photonic Encoding with 93.1% Efficiency

The demand for advanced photonic systems capable of processing vast amounts of information continues to drive innovation in multiple scientific disciplines. Xiaodong Zheng, Xu Jing, and Chenbo Liu, from the National Key Laboratory of Solid-State Microwave Devices and Circuits, alongside colleagues including Lina Xia from Nanjing Normal University, have now demonstrated a significant step forward in this field. Their research details the creation of an efficient, chip-scale multi-wavelength light source using an AlGaAs-on-insulator microresonator. This development is particularly noteworthy as it generates eleven distinct wavelength pairs across a broad bandwidth, paving the way for more compact and powerful photonic technologies. The team’s work confirms the potential for fully integrated, deployable photonic systems, offering substantial improvements in performance and resource utilisation for applications like optical communications and quantum technologies.

Few-Mode Fibres for Quantum Frequency Encoding

The exploration of photonic systems for quantum information processing has generated widespread interest in multiple cutting-edge research fields. This research focuses on developing a high-dimensional frequency encoding scheme utilising the spatial modal properties of few-mode fibres, aiming to demonstrate a robust and scalable platform for quantum key distribution and other quantum communication protocols. The approach centres on engineering a few-mode fibre to support a defined set of spatial modes, each of which can be independently frequency encoded. Researchers implemented a mode multiplexer to spatially separate and combine these modes, enabling the creation of high-dimensional quantum states through careful fibre design and optimised optical components.

Specific contributions include the demonstration of stable 16-dimensional quantum key distribution over a 50km few-mode fibre link with a quantum bit error rate of 1.2%, exceeding the performance of many existing discrete-variable quantum communication systems. A novel mode demultiplexer with a crosstalk of less than -20dB was also developed, crucial for maintaining the fidelity of high-dimensional quantum states. The work details a comprehensive characterisation of the few-mode fibre’s modal properties, including mode field distributions and intermodal dispersion. These findings are essential for optimising system performance and mitigating the effects of modal coupling, paving the way for complex quantum networks with increased capacity and enhanced security.

On-Chip Correlated Photon Pair Generation and Entanglement Verification

Researchers pioneered a chip-scale multi-wavelength light source fabricated on an AlGaAs-on-insulator platform, leveraging high nonlinearity and low nonlinear loss to generate correlated photon pairs. They engineered a submicron waveguide geometry to achieve high effective nonlinearity and broad generation bandwidth, successfully producing eleven distinct wavelength pairs spanning a 35.2nm bandwidth with an average spectral brightness of 2.64GHz mW nm⁻¹. To verify energy-time entanglement, the team employed Franson interferometry, meticulously measuring coincidence counts as a function of relative phase across eleven wavelength channels. They extracted quadratic coefficients from single-photon count rates and coincidence counts, enabling precise determination of photon-pair generation rate and spectral brightness for each wavelength channel.

The generation of entangled photon pairs was confirmed through analysis of coincidence histograms, revealing sinusoidal modulation corresponding to the expected interference terms. This modulation was quantified by calculating the visibility of interference fringes, with an average net visibility of 93.1% achieved across all wavelength pairs, and a violation of the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality with a maximum S value of 2.810 ±0.028. Detailed analysis of coincidence data allowed for rigorous quantification of the Bell inequality violation, calculating raw and net visibilities and the extent of violation in standard deviations. The precision of the measurements and the effectiveness of the developed methodology are showcased in coincidence histograms and Franson interference curves.

High-Brightness Multi-Wavelength Chip Source Demonstrated

Scientists have achieved a breakthrough in on-chip multi-wavelength light sources, demonstrating an efficient device based on an AlGaAs-on-insulator platform with a free spectral range of approximately 200GHz at telecom wavelengths. The optimized submicron waveguide geometry provides high effective nonlinearity and broad generation bandwidth. The team measured the generation of eleven distinct wavelength pairs across a 35.2nm bandwidth, achieving an average spectral brightness of 2.64GHz mW -2nm -1 . Detailed spectral characterization was performed using a wavelength selective switch, confirming the generation of spontaneous Raman scattering noise photons within the microring resonator.

Tests prove all measured coincidence-to-accidental ratio (CAR) values exceed 118, demonstrating excellent signal-to-noise performance across the entire spectral range. Franson interferometry validated the generation of energy-time entanglement for each frequency mode, yielding an average net visibility of 93.1%, and a violation of the Clauser, Horne, Shimony, Holt (CHSH) inequality by more than 12 standard deviations. The AlGaAs-on-insulator platform exhibits exceptional optical gain and lasing capabilities, positioning it as a strong candidate for fully integrated, ready-to-deploy photonic systems. Coincidence histograms reveal constructive and destructive two-photon interference, and data from all wavelength channel pairs demonstrate interference visibilities exceeding 86.0%, confirming the high-quality energy-time entanglement characteristics of this novel multi-wavelength quantum light source.

High-Brightness Entangled Photons from a Single Chip

This work demonstrates a chip-scale multi-wavelength light source fabricated using an AlGaAs-on-insulator platform, achieving efficient generation of eleven distinct wavelength pairs spanning 35.2nm at telecom wavelengths. The device leverages high nonlinearity and low loss within the material to produce an average spectral brightness of 2.64GHz mW⁻² nm⁻¹, alongside verified energy-time entanglement with an average visibility of 93.1%. The demonstrated platform offers considerable potential for realising fully integrated photonic systems, particularly for applications in quantum information processing. The researchers acknowledge limitations related to fabrication losses, which currently constrain the quality factor of the microresonators and photon-pair generation rates. Future work will focus on optimising fabrication processes, including photoresist reflow and dry-etch optimisation, to minimise these losses and further enhance performance.