The paper by Alberto Politi, Martin J. Cryan, John G. Rarity, Siyuan Yu, and Jeremy L. O'Brien presents the development of silica-on-silicon integrated optical quantum photonic circuits. These circuits are designed to improve the performance, miniaturization, and scalability of quantum technologies based on photons. Key demonstrations include: 1. **High-Fidelity Two-Photon Quantum Interference**: A visibility of \(94.8 \pm 0.5\%\). 2. **Controlled-NOT Gate**: Logical basis fidelity of \(94.3 \pm 0.2\%\). 3. **Path Entangled State**: Fidelity greater than \(92\%\). The researchers used silica waveguides on a silicon chip to achieve these results, leveraging the low loss, high refractive index contrast, and standard lithography fabrication techniques of silica. The monolithic nature of the devices ensures stable phase realization, simplifying the implementation of complex photonic quantum circuits. The team fabricated hundreds of devices on a single wafer, demonstrating robust and repeatable performance. The paper also discusses the challenges of achieving high visibility quantum interference and classical interference in bulk optical implementations, which are overcome by using waveguide devices. This approach reduces the need for sophisticated interferometers, making it easier to realize photonic quantum circuits for practical applications and future quantum technologies.The paper by Alberto Politi, Martin J. Cryan, John G. Rarity, Siyuan Yu, and Jeremy L. O'Brien presents the development of silica-on-silicon integrated optical quantum photonic circuits. These circuits are designed to improve the performance, miniaturization, and scalability of quantum technologies based on photons. Key demonstrations include: 1. **High-Fidelity Two-Photon Quantum Interference**: A visibility of \(94.8 \pm 0.5\%\). 2. **Controlled-NOT Gate**: Logical basis fidelity of \(94.3 \pm 0.2\%\). 3. **Path Entangled State**: Fidelity greater than \(92\%\). The researchers used silica waveguides on a silicon chip to achieve these results, leveraging the low loss, high refractive index contrast, and standard lithography fabrication techniques of silica. The monolithic nature of the devices ensures stable phase realization, simplifying the implementation of complex photonic quantum circuits. The team fabricated hundreds of devices on a single wafer, demonstrating robust and repeatable performance. The paper also discusses the challenges of achieving high visibility quantum interference and classical interference in bulk optical implementations, which are overcome by using waveguide devices. This approach reduces the need for sophisticated interferometers, making it easier to realize photonic quantum circuits for practical applications and future quantum technologies.