Photonic chip design has in recent years seen significant advancements with the adoption of inverse design methodologies largelyenabled by the increasing computational efficiency of electromagnetic solvers. However, the often black‐box nature of this optimization method presents new challenges in understanding their outputs, particularly in the case of topological inverse design implementations. This challenge is prevalent in machine learning–based optimization methods, which can suffer from the same lack of transparency. To this end, interpretability techniques address the opacity of optimization models. In this work, we apply interpretability techniques from machine learning, with the aim of gaining understanding of inverse design optimization used in designing photonic components, for the specific case of a two‐mode (de)multiplexer. We base our methodology on the widely used interpretability technique known as local interpretable model‐agnostic explanations, or LIME. As a result, LIME‐informed insights point us to more effective initial conditions, directly improving device performance. This demonstrates that interpretability methods can do more than explain models—they can actively guide and enhance the inverse‐designed photonic components. Our results demonstrate the ability of interpretable techniques to reveal underlying patterns in the inverse design process, leading to the development of better‐performing components.

Transverse spatial modes as a degree of freedom is an untapped resource for scaling up integrated photonic quantum computing. To be practically useful for improving scalability, reliable and high-visibility quantum interference between transverse spatial modes on-chip needs to be demonstrated. We show repeatable quantum interference using inverse-designed transverse mode beam splitters that have an ultra-compact footprint of 3 × 3 μm2—the smallest transverse mode beam splitters for 1550 nm photons to date. We measure a Hong–Ou–Mandel visibility of up to 99.56% ± 0.62% from a single device, with an average visibility across three identical devices of 99.38% ± 0.41%, indicating a high degree of reproducibility. Our work demonstrates that inverse-designed components are suitable for engineering quantum interference on-chip of multimode devices, paving the way for future compact integrated quantum photonic devices that exploit transverse spatial modes as a degree of freedom for high-dimensional quantum information.

A quantum bit, qubit, can be extended to higher dimensions with discrete d- level quantum systems-qudits. Qudits offer increased channel capacity for quantum communications and increased robustness for quantum computing. Encoding of quantum information in the transverse degree of freedom (DoF) of photons has the potential to make integrated photonic systems more compact and scalable. Multiple transverse modes can propagate simultaneously within a single multimode waveguide, and manipulating these modes on-chip readily builds on technology already available for path-encoding. In this work, we focus on developing compact sources of photons entangled in their transverse modes, one of the several photonic properties amenable to a qudit description. We exploit spontaneous four-wave mixing (SFWM) on multimode silicon-on-insulator (SOI) waveguides. While the long-term objective is to realise entangled qudits, the current work investigates two-mode (TE0 and TE1) sources as a first step. Aside from multimode waveguides, our photonic integrated circuit features novel inverse-designed mode (de)multiplexers. We experimentally characterise intramodal (Type I) and intermodal (Type III) FWM in SOI waveguides, systematically investigating coincidence-to-accidental ratio (CAR) by varying waveguide lengths. For Type I (TE0-TE0 and TE1-TE1) and Type III (TE0-TE1) processes in 750 nm wide, 220 nm high and 4 mm long waveguide, we demonstrate CARs exceeding 600 and 25, respectively. These results demonstrate high-quality photon-pair generation and establish a pathway toward fully integrated, high-dimensional entangled photon sources using transverse modes.

While the standard interpretation of quantum mechanics does not assign definite trajectories to particles, the Bohmian interpretation does. Only recently has an operational method for reconciling Bohmian mechanics with relativity been proposed. Here, we experimentally reconstruct relativistic Bohmian trajectories of a single photon in a Michelson-Sagnac interferometer, where counter-propagating probability amplitudes interfere head-on at the speed of light. As predicted by the relativistic Bohmian theory, we observe subluminal and superluminal features of the Bohmian trajectories of the photon traversing through the fringes. Our work provides experimental access to relativistic Bohmian mechanics and enables exploration of its unusual and counterintuitive properties.

High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices – including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of qudits, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we review and introduce the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.

Photonic chip design has seen significant advancements with the adoption of inverse design methodologies, offering flexibility and efficiency in optimizing device performance. However, the black-box nature of the optimization approaches, such as those used in inverse design in order to minimize a loss function or maximize coupling efficiency, poses challenges in understanding the outputs. This challenge is prevalent in machine learning-based optimization methods, which can suffer from the same lack of transparency. To this end, interpretability techniques address the opacity of optimization models. In this work, we apply interpretability techniques from machine learning, with the aim of gaining understanding of inverse design optimization used in designing photonic components, specifically two-mode multiplexers. We base our methodology on the widespread interpretability technique known as local interpretable model-agnostic explanations, or LIME. As a result, LIME-informed insights point us to more effective initial conditions, directly improving device performance. This demonstrates that interpretability methods can do more than explain models – they can actively guide and enhance the inverse-designed photonic components. Our results demonstrate the ability of interpretable techniques to reveal underlying patterns in the inverse design process, leading to the development of better-performing components.

The black-box nature of topological inverse design makes intuitive optimisation of device performance challenging. Our local interpretable model identifies patterns underlying inverse-designed broadband multiplexers and helps increase the 0.5-dB bandwidth beyond 200 nm.

Transverse spatial modes have become an increasingly popular degree-of-freedom for encoding quantum information owing to the compact implementation when compared to others such as path. Here we present the demonstration of two photon Hong-Ou-Mandel interference between the different transverse modes in a multimode silicon photonic chip using a compact inverse designed beam splitter with up to 99.56 0.64% interference visibility. We achieve high visibilities across both multiple copies of ±the same design and among different designs. This work demonstrates the potential of inverse designed devices for quantum information processing with transverse spatial modes.

We present an integrated switch that combines plasmonic and neuromorphic technologies with a single sub-stoichiometric VO2-x nanoparticle. The presented device acts as a versatile plasmonic switch with dual thermal and electrical reconfigurability leveraging the near-room temperature phase transition of the VO2-x nanoparticles combined with the rapid phase recovery to drive the device. The change in both the optical and electrical properties of the VO2-x nanoparticle enables simultaneous optical and electrical readouts making the plasmonic device suitable as a phase change memory cell which is crucial in the convergence of computing and communication technologies. Our demonstration of reversible electrical switching, evidenced by a 6dB modulation depth and concurrent optical and electrical outputs, signifies a major stride in merging electronic and photonic functionalities within phase-change material devices. This novel strategy not only harmonizes optical communication with electronic computing but also advances the development of sophisticated integrated neuromorphic devices.

Photonic switches play a vital role in optical communications and computer networks for establishing and releasing connections of optical signals. With the growing demand for ultra‐compact switches in high‐speed optical computing and communications, thermally reconfigurable optical switches have gained significant attention. These switches offer simplicity, ease of fabrication, and leverage a wide range of thermo‐optic materials. Silicon remains an ideal platform for making photonic devices including the switches due to its compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology and cost‐effectiveness. The article presents a drop cast sub‐stoichiometric vanadium oxide (VO2−x) nanoparticles combined with a silicon ridge waveguide to make a compact thermally reconfigurable optical switch with low transition temperature and accelerated phase transition. Furthermore, the design achieves high modulation depth in addition to its scalability and simplicity. This study demonstrates the potential of solution‐based VO2−x nanoparticles in combination with silicon waveguides for efficient optical switch design for various applications.

Photonic switches are crucial for optical communications, facilitating the connection and disconnection of optical signals. The demand for compact, high-speed switches has made thermally reconfigurable switches increasingly popular due to their simplicity and compatibility with various thermo-optic materials. Silicon, favored for its CMOS compatibility and cost-effectiveness, serves as an excellent base for these devices. This paper introduces a compact, efficient optical switch combining drop-cast sub-stoichiometric vanadium oxide (VO2-x) nanoparticles with a silicon ridge waveguide with a minimum extinction ratio of 7dB in the C band. The switch is characterized by a broad wavelength of operation as well as a simple and scalable fabrication methodology. The research highlights the effectiveness of VO2-x nanoparticles and silicon waveguides in creating versatile optical switches for diverse applications.

Integrated photonics is at the heart of many classical technologies, from optical communications to biosensors, LIDAR, and data center fiber interconnects. There is strong evidence that these integrated technologies will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying laser and optical quantum technologies, with the required functionality and performance, can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration and a dramatic reduction in optical losses have enabled benchtop experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. The reduction in size, weight, power, and improvement in stability that will be enabled by QPICs will play a key role in increasing the degree of complexity and scale in quantum demonstrations. In the next decade, with sustained research, development, and investment in the quantum photonic ecosystem (i.e. PIC-based platforms, devices and circuits, fabrication and integration processes, packaging, and testing and benchmarking), we will witness the transition from single- and few-function prototypes to the large-scale integration of multi-functional and reconfigurable QPICs that will define how information is processed, stored, transmitted, and utilized for quantum computing, communications, metrology, and sensing. This roadmap highlights the current progress in the field of integrated quantum photonics, future challenges, and advances in science and technology needed to meet these challenges.

Achieving a high level of pulsed squeezing, in a platform which offers integration and stability, is a key requirement for continuous-variable quantum information processing. Typically highly squeezed states are achieved with narrow band optical cavities and bulk crystals, limiting scalability. Using single-pass parametric down conversion in an integrated optical device, we demonstrate quadrature squeezing of picosecond pulses in a thin-film lithium niobate strip-loaded waveguide. For on-chip peak powers of less than 0.3 W, we measure up to −0.33 ± 0.07 dB of squeezing with an inferred on-chip value of −1.7 ± 0.4 dB. This work highlights the potential of the strip-loaded waveguide platform for broadband squeezing applications and the development of photonic quantum technologies.

In the 1960s, computer engineers had to address the tyranny of numbers problem in which improvements in computing and its applications required integrating an increasing number of electronic components. From the first computers powered by vacuum tubes to the billions of transistors fabricated on a single microprocessor chip today, transformational advances in integration have led to remarkable processing performance and new unforeseen applications in computing. Today, quantum scientists and engineers are facing similar integration challenges. Research labs packed with benchtop components, such as tunable lasers, tables filled with optics, and racks of control hardware, are needed to prepare, manipulate, and read out quantum states from a modest number of qubits. Analogous to electronic circuit design and fabrication nearly five decades ago, scaling quantum systems (i.e. to thousands or millions of components and quantum elements) with the required functionality, high performance, and stability will only be realized through novel design architectures and fabrication techniques that enable the chip-scale integration of electronic and quantum photonic integrated circuits (QPIC). In the next decade, with sustained research, development, and investment in the quantum photonic ecosystem (i.e. PIC-based platforms, devices and circuits, fabrication and integration processes, packaging, and testing and benchmarking), we will witness the transition from single- and few-function prototypes to the large-scale integration of multi-functional and reconfigurable QPICs that will define how information is processed, stored, transmitted, and utilized for quantum computing, communications, metrology, and sensing. This roadmap highlights the current progress in the field of integrated quantum photonics, future challenges, and advances in science and technology needed to meet these challenges.

We investigate a coherent feedback squeezer that uses quantum coherent feedback (measurement-free) control. Our squeezer is simple, easy to implement, robust to the gain fluctuation, and broadband compared to the existing squeezers because of the negative coherent feedback configuration. We conduct a feasibility study that looks at the stability conditions for a feedback system to optimize the designs of real optical devices. The feasibility study gives fabrication tolerance necessary for designing and realizing the actual device. Our formalism for the stability analysis is not limited to optical systems but can be applied to the other bosonic systems.

Fast secure random number generation is essential for high-speed encrypted communication, and is the backbone of information security. Generation of truly random numbers depends on the intrinsic randomness of the process used and is usually limited by electronic bandwidth and signal processing data rates. Here we use a multiplexing scheme to create a fast quantum random number generator structurally tailored to encryption for distributed computing, and high bit-rate data transfer. We use vacuum fluctuations measured by seven homodyne detectors as quantum randomness sources, multiplexed using a single integrated optical device. We obtain a real-time random number generation rate of 3.08 Gbit/s, from only 27.5 MHz of sampled detector bandwidth. Furthermore, we take advantage of the multiplexed nature of our system to demonstrate an unseeded strong extractor with a generation rate of 26 Mbit/s.