Photonic molecules are systems of coupled optical resonators, or of deliberately coupled modes within one resonator, whose collective states have engineered frequencies, lifetimes and optical access. Integration on silicon, silicon nitride, thin-film lithium niobate and hybrid platforms has turned them from spectroscopic objects into circuit elements for filtering, frequency conversion, nonlinear light generation, lasing and sensing. This review covers the physics and the state of the art of integrated photonic molecules. We first introduce the temporal coupled-mode description of a two-resonator molecule and the quantities used to characterize it: mode splitting, linewidth and their ratio. We then review each platform: thermally tuned and photonic-crystal molecules in silicon for compact spectral processing, spectrometry and sensing; low-loss silicon nitride molecules for soliton and parametric generation, squeezed light and narrow-linewidth Brillouin lasing; and electro-optic molecules in lithium niobate and hybrid stacks for programmable two-level dynamics, frequency shifting, comb generation and transduction. Non-Hermitian and active molecules, including exceptional-point devices and coupled lasers, are treated separately. Comparison tables collect the reported metrics together with their operating conditions. We close with the open problems of calibration, stability and packaging and with prospects in microwave photonics and quantum technologies.
Photonic molecules are systems of coupled optical resonators, or of deliberately coupled modes within one resonator, whose collective states have engineered frequencies, lifetimes and optical access. Integration on silicon, silicon nitride, thin-film lithium niobate and hybrid platforms has turned them from spectroscopic objects into circuit elements for filtering, frequency conversion, nonlinear light generation, lasing and sensing. This review covers the physics and the state of the art of integrated photonic molecules. We first introduce the temporal coupled-mode description of a two-resonator molecule and the quantities used to characterize it: mode splitting, linewidth and their ratio. We then review each platform: thermally tuned and photonic-crystal molecules in silicon for compact spectral processing, spectrometry and sensing; low-loss silicon nitride molecules for soliton and parametric generation, squeezed light and narrow-linewidth Brillouin lasing; and electro-optic molecules in lithium niobate and hybrid stacks for programmable two-level dynamics, frequency shifting, comb generation and transduction. Non-Hermitian and active molecules, including exceptional-point devices and coupled lasers, are treated separately. Comparison tables collect the reported metrics together with their operating conditions. We close with the open problems of calibration, stability and packaging and with prospects in microwave photonics and quantum technologies.
Integrated thin-film lithium niobate (TFLN) photonics has emerged as a powerful platform for quantum information science, offering its outstanding nonlinear, electro-optic (EO), and integration capabilities. In this review, we present the latest advances in TFLN-based integrated photonics tailored to quantum technologies. We first explore state-of-the-art quantum light sources realized in both straight waveguide and resonator configuration, including high-brightness photon pair generations, squeezed light, and versatile entanglement schemes. Next, we detail progress in integrated photonic processors, with a focus on programmable interferometric networks, ultrafast EO modulators, and essential passive components for photonic qubit processing. We then address critical challenges in optical interfacing and detection technologies, discussing recent innovations in low-loss fiber-to-chip and grating coupler designs, as well as the integration of on-chip single photon detectors. This review provides a forward-looking perspective on scalable quantum photonic systems that could underpin future advances in quantum communication, computing, and sensing.
Quantum technologies have surpassed classical systems by leveraging the unique properties of superposition and entanglement in photons and matter. Recent advancements in integrated quantum photonics, especially in silicon-based and lithium niobate platforms, are pushing the technology toward greater scalability and functionality. Silicon circuits have progressed from centimeter-scale, dual-photon systems to millimeter-scale, high-density devices that integrate thousands of components, enabling sophisticated programmable manipulation of multi-photon states. Meanwhile, lithium niobate, thanks to its wide optical transmission window, outstanding nonlinear and electro-optic coefficients, and chemical stability, has emerged as an optimal substrate for fully integrated photonic quantum chips. Devices made from this material exhibit high efficiency in in generating, manipulating, converting, storing, and detecting photon states, thereby establishing a basis for deterministic multi-photon generation and single-photon quantum interactions, as well as comprehensive frequency-state control. This review explores the development of integrated photonic quantum technologies based on both silicon and lithium niobate, highlighting invaluable insights gained from silicon-based systems that can assist the scaling of lithium niobate technologies. It examines the functional integration mechanisms of lithium niobate in electro-optic tuning and nonlinear energy conversion, showcasing its transformative impact throughout the photonic quantum computing process. Looking ahead, we speculate on the developmental pathways for lithium niobate platforms and their potential to revolutionize areas such as quantum communication, complex system simulation, quantum sampling, and optical quantum computing paradigms.
Quantum sensors have the potential to transform numerous applications across a broad range of fields from precision timing to electro-magnetic field sensing, medical imaging, gravimetry, to inertial sensing. Despite promising performances in laboratory experiments, quantum sensors have yet to be widely deployed in real-world applications at large scale. The large-scale adoption of quantum sensors has been hampered by the high SWaP-c and their sensitivity to environmental factors. Photonic integrated circuits have emerged as an enabling technology for quantum sensors to reduce SWaP-c, enhance fieldability, and add enhanced functionality. Honeywell has developed state-of-the-art capabilities in Silicon Nitride (SiN) and other photonics platforms, to enable low-swap quantum sensors ranging from atomic clocks, Rydberg electromagnetic field sensors, and NV diamond magnetometers among others. In this talk we will give an overview of how Honeywell is using photonic integrated circuits to enable a variety of quantum sensing and timing technologies for aerospace applications. Low-loss SiN waveguides can be used to deliver light to the quantum system in a compact form factor, produce high Q-optical cavities for narrow-linewidth lasers, tunably filter frequency components, or to generate optical combs. Mode-matched edge couplers can efficiently transfer light from fiberto- chip or chip-to-chip. Other photonic platforms such as diamond or thin-film lithium niobate (TFLN) can be integrated with a SiN platform in either hybrid or heterogenous configurations to enable quantum magnetometry and laser frequency tuning or modulation in ultra-compact form factors. We will discuss the advantages and remaining challenges for using photonic integrated circuits to advance quantum sensors to the point where they can be adopted in mainstream applications.
Quantum photonics has advanced rapidly and become a leading approach for developing a broad range of quantum technologies, including quantum communications, quantum information processing, quantum computing, and quantum sensing. Besides, the emerging photonic integrated circuits (PIC) technology has been widely used to implement versatile optical elements advantageous for different quantum applications. For example, PIC-based devices have functioned as building blocks, such as photonic qubit sources, linear optical circuits, modulators, and even single-photon detectors, for realizing highly integrated, large-scale quantum computing schemes. In such a powerful quantum photonic integrated circuits (QPIC) scheme, the creation, manipulation, and testing of quantum entanglement states from the generated qubits via a reconfigurable photonic circuitry are essential. Though the state-of-the-art PIC technology is established based on Si photonics due to its widely mature nanophotonic circuit (waveguide) fabrication technology supported by the existing immense semiconductor industry, Si-based PIC has limitations in implementing efficient and fast active elements that usually demand a quadratic nonlinearity. Lithium niobate (LN), renowned as “the silicon of photonics”, has long been a popular material widely used in integrated-optical and nonlinear-optical applications. A great variety of passive and active photonic elements have been realized in the LN platform. Recently, a novel material based on high-quality single-crystalline films of LN, known as thin-film LN on insulator (LNOI), has been developed and has soon drawn enormous attention for its greatly promising ability to realize nanoscale PIC/QPIC in this iconic optical material platform. In this presentation, we report our recent studies on several key LN PIC building blocks for developing novel quantum photonic sources and circuits, including highly integrated photonic-circuit periodically poled lithium niobate (PPLN) chips for photon-pair generation and manipulation, for high-dimensional frequency-entangled qudit production, for time-frequency grid-state generation, for continuous-variable squeezed vacuum state generation, and their potential applications to quantum computing/communication/sensing technology. Besides, a PP-LNOI electro-optic (EO) polarization mode converter (PMC) useful for demonstrating fast control of quantum polarization states and other LNOI circuit elements as QPIC building blocks will also be presented.
I will discuss progress on the development of ultra-low loss thin film lithium niobate (TFLN) photonic platform and its application in classical and quantum information processing. Examples of devices to be presented include low power consumption and high bandwidth electro-optic (EO) modulators and EO frequency comb sources, as well as their integration with high power lasers, of interest for applications in optical communications and microwave photonics. TFLN is also promising for spectral and temporal control of non-classical light. In particular, I will discuss the development of quantum transducers and photonics needed for realization of multiplexed quantum repeaters.
Photonics is poised to play a unique role in quantum technology for computation, communications and sensing. Meanwhile, integrated photonic circuits-with their intrinsic phase stability and high-performance, nanoscale components-offer a route to scaling. However, each integrated platform has a unique set of advantages and pitfalls, which can limit their power. So far, the most advanced demonstrations of quantum photonic circuitry has been in silicon photonics. However, thin-film lithium niobate (TFLN) is emerging as a powerful platform with unique capabilities; advances in fabrication have yielded loss metrics competitive with any integrated photonics platform, while its large second-order nonlinearity provides efficient nonlinear processing and ultra-fast modulation. In this short review, we explore the prospects of dynamic quantum circuits-such as multiplexed photon sources and entanglement generation-on hybrid TFLN on silicon (TFLN/Si) photonics and argue that hybrid TFLN/Si photonics may have the capability to deliver the photonic quantum technology of tomorrow.