Lithium niobate (LiNbO3) serves as one of the most competitive materials in nonlinear integrated photonics and becomes a key platform for the realization of integrated optical quantum technology, because it is physically and chemically very stable and it has a larger refractive index, a wider transparent wavelength range and better nonlinear optical characteristics than most of the traditional materials. Using the second-order nonlinear effect of periodically polarized lithium niobate (PPLN) crystal, single-photon frequency conversion was successfully realized to satisfy the quantum measurement requirements. LiNbO3and PPLN Waveguides were widely developed to solve the problems such as easy beam divergence in crystals, so that the improved nonlinear conversion effect promotes the successful fabrication of many quantum devices such as single photon sources, entangled photon sources and single photon detectors etc. By combining LiNbO3waveguides with other waveguide technologies, one can fabricate various LiNbO3integrated optical components serving as the core devices of quantum interfaces, quantum memories and etc. Based on these progresses, we see the attractive potential of LiNbO3material to play a deterministic role in quantum information technology.
Lithium niobate (LiNbO3) serves as one of the most competitive materials in nonlinear integrated photonics and becomes a key platform for the realization of integrated optical quantum technology, because it is physically and chemically very stable and it has a larger refractive index, a wider transparent wavelength range and better nonlinear optical characteristics than most of the traditional materials. Using the second-order nonlinear effect of periodically polarized lithium niobate (PPLN) crystal, single-photon frequency conversion was successfully realized to satisfy the quantum measurement requirements. LiNbO3and PPLN Waveguides were widely developed to solve the problems such as easy beam divergence in crystals, so that the improved nonlinear conversion effect promotes the successful fabrication of many quantum devices such as single photon sources, entangled photon sources and single photon detectors etc. By combining LiNbO3waveguides with other waveguide technologies, one can fabricate various LiNbO3integrated optical components serving as the core devices of quantum interfaces, quantum memories and etc. Based on these progresses, we see the attractive potential of LiNbO3material to play a deterministic role in quantum information technology.
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.
Entanglement is the hallmark of quantum mechanics. Quantum entanglement -- putting two or more identical particles into a non-factorable state -- has been leveraged for applications ranging from quantum computation and encryption to high-precision metrology. Entanglement is a practical engineering resource and a tool for sidestepping certain limitations of classical measurement and communication. Engineered nonlinear optical waveguides are an enabling technology for generating entangled photon pairs and manipulating the state of single photons. This dissertation reports on: i) frequency conversion of single photons from the mid-infrared to 843nm as a tool for incorporating quantum memories in quantum networks, ii) the design, fabrication, and test of a prototype broadband source of polarization and frequency entangled photons; and iii) a roadmap for further investigations of this source, including applications in quantum interferometry and high-precision optical metrology. \n \nThe devices presented herein are quasi-phase-matched lithium niobate waveguides. Lithium niobate is a second-order nonlinear optical material and can mediate optical energy conversion to different wavelengths. This nonlinear effect is the basis of both quantum frequency conversion and entangled photon generation, and is enhanced by i) confining light in waveguides to increase conversion efficiency, and ii) quasi-phase matching, a technique for engineering the second-order nonlinear response by locally altering the direction of a material's polarization vector. Waveguides are formed by diffusing titanium into a lithium niobate wafer. Quasi-phase matching is achieved by electric field poling, with multiple stages of process development and optimization to fabricate the delicate structures necessary for broadband entangled photon generation. \n \nThe results presented herein update and optimize past fabrication techniques, demonstrate novel optical devices, and propose future avenues for device development. Quantum frequency conversion from 1848nm to 843nm is demonstrated for the first time, with >75% single-photon conversion efficiency. A new electric field poling methodology is presented, combining elements from multiple historical techniques with a new fast-feedback control system. This poling technique is used to fabricate the first chirped-and-apodized Type-II quasi-phase-matched structures in titanium-diffused lithium niobate waveguides, culminating in a measured phasematching spectrum that is predominantly Gaussian (R^2 = 0.80), nearly eight times broader than the unchirped spectrum, and agrees well with simulations.
The quantum nature of light at the single photon level allows for unique applications that classical physics neither predicts nor describes. Most notably, appropriate conditions may cause the states of photons with indistinguishable properties to become entangled, enabling novel approaches to quantum computation, secure communications, and metrology. \n \nAny operational quantum information network transporting entangled-photon states must establish a high-fidelity link between its distant nodes despite the inherent fragility of entangled states. However, the lack of a universal operating wavelength for all optical devices makes this a substantial challenge. Telecommunication fibers, entangled photon pair sources, quantum optical memories, and quantum repeaters all function in disparate spectral windows dictated by the properties of the materials used to fabricate them. Quantum frequency conversion (QFC), the single-photon limit of nonlinear parametric sum and difference frequency generation in optics, offers a bridge between spectral regions with full preservation of quantum state character, including entanglement. \n \nPeriodically poled optical waveguides in ferroelectric crystals are versatile tools in nonlinear optics. Confining the nonlinear interaction to a waveguide greatly enhances its efficiency compared to bulk optics. Periodic poling, the process of periodically inverting the domains of a ferroelectric medium using a strong electric field, enables a variety of quasi-phase matching configurations to engineer a desired nonlinear interaction. \n \nThis work concentrates on the design, development, fabrication, and characterization of a titanium-diffused periodically poled lithium niobate (Ti:PPLN) waveguide device. This device is designed to execute single-step quantum frequency conversion from 369.5nm to 1550nm in order to facilitate a quantum state transfer between standard fiber telecommunication wavelengths and an atomic quantum memory system employing trapped ytterbium ions. \n \nThe creation of phase matching conditions for such an extreme distance in frequency demands precise control of high magnitude electric fields on the single-micron scale. As the first demonstration of its kind in Ti:PPLN, the development of these devices included novel improvements to existing fabrication methods, improving the state-of-the-art of precision, quality, and yield for poling of ferroelectric crystals.
Lithium niobate-on-insulator (LNOI) is a leading platform for high-speed optical communication technologies owing to its large electro-optic bandwidth, which enables light modulation above 100 GHz [1]. The wide transparency window is being leveraged to produce photonic integrated circuits operating at different wavelengths, and the strong second-order nonlinearity allows efficient frequency conversion via periodic poling of the crystal on the same waveguide network [2]. LNOI has potential to revolutionise optical communication, and nonlinear conversion processes can be used to generate highly pure quantum states acting as bits of information for the next generation of optical quantum information technologies.
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.
We present the use of quasi-phase matching technique and integrated optical technology on lithium niobate for quantum optical information. Based on recent achievements, we will discuss the large potential of nonlinear integrated quantum optics. The use of integrated optical technology and quasi-phase matching technique, extensively developped in our laboratory on lithium niobate, is very attractive for quatum optics as it allows the integration of various elements such as correlated pair sources, electrically controllable beam splitters, phase modulators, and wavelength selectors, on a single wafer, eventually eliminating the need for bulk optical interconnections. Such integrated optical components should permit the realization of compact, performant, elements for various quantum optical experiments.