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.
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.
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.
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.
Within the last decade, thin-film lithium niobate (TFLN) has emerged as a leading integrated photonics platform with immediate applications in classical communications. Concurrently, TFLN components tailored to the requirements of photonic quantum information processing have also garnered considerable interest. Chief among these is the quasi-phase matched (QPM) nonlinear frequency mixer—implemented via periodic domain inversion in ferroelectric lithium niobate—which can be used to generate photon pairs, squeezed states of light, and to perform single photon frequency conversion. Here, we demonstrate progress towards realizing a spectrally separable photon pair source in lithium niobate via waveguide dispersion engineering—a technique uniquely enabled by sub-wavelength optical mode confinement in the thin-film platform. Subsequently, we design optimize a scalable fabrication process to produce QPM TFLN devices for applications that require strict adherence to a specified operating wavelength, such as quantum frequency conversion in a quantum communications network. Finally, we explore how high-performance electro-optic devices can be combined with these nonlinear optical devices to realize a multi-functional platform in which quantum states of light can be generated and manipulated within a single, compact photonic integrated circuit.
Ion-sliced thin-film lithium niobate (LN) compact waveguide technology has facilitated the resurgence of integrated photonics based on lithium niobate. These thin-film LN waveguides offer over an order of magnitude improvement in optical confinement, and about two orders of magnitude reduction in waveguide bending radius, compared to conventional LN waveguides. Harnessing the improved confinement, a variety of miniaturized and efficient photonic devices are demonstrated in this work. First, two types of compact electrooptic modulators are presented – microring modulators, and Mach-Zehnder modulators. Next, two distinct approaches to nonlinear optical frequency converters are implemented – periodically poled lithium niobate, and mode shape modulation (grating assisted quasi-phase matching). Following this, stochastic variations are added to the mode shape modulation approach to demonstrate random quasi-phase matching. Afterward, broadband photon-pair generation is demonstrated in the miniaturized periodically poled lithium niobate, and spectral correlations of the biphoton spectrum are reported. Finally, extensions of the aforementioned results suitable for future work are discussed.
This thesis describes the establishment of a new fabrication facility for the realization of annealed and reverse proton exchanged waveguides in congruent lithium niobate at Grith University{Centre for Quantum Dynamics, and their application in dierent contexts of integrated quantum photonics technologies. We demonstrated the development of an accurate numerical model for the design of waveguides in X-cut and Z-cut substrates and high quality fabrication of both quasi-phase matched quadratic nonlinear waveguides and fast electro-optic waveguide modulators. Within the context of nonlinear optical processes, we demonstrated a newly proposed method which eciently reconstruct the biphoton state produced through spontaneous parametric down-conversion by the use of only classical measurements. The validity of the method is tested by a direct comparison with photon-coincidence counting measurements in a multi-channel integrated nonlinear device fabricated in our facility. We also implemented the rst frequency conversion of 369:5 nm light {resonant with the transition of Yb+ trapped ions{ to the 1550 nm telecom wavelength range with a periodically poled waveguide fabricated by the reverse proton exchange technique. Within the context of electro-optical processes, we showed the rst active demultiplexing of single-photons emitted from a quantum dot source with a single integrated device made of a network of electro-optically tunable directional couplers.
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.