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Two-Photon Quantum Interferometry and Its Applications in Quantum-Enhanced Metrology

Two-Photon Quantum Interferometry and Its Applications in Quantum-Enhanced Metrology
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Quantum entanglement is an important resource of the quantum information research and one of the most nonclassical features of quantum theory. In photonic quantum information research, entangled photon states are most often generated via the process of spontaneous parametric downconversion (SPDC), in which a higher energy pump photon is split into a pair of lower energy photons. In this thesis, I have studied how to anipulate entangled photons and how to exploit them for quantum information research. First, I studied how to generate entangled photons via SPDC and their properties such as polarization, frequency, etc. of entangled photon pairs generated via SPDC in detail. And I report a series of experimental studies on single-mode optical fiber coupling of entangled photon pairs of type-II SPDC. Type-II SPDC emission pattern has two rings, one belonging to the extraordinary ray and the other to the ordinary ray of the crystal. So, we explicitly compare the biphoton singlemode optical fiber coupling efficiencies of the entangled photon pairs in type-II SPDC for the three phase matching conditions: collinear, non-collinear, and beam-like. Second, I study quantum interference effects of multimode cw-laser pumped spontaneous parametric down-conversion. Here we show that inexpensive blue diode lasers can be used for building engineered entangled photon sources. Next, we study the photonic de Broglie waves experimentally. The photonic de Broglie wave generally has been known to be a unique feature exhibited by the photon-numberpath–entangled state or the N00N state. In contrast, I report the observation of the photonic de Broglie wave without N00N state. And I also show that the photonic de Broglie wave can even be observed for a pair of photons that are completely separable (i.e., no entanglement in all degrees of freedom) and distinguishable. In fact the results suggest that the photonic de Broglie wave is not related to the entanglement of the photons, rather it is related to the indistinguishable pathways established by the measurement scheme. The phase sensitivity surpassing the standard quantum limit, however, is shown to be closely related to the N00N state. We experimentally demonstrate that two separable ququarts can be entangled by interfering them at PBS and post-selecting the output state of PBS. The ququart is realized by exploiting polarization and path modes of a single-photon. In addition, we will show that postselected entangled state is easily changed to another form of two ququarts entangled state. Finally, as a practical application of quantum effect, I report a quantum random number generator based on the photon-number and path entangled state that is prepared by means of two-photon quantum interference at a beam splitter.
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