基于 HOM 干涉的光子计量技术研究(特邀)

This thesis provides a comprehensive and in-depth investigation into photon metrology techniques that are centered on the foundational principles and diverse applications of Hong-Ou-Mandel (HOM) interference. The research begins by establishing the context for the evolution toward advanced photon metrology. Traditional optical measurement techniques, which have served as the bedrock of metrology for centuries, typically rely on the classical properties of light, treating it as a continuous wave whose intensity is the primary carrier of information. The precision of these classical methods is fundamentally constrained by the shot-noise limit, an intrinsic statistical fluctuation where the measurement signal-to-noise ratio improves only with the square root of the number of photons detected. In contrast, modern photon metrology marks a paradigm shift by harnessing the quantum nature of light itself, treating it as a stream of discrete energy quanta—photons. This approach leverages uniquely quantum phenomena, such as the entanglement and interference of individual, indistinguishable photons, to unlock new measurement capabilities. By doing so, it becomes possible to systematically overcome the classical shot-noise limit and pursue ultra-sensitive measurement protocols that can, in principle, approach the ultimate Heisenberg limit. At this limit, precision scales directly and linearly with the number of photons, promising improvements in sensitivity of several orders of magnitude. This profound transition from classical to quantum-based measurement necessitates the parallel development of sophisticated enabling technologies, especially high-performance single-photon sources capable of producing pure and indistinguishable photons on demand, and highly efficient single-photon detectors. The thesis reviews the essential tools of the trade, including direct photon counting, Time-Correlated Single Photon Counting (TCSPC) for resolving ultrafast temporal dynamics, and photon correlation measurements, which are crucial for certifying the non-classical nature of a light source. The core of this research is a detailed theoretical and experimental exploration of the Hong-Ou-Mandel interference effect, a canonical example of two-photon quantum interference. Theoretically, the phenomenon is derived from a fundamental principle of quantum mechanics: when two perfectly identical (indistinguishable) photons arrive simultaneously at the two input ports of a balanced 50∶50 beam splitter, the quantum probability amplitudes for the two possible outcomes where the photons exit through different ports destructively interfere and cancel each other out. Consequently, the photons are forced to always exit together, or "coalesce", into one of the two output paths. The quality of this non-classical effect is quantified by the interference visibility, a metric that can reach 100% for perfectly indistinguishable photons. An experimental apparatus was meticulously constructed to demonstrate and analyze the HOM effect, using photon pairs generated via Type-Ⅱ Spontaneous Parametric Down-conversion(SPDC) in a nonlinear crystal. These photons were then directed into a HOM interferometer equipped with a high-precision delay stage. When high-quality Single-mode Fibers (SMFs) were employed to filter the spatial modes of the photons, ensuring they were spatially identical before meeting at the beam splitter, a high interference visibility of 85.3% was achieved. This result confirmed a high degree of indistinguishability for the generated photons. In a crucial comparative experiment, the SMFs were replaced with multi-mode fibers (MMFs). This change resulted in a drastic reduction in visibility, causing the characteristic HOM dip to become shallow and indistinct. This powerful comparison compellingly illustrates the critical and decisive role of spatial mode purity in achieving high-quality quantum interference; the multiple spatial modes supported by the MMFs effectively "mark" the photons, destroying their indistinguishability and thereby suppressing the underlying quantum effect. Building upon this foundational experiment, the thesis surveys the diverse and powerful applications of HOM interferometry in modern science and technology. In the realm of fundamental quantum characterization, it remains the gold standard for quantifying the degree of indistinguishability between photons from one or multiple sources. Furthermore, the temporal width of the HOM dip provides a direct measure of the photons’ coherence time. In the field of high-precision measurement, the extremely steep slope of the HOM dip as the time delay passes through zero is exploited for ultra-fine time delay sensing, enabling resolutions down to the attosecond level, far beyond the reach of conventional electronics. This principle is extended to Quantum Optical Coherence Tomography (QOCT), a powerful biomedical imaging technique that uses the HOM effect to achieve high axial resolution for cross-sectional imaging of biological tissues, while simultaneously benefiting from an inherent and automatic cancellation of even-order chromatic dispersion— a significant advantage over its classical counterpart. The thesis further explores its transformative potential in quantum-enhanced spectroscopy, detailing techniques like entanglement-assisted absorption, which can probe complex material interactions, and sub-shot-noise spectroscopy, which leverages the strong correlations of photon pairs to perform measurements with a precision that fundamentally bypasses the standard quantum limit. Finally, the thesis concludes by summarizing the key findings and providing a forward-looking perspective on the future of this vibrant field. The primary practical challenges remain the persistent mitigation of system losses(photon absorption and scattering) and the reduction of background noise, both of which degrade interference visibility and compromise measurement precision. The future direction points towards greater integration, sophistication, and robustness. The development of chip-scale integrated quantum photonic circuits, for instance, promises to miniaturize entire HOM interferometers onto a single chip, dramatically enhancing phase stability, scalability, and portability for real-world applications. The field is also vigorously advancing towards more complex multi-photon interference schemes, which are essential for pushing measurement precision ever closer to the true Heisenberg limit. These future advancements promise to unlock unprecedented capabilities in fundamental science, non-invasive biomedical imaging, and advanced materials science, solidifying the role of Hong-Ou-Mandel interference as a pivotal and indispensable technology in the toolkit of modern photon metrology.