The generation and manipulation of single- and multi-photon states play a central role in numerous quantum optics applications, including quantum communication, imaging, sensing, and information processing. The transverse spatial degrees of freedom of photons are particularly attractive because they provide access to high-dimensional quantum states. Optical systems that inherently support spatially structured modes therefore provide compact and versatile platforms for manipulating such states. Ring-core fibers naturally support azimuthal modes whose interference produces cylindrical Talbot self-imaging. At half the Talbot length, this propagation performs a transformation analogous to that of a balanced beam splitter between diametrically opposite angular positions. In this thesis, the cylindrical Talbot effect is experimentally investigated using the full transverse output of a spontaneous parametric down conversion source, with the goal of observing two-photon interference occurring inside a ring-core fiber. Degenerate photon pairs were generated in a periodically poled potassium titanyl phosphate crystal. The transverse momentum anti-correlations of the generated photons were determined with a single-photon avalanche diode camera utilizing the reconstructed joint probability distributions from sequences of short-exposure frames. Near- and far field measurements after the nonlinear crystal verified the expected transverse momentum anti-correlations and transverse position correlation properties of the generated biphoton state. The annular far field distribution was subsequently shaped and coupled into a ring-core fiber with a length corresponding to half of the cylindrical Talbot length. Intensity acquisitions at the fiber output confirmed the expected beam splitting of an angularly localized input distribution into two diametrically opposite outputs. Two-photon interference was investigated at discrete temporal delays by observing the coincidence counts between opposite output ports of the ring-core fiber. No reduction in coincidences between opposite outputs was observed at zero temporal delay. The statistical uncertainty, poor signal-to-noise ratio, and deviation from the correct Talbot length most likely prevent the observation of the interference effect. The correlation structure of the biphoton state after the fiber was likewise insufficient to determine an expected transformation from momentum anti-correlation to correlation. These results establish that the cylindrical Talbot effect can be employed for manipulating the spatial output of an SPDC source to observe beam splitting. The two-photon interference measurements can be utilized for identifying the main experimental limitations affecting the visibility of the observed effect. These results provide a framework for further investigations of the cylindrical Talbot effect for compact beam splitting and quantum state manipulation.
The generation and manipulation of single- and multi-photon states play a central role in numerous quantum optics applications, including quantum communication, imaging, sensing, and information processing. The transverse spatial degrees of freedom of photons are particularly attractive because they provide access to high-dimensional quantum states. Optical systems that inherently support spatially structured modes therefore provide compact and versatile platforms for manipulating such states. Ring-core fibers naturally support azimuthal modes whose interference produces cylindrical Talbot self-imaging. At half the Talbot length, this propagation performs a transformation analogous to that of a balanced beam splitter between diametrically opposite angular positions. In this thesis, the cylindrical Talbot effect is experimentally investigated using the full transverse output of a spontaneous parametric down conversion source, with the goal of observing two-photon interference occurring inside a ring-core fiber. Degenerate photon pairs were generated in a periodically poled potassium titanyl phosphate crystal. The transverse momentum anti-correlations of the generated photons were determined with a single-photon avalanche diode camera utilizing the reconstructed joint probability distributions from sequences of short-exposure frames. Near- and far field measurements after the nonlinear crystal verified the expected transverse momentum anti-correlations and transverse position correlation properties of the generated biphoton state. The annular far field distribution was subsequently shaped and coupled into a ring-core fiber with a length corresponding to half of the cylindrical Talbot length. Intensity acquisitions at the fiber output confirmed the expected beam splitting of an angularly localized input distribution into two diametrically opposite outputs. Two-photon interference was investigated at discrete temporal delays by observing the coincidence counts between opposite output ports of the ring-core fiber. No reduction in coincidences between opposite outputs was observed at zero temporal delay. The statistical uncertainty, poor signal-to-noise ratio, and deviation from the correct Talbot length most likely prevent the observation of the interference effect. The correlation structure of the biphoton state after the fiber was likewise insufficient to determine an expected transformation from momentum anti-correlation to correlation. These results establish that the cylindrical Talbot effect can be employed for manipulating the spatial output of an SPDC source to observe beam splitting. The two-photon interference measurements can be utilized for identifying the main experimental limitations affecting the visibility of the observed effect. These results provide a framework for further investigations of the cylindrical Talbot effect for compact beam splitting and quantum state manipulation.
Wave propagation on the surface of cylinders exhibits interferometric self-imaging, much like the Talbot effect in the near-field diffraction at periodic gratings. We report the experimental observation of the cylindrical Talbot carpet in weakly guiding ring-core fibers for classical light fields. We further show that the ring-core fiber acts as a higher-order optical beamsplitter for single photons, whose output can be controlled by the relative phase between the input light fields. By also demonstrating high-quality two-photon interference between indistinguishable photons sent into the ring-core fiber, our findings open the door to applications in optical telecommunications as a compact beam multiplexer as well as in quantum information processing tasks as a scalable realization of a linear optical network.
The Talbot effect, a wave-optical self-imaging phenomenon originally observed in real-space, has developed into a powerful framework for programmable modal transformations across diverse physical platforms. Here, we report the experimental demonstration of nonlocal Talbot transformations within a high-dimensional, orbital angular momentum (OAM) entangled two-photon system. By exploiting the intrinsic nonlocal correlations generated through spontaneous parametric down-conversion, we show that applying an azimuthal Talbot phase on one photon of an entangled pair is sufficient to coherently reshape the OAM modal distribution of its spatially separated partner, without any direct local operation on the latter. As a proof of principle, we further demonstrate the inverse Talbot transformation within the same platform by incorporating the required quadratic phase during the state-preparation stage. These results extend Talbot physics from local mode manipulation to entanglement-assisted nonlocal transformations in a high-dimensional discrete basis. The demonstrated platform opens versatile routes toward conditional remote OAM-mode conversion, structured entangled-state engineering, and high-dimensional quantum information processing.
In recent years, quantum entanglement has attracted much; attention, because its unique properties provide potential; applications, which could not be achieved using conventional; techniques, such as quantum computing, quantum imaging and; lithography. To realize these advancements, one has to obtain an; entanglement-generation source, thoroughly master its physical; properties, and fully understand the light-matter interaction.; This dissertation is an attempt to address such issues as stated; above.; Conventionally, paired photons are created from; \textit{spontaneous parametric down-conversion} (SPDC). It is; known that the transverse correlation in biphotons may improve the; visibility and resolution in quantum imaging and lithography. In; this thesis, we described an alternative biphoton source --; Raman-EIT (\textit{electromagnetically induced transparency}); generator, and emphasize on its geometrical and optical; properties. We found that to utilize the transverse effects in; paired Stokes-anti-Stokes, it is necessary to make the product of; the EIT window times the group delay much greater than unity.; To gain further insight into quantum imaging and lithography, we; studied the transverse correlation in triphoton entanglement; theoretically. We found that in the two-image process, the quality; of images is determined by the optical path-lengths, even though; the Gaussian thin lens equations are satisfied. The ghost; interference-diffraction patterns of double slits show one more; fold interference, which is essentially different from the; biphoton case. Klyshko's advanced-wave model is still applicable,; with some modifications. We also generalized the transverse; correlation to the case of multi-photon entangled states.; To implement quantum computing, one key element is quantum memory.; In this thesis, we have theoretically explored the feasibility of; such a memory by using nonclassical SPDC light in an EIT system at; the single-photon level. We found that both the quantum coherence; of SPDC and atomic coherence of EIT can survive after interacting; within a vapor cell. Due to the inherent mismatch of magnitude; between the spectral bandwidth of SPDC and the very narrow; transmission width of EIT, the coincidence counts of the; two-photon interference is reduced to one pair per second, which; is barely doable in the current experimental situation.
In the realm of quantum information processing, harnessing high-dimensional photonic systems provides a pathway to overcome limitations of traditional two-level systems. Orbital angular momentum (OAM) of light has emerged as a powerful tool for creating and manipulating high-dimensional entanglement, promising increased information capacity and enhanced security in quantum communication protocols. However, conventional methods like spontaneous parametric downconversion encounter challenges due to non-uniform production rates of Laguerre–Gaussian modes. This study explores the potential of spontaneous four-wave mixing in ring-core fibers (RCFs) as a viable platform for generating OAM photon pairs with tailored spectral and spatial properties. We show that by controlling the topological charge of pump photons, correlated, uncorrelated, and anti-correlated photon pairs can be engineered across arbitrary spectral ranges, essential for diverse quantum applications. Experimental noise characterization of the RCF-based source demonstrates a high coincidence-to-accidental ratio exceeding 4000, and a low heralded second-order correlation function ( g H (2) <0.005), which confirms its operation well into the single-photon regime. This work demonstrates the potential of RCFs as a versatile platform for generating structured photon pairs, paving the way for future high-dimensional quantum communication and information processing applications.
Spontaneous parametric down-conversion (SPDC) is one of the most important sources of entangled photons in quantum optics and underpins a wide range of applications in quantum communication, quantum imaging, quantum sensing, and photonic quantum information processing. In this nonlinear optical process, a pump photon is converted into a pair of lower-energy photons that exhibit strong correlations in multiple degrees of freedom. While the generation of photon pairs is governed by energy and momentum conservation, the resulting biphoton state possesses a rich spatial structure whose properties are determined by the interplay among the pump field, the nonlinear medium, and subsequent propagation. This thesis investigates the spatial properties of entangled photon pairs generated through SPDC, focusing on the evolution of their correlations in the near field and on the behavior of orbital-angular-momentum (OAM) entangled states under tight focusing conditions. A common theme throughout this work is the examination of physical effects that emerge when the full biphoton wavefunction is retained rather than reduced through the simplifying approximations often employed in theoretical treatments. The principal contribution of this thesis concerns the near-field propagation of SPDC photon pairs. The spatial correlations of SPDC-generated photons are commonly analyzed within the thin-crystal approximation, where the biphoton wavefunction is assumed to factorize into independent functions of the sum and difference coordinates. We demonstrate that birefringence-induced transverse walk-off breaks this factorization and introduces an intrinsic coupling between these coordinates. More importantly, this coupling survives free-space propagation and remains observable even for crystals that nominally satisfy the thin-crystal condition. As a result, the joint spatial correlations develop a characteristic tapering near the crystal image plane that cannot be captured by conventional factorized models. Numerical simulations and experimental measurements are found to be in excellent agreement, revealing a previously overlooked feature of SPDC and providing a more complete description of photon-pair generation and propagation in birefringent nonlinear media. The second part of the thesis explores the propagation of OAM-entangled photon pairs beyond the paraxial regime. While OAM correlations are typically studied under weak-focusing conditions, many emerging quantum technologies require structured quantum states to be manipulated in highly confined optical systems. We show that tight focusing modifies the OAM correlation spectrum, leading to a broadening of the accessible mode distribution. This broadening exhibits distinct transverse and longitudinal characteristics and arises from the emergence of nonparaxial field components absent in conventional scalar descriptions. These results provide new insight into the behavior of structured quantum light in strongly focused optical fields and establish a framework for understanding OAM entanglement beyond the paraxial approximation. Together, the results presented in this thesis reveal previously unexplored aspects of the structure and propagation of entangled photon pairs. By moving beyond widely used approximations, this work uncovers physical effects that influence the generation, evolution, and measurement of spatially entangled states, with implications for quantum imaging, high-dimensional quantum communication, and the broader development of structured-light-based quantum technologies.
High-dimensional entanglement in the form of transverse spatial correlation between a pair of photons generated via spontaneous parametric downconversion is not only a valuable resource in many academic and real-life applications but also provides access to several intriguing quantum phenomena. One such non-intuitive phenomenon is phase entanglement, in which the biphoton state is correlated in the complex phase of its wavefunction. This state, which emerges during the propagation of the biphoton wavefunction, exhibits neither position nor momentum correlation, yet retains full entanglement. In this work, we experimentally explore this state in two distinct ways. The first is by tracking the vanishing spatial photon number correlation over propagation distances lying in [0,∞), folded into a finite range using single-lens imaging. These observations show excellent agreement with our theoretical predictions based on the Double Gaussian (DG) approximation of the biphoton state. The second approach involves performing a two-photon interference experiment using a double slit and this state, which reveals the correlated phase front. We show, both theoretically and experimentally, that the observed two-photon interference structure is markedly different from that produced by position-correlated photons, as confirmed by computing the joint probability distribution of photons (JPD) and related metrics. Such interference using phase-entangled light has not been attempted before and opens avenues for advanced experiments and applications in the field of spatial entanglement.