We present an experimental study of polarization-entangled photon pairs, investigating the dependence of the measurement outcomes on different polarization rotation angles and the slow temporal fluctuations of the generated quantum state. Entangled photon pairs are produced and analyzed using single-photon detectors while systematically varying polarization settings on both sides. Bell tests yield an S parameter of approximately 2.52, confirming strong entanglement. The measured coincidence counts between identical measurement outcomes exhibit the expected sinusoidal dependence on the combined rotation angles, but systematic deviations appear as pronounced peaks and variations in certain configurations. Long-term monitoring over twenty minutes shows correlated fluctuations on both sides in individual detection outcomes of up to 25%, while the total photon flux remains constant, indicating that these deviations originate from slow intrinsic changes in the generated quantum state. These temporal fluctuations can reduce the effective time window over which optimal polarization and phase alignment is maintained, highlighting the importance of implementing real-time compensation or feedback mechanisms in experiments requiring high-fidelity entanglement over extended durations. Our results provide insight into the stability limits of entangled photon sources and underscore the importance of accounting for temporal fluctuations in precision entanglement experiments.
We present an experimental study of polarization-entangled photon pairs, investigating the dependence of the measurement outcomes on different polarization rotation angles and the slow temporal fluctuations of the generated quantum state. Entangled photon pairs are produced and analyzed using single-photon detectors while systematically varying polarization settings on both sides. Bell tests yield an S parameter of approximately 2.52, confirming strong entanglement. The measured coincidence counts between identical measurement outcomes exhibit the expected sinusoidal dependence on the combined rotation angles, but systematic deviations appear as pronounced peaks and variations in certain configurations. Long-term monitoring over twenty minutes shows correlated fluctuations on both sides in individual detection outcomes of up to 25%, while the total photon flux remains constant, indicating that these deviations originate from slow intrinsic changes in the generated quantum state. These temporal fluctuations can reduce the effective time window over which optimal polarization and phase alignment is maintained, highlighting the importance of implementing real-time compensation or feedback mechanisms in experiments requiring high-fidelity entanglement over extended durations. Our results provide insight into the stability limits of entangled photon sources and underscore the importance of accounting for temporal fluctuations in precision entanglement experiments.
Bell inequality experiments have demonstrated that quantum entanglement violates the predictions of local realism. However, all Bell experiments are based on joint measurements of two photons, and their analysis implicitly assumes that "the polarization state of a single photon is ideally linear." This paper proposes an experimental scheme to directly test this assumption: using type-II spontaneous parametric down-conversion (SPDC) to generate polarization-entangled photon pairs, employing fixed-angle measurement of the e-light channel as a coincidence gate to filter entangled photons in the o-light channel, then scanning the o-light analyzer angle with high precision to measure the coincidence count rate as a function of the analyzer angle, testing whether it strictly follows Malus' law I(\theta)=I_0\cos^2(\theta-\theta_0). If systematic deviation is observed in the curve, it would indicate the existence of asymmetric secondary polarization structures in the polarization state of entangled photons, providing an alternative explanatory pathway for Bell inequality violation distinct from nonlocality. This experiment employs a high-precision coincidence measurement scheme to ensure statistical significance and designs multiple control experiments to eliminate systematic errors. The experiment can be implemented in laboratories equipped with standard quantum optics equipment.
Time synchronization is a fundamental requirement in entanglement-based quantum networks, where the indistinguishability of photons in the time domain is essential for enabling Hong-Ou-Mandel interference and entanglement swapping. In addition to precise temporal alignment, it is equally crucial to ensure the stability of the reference clock over time, as even small fluctuations can degrade the overall performance of the network. In this work, we investigate the stability of clock synchronization for entanglement distribution based on entangled-photon sources operating in the telecommunication C-band. Temporal correlations between photon detection events are analyzed using time-tagged coincidence measurements, enabling the extraction of synchronization peaks and their long-term stability. Experimental results demonstrate that, once locked, the sources exhibit stable temporal correlations over an 8-hour acquisition period, with a maximum observed drift of approximately 120 ps, primarily associated with long fiber links. The width of the correlation peak remains consistent with detector jitter, indicating negligible additional system-induced temporal broadening. A central result of this work is the experimental synchronization between two entanglement-photon sources in a realistic metropolitan deployment, where entanglement distribution is performed over existing telecommunication infrastructure characterized by non-negligible losses and background noise. In this scenario, despite the presence of significant imperfections introduced by the metropolitan-scale fiber network, the observed correlations remain clearly detectable and are consistently well-approximated by Gaussian statistics. This confirms that the two sources can be reliably synchronized not only in controlled laboratory conditions but also under real-world operating constraints.
The nascent field of quantum information offers the promise of dramatic speed increases for certain types of computation and logical protocols which are classically impossible. The phenomenon of nonlocality is fundamental to this study, and polarization-entangled photons o er a clean, bright, and stable system for its experimental investigation. This thesis investigates, both experimentally and theoretically, the creation, manipulation, and measurement of polarization-entangled photon pairs. Our entangled photon pairs are created through spontaneous parametric down-conversion within a pair of adjacent, orthogonally oriented nonlinear crystals. The quality and brightness of this source is then dramatically improved through the use of birefringent crystals which compensate for an angle-dependent phase shift. The use of these compensation crystals has allowed measured count rates of two million pairs per second with 97.7% fidelity (with a maximally entangled state) or alternately, ten thousand pairs per second with 99.5% fidelity. By manipulating these entangled states, it is possible to study both how they change and how they can be used. We discuss the theory of state manipulation and the experimental implementation of extremely precise single-qubit operations. To study the operations themselves, we use quantum process tomography to characterize them and have successfully implemented the first experimental realization of ancilla-assisted process tomography. To study decoherence, we implemented the first experimental decoherence-free subspace. By using techniques developed during these investigations, we are able to transform our source of entangled photons into a source of any two-photon polarization state, mixed or pure. Developing new techniques for state creation and manipulation is possible because of simultaneous development in state measurement. In addition to detailing both a theoretical analysis and experimental instructions for state tomography, we experimentally and theoretically compared state tomography with tests of local realism (Bell inequalities) and entanglement witnesses (an entanglement detection technique). In the process, we have measured the largest violations of local realism to date, both statistical (over 2400-sigma) and absolute (S = 2:826 ± 0:005 -- within 0:2% of a maximal violation).
Summary form only given. In a Bell experiment, one prepares pairs of entangled particles and sends them to two observers, Alice and Bob, for measurement and detection [1]. For some choices of their measurement settings, Alice and Bob may observe strong correlations between their results in accordance with the predictions of quantum mechanical theory. Conversely, any physical theory that assumes no physical influence can be faster than the speed of light and that properties of physical systems are elements of reality [2] - a local realistic theory - can predict only a limited amount of correlation between Alice's and Bob's measurement outcomes. Upon observing correlations sufficient to violate Bell's inequality, Alice and Bob must abandon local realism.It is common that in an experiment, some particles emitted by the source will not be detected. Then the subset of detected particles might display correlations that violate the Bell inequality although the entire ensemble can be described by a local realistic theory [3]. To achieve a conclusive Bell violation without assuming that the detected particles are a “fair” sample, a highly efficient setup is necessary. Due to experimental limitations, fair sampling has been assumed in most Bell experiments performed to date, and it has never been possible to avoid this assumption with photons due to the absence of efficient sources and detectors. Here we report the first Bell experiment with photons that does not rely on any fair-sampling assumption. In our experiment we employ the Eberhard inequality, a Bell inequality that by construction does not rely on the fair-sampling assumption [4]. Our source of entangled photon pairs utilizes bulk-crystal spontaneous parametric downconversion in a Sagnac configuration, which has shown to be very efficient [5, 6]. For photon detection, we employ superconducting transition-edge sensors, which not only offer high efficiency but also are intrinsically free of dark counts [7]. This combination of features is imperative for an experiment in which no correction of count rates can be tolerated. We achieve uncorrected coupling efficiencies over 73% in each arm of our source, facilitating a violation of the inequality by nearly 70 standard deviations after five minutes of measurement. Since the first experimental Bell test, a satisfactory laboratory realization of the motivating gedankenexperiment has remained an outstanding challenge. The two other main assumptions include “locality” [8, 9] and “freedom of choice” [10]. Invoking any of these renders an experiment vulnerable to explanation by a local realistic theory. The realization of an experiment that is free of all three assumptions-a so-called loopholefree Bell test-remains an important outstanding goal for the physics community with strong implications for quantum technologies. We note that with our experiment, photons are the first physical system for which each of these three assumptions has been successfully addressed, albeit in different experiments. This represents promise for practical applications like one-sided device-independent quantum key distribution [11], and in an additional test, we also demonstrate that our experimental apparatus is capable of implementing this protocol on both sides.
Advancements in the field of compact quantum photonics have enabled researchers to reliably generate and detect spectral and temporal correlated biphotons, opening avenues for diverse applications in quantum technology. Although many routine quantum interferometry and entanglement generation experiments are performed worldwide, only a few studies discuss how different experimental detection and correlation measurement parameters can significantly influence the raw outcomes of these experiments. In this work, we analytically and experimentally elucidate the impact of quantum efficiency and dead time of single-photon detectors, and bin width of the time correlator on the coincidence-to-accidental ratio (CAR) and hence the quantum temporal cross-correlations (gsi2(0)) of photon pairs generated at 1560 nm via a type-II MgO:ppLN ridge waveguide-based source. We experimentally achieved a high cross-correlation of up togsi2(0)=∼104at different detection settings, thus revealing the high-quality of photon-pair correlations, surpassing several state-of-the-art biphoton sources. Additionally, we demonstrate how this analysis anticipates the effect of afterpulsing, temporal distinguishability between signal and idler photons, and temporal resolution of the correlation analysis. We further use this analysis for application in Hong-Ou-Mandel (HOM) interferometry and achieve raw visibility∼99±1%. To the best of our knowledge, such high raw HOM visibility has not been previously reported in a fiber-based type-II SPDC configuration. This study is important for understanding the quantum source and finding the optimal detection settings to operate it on for its real-time practical implementation in various quantum optical experiments.
Polarization-entangled photon pairs generated by spontaneous parametric down-conversion are pivotal for quantum information applications, but multi-photon pair emissions can degrade entanglement quality and threaten communication security. Here, we present a comprehensive theoretical and experimental study of how multi-photon pair generation affects the Bell parameter in a Sagnac-based polarization-entangled photon source. By employing photon number resolving (PNR) parallel superconducting nanowire single-photon detectors, we successfully isolate and quantify the impact of single, double, and triple photon pair events on the entanglement parameter. Our results reveal that increased multi-photon contributions lead to a marked reduction in the Bell parameter, with experimental data aligning closely with our theoretical predictions. This degradation arises primarily from accidental coincidences associated with multi-photon events. The findings underscore the critical role of PNR detection in mitigating vulnerability, such as photon number splitting attacks in entanglement-based quantum key distribution protocols, suggesting that minimizing multi-photon pair generation and real-time photon number resolution are essential for achieving robust, secure quantum communication.