Commercial sources of polarization entanglement at telecommunication wavelengths are already available on the market, but they lack proper certification or third-party testing. We aim to provide a comprehensive testing framework for photon counting and correlation measurements to characterize the parameters of these sources in a scalable and repeatable manner. The detection setup is included in our considerations, as the non-idealities of the components negatively affect the relevance of the measurement results. We discuss bounds for both true and false coincidences with rigorous probabilistic approach, as their ratio directly impacts the resolution of coincidence measurements and is reflected in Quantum Bit Error Rate (QBER) in the quantum telecommunication system. Quantum State Tomography (QST), polarization visibility measurements, temporal correlations measurements, and computations of other statistics are to be performed and compared at the state-of-the-art level for a three commercially available sources. Given that QST is demanding in terms of number of measurements and post-processing analysis, we discuss the relevance of determining the degree of polarization entanglement considering solely other statistics of direct measurement approach.
Commercial sources of polarization entanglement at telecommunication wavelengths are already available on the market, but they lack proper certification or third-party testing. We aim to provide a comprehensive testing framework for photon counting and correlation measurements to characterize the parameters of these sources in a scalable and repeatable manner. The detection setup is included in our considerations, as the non-idealities of the components negatively affect the relevance of the measurement results. We discuss bounds for both true and false coincidences with rigorous probabilistic approach, as their ratio directly impacts the resolution of coincidence measurements and is reflected in Quantum Bit Error Rate (QBER) in the quantum telecommunication system. Quantum State Tomography (QST), polarization visibility measurements, temporal correlations measurements, and computations of other statistics are to be performed and compared at the state-of-the-art level for a three commercially available sources. Given that QST is demanding in terms of number of measurements and post-processing analysis, we discuss the relevance of determining the degree of polarization entanglement considering solely other statistics of direct measurement approach.
Detecting and quantifying quantum entanglement of a given unknown state poses problems that are fundamentally important for quantum information processing. Surprisingly, no direct (i.e., without quantum tomography) universal experimental implementation of a necessary and sufficient test of entanglement has been designed even for a general two-qubit state. Here we propose an experimental method for detecting a collective universal witness, which is a necessary and sufficient test of two-photon polarization entanglement. It allows us to detect entanglement for any two-qubit mixed state, and to establish tight upper and lower bounds on its amount. A novel element of this method is the sequential character of its main components, which allows us to obtain relatively complicated information about quantum correlations with the help of simple linear-optical elements. As such, this proposal is the first to realize a universal two-qubit entanglement test within the present state of the art of quantum optics. We show the optimality of our setup with respect to the minimal number of measured quantities.
Detecting and quantifying quantum entanglement of a given unknown state poses problems that are fundamentally important for quantum information processing. Surprisingly, no direct (i.e., without quantum tomography) universal experimental implementation of a necessary and sufficient test of entanglement has been designed even for a general two-qubit state. Here we propose an experimental method for detecting a collective universal witness, which is a necessary and sufficient test of two-photon polarization entanglement. It allows us to detect entanglement for any two-qubit mixed state and to establish tight upper and lower bounds on its amount. A different element of this method is the sequential character of its main components, which allows us to obtain relatively complicated information about quantum correlations with the help of simple linear-optical elements. As such, this proposal realizes a universal two-qubit entanglement test within the present state of the art of quantum optics. We show the optimality of our setup with respect to the minimal number of measured quantities.
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.
In the past two decades, the union of quantum mechanics and information science has led to significant progress in our understanding of the quantum world and in our ability to coherently control individual quantum systems. New technologies have appeared, and some of them, such as quantum key distribution (QKD), are now commercially available. We are now envisioning quantum networks that could allow us to communicate over intercontinental scales, as the internet does, but with a provable security. An open question in this framework is how to certify that a given quantum network performs well, i.e. how to detect entanglement between 2, 5, 10 or 100 nodes located far away from each other. Bell tests are appealing for such a task as they allow one to certify the presence of entanglement deviceindependently, i.e. without assumptions on the Hilbert space dimension or on the proper alignment of the measurement devices. In the first chapter, I start by examining in depth the most standard approach to perform a Bell test in which a source based on spontaneous parametric down conversion (SPDC) is used to create photon pairs entangled in polarization. I show, in particular, that in this case the Bell inequality violation is fundamentally limited by vacuum and multi-photon emissions. In the second part of this thesis, I show how to overcome this limitation by using the SPDC source to create single photon entanglement, i.e. entanglement between two paths sharing a single photon, or two-mode squeezed states. Here, the non local content of the states is revealed using photon counting techniques preceded by small displacements in phase space. In the third part, I show how these measurements can be used for an optomechanical Bell test. This opens the way for device-independent quantum information processing with optomechanical systems and might be relevant to test collapse models in a theory independent manner. In the fourth chapter, the same measurements are used to build up a witness that can reveal genuine path entanglement. This witness has triggered an experiment which highlights its potential for network certification. I conclude the thesis by showing how path entanglement can be revealed with the human eye by exploiting these displacement operations.
The polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber-optic lines.
Device-independent quantum random number generators (DI-QRNGs) are crucial for many information-processing applications, requiring certified quantumness to ensure genuine randomness. However, inherent technical challenges in implementing quantumness tests often result in a low bit rate. Here, we present a high-bit-rate DI-QRNG with live quantumness certification through the Bell test. By employing spontaneous parametric down-conversion (SPDC) in a polarization Sagnac interferometer, which generates entangled degenerate pair-photons in annular ring distribution with strong temporal and spatial correlations due to energy and momentum conservation, we divide the annular ring into six diametrically opposite sections, forming three robust entangled photon sources from a single laser and nonlinear crystal. The pair-photons from these three sources exhibit bias-free quantum mechanical randomness. By utilizing the coincidence counts of pair-photons from two sources, we generate raw bits, while the third source simultaneously measures the Bell’s parameter without any loss of QRNG bits. We have generated 90 million raw bits in 46.4 seconds with the Bell parameter (S > 2), with a minimum entropy extraction ratio exceeding 97%. Post-processed using a Toeplitz matrix, the DI-QRNG achieves a bit rate of 1.8 Mbps, passing all NIST 800-22 and TestU01 tests. In the absence of Bell’s parameter for a non-maximally entangled state, g (2) (0) can be the metric for quantumness measure. Free from the requirement of any physical devices such as beam-splitters, this scalable DI-QRNG scheme promises high bit-rates and trustworthy certifications essential for practical applications.