Photonic entanglement sources: high quality and high-dimensional implementations.

Abstract

Entanglement is a physical phenomenon described by the quantum theory. When the system is composed by two or more entangled particles, their quantum state must be understood as a whole, not as independent quantum states, even when the two particles were spatially separated. Hence, when we measure over entangled systems, leads to correlations between outcomes. These correlations, called non-local, are detected using Bell inequalities. This new concept did not only bring a change in the way nature is understood, but also new applications in quantum information. For example, the device-independent protocol that is based on the observation of non-local correlation. For this reason, the implementation and detection of entangled states are essential in quantum information. The goal of this thesis is to introduce two sources of entangled photons, and through these carry out the following experiments: Experimental nonlocality-based randomness generation with non-projective measurements, Experimental investigation of partially entangled states for deviceindependent randomness and Multi-dimensional entanglement generation with multi-core optical fibers. Each experiment provides a solution to problems in different fields of quantum information, the two first in randomness certification, and the last in high-dimension entangled state generation. To implement these three experiments, we use high-quality down-conversion sources, which are explained from Chapter 3 to Chapter 5. In Chapter 3, we propose the first experiment, which consists of an optical setup generating more than one bit of randomness from one entangled bit (i.e., a maximally entangled state of two qubits). To attain this result, we implemented a high-purity entanglement source and a nonprojective three-outcome measurement. Our implementation achieves a gain of 27% of randomness as compared with the standard methods using projective measurements. Additionally, we estimate the amount of randomness certified in a one-sided device-independent scenario, through the observation of Einstein-Podolsky-Rosen steering. Our results prove that non-projective quantum measurements allow extending the limits for nonlocality-based certified randomness generation using current technology.In Chapter 4, we explain the second xperiment. This research is base on previous theoretical pieces of works, which shows that all pure two-qubit entangled states can generate one bit of local randomness and can be self-tested through the violation of proper Bell inequalities. Using the same source of photons, proposed in the previous experiment, we produce pure partially entangled states of photonic qubits to investigate these protocols in a practical scenario. We show that small deviations from the ideal situation make low entangled states impractical to self-testing and randomness generation using the available techniques. Our results show that in practice lower entanglement implies lower randomness generation, recovering the intuition that maximally entangled states are better candidates for device-independent quantum information processing. Furthermore, we are interested in the uses that high-dimensional entangled states have in quantum information protocols. In Chapter 5, we develop a parametric down-conversion source of entangled qudits that is fully based on state-of-the-art multi-core fiber technology. The source design uses modern multi-core fiber beam splitters to prepare the pump laser beam as well as to measure the generated entangled state, achieving high spectral brightness while providing a stable architecture. It can also be readily used with any core geometry, which is crucial since standards for multi-core fibers in telecommunications have yet to be established. Our source represents an important step towards the compatibility of quantum communications with the next-generation optical networks.