The aim of this work is to explore the characterization of various entangled parameters of the two-photon state that is created in the process of spontaneous parametric down-conversion, as well as to investigate the potential application of these two-photon states to quantum communication and quantum information processing. The parameters fall into two natural divisions, the discrete-variable and continuous-variable regimes. Polarization-correlated photon pairs are used to explore the discrete-variable regime. Using these polarization-correlated photon pairs we investigate phasecovariant quantum cloning, sum-variance entanglement measures, and unambiguous state-discrimination. Phase-covariant quantum cloning is experimentally demonstrated to provide higher cloning fidelity than a universal quantum cloner. The simplicity of the practical implementation of this cloning method makes this cloner a useful addition to the quantum information and communication toolbox. Next, it is experimentally demonstrated that three, concatenating, sum-variance entanglement measures possess higher sensitivities than the popular Bell entanglement measure, while each requires fewer measurements than a Bell measurement
The aim of this work is to explore the characterization of various entangled parameters of the two-photon state that is created in the process of spontaneous parametric down-conversion, as well as to investigate the potential application of these two-photon states to quantum communication and quantum information processing. The parameters fall into two natural divisions, the discrete-variable and continuous-variable regimes. Polarization-correlated photon pairs are used to explore the discrete-variable regime. Using these polarization-correlated photon pairs we investigate phasecovariant quantum cloning, sum-variance entanglement measures, and unambiguous state-discrimination. Phase-covariant quantum cloning is experimentally demonstrated to provide higher cloning fidelity than a universal quantum cloner. The simplicity of the practical implementation of this cloning method makes this cloner a useful addition to the quantum information and communication toolbox. Next, it is experimentally demonstrated that three, concatenating, sum-variance entanglement measures possess higher sensitivities than the popular Bell entanglement measure, while each requires fewer measurements than a Bell measurement
Quantum computation and quantum communication require sources of entangled photons, especially from low-cost diode lasers. This study considered a polarization entangled photons pulse source produced via a spontaneous parametric down-conversion process in the type-I phase-matching of two nonlinear crystals, incorporating a continuous wave laser diode operating in pulse mode as a pump. The generated photons were measured based on visibility as a function of the pump frequency and collection apertures. A polarization correlation between photon pairs of approximately 93% and a Bell parameter S value of 2.56 ± 0.05 were obtained using the source described here.
Entanglement is a key resource for fundamental tests of physics and emerging quantum technologies. In quantum optics, two perspectives on entanglement coexist. In the continuous-variable framework, entanglement is understood as holding between optical modes. In contrast, discrete-variable quantum optics focuses on quantum correlations in degrees of freedom such as polarization that label fixed numbers of photons. In this paper, we show that entanglement can transcend this separation. Spontaneous parametric down-conversion inherently generates correlations in optical phase space, photon number, and labelling degrees of freedom simultaneously. In polarization, this structure is traditionally described by macroscopic Bell states. Existing witnesses, however, fail to detect the genuine hybrid entanglement of these states, which goes beyond the continuous-discrete-variable categorization. Here, we lay the groundwork for a general framework unifying continuous- and discrete-variable notions of entanglement. In particular, we derive an operational witness providing a sufficient criterion for genuine hybrid number-polarization entanglement and outline its experimental implementation. Finally, we discuss exemplary states which, together with our results on macroscopic Bell states, motivate a broader classification of genuine hybrid quantum correlations.
The photon is a quantum system that is by nature robust against noise, and fast travelling, providing an ideal information carrier for quantum information science. The field of quantum information science has generated a great deal of interest for both researchers and the public due to the advantages it can have over classical technologies in areas including communication, computing and cryptography. In this thesis, I explore several different ways that photons can be used to realize quantum information advantages and enable quantum technologies, information tasks and fundamental studies of physics. A strong emphasis is placed on their practical implementation in optical experiments, where the challenges of noise, loss and device imperfections must be considered and combated. I utilize spontaneous parametric downconversion-based sources of high-quality single photons, and their polarization, path, and temporal degrees of freedom for encoding quantum information. In my first experimental project, I demonstrate the first violation of noncontextuality with time-polarization intraparticle entangled states encoded in a single photon by implementing a powerful interference-based measurement technique for photonic states. I perform a test of noncontextuality, acquiring a parameter |SCHSH| = 2.744 ± 0.006, which violates the classical bound of 2 by over 110 standard deviations, rigorously certifying both entanglement and contextuality of the system. We propose a new approach for preparing and measuring high-dimensional time-bin states, that overcomes many of the limitations of previous techniques, and demonstrate it for time-bin qubits and qutrits. In the next experimental project, I investigate the topic of universal cloning in quantum steering-with cloning and steering being two fundamental aspects of quantum physics deeply involved in the security of quantum protocols. I report the first experimental demonstration of cloned quantum steering, showing a simultaneous violation in a steering test by two clones made from one part of an entangled pair of photons. This provides a conclusive result on the question of whether the cloning of quantum steering is possible. We also present theoretical results that clarify our understanding of cloned steering for qubits: we find a no-go theorem where quantum steering can only be copied by a qubit universal cloning machine for 2 and 3 clones, and a monogamy relation where for N clones, steering is only possible if a minimum of N + 1 measurements are performed on each clone. I discuss the field of random number generation, and study how quantum physics allows for certifiably unpredictable random sequences, without needing complete trust in all of the physical devices, offering high levels of security over classical approaches. I report the first discrete-variable detection-loophole-free implementation of one-sided device-independent randomness expansion of certified random bits, based on the quantum steering task. These results illustrate an approach to secure randomness with greater tolerance to noise and loss compared to Bell test-based protocols. The final experimental project looks at quantum autoencoders, which are a quantum machine learning tool that we investigate for the purpose of denoising quantum states. I model and simulate a denoising quantum autoencoder which compresses ququarts into a qubit space and reconstructs noise-reduced ququart states. I construct an experimental apparatus that implements this device, and demonstrate quantum state tomography of ququart states, and the training of the autoencoder.
The experimental realization of optimal symmetric phase-covariant $1\ensuremath{\rightarrow}2$ cloning of qubit states is presented. The qubits are represented by polarization states of photons generated by spontaneous parametric down-conversion. The experiment is based on the interference of two photons on a custom-made beam splitter with different splitting ratios for vertical and horizontal polarization components. From the measured data we have estimated the implemented cloning transformation using the maximum-likelihood method. The result shows that the realized transformation is very close to the ideal one and the map fidelity reaches 94%.
Entangled photons are at the heart of experimental quantum physics. They were used for the first fundamental tests of quantum theory, and became a basic building block for many novel quantum protocols, such as quantum cryptography, dense coding or teleportation. Therefore, the efficient generation of entangled photons, as well as their distribution and accurate analysis are of paramount importance, particularly with regard to the practicability of many applications of quantum communication. This thesis deals largely with the problem of efficient generation of photonic entanglement with the principal aim of developing a bright source of polarization-entangled photon pairs, which meets the requirements for reliable and economic operation of quantum communication prototypes and demonstrators. Our approach uses a correlated photon-pair emission in nonlinear process of spontaneous parametric down-conversion pumped by light coming from a compact and cheap blue laser diode. Two alternative source configurations are examined within the thesis. The first makes use of a well established concept of degenerate non-collinear emission from a single type-II nonlinear crystal and the second relies on a novel method where the emissions from two adjacent type-I phase-matched nonlinear crystals operated in collinear non-degenerate regime are coherently overlapped. The latter approach showed to be more effective, yielding a total detected rate of almost 10^6 pairs/s at >98 % quantum interference visibility of polarization correlations. This performance, together with the almost free of alignment operation of the system, suggest that it is an especially promising candidate for many future practical applications, including quantum cryptography, detector calibration or use in undergraduate lab courses. The second issue addressed within the thesis is the simplification and practical implementation of quantum-assisted solutions to multiparty communication tasks. While the recent rapid progress in the development of bright entangled photon-pair sources has been followed with ample experimental reports on two-party quantum communication tasks, the practical implementations of tasks for more than two parties have been held back, so far. This is mainly due to the requirement of multiparty entangled states, which are very difficult to be produced with current methods and moreover suffer from a high noise. We show that entanglement is not the only non-classical resource endowing the quantum multiparty information processing its power. Instead, only the sequential communication and transformation of a single qubit can be sufficient to accomplish certain tasks. This we prove for two distinct communication tasks, secret sharing and communication complexity. Whereas the goal of the first is to split a cryptographic key among several parties in a way that its reconstruction requires their collaboration, the latter aims at reducing the amount of communication during distributed computational tasks. Importantly, our qubit-assisted solutions to the problems are feasible with state-of-the-art technology. This we clearly demonstrate in the laboratory implementation for 6 and 5 parties, respectively, which is to the best of our knowledge the highest number of actively performing parties in a quantum protocol ever implemented. Thus, by successfully solving and implementing a cryptographic task as well as a task originating in computer science, we clearly illustrate the potential to introduce multiparty communication problems into real life.
Quantum entanglement has been a topic of much research in modern physics, and an application in quantum computing is envisioned. This thesis address the use of polarization entangled photon pairs produced by spontaneous parametric down-conversion (SPDC) in entanglement transfer process and quantum computing. The simplicity of SPDC entanglement relative to other quantum phenomena is first emphasized by developing a complete classical theory of the photon-crystal interactions characteristic of SPDC. The methods of achieving polarization superposition via the Type-I and Type-II SPDC are described. A combination of superposition and polarization correlation between down-converted photons is shown to violate Bell’s inequality, thus revealing the entangled nature of the photon pairs. Finally, the possibility of controlling entangled photons through the manipulation of surface plasmons is highlighted as a way to transfer entanglement from photons to material particles.