The scope of this thesis is to explore different ways of bringing the phenomenon of quantum entanglement to our classical world. A first manner is to make entanglement an element of everyone's cultural background. I show that nontrivial quantum nonlocality tests can be performed with do-it-yourself optical sources. Furthermore, entanglement can be brought to the real world as a technological resource for improving everyday activities, like communication. Sources of entangled photons, suited to long distance quantum communication, need to be developed. I present a source of entangled photons realized via Spontaneous Parametric Down Conversion in an integrated cavity-waveguide. I show how this kind of sources can be engineered. I also report the realization of low-loss sources for the implementation of an experiment of faithful entanglement swapping adopting a nonlinear interaction between two single photons. Last, one can also attempt to physically generate and observe entanglement at macroscopic scales. I analyze the possibility of cloning a photon of an entangled pair and detecting the amplified state with human eyes. Bringing entanglement to scales one can see allows one to investigate ways of measuring light in the quantum-to-classical regime. I report the experimental demonstration of a method, based on optimal quantum cloning, for measuring low optical radiances in an absolute way. Moreover, I present the realization of photon number resolving detectors at infrared wavelengths and room temperature operation.
The scope of this thesis is to explore different ways of bringing the phenomenon of quantum entanglement to our classical world. A first manner is to make entanglement an element of everyone's cultural background. I show that nontrivial quantum nonlocality tests can be performed with do-it-yourself optical sources. Furthermore, entanglement can be brought to the real world as a technological resource for improving everyday activities, like communication. Sources of entangled photons, suited to long distance quantum communication, need to be developed. I present a source of entangled photons realized via Spontaneous Parametric Down Conversion in an integrated cavity-waveguide. I show how this kind of sources can be engineered. I also report the realization of low-loss sources for the implementation of an experiment of faithful entanglement swapping adopting a nonlinear interaction between two single photons. Last, one can also attempt to physically generate and observe entanglement at macroscopic scales. I analyze the possibility of cloning a photon of an entangled pair and detecting the amplified state with human eyes. Bringing entanglement to scales one can see allows one to investigate ways of measuring light in the quantum-to-classical regime. I report the experimental demonstration of a method, based on optimal quantum cloning, for measuring low optical radiances in an absolute way. Moreover, I present the realization of photon number resolving detectors at infrared wavelengths and room temperature operation.
This thesis examines the integration of two disparate technologies in order to perform experiments in single photon quantum optics with low loss. Many technologies and experiments in quantum optics, communication, or computing require a certain fraction of the photons involved to be received at the end of the experiment, and in many cases the required efficiency has not yet been reached. This includes the famous Einstein, Podolsky, and Rosen gedanken-experiment, now implemented in the laboratory, demonstrating the existence of entanglement to unconvinced observers. Also included is the nonlocality test of John Bell, as well as technological problems such as device-independent quantum key distribution. In this thesis I perform these experiments in the high-efficiency regime. This programme requires the integration of two lines of research: improving sources of single photons, and improving detectors thereof. Until recently detector research was focussed on development, with improvements being sought for their own sake, working towards the ultimate goal of perfect photon detection. Recent years have seen these devices move into quantum photonics laboratories, allowing for previously impossible experiments to be undertaken. In this thesis, I combine high-efficiency, number-resolving, detectors with a high-efficiency entangled photon pair source, based on another line of research going back decades: the use of spontaneous parametric down-conversion to create single-photon-like modes of light, and the entanglement of the output modes in a useful way. For small demonstrations, such as the experiments mentioned above, this can emulate a single photon with enough fidelity, and low enough loss, to successfully perform the procedure. Here I push both of these technologies, and the problems of combining them, as far as I can, and solve the problems inherent with any marriage of disparate devices. I also examine the relative performance of two different steering inequalities, one linear and one quadratic, in the presence of noise, analysing my experimental data with each of them. I perform a detection- loophole free demonstration of EPR steering, violating the local bound by 200s, setting a then world record for efficiency at 62.5%.
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.
The scope of this thesis is to present and discuss some experiments related to the quantum phenomenon of entanglement. We start by introducing the two forms of entanglement that we use: energy-time entanglement and single-photon entanglement. We explain how entanglement can be created through the process of spontaneous parametric down conversion in non-linear waveguides. We explain the importance of Bell inequalities and how we can use them to test the nonlocal nature of quantum correlations. The first experiment is a Bell test with spacelike separation, where we explore the possibility that a change in the gravitational field around one of the detection devices could trigger the collapse of the wavefunction, ending the entangled state. In the second one, we test the hypothesis that a superluminal signal defined in a universal privileged reference frame travelling between the detectors could influence the results. Last, we present the first experimental implementation of an entanglement purification protocol using single-photon entanglement and linear optics elements.
"Quantum communication is the art of transferring a quantum state from one place to another". The capability of distributing quantum states allows, for instance, provably secure communication and, in the near future, will enable the interconnection of quantum computers to establish a quantum internet. A convenient way to send quantum states from one place to another is to encode them into photons and distribute them using optical bres. The distance over which this can be achieved is still limited to a few hundred of kilometers. However, it can be extended in principle to arbitrary long distances by means of quantum repeaters. The realization of such quantum repeaters will require the engineering and interfacing of heterogeneous elements such as entangled photon-pair sources, quantum memories and entanglement swapping operations. The main goal of this thesis is to develop and study the compatibility of the main elements required to build a frequency multiplexed quantum repeater. This architecture requires a frequency multimode entangled photon pair source which, for example, could be implemented exploiting the process that involves the interaction of a strong light pulse with a non-linear crystal. Such a process is versatile and allows easy tailoring of the photon-pair spectral properties in order to match the requirements of the frequency multiplexed quantum repeater architecture. First, as a precursor of such a photon pair source, we developed a novel heralded single photon source that involves spectrally resolved single photon detection and on-demand frequency shifting. These capabilities are also essential tools in a frequency multiplexed quantum repeater. Next, we demonstrated the compatibility of such a sources with frequency multimode photonic quantum memories by performing spectrally multiplexed storage of single photons. We also tested the capability of high- delity storage of photons encoding quantum information in their polarization degree of freedom. Finally, we showed that entanglement swapping operations, i.e. Bell-state measurements, can be performed using photon pair sources compatible with quantum memories, quantum repeater architectures, and real-world scenarios. While a lot of e ort remains to be done, in the work presented in this thesis we demonstrated that all the key elements required to build a frequency multiplexed quantum repeater can be built and eventually be interfaced. The main obstacles towards that goal and possible ways to overcome them are also discussed.
Optical quantum systems are ideal for testing quantum mechanical laws, but they also find many uses into possible applications to our everyday life related to quantum information science, such as quantum key distribution (QKD) or, in general, quantum communication. Nonlinear and quantum optics and their applications both to quantum communication and to tests of fundamental physics are described in this Thesis. In particular, we have developed and characterised heralded single photon sources at telecommunication wavelengths (1550 nm), based on nonlinear interactions of the second order between electric fields in dielectric materials. This is the starting point for the preparation of single photon entangled states, that we used as a resource for heralded amplification of photonic qubits, with a direct application in Device Independent Quantum Key Distribution (DIQKD) protocols, but also for testing fundamental questions in quantum physics, demonstrating the presence of entanglement in a state that counts a macroscopic number of particles, 500 photons.
The quantum description of light offers a unique set of optical effects that has led to promising applications beyond those described by classical physics. Although well-defined quantum states of light do not persist in typical classical environments, phenomena such as entanglement often enhance optical approaches to communication, measurement, and sensing. With the emergence of new tasks in classical and quantum optical technology, new tools are required that must be specifically engineered including the generation of quantum states. This thesis is concerned with three principle tasks in engineering and implementing entangled photonic states. First, the use of frequency anti-correlated and polarization entangled two-photon states generated during spontaneous parametric down conversion (SPDC) to precisely evaluate optical delays with quantum interferometry is demonstrated in a realistic commercially available optical telecommunication device. Second, the study of correlated orbital angular momentum (OAM) states for efficient object identification is presented. Finally, experimental efforts towards the development of sources for entangled weak coherent states are discussed. The generation of broadband entangled states leading to well-defined second order interference patterns is a necessary step for the application of low coherence quantum interferometry as a metrological device. The flexibility of non-uniformly chirped periodically poled nonlinear crystals offers a rich set of tools for precise state engineering. The experimental evaluation of a broadband source of polarization entanglement is presented. In addition, design considerations for applications that require optimized quantum interference features are discussed along with a numerical investigation of the limits of quantum interferometry with even order dispersion cancellation. We present an experimental demonstration of correlated OAM sensing exploiting the two-dimensional and correlated nature of states produced during SPDC projected onto the OAM basis. Efficient object recognition through the identification of azimuthal symmetries of arbitrary objects is achieved by observing the full two-photon joint OAM spectrum and focusing on non-conserved OAM components not found in the natural OAM spectrum of SPDC. Finally, quantum key distribution (QKD) is currently the most successful quantum optical application; however, a limiting trade off between the achievable rates and distances confines the approach to niche applications. The generation of entangled coherent states has been proposed to transition QKD into a new regime that would set aside single photons and two-photon entangled states for higher intensity coherent pulses. The key technical limitation that has prohibited the demonstration of such states is a reliable source of single-photon cross phase modulation. The plausibility and experimental efforts towards creating such an environment in a solid state device is presented.