Advances in photonics will lead to breakthroughs in technology with a similar impact to that resulted from the development of electronics. At the quantum level, photons exhibit some unique features, such as non-classical correlations and bosonic bunching. The manipulation of these quanta of light foresee the construction of quantum devices capable of outperforming their classical counterparts in the purposes for which they are built. Despite a significant progress in developing quantum technologies, several unsolved issues certainly remain: Efficient quantification of quantum correlations in a system, and manipulation of multiple photons at once, are two of such hurdles to be overcome. In this thesis, we tackle these two issues and present experimental progress towards their solution. We first explore quantum entanglement between two single-photons and its effect on phases appearing on the system's wavefunction. The observed phases revealed an intimate connection between entanglement––a purely quantum phenomena––and geometric and topological structures of abstract manifolds: A special case of geometric phases was found to increase monotonically with the amount of entanglement in the system, thus serving as an alternative measure of quantum correlations. We then employed a system of three single-photons, and two concatenated entangling gates, as the basis of a quantum simulation protocol: A three-particle quantum simulator, mimicking the entangling evolution of two particles, is used to experimentally demonstrate that the overhead in measuring concurrence––a measure of two-particle entanglement––can be significantly reduced down to measuring only two observables. At this point, increasing the number of photons being handled proved extremely difficult; the low efficiency of photon sources based on parametric downconversion––the gold-standard and only source at-the-time used for multi-photon experiments––imposed a hard limit on countrates and hence the number of photons to be manipulated. We thus then focused on overcoming this obstacle: We studied a solid-state emitter and its performance as a multi-photon source. We employed a quantum dot-cavity system to show that streams of tens of indistinguishable single-photons are produced, whose temporal-to-spatial demultiplexing serves as a novel kind of multi-photon source. A three-photon source was then built and employed in the first demonstration of multi-photon interference from a solid-state source: We implemented a BosonSampling device with a source that is between one and two orders-of-magnitude more efficient than its downconversion counterpart. The main limiting factors for scaling to higher photon numbers were identified to be not intrinsic to the solid-state emitter, but determined by low detector efficiencies and the probabilistic nature of the demultiplexing scheme. Future improvements on these limitations are expected to allow the generation of sources with a higher number of single-photons, a task that has to date remained otherwise impossible.
Advances in photonics will lead to breakthroughs in technology with a similar impact to that resulted from the development of electronics. At the quantum level, photons exhibit some unique features, such as non-classical correlations and bosonic bunching. The manipulation of these quanta of light foresee the construction of quantum devices capable of outperforming their classical counterparts in the purposes for which they are built. Despite a significant progress in developing quantum technologies, several unsolved issues certainly remain: Efficient quantification of quantum correlations in a system, and manipulation of multiple photons at once, are two of such hurdles to be overcome. In this thesis, we tackle these two issues and present experimental progress towards their solution. We first explore quantum entanglement between two single-photons and its effect on phases appearing on the system's wavefunction. The observed phases revealed an intimate connection between entanglement––a purely quantum phenomena––and geometric and topological structures of abstract manifolds: A special case of geometric phases was found to increase monotonically with the amount of entanglement in the system, thus serving as an alternative measure of quantum correlations. We then employed a system of three single-photons, and two concatenated entangling gates, as the basis of a quantum simulation protocol: A three-particle quantum simulator, mimicking the entangling evolution of two particles, is used to experimentally demonstrate that the overhead in measuring concurrence––a measure of two-particle entanglement––can be significantly reduced down to measuring only two observables. At this point, increasing the number of photons being handled proved extremely difficult; the low efficiency of photon sources based on parametric downconversion––the gold-standard and only source at-the-time used for multi-photon experiments––imposed a hard limit on countrates and hence the number of photons to be manipulated. We thus then focused on overcoming this obstacle: We studied a solid-state emitter and its performance as a multi-photon source. We employed a quantum dot-cavity system to show that streams of tens of indistinguishable single-photons are produced, whose temporal-to-spatial demultiplexing serves as a novel kind of multi-photon source. A three-photon source was then built and employed in the first demonstration of multi-photon interference from a solid-state source: We implemented a BosonSampling device with a source that is between one and two orders-of-magnitude more efficient than its downconversion counterpart. The main limiting factors for scaling to higher photon numbers were identified to be not intrinsic to the solid-state emitter, but determined by low detector efficiencies and the probabilistic nature of the demultiplexing scheme. Future improvements on these limitations are expected to allow the generation of sources with a higher number of single-photons, a task that has to date remained otherwise impossible.
Controlling quantum states of light is of central importance to many fields of modern physics. The technology to do so underpins the only feasible means for long-distance quantum communication using quantum-key-distribution, and plays a pivotal role in the application of quantum computing, quantum metrology and investigations of fundamental physics. Motivated by these applications, in this thesis we present a number of experimental results that demonstrate an enhanced level of control over photonic states. First, we tackle one of the most important technological issues currently facing the field, namely the creation of pure multi-photon Fock states from pulsed parametric downconversion. Our technique shows a marked improvement over previous photon sources when employed for quantum information tasks. At a more foundational level, we experimentally examine the properties of quantum correlations in the temporal domain, where our results highlight surprising differences between its spatial-domain counterpart of multipartite entanglement. Finally, we experimentally implement single- and multi-photon quantum walks in the discrete- and continuous-time regimes respectively. Quantum walks have received much attention in recent years due to their vast applicability in quantum information science, especially for quantum simulation and emulation. On this front we use the quantum walk formalism to perform a full experimental simulation of topological phases in a 1-dimensional configuration. Aside from being the first demonstration of topological phases in this regime, our system exhibits unprecedented control over the topological properties in a quantum system. As such we are able to observe the exotic behaviour of trapped bound states, and discover the new phenomenon of paired bound states—a feature unique to periodically driven systems.
Multi-particle entangled states are becoming more and more interesting for different applications in quantum information, such as quantum multi-particle teleportation, entangled based cryptography and quantum communication schemes based on multi-particle states, but also for fundamental reasons such as testing local realism by means of the Greenberger-Horne-Zeilinger states (GHZ) arguments . The advantage of multiparticle entanglement versus the most common two particle one lies in the fact that the greater the number of particles involved in the entangled state, the more clearly the quantum effects are exhibited. Our final goal is to exploit the multi-photon entangled states for quantum metrology. By the interaction of those states with nanostructures (mostly nanoholes, but in a future also nanospheres), we expect to reach sub-wavelength sensitivity phase measurements beating the Standard Quantum Limit as well as to detect plasmonic effects. In this thesis we present our work consisting on designing and building an experimental setup for the generation of states of 4-photon entangled in the transverse linear momentum. In the first part we explain some basics in the form of a technical introduction. We then adapt a general theoretical model to our particular case, explaining the physics of the process of generation of entangled states of photons. We proceed explaining the details of our setup, as well as the procedure of measurement. We then present the results of our tests in a "standard" 2-photon regime to show the correct functioning of the setup itself. Some more sophisticated measurements using structured light in the form of a first-order Hermite-Gaussian mode has been performed to generate entanglement in higher spatial modes are also presented. We conclude with theoretical and experimental details regarding the 4-photon experiments feasible with our setup. Our source is based on Spontaneous Parametric Down-Conversion (SPDC). In the last two decades it has been extensively studied for the generation of two-photon entangled states, since Kwiat et al. published in 1995 the new scheme based on polarization entangled photons. Most of the studies regarding entangled states involving more than two photons are indeed based on quantum correlations in the polarization or in the path of the down converted beams. The great advantage of entangling a spatial degree of freedom (as we do) is related to the dimensionality of the Hilbert space in which the state is defined. In the particular case of linear momentum, which is a continuous variable, we are dealing with infinite dimensions. Since in each dimension of the state one quanta of information can be encoded, the advantage of having multidimension entanglement for the field of quantum communication and computing is clear. The specs of our setup include a periodically-poled KTP crystals offering high conversion efficiency and cut for type-0 SPDC. The photons generated are well known to be inherently entangled in different degrees of freedom such as polarization, energy-time, linear momentum, and orbital angular momentum. Since each single SPDC event generates one photon pair, in order to reach four photon we entangle two consecutive SPDC events per each state. Furthermore, it is possible to manipulate (to enhance) the dimensionality of the entanglement by properly tailoring the pump spatial profile. The key point for the generation of four-photon entangled states is that, under some conditions, two consecutive pairs generated via SPDC cannot be described as two independent pairs but necessarily as a four-photon state showing genuine entanglement. This can happen when using a pulsed laser to generate the entangled photons; in particular, in the regime in which the coherence time of the down-converted photons τc is (much) longer than the temporal length of the pumping pulse Δt. In this condition the emission of a second pair is stimulated by the presence of the first one. That effect can be ultimately seen as a constructive multi-particle interference effect, the deep origin of which has to be sought in the boson statistic of photons.
Semiconductor quantum dots are currently emerging as one of the most promising sources of non-classical light on which to base future quantum networks. They can generate single photons as well as pairs of entangled photons with unprecedented brightness, indistinguishability, and degree of entanglement. These features have very recently opened up the possibility to perform advanced quantum optics protocols that were previously inaccessible to single quantum emitters. In this work, we report on two experiments that use the non-local properties of entanglement to teleport quantum states: three-photon state teleportation and four-photon entanglement teleportation. We discuss all the experimental results in light of a theoretical model that we develop to account for the non-idealities of the quantum source. The excellent agreement between theory and experiment enables a deep understanding of how each parameter of the source affects the teleportation fidelities and it pinpoints the requirements needed to overcome the classical limits. Finally, our model suggests how to further improve quantum-dot entangled-photon sources for practical quantum networks.
High quality entangled photon sources are a key requirement for many promising quantum optical technologies. However, the production of multi-photon entangled states with good fidelity is challenging. Current sources of multi-photon entanglement require the use of post-selection, which limits their usefulness for some applications. It has been an open challenge to create a source capable of directly producing three-photon entanglement. An important step in this direction was achieved with the demonstration of photon triplets produced by a new process called cascaded downconversion, but these previous measurements were not sufficient to show whether these photons were in an entangled state and only had detection rates of five triplets per hour. In this thesis, we show the first demonstration of a direct source of three-photon entanglement. Our source is based on cascaded downconversion, and we verify that it produces genuine tripartite entanglement in two degrees of freedom: energy-time and polarization. \n \nThe energy-time entanglement is similar to a three-particle generalization of an Einstein-Podolski-Rosen state; the three photons are created simultaneously, yet the sum of their energies is well defined, which is an indication of energy-time entanglement. To prove it, we use time-bandwidth inequalities which check for genuine tripartite entanglement. Our measurements show that the state violates the inequalities with what constitute, to the best of our knowledge, the strongest violation of time-bandwidth inequalities in a tripartite continuous-variable system to date. \n \nWe create polarization entanglement by modifying our experimental setup so that two downconversion processes producing orthogonally polarized triplets interfere to create Greenberger-Horne-Zeilinger states. By using highly efficient superconducting nanowire single photon detectors, we improve the detected triplet rate by 2 orders of magnitude to 660 triplets per hour. We characterize the state using quantum state tomography, and find a fidelity of 86\\% with the ideal state, beating the previous best value for a three-photon entangled state fidelity measured by tomography. We also use the state to perform two tests of local realism. We violate the Mermin and Svetlichny inequalities by 10 and 5 standard deviations respectively, the latter being the strongest violation to date. Finally, we show that, unlike previous sources of tree-photon entanglement, our source can be used as a source of heralded Bell pairs. We demonstrate this by measuring a CHSH inequality with the heralded Bell pairs, and by reconstructing their state using quantum state tomography.
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.
Major efforts in quantum information science are devoted to the development of methods that are superior to the one of classical information processing, for example the quantum computer or quantum simulations. For these purposes, superposition and entangled states are considered a decisive resource. Furthermore, since the early days of quantum mechanics, entanglement has revealed the discrepancy between the quantum mechanical and the everyday life perception of the physical world. This combination of fundamental science and application-oriented research makes the realization, characterization, and application of entanglement a challenge pursued by many researchers. In this work, the observation of entangled states of polarization encoded photonic qubits is pushed forward in two directions: flexibility in state observation and increase in photon rate. To achieve flexibility two different schemes are developed: setup-based and entanglement-based observation of inequivalent multi-photon states. The setup-based scheme relies on multi-photon interference at a polarizing beam splitter with prior polarization manipulations. It allows the observation of a family of important four-photon entangled states. The entanglement-based scheme exploits the rich properties of Dicke states under particle projections or loss in order to obtain inequivalent multi-photon entangled states. The observed states are characterized using the fidelity and entanglement witnesses. An increase in photon rate is crucial to achieve entanglement of higher photon numbers. This holds especially, when photon sources are utilized that emit photons spontaneously. To this end, a new photon source is presented based on a femtosecond ultraviolet enhancement cavity and applied to the observation of the six-photon Dicke state with three excitations. The implemented schemes not only allow the observation of inequivalent types of entanglement, but also the realization of various quantum information tasks. In this work, the four-photon GHZ state has been used in a quantum simulation of a minimal instance of the toric code. This code enables the demonstration of basic properties of anyons, which are quasiparticles distinct from bosons and fermions. Further, the six-photon Dicke state has been applied for quantum metrology: It allows one to estimate a phase shift with a higher precision than by using only classical resources. Altogether, a whole series of experiments for observing inequivalent multi-photon entangled states can now be substituted by a single experimental setup based on the designs developed in this work. In addition to this new approach of photon processing, a novel photon source has been implemented, paving the way to realizations of applications requiring higher photon numbers.