Quantum optics has offered new possibilities for experimental tests of basic principles of quantum mechanics. It enables us to experimentally investigate such phenomena as quantum interference and quantum non-locality. By means of cascade transitions or spontaneous parametric down- conversion, entangled photon pairs can be prepared. It makes possible to test the violation of Bell's inequalities and to study other issues connected with the question of completeness of quantum theory. The results of experiments of this kind pronounce in favor of quantum mechanics. The entangled pairs can further be used to demonstrate the violation of other classical inequalities, `non-local' interference in frequency domain, induced interference without induced emission, etc. They are also employed for quantum teleportation. Another family of interesting quantum optical experiments relates to single-photon interference. These experiments demonstrate wave-particle duality and correspondence between the interference visibility and the degree of knowledge of the photon's path in an interferometer. A spectacular example is the so-called interaction-free measurement. Quantum optical experiments open a new window on the quantum world and help us understand it. Further, quantum optics provides the ground for new interesting applications: quantum cryptography, quantum communications, and quantum computation.
Quantum optics has offered new possibilities for experimental tests of basic principles of quantum mechanics. It enables us to experimentally investigate such phenomena as quantum interference and quantum non-locality. By means of cascade transitions or spontaneous parametric down- conversion, entangled photon pairs can be prepared. It makes possible to test the violation of Bell's inequalities and to study other issues connected with the question of completeness of quantum theory. The results of experiments of this kind pronounce in favor of quantum mechanics. The entangled pairs can further be used to demonstrate the violation of other classical inequalities, `non-local' interference in frequency domain, induced interference without induced emission, etc. They are also employed for quantum teleportation. Another family of interesting quantum optical experiments relates to single-photon interference. These experiments demonstrate wave-particle duality and correspondence between the interference visibility and the degree of knowledge of the photon's path in an interferometer. A spectacular example is the so-called interaction-free measurement. Quantum optical experiments open a new window on the quantum world and help us understand it. Further, quantum optics provides the ground for new interesting applications: quantum cryptography, quantum communications, and quantum computation.
From the point of view of classical theory and our everyday experience, quantum phenomena are surprising and the concepts of quantum mechanics are incomprehensible. Since the beginning of quantum theory scientists questioned its very basic concepts, such as locality, reality, and complementarity, and proposed gedankenexperiments , or thought experiments, to probe its foundations. In the light of new optical technology we have been able to realize some of these experiments. This chapter will focus on three of them: (1) EPR-Bohm-Bell correlation and Bell’s inequality; (2) Scully’s quantum eraser; (3) Popper’s experiment. The results of these experiments are very interesting. On one hand, the experimental observations confirm the predictions of EPR-Bell, Scully, and Popper. On the other hand, the calculations from quantum theory perfectly agree with the experimental data. Moreover, apparently, the experimental observations do not lead to any “violations” of the principles of quantum mechanics. One important conclusion we may draw from these optical tests is that all the observations are the results of multi-photon interference: a group of photons interferes with the group itself at distance. The nonlocal multi-photon interference phenomena may never be understood in classical theory, however, it is legitimate in quantum mechanics. The superposition principle of quantum theory supports the superposition of multi-photon amplitudes, whether the photons are entangled or randomly grouped and despite the distances between these individual photodetection events. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Over the past three decades or so, experiments of the type called Gedanken have become real. Recall the Schrödinger quote from Chapter 8: “… we never experiment with just one electron or atom or (small) molecule.” This is no longer true. We can do experiments involving single atoms or molecules and even on single photons, and thus it becomes possible to demonstrate that the “ridiculous consequences” alluded to by Schrödinger are, in fact, quite real. We have already discussed some examples of single-photon experiments in Chapter 6, and in Chapter 10 we shall discuss experiments performed with single atoms and single trapped ions. In the present chapter, we shall elaborate further on experimental tests of fundamentals of quantum mechanics involving a small number of photons. By fundamental tests we mean tests of quantum mechanics against the predictions of local realistic theories (i.e. hidden variable theories). Specifically, we discuss optical experiments demonstrating violations of Bell's inequalities, violations originally discussed by Bell in the context of two spin-one-half particles. Such violations, if observed experimentally, falsify local realistic hidden-variable theories. Locality refers to the notion, familiar in classical physics, that there cannot be a causal relationship between events with space-like separations. That is, the events cannot be connected by any signal moving at, or less than, the speed of light; i.e. the events are outside the light-cone. But in quantum mechanics, it appears that nonlocal effects, effects seemly violating the classical notion of locality in a certain restricted sense, are possible.
The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single- and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that nature cannot be local, that objective randomness exists and about the emergence of a classical world. It is suggestive that information plays a fundamental role also in the foundations of quantum physics.
Although most physics experiments are carried out with independent particles, it is the collective nature of entangled particles that reveals the most fascinating and unexpected aspects of the quantum world. It was Erwin Schrodinger who first said “entanglement is not one but rather the characteristic trait of quantum mechanics”. One curious aspect of the behavior of a pair of particles in an entangled state is that, though each individual particle exhibits an inherent uncertainty, the joint entity of an entangled pair can exhibit no such uncertainty. As an example, while the time of arrival of an individual particle may be totally random, an entangled pair must always arrive simultaneously. This property offers a unique tool for carrying out absolute measurements. Our goal here is to explore the myriad implications and significance of entanglement and to exploit it for the development of a new type of optical measurement—quantum optical metrology. The existence of unique non-classical correlations between twin photons generated in the nonlinear process of spontaneous parametric origins. The non-classical link between such twin quanta is not diminished by arbitrarily large separations between the twins, even when they lie outside the light cone. Twin states have been used with great effectiveness over the past two decades for carrying out definitive quantum experiments that lead to counterintuitive results; among these are those arising from the EinsteinPodolsky-Rosen (EPR) paradox such as various tests of Bell’s inequalities [1-12], as well as non-local dispersion cancellation, entangled-photon–induced transparency, and entangled-photon spectroscopy with monochromatic light. The availability of these twin beams has made it possible to conduct such experiments without having to resort to
Single-particle interference phenomena can be observed with neutrons, and the "entanglement of degrees of freedom", i.e. the contextuality, can be verified and used in further experiments. Entanglement of two photons or atoms is a complementary situation to the double-slit diffraction of a single photon, neutron, or atom. In this respect, neutrons are proper tools for testing quantum mechanics, because they are massive, they couple to electromagnetic fields due to their magnetic moment, and they are subject to all basic interactions, and they are sensitive to topological effects as well. The4π-symmetry of spinor wave functions, the spin-superposition law, and many topological phenomena can be made visible, which shows interesting intrinsic features of quantum physics. Related experiments will be discussed. Deterministic and stochastic partial absorption experiments can be described by Bell-type inequalities. Recent neutron interferometry experiments based on post-selection methods have renewed the discussion about quantum non-locality and the quantum measuring process. It has been shown that interference phenomena can be revived even when the overall interference pattern has lost its contrast. This indicates a persisting coupling in the phase space even in cases of spatially separated Schrödinger cat-like situations. These states are extremely fragile and sensitive against any kind of fluctuations and other de-coherence processes. More complete quantum experiments also show that a complete retrieval of quantum states behind an interaction volume becomes impossible in principle.
Multiphoton interference reveals strictly nonclassical phenomena. Its applications range from fundamental tests of quantum mechanics to photonic quantum information processing, where a significant fraction of key experiments achieved so far comes from multiphoton state manipulation. The progress, both theoretical and experimental, of this rapidly advancing research is reviewed. The emphasis is given to the creation of photonic entanglement of various forms, tests of the completeness of quantum mechanics (in particular, violations of local realism), quantum information protocols for quantum communication (e.g., quantum teleportation, entanglement purification, and quantum repeater), and quantum computation with linear optics. The scope of the review is limited to ``few-photon'' phenomena involving measurements of discrete observables.