In this thesis, we will present different theoretical tools for demonstrating the quantum features of possibly large photonic and spin systems. Our aim is not only to propose experiments demonstrating entanglement/non-locality in possibly large spin and photonic systems, but to clarify on the requirements that the detection systems need to fulfil to detect the quantum nature of these large systems. We will see that collective projections in which the subsystems are measured along the same direction, does not prevent the detection of entanglement and non-locality. Even threshold detectors with low efficiencies can be used to reveal non-classical features of states involving many photons. In parallel, we will make steps to clarify on the meaning of large/macroscopic when describing quantum states.
In this thesis, we will present different theoretical tools for demonstrating the quantum features of possibly large photonic and spin systems. Our aim is not only to propose experiments demonstrating entanglement/non-locality in possibly large spin and photonic systems, but to clarify on the requirements that the detection systems need to fulfil to detect the quantum nature of these large systems. We will see that collective projections in which the subsystems are measured along the same direction, does not prevent the detection of entanglement and non-locality. Even threshold detectors with low efficiencies can be used to reveal non-classical features of states involving many photons. In parallel, we will make steps to clarify on the meaning of large/macroscopic when describing quantum states.
In this work, three applications aimed at studying or exploiting various aspects of entanglement are considered. In the first project, the entanglement between two atoms inside a multimode resonator is investigated in the presence of retardation. Retardation is associated with the finite time required by a photon to propagate between atoms and cavity boundaries. It is found that retardation affects the atomic populations as well as the entanglement dynamics to a large degree. The second project is a study of entangled states of light to obtain an enhanced resolution. We have simulated optical centroid measurements for spatial resolution enhancement with various types of non-classical input states. By numerically simulating the measurement scheme, we optimize the detection parameters for an experimental implementation and also study the multiphoton absorption required for quantum lithography. The third project uses the scattered light from a resonantly driven correlated system to obtain information about the system. Techniques have been proposed using which in certain detection directions, n-atom correlations can be directly accessed in an experiment via light scattering with a significant count rate. Moreover, such detection of correlations is not limited to a particular spatial geometry but can be utilized for generalized geometries, too.
This dissertation deals with theoretical and experimental research on nonclassical, or entangled, states of atoms and photons. In the first part, I describe two approaches to the preparation of entangled states of a large number of atoms. The first approach is based on the transfer of quantum correlations from non-classical light to the atomic spins. I consider three different situations: (a) the atoms are placed in a loss-free cavity and no relaxation of any kind is present; (b) the atoms are placed in a cavity with input and output for the electromagnetic field and the atoms spontaneously decay from the upper state; (c) there is no cavity around the atoms and spontaneous emission is present. I show that in all three situations with judicious choice of parameters, entangled samples of atoms can be produced. The second approach is based on quantum-nondemolition (QND) measurements of collective atomic operators. I describe experiments with pulsed and cw light as the probe, that result in squeezed spin states of atoms. When a rf magnetic field is applied to the atomic sample, sub- shot noise performance of an atomic spin interferometer is demonstrated. In the second part of the thesis I describe investigations of phase properties of two-photon and single-photon states. These experiments make use of spontaneous type-II down-conversion in a nonlinear crystal. First, interferometric measurement of a phase shift at the Heisenberg limit for a two-photon state is described. Next, a violation of Bell-type inequalities in phase space is demonstrated for the quantum optical version of the Einstein-Podolsky-Rosen state. Finally, some recent work directed toward homodyning a single photon against a strong local oscillator field is described.
In this thesis we present methods to efficiently verify quantum many-body experiments. The first part focuses on the quantitative verification of graph state experiments from simple measurements. In particular, optimal bounds on the fidelity, purity and entropy from stabilizer measurements are derived as well as high-quality bounds on entanglement measures. Furthermore, we show how entanglement may be quantified in spin and cold atom many-body systems using standard experimental techniques only. The scheme requires no assumptions on the state in the laboratory, and a lower bound to the entanglement can be read off directly from the scattering cross section of neutrons deflected from solid state samples or the time-of-flight distribution of cold atoms in optical lattices, respectively. Quantum mechanics offers many interesting properties such as entanglement and non-locality. However, one can easily think of more general correlations than quantum correlations. This gives rise to the question if quantum correlations are the most general allowed in nature? In the last part we propose a new axiom we call macroscopic locality. The idea behind this axiom is that any physical theory should recover classical physics in the continuum limit (i.e., when a large number of particles is involved and our measurement devices fail to resolve discrete particles). We will show that this very intuitive axiom, together with the no-signaling principle, allows to recover many important results in quantum mechanics, like the universality of the Tsirelson bound, or the set of accessible two-point correlators. Moreover, we will provide a complete characterization of the correlations between two distant physical systems that arise out of both axioms and comment on its consistency.
In quantum physics, several concepts have been developed to capture the distinctive characteristics of quantum states that have no classical equivalent. Among the most important are non-locality, entanglement and non-classicality, the latter generally identi- fied with the negativity of quasi-probability distributions in phase space. More recently, the idea of anticoherence, defined by the isotropy of spin states resulting from quantum superpositions, has emerged as another distinctive feature of quantum states. Although these different forms of “quantumness” are becoming increasingly central to the understanding and development of quantum foundations and quantum technologies, and are becoming experimentally accessible, their fundamental nature and interconnec- tions are still only partially understood. The aim of this thesis is two-fold. First, we study and characterise the quantum nature of spin states through entanglement, non-classicality and anticoherence, with a particular focus on the relationships between these concepts. Second, we propose and analyse experimental protocols for estimating the entanglement of multiqubit states and for generating maximally entangled anticoherent states, thereby paving the way for their use in future quantum applications.
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 rapidly developing interdisciplinary field of quantum information, which merges quantum and information science, studies non-classical aspects of quantum systems. These studies are motivated by the promise that the non-classicality can be used to solve tasks more efficiently than classical methods would allow. In many quantum informational studies, non-classical behaviour is attributed to the notion of entanglement. In this thesis we use photons to experimentally investigate fundamental questions such as: What happens to the entanglement in a system when it is affected by noise? In our study of noisy entanglement we pursue the challenging task of creating bound entanglement. Bound entangled states are created through an irreversible process that requires entanglement. Once in the bound regime, entanglement cannot be distilled out through local operations assisted by classical communication. We show that it is possible to experimentally produce four-photon bound entangled states and that a violation of a Bell inequality can be achieved. Moreover, we demonstrate an entanglement-unlocking protocol by relaxing the condition of local operations. We also explore the non-classical nature of quantum mechanics in several single-photon experiments. In these experiments, we show the violation of various inequalities that were derived under the assumption of non-contextuality. Using qutrits we construct and demonstrate the simplest possible test that offers a discrepancy between classical and quantum theory. Furthermore, we perform an experiment in the spirit of the Kochen-Specker theorem to illustrate the state-independence of this theorem. Here, we investigate whether or not measurement outcomes exhibit fully contextual correlations. That is, no part of the correlations can be attributed to the non-contextual theory. Our results show that only a small part of the experimental generated correlations are amenable to a non-contextual interpretation.