Littérature scientifique sur le sujet « Quantum phase transitions, Entanglement, Information theory »

Invariance under Lorentz transformations is fundamental to both the standard model and general relativity. Testing Lorentz-symmetry violation (LSV) via atomic systems attracts extensive interests in both theory and experiment. In several test proposals, the LSV violation effects are described as a local interaction and the corresponding test precision can asymptotically reach the Heisenberg limit via increasing quantum Fisher information (QFI), but the limited resolution of collective observables prevents the detection of large QFI. Here, we propose a multimode many-body quantum interferometry for testing the LSV parameter κ via an ensemble of spinor atoms. By employing an N-atom multimode GHZ state, the test precision can attain the Heisenberg limit Δκ∝1/(F2N) with the spin length F and the atom number N. We find a realistic observable (i.e. practical measurement process) to achieve the ultimate precision and analyze the LSV test via an experimentally accessible three-mode interferometry with Bose condensed spin-1 atoms for example. By selecting suitable input states and unitary recombination operation, the LSV parameter κ can be extracted via realizable population measurement. Especially, the measurement precision of the LSV parameter κ can beat the standard quantum limit and even approach the Heisenberg limit via spin mixing dynamics or driving through quantum phase transitions. Moreover, the scheme is robust against nonadiabatic effect and detection noise. Our test scheme may open up a feasible way for a drastic improvement of the LSV tests with atomic systems and provide an alternative application of multi-particle entangled states.

Entanglement is one of the most intriguing features of quantum theory and a main resource in quantum information science. Ground states of quantum many-body systems with local interactions typically obey an “area law” which means that the entanglement entropy is proportional to the boundary length. It is exceptional when the system is gapless, and the area law had been believed to be violated by at most a logarithm over two decades. Recent discovery of Motzkin and Fredkin spin chain models is striking, since these models provide significant violation of the entanglement beyond the belief, growing as a square root of the volume in spite of local interactions. In this paper, we first analytically compute the Rényi entropy of the Motzkin and Fredkin models by careful treatment of asymptotic analysis. The Rényi entropy is an important quantity, since the whole spectrum of an entangled subsystem is reconstructed once the Rényi entropy is known as a function of its parameter. We find nonanalytic behavior of the Rényi entropy with respect to the parameter, which is a novel phase transition never seen in any other spin chain studied so far. Interestingly, similar behavior is seen in the Rényi entropy of Rokhsar–Kivelson states in two dimensions.

We provide an introduction to the use of ion crystals in a Penning trap for experiments in quantum information. Macroscopic Penning traps allow for the containment of a few to a few million atomic ions whose internal states may be used in quantum information experiments. Ions are laser Doppler cooled, and the mutual Coulomb repulsion of the ions leads to the formation of crystalline arrays. The structure and dimensionality of the resulting ion crystals may be tuned using a combination of control laser beams and external potentials. We discuss the use of two-dimensional $^{9}$Be$^{+}$ ion crystals for experimental tests of quantum control techniques. Our primary qubit is the 124 GHz ground-state electron spin flip transition, which we drive using microwaves. An ion crystal represents a spatial ensemble of qubits, but the effects of inhomogeneities across a typical crystal are small, and as such we treat the ensemble as a single effective spin. We are able to initialize the qubits in a simple state and perform a projective measurement on the system. We demonstrate full control of the qubit Bloch vector, performing arbitrary high-fidelity rotations ($\tau_{\pi}\sim$200 $\mu$s). Randomized Benchmarking demonstrates an error per gate (a Pauli-randomized $\pi/2$ and $\pi$ pulse pair) of $8\pm1\times10^{-4}$. Ramsey interferometry and spin-locking measurements are used to elucidate the limits of qubit coherence in the system, yielding a typical free-induction decay coherence time of $T_{2}\sim$2 ms, and a limiting $T_{1\rho}\sim$688 ms. These experimental specifications make ion crystals in a Penning trap ideal candidates for novel experiments in quantum control. As such, we briefly describe recent efforts aimed at studying the error-suppressing capabilities of dynamical decoupling pulse sequences, demonstrating an ability to extend qubit coherence and suppress phase errors. We conclude with a discussion of future avenues for experimental exploration, including the use of additional nuclear-spin-flip transitions for effective multiqubit protocols, and the potential for Coulomb crystals to form a useful testbed for studies of large-scale entanglement.

Multi-Species entanglement, defined for a many-particle system as the entanglement between different species of particles, is shown to exist in the thermodynamic limit of the system size going to infinity. This macroscopic entanglement, as it can exhibit singular behavior, is capable of tracking quantum phase transitions. The entanglement between up and down spins has been analytically calculated for the one-dimensional Ising model in a transverse magnetic field. As the coupling strength is varied, the first derivative of the entanglement shows a jump discontinuity and the second derivative diverges near the quantum critical point.

We study ground state pairwise entanglement within one-dimensional spin-1/2 antiferromagnetic J1–J2 model with competing interactions. Contrary to some claims we found that frustration does not increase entanglement. Concurrence of nearest and next nearest neighbors are found to show abrupt change at phase transition points. We also show that the concurrence can be used to classify the phase diagram of the model in anisotropy–frustration plane.

A microscopic calculation of ground state entanglement for the XY and Heisenberg models shows the emergence of universal scaling behavior at quantum phase transitions. Entanglement is thus controlled by conformal symmetry. Away from the critical point, entanglement gets saturated by a mass scale. Results borrowed from conformal field theory imply irreversibility of entanglement loss along renormalization group trajectories. Entanglement does not saturate in higher dimensions which appears to limit the success of the density matrix renormalization group technique. A possible connection between majorization and renormalization group irreversibility emerges from our numerical analysis.

The nature of entanglement in many-body systems is a focus of intense research with the observation that entanglement holds interesting information about quantum correlations in large systems and their relation to phase transitions. In particular, it is well known that although generic, many-body states have large, extensive entropy, ground states of reasonable local Hamiltonians carry much smaller entropy, often associated with the boundary length through the so-called area law. Here we introduce a continuous family of frustration-free Hamiltonians with exactly solvable ground states and uncover a remarkable quantum phase transition whereby the entanglement scaling changes from area law into extensively large entropy. This transition shows that entanglement in many-body systems may be enhanced under special circumstances with a potential for generating “useful” entanglement for the purpose of quantum computing and that the full implications of locality and its restrictions on possible ground states may hold further surprises.

We develop entanglement perturbation theory (EPT) for infinite Quasi-1D quantum systems. The spin-1/2 Heisenberg chain with ferromagnetic nearest neighbor (NN) and antiferromagnetic next nearest neighbor (NNN) interactions with an easy-plane anisotropy is studied as a prototypical system. The obtained phase diagram is compared with a recent prediction [Phys. Rev. B 81, 094430 (2010)] that dimer and Néel orders appear alternately as the XXZ anisotropy Δ approaches the isotropic limit Δ = 1. The first and second transitions (across dimer, Néel and dimer phases) are detected with improved accuracy at Δ ≈ 0.722 and 0.930. The third transition (from dimer to Néel phases), previously predicted to be at Δ ≈ 0.98, is not detected at this Δ in our method, strongly indicating that the second Néel phase is absent.

The entanglement properties of correlated wave functions commonly employed in theories of strongly correlated many-body systems are studied. The variational treatment of the transverse Ising model within correlated-basis theory is reviewed, and existing calculations of the one- and two-body reduced density matrices are used to evaluate or estimate established measures of bipartite entanglement, including the Von Neumann entropy, the concurrence, and localizable entanglement, for square, cubic, and hypercubic lattice systems. The results discussed in relation to the findings of previous studies that explore the relationship of entanglement behaviors to quantum critical phenomena and quantum phase transitions. It is emphasized that Jastrow-correlated wave functions and their extensions contain multipartite entanglement to all orders.

Quantum discord is a more general measure of quantum correlations than entanglement and has been proposed as a resource in certain quantum information processing tasks. The computation of discord is mostly confined to two-qubit systems for which an analytical calculational scheme is available. The utilization of quantum correlations in quantum information-based applications is limited by the problem of decoherence, i.e., the loss of coherence due to the inevitable interaction of a quantum system with its environment. The dynamics of quantum correlations due to decoherence may be studied in the Kraus operator formalism for different types of quantum channels representing system-environment interactions. In this review, we describe the salient features of the dynamics of classical and quantum correlations in a two-qubit system under Markovian (memoryless) time evolution. The two-qubit state considered is described by the reduced density matrix obtained from the ground state of a spin model. The models considered include the transverse-field XY model in one dimension, a special case of which is the transverse-field Ising model, and the XXZ spin chain. The quantum channels studied include the amplitude damping, bit-flip, bit-phase-flip and phase-flip channels. The Kraus operator formalism is briefly introduced and the origins of different types of dynamics discussed. One can identify appropriate quantities associated with the dynamics of quantum correlations which provide signatures of quantum phase transitions in the spin models. Experimental observations of the different types of dynamics are also mentioned.

From the seminal ideas of Feynman and until now, quantum information and computation has been a rapidly evolving field. While at the beginning, physicists looked at quantum mechanics as a theoretical framework to describe the fundamental processes that take place in Nature, it was during the 80’s and 90’s that people began to think about the intrinsic quantum behavior of our world as a tool to eventually develop powerful information technologies. As Landauer pointed out, information is physical, so it should not look strange to try to bring together quantum mechanics and information theory. Indeed, it was soon realized that it is possible to use the laws of quantum physics to perform tasks which are unconceivable within the framework of classical physics. For instance, the discovery of quantum teleportation, superdense coding, quantum cryptography, Shor’s factorization algorithm or Grover’s searching algorithm, are some of the remarkable achievements that have attracted the attention of many people, both scientists and non-scientists. This settles down quantum information as a genuine interdisciplinary field, bringing together researchers from different branches of physics, mathematics and engineering. While until recently it was mostly quantum information science that benefited from other fields, today the tools developed within its framework can be used to study problems of different areas, like quantum many-body physics or quantum field theory. The basic reason behind that is the fact that quantum information develops a detailed study of quantum correlations, or quantum entanglement. Any physical system described by the laws of quantum mechanics can then be considered from the perspective of quantum information by means of entanglement theory. It is the purpose of this introduction to give some elementary background about basic concepts of quantum information and computation, together with its possible relation to other fields of physics, like quantum many-body physics. We begin by considering the definition of a qubit, and move then towards the definition of entanglement and the convertibility properties of pure states by introducing majorization and the von Neumann entropy. Then, we consider the notions of quantum circuit and quantum adiabatic algorithm, and move towards what is typically understood by a quantum phase transition, briefly sketching how this relates to renormalization and conformal field theory. We also comment briefly on some possible experimental implementations of quantum computers
Desde las pioneras ideas de Feynman hasta el día de hoy, la información y computación cuánticas han evolucionado de forma veloz. Siendo la mecánica cuántica en sus orígenes considerada esencialmente como un marco teórico en el que poder explicar ciertos procesos fundamentales que acontecían en la Naturaleza, fue durante los años 80 y 90 cuando se empezó a pensar sobre el comportamiento intrínsecamente cuántico del mundo en el que vivimos como una herramienta con la que poder desarrollar tecnologías de la información más potentes, basadas en los mismos principios de la física cuántica. Tal y como Landauer dijo, la información es física, por lo que no debe en absoluto extrañarnos el que se intentara comulgar la mecánica cuántica con la teoría de la información. Y nada más lejos de la realidad, pues pronto se vio que era posible utilizar las leyes de la física cuántica para realizar tareas inconcebibles desde un punto de vista clásico. Por ejemplo, el descubrimiento de la teleportación, la codificación superdensa, la criptografía cuántica, el algoritmo de factorización de Shor o el algoritmo de búsqueda de Grover, constituyen algunos de los logros remarcables que han atraído la atención de mucha gente, dentro y fuera de la ciencia. Queda la información cuántica, pues, constituida como un campo genuinamente pluridisciplinar, en el que se concentran investigadores provenientes de diferentes ramas de la física, las matemáticas y la ingeniería. Mientras en sus orígenes era la información cuántica quien se beneficiaba del conocimiento de otros campos, a día de hoy las herramientas desarrolladas en el marco de la teoría cuántica de la información pueden ser asimismo usadas en el estudio de problemas de diferentes áreas, como la física de muchos cuerpos o la teoría cuántica de campos. Ello es debido al estudio detallado que la información cuántica desarrolla de las correlaciones cuánticas, o entrelazamiento cuántico. Cualquier sistema físico descrito por las leyes de la mecánica cuántica se puede por lo tanto considerar bajo la perspectiva de la teoría cuántica de la información a través de la teoría del entrelazamiento.

We investigate the geometric phase (GP) acquired by the states of a spin-1/2 nucleus which is subject to a static magnetic field. This nucleus as the carrier system of GP, is taken as coupled to a dissipative environment, so that it evolves non-unitarily. We study the effects of different characteristics of different environments on GP as nucleus evolves in time. We showed that magnetic field strength is the primary physical parameter that determines the stability of GP
its stability decreases as the magnetic field strength increases. (By decrease in stability what we mean is the increase in the time rate of change of GP.) We showed that this decrease can be very rapid, and so it could be impossible to make use of it as a quantum logic gate in quantum information theory (QIT). To see if these behaviors differ in different environments, we analyze the same system for a fixed temperature environment which is under the influence of an electromagnetic field in a squeezed state. We find that the general dependence of GP on magnetic field does not change, but this time the effects are smoother. Namely, increase in magnetic field decreases the stability of GP also for in this environment
but this decrease is slower in comparison with the former case, and furthermore it occurs gradually. As a second problem we examine the entanglement of two atoms, which can be used as a two-qubit system in QIT. The entanglement is induced by an external quantum system. Both two-level atoms are coupled to a third two-level system by dipole-dipole interaction. The two atoms are assumed to be in ordinary vacuum and the third system is taken as influenced by a certain environment. We examined different types of environments. We show that the steady-state bipartite entanglement can be achieved in case the environment is a strongly fluctuating, that is a squeezed-vacuum, while it is not possible for a thermalized environment.

On a compris ces dernières années que certaines mesures d'intrications sont un outil efficace pour la compréhension et la caractérisation de phases nouvelles et exotiques de la matière, en particulier lorsque les méthodes traditionnelles basées sur l'identification d'un paramètre d'ordre sont insuffisantes. Cette thèse porte sur l'étude de quelques systèmes quantiques à basse dimension où un telle approche s'avère fructueuse. Parmi ces mesures, l'entropie d'intrication, définie via une bipartition du système quantique, est probablement la plus populaire, surtout à une dimension. Celle-ci est habituellement très difficile à calculer en dimension supérieure, mais nous montrons ici que le calcul se simplifie drastiquement pour une classe particulière de fonctions d'ondes, nommées d'après Rokhsar et Kivelson. L'entropie d'intrication peut en effet s'exprimer comme une entropie de Shannon relative à la distribution de probabilité générée par les composantes de la fonction d'onde du fondamental d'un autre système quantique, cette fois-ci unidimensionnel. Cette réduction dimensionnelle nous permet d'étudier l'entropie aussi bien par des méthodes numériques (fermions libres, diagonalisations exactes, ...) qu'analytiques (théories conformes). Nous argumentons aussi que cette approche permet d'accéder facilement à certaines caractéristiques subtiles et universelles d'une fonction d'onde donnée en général.Une autre partie de cette thèse est consacrée aux trempes quantiques locales dans des systèmes critiques unidimensionnels. Nous insisterons particulièrement sur une quantité appelée écho de Loschmidt, qui est le recouvrement entre la fonction d'onde avant la trempe et la fonction d'onde à temps t après la trempe. En exploitant la commensurabilité du spectre de la théorie conforme, nous montrons que l'évolution temporelle doit être périodique, et peut même être souvent obtenue analytiquement. Inspiré par ces résultats, nous étudions aussi la contribution de fréquence nulle à l'écho de Loschmidt après la trempe. Celle-ci s'exprime comme un simple produit scalaire -- que nous nommons fidélité bipartie -- et est une quantité intéressante en elle-même. Malgré sa simplicité, son comportement se trouve être très similaire à celui de l'entropie d'intrication. Pour un système critique unidimensionnel en particulier, notre fidélité décroît algébriquement avec la taille du système, un comportement rappelant la célèbre catastrophe d'Anderson. L'exposant est universel et relié à la charge centrale de la théorie conforme sous-jacente
In recent years, it has been understood that entanglement measures can be useful tools for the understanding and characterization of new and exotic phases of matter, especially when the study of order parameters alone proves insufficient. This thesis is devoted to the study of a few low-dimensional quantum systems where this is the case. Among these measures, the entanglement entropy, defined through a bipartition of the quantum system, has been perhaps one of the most heavily studied, especially in one dimension. Such a quantity is usually very difficult to compute in dimension larger than one, but we show that for a particular class of wave functions, named after Rokhsar and Kivelson, the entanglement entropy of an infinite cylinder cut into two parts simplifies considerably. It can be expressed as the Shannon entropy of the probability distribution resulting from the ground-state wave function of a one-dimensional quantum system. This dimensional reduction allows for a detailed numerical study (free fermion, exact diagonalizations, \ldots) as well as an analytic treatment, using conformal field theory (CFT) techniques. We also argue that this approach can give an easy access to some refined universal features of a given wave function in general.Another part of this thesis deals with the study of local quantum quenches in one-dimensional critical systems. The emphasis is put on the Loschmidt echo, the overlap between the wave function before the quench and the wave function at time t after the quench. Because of the commensurability of the CFT spectrum, the time evolution turns out to be periodic, and can be obtained analytically in various cases. Inspired by these results, we also study the zero-frequency contribution to the Loschmidt echo after such a quench. It can be expressed as a simple overlap -- which we name bipartite fidelity -- and can be studied in its own right. We show that despite its simple definition, it mimics the behavior of the entanglement entropy very well. In particular when the one-dimensional system is critical, this fidelity decays algebraically with the system size, reminiscent of Anderson's celebrated orthogonality catastrophe. The exponent is universal and related to the central charge of the underlying CFT

Pour d non puissance d'un nombre premier, le nombre maximal de bases deux à deux décorrélées d'un espace de Hilbert de dimension d n'est pas encore connu. Dans ce mémoire, nous commençons par donner une construction de bases décorrélées en lien avec une famille de représentations irréductibles de l'algèbre de Lie su(2) et faisant appel aux sommes de Gauss.Puis nous étudions de façon systématique la possibilité de construire de telle bases au moyen des opérateurs de Pauli. 1) L'étude de la droite projective sur Zdm montre que, pour obtenir des ensembles maximaux de bases décorrélées à l'aide d'opérateurs de Pauli, il est nécessaire de considérer des produits tensoriels de ces opérateurs. 2) Les sous-modules lagrangiens de Zd2n, dont nous donnons une classification complète, rendent compte des ensembles maximalement commutant d'opérateurs de Pauli. Cette classification permet de savoir lesquels de ces ensembles sont susceptibles de donner des bases décorrélées : ils correspondent aux demi-modules lagrangiens, qui s'interprètent encore comme les points isotropes de la droite projective (P(Mat(n, Zd)²),ω). Nous explicitons alors un isomorphisme entre les bases décorrélées ainsi obtenues et les demi-modules lagrangiens distants, ce qui précise aussi la correspondance entre sommes de Gauss et bases décorrélées. 3) Des corollaires sur le groupe de Clifford et l'espace des phases discret sont alors développés.Enfin, nous présentons quelques outils inspirés de l'étude précédente. Nous traitons ainsi du rapport anharmonique sur la sphère de Bloch, de géométrie projective en dimension supérieure, des opérateurs de Pauli continus et nous comparons l'entropie de von Neumann à une mesure de l'intrication par calcul d'un déterminant.

In this thesis we have studied different aspects of the novel field of quantum information with continuous variables. The higher efficiency and bandwidth of homodyne detection combined with the easiness of generation and manipulation of Gaussian states makes continuous-variable quantum information a promising and flourishing field of research. This dissertation is divided in two parts. The first part explores two applications of the “photon subtraction” operation; Firstly, a technique to generate highly non-Gaussian single-mode states of light; Secondly, an experimental setup capable of realizing a loophole-free Bell test. The second part of this dissertation develops a detailed analysis of an important family of continuous-variable quantum key distribution protocols, namely those based on Gaussian modulation of Gaussian states./Dans cette thèse on a étudié différents aspects de l'information quantique à variables continues. Les meilleures efficacité et bande passante de la détection homodyne combinées à la simplicité de génération et de manipulation d'états gaussiens rend l'information quantique à variables continues un domaine de recherche très prometteur, qui est actuellement en plein essor. La dissertation est divisée en deux parties. La première explore deux applications de l'opération “soustraction de photon”; en premier lieu on présente une nouvelle technique capable de générer des états mono-modaux de la lumière hautement non-gaussiens; deuxiemement on présente un schéma expérimental capable de réaliser un test de Bell sans faille logique. La deuxième partie de cette dissertation développe une étude détaillée d'une famille très importante de protocoles de distribution quantique de clé à variables continues, ceux basés sur la modulation gaussienne d'états gaussiens.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Dans cette thèse, nous avons répondu à certaines questions ouverts dans le domaine de la dynamique hors équilibre des systèmes quantiques unidimensionnels fermés. Durant ces dernières années, les avancées dans les techniques expérimentales ont revitalisé la recherche théorique en physique de la matière condensée et dans l'optique quantique. Nous avons traité trois sujets différents et en utilisant des techniques à la fois numériques et analytiques. Dans le cadre des techniques numériques, nous avons utilisé des méthodes de diagonalisation exacte, l'algorithme du groupe de renormalisation de la matrice densité en fonction du temps (t-DMRG) et l'algorithme de Lanczos. Au début, nous avons étudié la dynamique quantique adiabatique d'un système quantique près d'un point critique. Nous avons démontré que la présence d'un potentiel de confinement modifie fortement les propriétés d'échelle de la dynamique des observables en proximité du point critique quantique. La densité d'excitations moyenne et l'excès d'énergie, après le croisement du point critique, suivent une loi algébrique en fonction de la vitesse de la trempe avec un exposant qui dépend des propriétés spatio-temporelles du potentiel. Ensuite, nous avons étudié le comportement de bosons ultra-froids dans un réseau optique incliné. En commençant par l'hamiltonien de Bose-Hubbard, dans la limite de Hard-Core bosons, nous avons développé une théorie hydrodynamique qui reproduit exactement l'évolution temporelle d'une partie des observables du système. En particulier, nous avons observé qu'une partie de bosons reste piégée, et oscille avec une fréquence qui dépend de la pente du potentiel, au contraire, une autre partie est expulsée hors de la rampe. Nous avons également analysé la dynamique du modèle de Bose-Hubbard en utilisant l'algorithme t-DMRG et l'algorithme de Lanczos. De cette façon, nous avons mis en évidence le rôle de la non-intégrabilité du modèle dans son comportement dynamique. Enfin, nous avons abordé le problème de la thermalisation dans un système quantique étendu. À partir de considérations générales, nous avons introduit la notion de profil de température hors équilibre dans une chaîne des bosons à coeur dure. Nous avons analysé la dynamique du profil de temperature et, notamment, ses propriétés d'échelle
In this thesis we have addressed some open questions on the out-of-equilibrium dynamics of closed one-dimensional quantum systems. In recent years, advances in experimental techniques have revitalized the theoretical research in condensed matter physics and quantum optics. We have treated three different subjects using both numerical and analytical techniques. As far as the numerical techniques are concerned, we have used essentially exact diagonalization methods, the adaptive time-dependent density-matrix renormalization-group algorithm (t-DMRG) and the Lanczos algorithm. At first, we studied the adiabatic quantum dynamics of a quantum system close to a critical point. We have demonstrated that the presence of a confining potential strongly affects the scaling properties of the dynamical observables near the quantum critical point. The mean excitation density and the energy excess, after the crossing of the critical point, follow an algebraic law as a function of the sweeping rate with an exponent that depends on the space-time properties of the potential. After that, we have studied the behavior of ultra-cold bosons in a tilted optical lattice. Starting with the Bose-Hubbard Hamiltonian, in the limit of Hard-Core bosons, we have developed a hydrodynamic theory that exactly reproduces the temporal evolution of some of the observables of the system. In particular, it was observed that part of the boson density remains trapped, and oscillates with a frequency that depends on the slope of the potential, whereas the remaining packet part is expelled out of the ramp. We have also analyzed the dynamics of the Bose-Hubbard model using the tDMRG algorithm and the Lanczos algorithm. In this way we have highlighted the role of the non-integrability of the model on its dynamical behavior. Finally, we have addressed the issue of thermalization in an extended quantum system. Starting from quite general considerations, we have introduced the notion of out-of-equilibrium temperature profile in a chain of Hard-Core bosons. We have analyzed the dynamics of the temperature profile and especially its scaling properties

Quantum phase transitions are collective phenomena that take place in large interacting many-body systems when tuning a nonthermal control parameter across a critical value. They occur at zero temperature, driven by genuinely quantum fluctuations, and manifest as a qualitative change in the structure (symmetry or topology) of the ground state due to the competition between incompatible terms in the many-body Hamiltonian describing the system. A rich variety of signatures flags the presence of the quantum critical point, ranging from the singular behaviour in some properties of the low-lying spectrum to the persistence of quantum features even at finite temperature. The latter footprints permits to observe the effects of quantum phase transitions experimentally. In particular, the low-temperature region on the phase diagram where the critical point strongly affects the thermal behaviour of the system has been intensively studied in recent years but many issues about its nature and extension are still open. Generally speaking, the investigation of critical systems from the perspective of information science advances our understanding of criticality beyond standard approaches developed in statistical mechanics. Moreover, it sheds new light on the preparation and processing of useful resources for quantum technologies. In fact, the considerable amount of knowledge accumulated in the study of quantum systems during the last decades led to a fundamental as well as technological revolution: entanglement -- the quintessence of quantum world -- has turned from a troublesome source of paradoxes into a useful resource. It allows to outperform classical tasks in information processing or even accomplish tasks that cannot be carried out through classical means. This change of perspective justifies the growing attention entanglement has been attracting. The characterization of quantum phases and quantum phase transitions through entanglement measures and witnesses is an intriguing problem at the verge of quantum information and many-body physics. Current studies have mainly focused on bipartite or pairwise entanglement in the ground state of critical Hamiltonians: these studies have emphasized a growth of entanglement in the vicinity of quantum critical points. However, bipartite and pairwise correlations are hardly accessible in systems with a large number of particles, that are the preferred platforms for quantum sensors and the natural targets of quantum simulators. Moreover, they cannot fully capture the richness of multiparticle correlations and the complex structure of a many-body quantum state. Much less attention has been devoted to witnessing multipartite entanglement in critical systems and it has been mainly limited to spin models. Yet, multipartite entanglement among hundreds of particles has been detected experimentally in atomic ensembles so far, and a variety of witnesses are available in the literature. Among these witnesses, the quantum Fisher information has proved to be especially powerful: it extends the class of entangled states detectable by popular methods such as the spin squeezing, it can be extracted from experimental data and it has an appealing physical meaning in terms of distinguishability of quantum states under external parametric transformations. In our work, we merge the two concepts -- somehow abstract though experimentally accessible -- of multipartite entanglement and quantum phase transitions. We investigate the behaviour of multipartite entanglement as detected and quantified by the quantum Fisher information, both at zero and finite temperature, in a collection of benchmark models displaying quantum critical points. All the selected models describe exemplary systems in the field of condensed-matter or nuclear physics and can be realized and tested in the laboratory as well. We find that the multipartite entanglement at zero temperature is a reliable detector of criticality: its enhanced susceptibility to any minute change of the parameter driving the transition sharply marks the boundary between different phases, not only in symmetry-breaking first-order and second-order transitions, but also in topological transitions. Moreover, multipartite entanglement distinguishes between phases with different symmetries or different topologies in terms of different scaling with the system size. When witnessing multipartite entanglement in topological systems, where no local order parameter exists, we address an open problem in the literature: the extension of the standard protocol based on the quantum Fisher information to nonlocal probe operators. The study of the interplay between thermal and quantum fluctuations in the vicinity of quantum critical points reveals the existence of a universal decay law of multipartite entanglement at sufficiently low temperature. Thermal fluctuations and noise pose a significant threat to the preparation, protection and usage of highly-entangled states and constitute a big challenge for future quantum technologies. Our work offers insights on the robustness of quantum correlations against decoherence, especially for applications in the field of quantum metrology.

This text distills the core ideas of statistical mechanics to make room for new advances important to information theory, complexity, active matter, and dynamical systems. Chapters address random walks, equilibrium systems, entropy, free energies, quantum systems, calculation and computation, order parameters and topological defects, correlations and linear response theory, and abrupt and continuous phase transitions. Exercises explore the enormous range of phenomena where statistical mechanics provides essential insight — from card shuffling to how cells avoid errors when copying DNA, from the arrow of time to animal flocking behavior, from the onset of chaos to fingerprints. The text is aimed at graduates, undergraduates, and researchers in mathematics, computer science, engineering, biology, and the social sciences as well as to physicists, chemists, and astrophysicists. As such, it focuses on those issues common to all of these fields, background in quantum mechanics, thermodynamics, and advanced physics should not be needed, although scientific sophistication and interest will be important.