The notion of coherence, intimately related to the notion of wave-particle duality, plays a central role in quantum mechanics. When quantum coherence extends over several particles inside a system, the description in terms of individual objects becomes impossible, due to the development of quantum correlations (or entanglement). In this manuscript, we focus on equilibrium systems, for which we show that coherent fluctuations add up to the fluctuations predicted by thermodynamic identities, valid for classical systems only. In the ground state, coherent fluctuations are the only ones to subsist, an in this case we study their relationship with entanglement entropy. We show in particular that an hypothesis of effective temperature, spatially modulated, captures the structure of entanglement in a many-body system, and we show how this temperature can be reconstructed from usual correlation functions. Our results also enable for a refined understanding of quantum phase transitions. We show in particular that the phase transition between a bosonic Mott insulator and a superfluid gives rise to a singularity of entanglement entropy induced by amplitude fluctuations of the phase of the condensate. We finally identify a coherence length governing the scaling behaviour of coherent fluctuations inside the quantum critical region in the finite-temperature vicinity of a quantum critical point, and open novel perspectives for the metrological advantage offered by the exceptional coherence which develops close to quantum critical points, based on the example of the quantum Ising model.
The notion of coherence, intimately related to the notion of wave-particle duality, plays a central role in quantum mechanics. When quantum coherence extends over several particles inside a system, the description in terms of individual objects becomes impossible, due to the development of quantum correlations (or entanglement). In this manuscript, we focus on equilibrium systems, for which we show that coherent fluctuations add up to the fluctuations predicted by thermodynamic identities, valid for classical systems only. In the ground state, coherent fluctuations are the only ones to subsist, an in this case we study their relationship with entanglement entropy. We show in particular that an hypothesis of effective temperature, spatially modulated, captures the structure of entanglement in a many-body system, and we show how this temperature can be reconstructed from usual correlation functions. Our results also enable for a refined understanding of quantum phase transitions. We show in particular that the phase transition between a bosonic Mott insulator and a superfluid gives rise to a singularity of entanglement entropy induced by amplitude fluctuations of the phase of the condensate. We finally identify a coherence length governing the scaling behaviour of coherent fluctuations inside the quantum critical region in the finite-temperature vicinity of a quantum critical point, and open novel perspectives for the metrological advantage offered by the exceptional coherence which develops close to quantum critical points, based on the example of the quantum Ising model.
This chapter addresses phase transitions and dynamic scaling occurring in systems comprised of interacting indistinguishable quantum particles, for which entanglement correlations are crucial. It first describes how the dynamics (in real time) and thermodynamics (in imaginary time) of quantum many-particle Hamiltonians can be mapped onto field theories based on coherent-state path integrals. While bosons are described by complex-valued fields, fermions are represented by anticommuting Grassmann variables. Since quantum-mechanical systems are of inherently dynamical nature, the corresponding field theory action entails d + 1 dimensions, with time playing a special role. For Hamiltonians that incorporate only two-particle interactions, we can make contact with the previously studied Langevin equations, yet with effectively multiplicative rather than additive noise. As an illustration, this formalism is applied to deduce fundamental properties of weakly interacting boson superfluids. Whereas Landau–Ginzburg theory already provides a basic hydrodynamic description, the Gaussian approximation allows the computation of density correlations, the Bose condensate fraction, and the normal- and superfluid densities from the particle current correlations. We next establish that quantum fluctuations are typically irrelevant for thermodynamic critical phenomena, provided that T c > 0, and readily extend finite-size scaling theory to the imaginary time axis to arrive at general scaling forms for the free energy. Intriguing novel phenomena emerge in the realm of genuine quantum phase transitions at zero temperature, governed by other control parameters such as particle density, interaction or disorder strengths.
Quantum Phase Transitions Quantum phase transitions, in contrast to temperature-driven critical phenomena, occur due to a competition between different parameters describing the basic interactions of the system. Their specific feature is the quantum character of the critical fluctuations. This implies, through the uncertainty principle, that energy fluctuations and time are coupled. Then at zero temperature time plays a crucial and fundamental role, the static properties being entangled with the dynamics (Continentino, 1994a; Sondhi et al ., 1997; Sachdev, 1999). In this book we are mainly interested in quantum phase transitions that occur in electronic many-body systems and how scaling concepts can be useful to understand their properties close to these transitions (Continentino, 1994a). Even though a similar approach has been used in the case of interacting bosons (Fisher et al ., 1989) the fermionic problem has its own idiosyncrasies and difficulties. For example, there is no natural order parameter associated with the localisation transition in the electronic case, while bosons at zero temperature are either localised or superfluid so that the superfluid order parameter can be used to distinguish between both phases. If the results of the study of quantum phase transitions were restricted to zero temperature, this would be an interesting but purely academic area of research. What is really exciting about this subject is the effect of quantum critical points (QCP) in the finite temperature phase diagram of actual physical systems, even far away from the QCP (Freitas, 2015). As we will show, there is a special line in this phase diagram, the quantum critical trajectory , where the temperature dependence of the thermodynamic and transport properties is governed by the quantum critical exponents, i.e. those associated with the QCP. This and the observed crossover effects induced by temperature in the non-critical side of the phase diagram of a material with a QCP are sufficient to make the study of quantum phase transitions an inevitable subject. We start this chapter by introducing the scaling theory of quantum critical phenomena, since scaling concepts are used throughout this book.
One of the distinguishing achievements of physics has been the ability to describe the Universe based upon underlying microscopic principles. When there are many participating objects being governed by these microscopic principles, the resultant behavior often harbors emergent collective phenomena that may not be obviously related to the microscopic description. Phase transitions, quantum or classical, are a canonical example of such many-body phenomena that are understood in equilibrium through a framework of thermodynamic constraints, statistical ensembles, and a growth of system-wide correlations at the transition. However, a new class of quantum phase transitions has recently emerged for non-equilibrium systems with behavior that is not captured by this traditional framework. In the case of isolated quantum systems, these transitions occur among excited eigenstates and requires knowledge of the structure of these individual eigenstates in the quantum system. Understanding what drives such a phase transition requires a deeper microscopic knowledge of the role of correlations at the transition. \nThe use of ultracold atoms in optical lattices has paved the way for faithfully realizing quantum phase transitions. In particular, the advent of the quantum gas microscope realized an experimental platform that is highly controllable and provides microscopic access to the many-bodied wave functions. All experiments presented in this thesis use such a quantum gas microscope architecture where a quantum degenerate gas of Rb is used to probe the behavior of non-equilibrium quantum systems in one spatial dimension.\nThis thesis briefly discusses some studies of entanglement in equilibrium quantum phase transitions near the ground state of the system. In the first experiment, we directly measure the second-order R\\'enyi entanglement entropy that develops as a Bose-Hubbard system transitions from a Mott insulator to a superfluid. This is followed by a study of entanglement growth at the transition in the transverse Ising model. \nThese studies progress to the investigation of two particular non-equilibrium quantum phases and their transition into one another: quantum thermalization and many-body localization. We investigate the development of entanglement entropy that mimics thermodynamic entropy for an isolated quantum system which locally thermalizes under its own unitary dynamics. We then investigate the only known robust exception to this thermalizing behavior by studying the onset of many-body localization due to disorder. We observe the breakdown of thermalization in this phase and the logarithmically slow growth of entanglement throughout the system, which identifies the localized system to be an interacting one. Lastly, we investigate the role of correlations in the critical dynamics that emerge at high-orders where the system transitions from thermalizing behavior to many-body localization.
Corrélations quantiques : une approche de physique statistique La notion de cohérence, intimement liée à celle de dualité onde-corpuscule, joue un rôle central en mécanique quantique. Lorsque la cohérence quantique s'étend sur plusieurs particules au sein d'un système, la description en termes d'objets individuels devient impossible en raison des corrélations quantiques (ou intrication) qui se développent. Dans ce manuscrit, nous étudions les systèmes à l'équilibre, pour lesquels nous montrons que les fluctuations cohérentes viennent s'ajouter aux fluctuations prédites par des identités thermodynamiques valides pour les systèmes classiques. Au zéro absolu, les fluctuations cohérentes sont les seules à subsister, et nous étudions dans ce cas leur lien avec l'entropie d'intrication. Nous montrons en particulier qu'une hypothèse de température effective modulée spatialement rend compte de la structure de l'intrication au sein d'un système à N corps, et montrons comment cette température peut être extraite des corrélations usuelles. Nos résultats permettent par ailleurs une compréhension affinée des transitions de phase quantiques. Nous montrons par exemple que la transition entre un isolant de Mott bosonique et un superfluide donne lieu à une singularité de l'entropie d'intrication induite par les fluctuations d'amplitude de la phase du condensat. Nous identifions enfin une longueur de corrélation pilotant les lois d'échelles des fluctuations cohérentes au sein de l'éventail critique avoisinant une transition de phase quantique du second ordre, et proposons une ouverture vers les potentielles applications métrologiques de ces fluctuations cohérentes exceptionnellement fortes en nous appuyant sur l'exemple du modèle d'Ising.
The interplay of quantum and thermal fluctuations in the vicinity of a quantum critical point characterizes the physics of strongly correlated systems. Here we investigate this interplay from a quantum information perspective presenting the universal phase diagram of the quantum Fisher information at a quantum phase transition. Different regions in the diagram are identified by characteristic scaling laws of the quantum Fisher information with respect to temperature. This feature has immediate consequences on the thermal robustness of quantum coherence and multipartite entanglement. We support the theoretical predictions with the analysis of paradigmatic spin systems showing symmetry-breaking quantum phase transitions and free-fermion models characterized by topological phases. In particular we show that topological systems are characterized by the survival of large multipartite entanglement, reaching the Heisenberg limit at finite temperature.
The dynamical critical exponent is introduced. The phase diagram and the phase transitions in the Bose–Hubbard model of interacting bosons on a lattice are determined. The concept of quantum fluctuations is introduced on the example of an interacting superfluid, and finally the special scaling of conductivity is discussed. Dynamical critical exponent The finite temperature phase transitions studied in previous chapters are the result of the competition between the entropy and the energy terms in the free energy: the weight of entropy increases with temperature, and ultimately destroys the order that may be existing in the system. A sharp phase transition between two phases exhibiting qualitatively different correlations may, however, occur even at zero temperature, by varying a coupling constant in the Hamiltonian. The transition then corresponds to a non-analyticity of the energy of the ground state. A simple example is provided by the interacting bosons, where the superfluid transition may be brought about by tuning the chemical potential at T = 0. Such T = 0 phase transitions are called quantum phase transitions , and will be the subject of the present chapter. In general, a quantum phase transition may lie at the end of a line of thermal phase transitions, as in the bosonic example mentioned above. It is possible, however, that the system may not even have an ordered state at finite temperatures, but still exhibits a quantum critical point. Two different situations are depicted in Fig. 8.1. Examples of such phase diagrams are provided by the system of interacting bosons which will be discussed in Sections 8.2 and 8.3.