In this thesis, I present my work during my Ph.D. in quantum Darwinism. It is a original project, which has been peer-reviewed and accepted for publication in a respected international journal. In the first chapters, I recalled the essential concepts to understand quantum Darwinism, and in the two last, I present my works. The quantum Darwinism concept was constructed to explain a possible quantum-to-classical transition. My main interests are to find the quantum Darwinism implications in the system’s dynamics and to find more realistic models where it applies. The first work was finished and published while the second is still in progress. In the first model I studied quantum Darwinism in a many-body system of quantum harmonic oscillators where the subenvironments neither interact nor correlate between themselves. Through analytical and numerical calculations, we observed quantum Darwinism from two different approaches. We show quantum Darwinism through the BPH approach in a model for the first time. In contrast with a recently published work, we also show that quantum Darwinism can be observed even in a system with a high degree of non-Markovianity, and proposed a more suitable way to quantify it. In the second work, we investigate quantum Darwinism in a more realistic model where the subenvironments can become strongly correlated. The model consists of a two-level atom interacting with a fermionic environment and was used to study orthogonality catastrophe. As both orthogonality catastrophe and quantum Darwinism comes with decoherence, we are investigating if these two concepts are correlated. Beyond that, we chose this model because it can be mapped in a bosonic model of the Tonks-Girardeau gas enabling an experimental implementation with cold atoms. We are performing numerical and analytical calculations in this work. In the sixth chapter, I present the first result and describe what is missing.
In this thesis, I present my work during my Ph.D. in quantum Darwinism. It is a original project, which has been peer-reviewed and accepted for publication in a respected international journal. In the first chapters, I recalled the essential concepts to understand quantum Darwinism, and in the two last, I present my works. The quantum Darwinism concept was constructed to explain a possible quantum-to-classical transition. My main interests are to find the quantum Darwinism implications in the system’s dynamics and to find more realistic models where it applies. The first work was finished and published while the second is still in progress. In the first model I studied quantum Darwinism in a many-body system of quantum harmonic oscillators where the subenvironments neither interact nor correlate between themselves. Through analytical and numerical calculations, we observed quantum Darwinism from two different approaches. We show quantum Darwinism through the BPH approach in a model for the first time. In contrast with a recently published work, we also show that quantum Darwinism can be observed even in a system with a high degree of non-Markovianity, and proposed a more suitable way to quantify it. In the second work, we investigate quantum Darwinism in a more realistic model where the subenvironments can become strongly correlated. The model consists of a two-level atom interacting with a fermionic environment and was used to study orthogonality catastrophe. As both orthogonality catastrophe and quantum Darwinism comes with decoherence, we are investigating if these two concepts are correlated. Beyond that, we chose this model because it can be mapped in a bosonic model of the Tonks-Girardeau gas enabling an experimental implementation with cold atoms. We are performing numerical and analytical calculations in this work. In the sixth chapter, I present the first result and describe what is missing.
To explain aspects of the quantum-to-classical transition, quantum Darwinism explores the fact that, due to interactions between a quantum open system and its surrounding environment, information about the system can be spread redundantly to the environment. Here we recall that there are in the literature two distinct and nonequivalent ways to make this statement precise and quantitative. We first point out the difference with some simple but illustrative examples. We then consider a model where Darwinism can be seen from both perspectives. Moreover, the non-Markovianity of our model can be varied with a parameter. In a recent work [F. Galve et al., Sci. Rep. 6, 19607 (2016)], the authors concluded that quantum Darwinism can be hindered by non-Markovianity. We depart from their analysis and argue that, from both perspectives to quantum Darwinism, there is no clear relationship between non-Markovianity and quantum Darwinism in our model.
An explanation for the quantum-to-classical transition has long been sought after since the haydays of the field. Decoherence theory and open quantum system dynamics have paved the way, showing how a system can become correlated with, and so lose information into the environment. Quantum Darwinism, and its more stringent cousins Strong Quantum Darwinism and Spectrum Broadcast Structures, seek to explain how this information loss is achieved in such a way as to be objective, classical and redundant. However there remains questions as to how the structure of the environment, states and types of interactions can give rise to this redundant encoding. In this thesis, we collect the work of 3 of our papers, exploring these questions. First, we show how information from one environment can spread to another. Then we show how commutativity can play a role in the emergence of objectivity. Lastly, we move beyond a single, pure system to a more realistic bipartite one, with both classical and quantum correlations between the subsystems. We shall show how interactions between only these subsystems can mediate the redundancy of others, and how these can lower bound and upper bound other forms of interactions between said subsystems and the environment.
In this thesis I present the work done during my PhD in the area of low dimensional quantum gases. The chapters of this thesis are self contained and represent individual projects which have been peer reviewed and accepted for publication in respected international journals. Various systems are considered, the first of which is a two particle model which possesses an exact analytical solution. I investigate the non-classical correlations that exist between the particles as a function of the tunable properties of the system. In the second work I consider the coherences and out of equilibrium dynamics of a one-dimensional Tonks-Girardeau gas. I show how the coherence of the gas can be inferred from various properties of the reduced state and how this may be observed in experiments. I then present a model which can be used to probe a one-dimensional Fermi gas by performing a measurement on an impurity which interacts with the gas. I show how this system can be used to observe the so-called orthogonality catastrophe using modern interferometry techniques. In the next chapter I present a simple scheme to create superposition states of particles with special emphasis on the NOON state. I explore the effect of inter-particle interactions in the process and then characterise the usefulness of these states for interferometry. Finally I present my contribution to a project on long distance entanglement generation in ion chains. I show how carefully tuning the environment can create decoherence-free subspaces which allows one to create and preserve entanglement.
Quantum mechanics describes the smallest systems, whilst classical physics describes macroscopic scales. However, the quantum-to-classical transition is a major unsolved problem. In this thesis, we develop further understanding of the transition through the concept of objectivity. The objects we perceive in our everyday world are objective: they exist regardless of observation, and their properties are independent of observers. This is because information about everyday objects came be found, for example, in the surrounding photon environment. Quantum Darwinism is a conceptual and theoretical framework that describes the emergence of objectivity. The “Darwinistic” part refers to how certain types of information tends to spread at the expense of others. The thesis addresses the quantitative description of objectivity, the dynamics of emergent objectivity, and the analysis of non-objectivity in experimental and biology-inspired systems. Firstly, we propose two new mathematical frameworks to describe objectivity, Strong Quantum Darwinism and invariant spectrum broadcast structure. Then we unite them with the pre-existing frameworks, Zurek's Quantum Darwinism and spectrum broadcast structure, by identifying their deep connections and differences. These conditions highlight how Each framework considers varying amounts of objectivity in the system and environments and together, they create a hierarchy of objectivity. We then analyse objectivity in a scenario where system-environment interactions are strong and non-Markovian, highlighting some practical differences between the Quantum Darwinism frameworks. Next, we describe the quantum channels that create objective states, leading us to discuss the contribution of non-Markovianity to the emergence of objectivity. Then, we introduce a witnessing scheme that simplifies the detection of non-objectivity to promote further experimental testing of Quantum Darwinism with larger environments. Finally, we examine the contribution of quantum coherence and non-objectivity to the performance of a biological-inspired quantum magnetic sensor that operates close to the quantum-classical boundary. We find that the sensor operates in a non-objective regime, and that quantum coherence is necessary for the sensor's performance. The quantum-to-classical transition is a longstanding open question: these findings deepen our knowledge of how objectivity emerges, and contributes to and the types of interactions that keep systems quantum, which is relevant for understanding quantum biological systems and exploiting environment effects in novel quantum technologies. While the quantum-to-classical transition remains unsolved, Quantum Darwinism and its frameworks contribute to the understanding of emergent objectivity and have much potential to further explore aspects of the transition within the structure of quantum mechanics.
Quantum Darwinism is a paradigm to understand how classically objective reality emerges from within a fundamentally quantum universe. Despite the growing attention that this field of research has been enjoying, it is currently not known what specific properties a given Hamiltonian describing a generic quantum system must have to allow the emergence of classicality. Therefore, in the present work, we consider a broadly applicable generic model of an arbitrary finite-dimensional system interacting with an environment formed from an arbitrary collection of finite-dimensional degrees of freedom via an unspecified, potentially time-dependent Hamiltonian containing at most two-body interaction terms. We show that such models support quantum Darwinism if the set of operators acting on the system which enter the Hamiltonian satisfy a set of commutation relations with a pointer observable and with one other. We demonstrate our results by analyzing a wide range of example systems: a qutrit interacting with a qubit environment, a qubit-qubit model with interactions alternating in time, and a series of collision models including a minimal model of a quantum Maxwell demon. Published by the American Physical Society 2024
In this article I aim to provide an intuitive and non-technical introduction to decoherence and quantum Darwinism. Together these theories explain how our classical reality emerges from an underlying quantum mechanical description. Here I focus on two aspects of this and explain, firstly, how decoherence can tell us why we never see macroscopic superpositions, such as dead-and-alive cats, in our classical surroundings; and secondly I describe and then provide a resolution to the so-called preferred basis problem. I then introduce recent results demonstrating that certain aspects of classicality are generic phenomena that emerge from the basic mathematical structure of quantum mechanics. This is in stark contrast to previous work in this field that focused on specific models that cannot realistically be scaled up to explicitly answer questions about the macroscopic world. Finally, I demonstrate how decoherence and quantum Darwinism can shed significant light on the measurement problem, and I discuss the implications for how we should interpret quantum mechanics.