In spite of all of its successes, quantum mechanics leaves us with a central problem: How does Nature create a ''foot-bridge'' from fragile quanta to the objective world of everyday experience? Here we identify within quantum mechanics a fundamental process leading to the perceived objectivity and called state information broadcasting. This is the trick that Nature uses instead of a simple cloning. We uncover it basing on minimal assumptions, without referring to any dynamical details or a concrete model. More specifically, we show how a crucial for quantum mechanics notion of non-disturbance due to Bohr and a natural definition of objectivity lead to a canonical structure of a quantum system-environment state, reflecting objective information records about the system stored in the environment.
In spite of all of its successes, quantum mechanics leaves us with a central problem: How does Nature create a ''foot-bridge'' from fragile quanta to the objective world of everyday experience? Here we identify within quantum mechanics a fundamental process leading to the perceived objectivity and called state information broadcasting. This is the trick that Nature uses instead of a simple cloning. We uncover it basing on minimal assumptions, without referring to any dynamical details or a concrete model. More specifically, we show how a crucial for quantum mechanics notion of non-disturbance due to Bohr and a natural definition of objectivity lead to a canonical structure of a quantum system-environment state, reflecting objective information records about the system stored in the environment.
In spite of all of its successes, quantum mechanics leaves us with a central problem: How does nature create a bridge from fragile quanta to the objective world of everyday experience? Here we find that a basic structure within quantum mechanics that leads to the perceived objectivity is a so-called spectrum broadcast structure. We uncover this based on minimal assumptions, without referring to any dynamical details or a concrete model. More specifically, working formally within the decoherence theory setting with multiple environments (called quantum Darwinism), we show how a crucial for quantum mechanics notion of nondisturbance due to Bohr [N. Bohr, Phys. Rev. 48, 696 (1935)] and a natural definition of objectivity lead to a canonical structure of a quantum system-environment state, reflecting objective information records about the system stored in the environment.
Quantum mechanics is one of the most successful theories, correctly predicting huge class of physical phenomena. Ironically, in spite of all its successes, there is a notorious problem: how does Nature create a ''bridge'' from fragile quanta to the robust, objective world of everyday experience? It is now commonly accepted that the most promising approach is the Decoherence Theory, based on the system-environment paradigm. To explain the observed redundancy and objectivity of information in the classical realm, Zurek proposed to divide the environment into independent fractions and argued that each of them carries a nearly complete classical information about the system. This Quantum Darwinism model has nevertheless some serious drawbacks: i) the entropic information redundancy is motivated by a priori purely classical reasoning; ii) there is no answer to the basic question: what physical process makes the transition from quantum description to classical objectivity possible? Here we prove that the necessary and sufficient condition for objective existence of a state is the spectrum broadcasting process, which, in particular, implies Quantum Darwinism. We first show it in general, using multiple environments paradigm, a suitable definition of objectivity, and Bohr's notion of non-disturbance, and then on the emblematic example for Decoherence Theory: a dielectric sphere illuminated by photons. We also apply Perron-Frobenius Theorem to show a faithful, ''decoherence-free'' form of broadcasting. We suggest that the spectrum broadcasting might be one of the foundational properties of Nature, which opens a ''window'' for life processes.
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.
Recently, the emergence of classical objectivity as a property of a quantum state has been explicitly derived for a small object embedded in a photonic environment in terms of a spectrum broadcast form-a specific classically correlated state, redundantly encoding information about the preferred states of the object in the environment. However, the environment was in a pure state and the fundamental problem was how generic and robust is the conclusion. Here, we prove that despite the initial environmental noise, the emergence of the broadcast structure still holds, leading to the perceived objectivity of the state of the object. We also show how this leads to a quantum Darwinism-type condition, reflecting the classicality of proliferated information in terms of a limit behavior of the mutual information. Quite surprisingly, we find "singular points" of the decoherence, which can be used to faithfully broadcast a specific classical message through the noisy environment.
The starting point of this dissertation is that a quantum state represents the observer's knowledge about the system of interest. As it has been pointed out several times by the opponents of this epistemic interpretation, it is difficult to reconcile this point of view with our common notion of “physical reality”, which exists independently of our monitoring, and can be discovered without disturbance. Indeed, if quantum theory is correct, it should apply to classical systems—including measurement devices—as well as to any other system. In this dissertation, we will study the quantum mechanisms responsible for our perception of the world and demonstrate how they lead to the emergence of an operational objective reality from within quantum theory: several observers gathering information through these mechanisms will arrive at a common consensus about the properties of the world. The two mechanisms we study in great detail are the redundant proliferation of information in the environment and the direct measurement of a macroscopic observable. An example of the first mechanism is the photon environment which provides us with our visual data about the world. Several independent observers learning about their surroundings in this indirect fashion will agree on their findings. An example of the second mechanism is our tactile information: when the tip of our finger touches an object, it interacts collectively with a very large number of molecules. Again, under realistic assumptions, this type of information acquisition will lead to a classical perception of the world.
How the objective everyday world emerges from the underlying quantum behavior of its microscopic constituents is an open question at the heart of the foundations of quantum mechanics. Quantum Darwinism and spectrum broadcast structure are two different frameworks providing key insight into this question. Recent works, however, indicate these two frameworks can lead to conflicting predictions on the objectivity of the state of a system interacting with an environment. Here, we provide a resolution to this issue by defining strong quantum Darwinism and proving that it is equivalent to spectrum broadcast structure when combined with strong independence of the subenvironments. We further show that strong quantum Darwinism is sufficient and necessary to signal state objectivity without the requirement of strong independence. Our Letter unveils the deep connection between strong quantum Darwinism and spectrum broadcast structure, thereby making fundamental progress toward understanding and solving the emergence of classicality from the quantum world. Together they provide us a sharper understanding of the transition in terms of state structure, geometry, and quantum and classical information.