Quantum Darwinism describes objectivity of quantum systems via their correlations with their environment---information that hypothetical observers can recover by measuring the environments. However, observations are done with respect to a frame of reference. Here we take the formalism of [Giacomini et al., Nat. Commun. 10, 494 (2019)] and consider the repercussions on objectivity when changing quantum reference frames. We find that objectivity depends on nondegenerative relative separations, conditional state localization, and environment macrofractions. There is different objective information in different reference frames due to the interchangeability of entanglement and coherence, and of statistical mixing and classical correlations. As such, objectivity is subjective across quantum reference frames.
Quantum Darwinism describes objectivity of quantum systems via their correlations with their environment---information that hypothetical observers can recover by measuring the environments. However, observations are done with respect to a frame of reference. Here we take the formalism of [Giacomini et al., Nat. Commun. 10, 494 (2019)] and consider the repercussions on objectivity when changing quantum reference frames. We find that objectivity depends on nondegenerative relative separations, conditional state localization, and environment macrofractions. There is different objective information in different reference frames due to the interchangeability of entanglement and coherence, and of statistical mixing and classical correlations. As such, objectivity is subjective across quantum reference frames.
Reference frames are of special importance in physics. They are usually considered to be idealized entities. However, in most situations, e.g. in laboratories, physical processes are described within reference frames constituted by physical systems. As new technological developments make it possible to demonstrate quantum properties of complex objects an interesting conceptual problem arises: Could one use states of quantum systems to define reference frames? Recently such a framework has been introduced in [F. Giacomini, E. Castro-Ruiz, and Č. Brukner, Nat Commun 10, 494 (2019)]. One of its consequences is the fact that quantum correlations depend on a physical state of an observers reference frame. The aim of this work is to examine the dynamical aspect of this phenomena and show that the same is true for correlations established during an evolution of a composite systems. Therefore, decoherence process is also relative: For some observers the reduced evolution of subsystems is unitary, whereas for others not. I also discuss implications of this results for modern developments of decoherence theory: Quantum Darwinism and Spectrum Broadcast Structures.
Quantum Darwinism identifies objective observables with the center of the accessible algebra — but observations are made from a reference frame, and when the frame is itself quantum, what counts as objective can change from one frame to another. This leaves a sharp question: what fact, if any, do all frames share? The paper answers this question structurally, first making "relational" precise: the frame-invariant observables are exactly the relational (Dirac) observables, the fixed points of averaging over the frame group. It then proves that the perspective-neutral fact is exactly the frame-invariant part of the objective center — the objective observables that every perspective assigns the same value — with an invariant fraction measuring how much of the objective content is absolute rather than frame-relative. We give a sharp criterion for when such a shared fact exists: it is present exactly when the frame group does not act transitively on the charge sectors, and the whole charge is shared when the frame group leaves that charge unchanged. We also correct a subtlety — "objective in every perspective" is strictly weaker than "assigned the same value in every perspective" — and we resolve an extended Wigner's-friend scenario, in which extensive broadcasting makes an outcome an absolute fact for every accessible frame. The results are finite-dimensional theorems, each reproduced by self-testing code. In short: even across different quantum reference frames, enough redundant broadcasting still makes a fact absolute for all — the shared reality is the frame-invariant center.
Quantum Darwinism tells us that some observable of a system becomes objective when the environment redundantly records it — but for a general, partly quantum system it does not say which observable that is. This paper proposes a precise structural answer: the objective observables are exactly the "center" of the system's algebra of observables — the quantities that are simultaneously compatible with everything else and that all observers can share. The proposal is given substance with three results, each proved in finite dimension and checked numerically. First, objectivity emerges as an algebraic order parameter: as broadcasting proceeds, the shared "consensus" center grows steadily from nothing to the full set of classical values, naming which observables have become objective at each stage — more information than any single number could give. Second, the abstract idea appears concretely in an ordinary lattice gauge model, where the center of a region's observables is exactly its boundary electric charge, with no gravity assumed. Third, the objective content is agreed by every observer but relative in degree: changing one's quantum reference frame never enlarges what is objective, though it can change how much microscopic detail is assigned. The paper connects this to the operator-algebra language of recent gravitational research, while being explicit that the gravitational completion is imported, not re-derived. In short: objectivity is not a vague notion but a sharp algebraic one — it is the center, the shared heart of what everyone can agree on.
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.
The aim in Chapter 7 is to take into account the role of the means of information transmission on the nature of the states that can be perceived. Our point of departure is the recognition that the information we obtain is acquired by observers who monitor fragments of the same environment that decohered the system, einselecting preferred pointer states in the process. Moreover, we only intercept a fraction of the environment. The only information about the system that can be transmitted by its fraction must have been reproduced in many copies in that environment. This process of amplification limits what can be found out to the states einselected by decoherence. Quantum Darwinism provides a simple and natural explanation of this restriction, and, hence, of the objective existence—the essence of classicality—for the einselected states. This chapter introduces and develops information-theoretic tools and concepts (including, e.g., redundancy) that allow one to explore and characterize correlations and information flows between systems, environments, and observers, and illustrates them on an exactly solvable yet non-trivial model.
Quantum Darwinism explains objectivity through the redundant copying of a system's record into many environmental fragments. This paper extends that account to observers in relativity — accelerated observers and observers near horizons, for whom part of the environment lies causally beyond reach. Rather than assuming the record structure, the paper derives it from the causal split of the environment. An accelerated observer's accessible records turn out to be thermal, warmed to the temperature associated with the horizon, and the same construction applies to accelerated frames, cosmological horizons, and black holes alike. On this foundation it proves several results: objectivity requires both that a system lose its quantum coherence and that its records stay legible, and these two demands depend oppositely on the horizon temperature; there is a critical acceleration beyond which no redundant, readable record survives even as decoherence continues; and whether an objective record exists is agreed by all observers, though they disagree on how redundant — how objective — it is. At a black-hole horizon this yields "information without objectivity," where a fact can be globally present yet no longer objective. All closed-form results are proved and numerically verified, and the central prediction — a temperature-tunable threshold where objectivity is lost — is testable on analog quantum simulators. In short: near a horizon a fact can still be there for everyone yet objective for no one — classicality, it turns out, depends on the observer and how fast they are moving.