The problem of "what is 'system'?" is in the very foundations of modern quantum mechanics. Here, we point out the interest in this topic in the information-theoretic context. E.g., we point out the possibility to manipulate a pair of mutually non-interacting, nonentangled systems to employ entanglement of the newly defined "(sub)systems" consisting the one and the same composite system. Given the different divisions of a composite system into "subsystems", the Hamiltonian of the system may generate in general non-equivalent quantum computations. Redefinition of "subsystems" of a composite system may be regarded as a method for avoiding decoherence in the quantum hardware. In principle, all the notions refer to a composite system as simple as the hydrogen atom.

For the standard Quantum Brownian Motion (QBM) model, we point out the occurrence of simultaneous (parallel), mutually irreducible and autonomous decoherence processes. Besides the standard, one Brownian particle, we show there is at least another one system undergoing the dynamics described by the QBM effect. We do this by selecting the two mutually irreducible, global structures (decompositions into subsystems) of the composite system of the QBM model. A generalization of this observation is a new, challenging task in the foundations of the decoherence theory. We do not place our findings in any interpretational context. PACS 03.65.Yz Decoherence; open systems; quantum statistical methods PACS 03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.) PACS 03.65.Ta Foundations of quantum mechanics; measurement theory one and the same composite system of the QBM model that cannot be obtained from each other via the "coarse graining" [6] operation.The LCTs is a universal physical method. Already the unitary evolution regroups or separates the constituent subsystems thus locally changing a structure of a composite system. In quantum decoherence, the LCTs are sometimes used to ease the calculation [5], while (kinematically) regrouping or decomposing the subsystems may shed some new light on the mechanism of decoherence [7,8]. A change in the environmental degrees of freedom reveals some subtleties such as the "system-size" dependence of decoherence [5,9] and can help in distinguishing the robust (the preferred "pointer basis" [4, 5, 10, 11]) states for the open system [12,13,14]. However, these models and considerations refer to the local structures that share the degrees of freedom or can be reduced to each other. This makes the structures considered mutually dynamically coupled or dependent, which is not our objective.Paradigmatic for our considerations is the Hydrogen Atom (HA) model. The quantum theory of the hydrogen atom relies on the transformations of the electron's (e) and the proton's (p) degrees of freedom to introduce the atom center of mass (CM) and the "relative position (R)" degrees of freedom. Due to the absence of the coupling between CM and R, one obtains the variables separation and the exact solution of the atomic internal energy and eigenstates. The two structures of HA, e + p and CM + R, are mutually both irreducible and global likewise those of the QBM setup we discuss below.In Section 2, we derive our main result on the parallel decoherence: at variance with the standard wisdom, we point out that a composite quantum system can be described by (may host) the different, simultaneously existing and mutually independent quasi-classical (global) structures. In Section 3 we generalize our considerations and we emphasize that the "parallel decoherence" launches a new task in the foundations of the decoherence theory. In general, this task can be formidable yet possibly of a wider scientific interest. Section 4 is Discussion, where we emphasize: as long...

We investigate dynamical stability of a single propeller-like shaped molecular cogwheel modelled as the fixed-axis rigid rotator. In the realistic situations, rotation of the finite-size cogwheel is subject to the environmentally-induced Brownian-motion effect that we describe by utilizing the quantum Caldeira-Leggett master equation. Assuming the initially narrow (classical-like) standard deviations for the angle and the angular momentum of the rotator, we investigate the dynamics of the first and second moments depending on the size, i.e. on the number of blades of both the free rotator as well as of the rotator in the external harmonic field. The larger the standard deviations, the less stable (i.e. less predictable) rotation. We detect the absence of the simple and straightforward rules for utilizing the rotator's stability. Instead, a number of the size-related criteria appear whose combinations may provide the optimal rules for the rotator dynamical stability and possibly control. In the realistic situations, the quantum-mechanical corrections, albeit individually small, may effectively prove non-negligible, and also revealing subtlety of the transition from the quantum to the classical dynamics of the rotator. As to the latter, we detect a strong size-dependence of the transition to the classical dynamics beyond the quantum decoherence process.

There is a solution to the problem of asymptotic completeness in many body scattering theory that offers a specific view of the quantum unitary dynamics which allows for the straightforward introduction of local time for every, at least approximately closed, many-particle system. In this approach, Time appears as a hidden classical parameter of the unitary dynamics of a many-particle system. We show that a closed many-particle system can exhibit behavior that is characteristic for open quantum systems and there is no need for the "state collapse" or environmental influence. On the other hand, closed few-particle systems bear high quantum coherence. This local time scheme encompasses concepts including "emergent time", "relational time" as well as the "hybrid system" models with possibly induced gravitational uncertainty of time.Comment: Rewritten (including the title). This version accepted by the Proceedings

It is shown that a choice of degrees of freedom of a bipartite continuous variable system determines amount of non-classical correlations (quantified by discord) in the system's state. Non-classical correlations (that include entanglement as a special kind of correlations) are ubiquitous for such systems. For a quantum state, if there are not non-classical correlations (quantum discord is zero) for one, there are in general non-classical correlations (quantum discord is non-zero) for another set of the composite system's degrees of freedom. The physical relevance of this 'quantum correlations relativity' is emphasized also in the more general context.