This work develops a conceptual framework for metastable quantum transport based on continuous, phase-controlled tunneling between coupled subsystems. In contrast to an earlier capture–deadband–release formulation, the revised architecture maintains finite coupling throughout the transport cycle and treats reverse flow not necessarily as a loss, but as part of the total bidirectional dynamical activity of the system. The framework combines metastable interfacial modulation, quantum tunneling, local entropy/information redistribution, open-system transport, and explicit energy transduction. A series of dimensionless proof-of-principle toy models is used to test thermodynamic consistency and to distinguish four quantities that can otherwise be easily conflated: local entropy redistribution, net current, bidirectional current activity, and extractable work. Closed-system simulations preserve global von Neumann entropy while allowing local entropies and correlations to evolve. Open-system models explicitly account for reservoir energy, external drive work, dissipation, and load extraction. The numerical tests show that metastable phase modulation can modify internal bidirectional current activity and energy exchange with the drive. In a selected parameter regime, a passive bidirectional transduction model also exhibits a small, phase-dependent increase in load output relative to a non-metastable control. The results do not imply entropy-to-energy conversion, over-unity operation, or a new fundamental entropy current. Rather, they identify a thermodynamically consistent transport architecture whose potential novelty lies in combining continuous metastable modulation with the operational use of bidirectional current activity as a possible transduction resource. The work concludes by defining a falsifiable research program toward a microscopic, platform-specific realization and experimental comparison with an equivalent non-metastable control. The revised manuscript also develops a separate exploratory semiclassical extension, examining the chain from metastable transport to stress-energy redistribution, linearized metric response, and recursive backreaction. The analysis indicates that a nonzero stress-energy perturbation can formally source a metric perturbation, while gravitational coupling at ordinary laboratory scales makes the resulting response extremely small and the recursive feedback strongly subcritical. The extension therefore defines a quantitative failure boundary for practical causal engineering rather than providing evidence of a macroscopic modification of spacetime causality.
This work develops a conceptual framework for metastable quantum transport based on continuous, phase-controlled tunneling between coupled subsystems. In contrast to an earlier capture–deadband–release formulation, the revised architecture maintains finite coupling throughout the transport cycle and treats reverse flow not necessarily as a loss, but as part of the total bidirectional dynamical activity of the system. The framework combines metastable interfacial modulation, quantum tunneling, local entropy/information redistribution, open-system transport, and explicit energy transduction. A series of dimensionless proof-of-principle toy models is used to test thermodynamic consistency and to distinguish four quantities that can otherwise be easily conflated: local entropy redistribution, net current, bidirectional current activity, and extractable work. Closed-system simulations preserve global von Neumann entropy while allowing local entropies and correlations to evolve. Open-system models explicitly account for reservoir energy, external drive work, dissipation, and load extraction. The numerical tests show that metastable phase modulation can modify internal bidirectional current activity and energy exchange with the drive. In a selected parameter regime, a passive bidirectional transduction model also exhibits a small, phase-dependent increase in load output relative to a non-metastable control. The results do not imply entropy-to-energy conversion, over-unity operation, or a new fundamental entropy current. Rather, they identify a thermodynamically consistent transport architecture whose potential novelty lies in combining continuous metastable modulation with the operational use of bidirectional current activity as a possible transduction resource. The work concludes by defining a falsifiable research program toward a microscopic, platform-specific realization and experimental comparison with an equivalent non-metastable control. The revised manuscript also develops a separate exploratory semiclassical extension, examining the chain from metastable transport to stress-energy redistribution, linearized metric response, and recursive backreaction. The analysis indicates that a nonzero stress-energy perturbation can formally source a metric perturbation, while gravitational coupling at ordinary laboratory scales makes the resulting response extremely small and the recursive feedback strongly subcritical. The extension therefore defines a quantitative failure boundary for practical causal engineering rather than providing evidence of a macroscopic modification of spacetime causality.
When driven by a potential bias between two finite reservoirs, the particle current across a quantum system evolves from an initial loading through a coherent, followed by a metastable phase, and ultimately fades away upon equilibration. We formulate a theory which fully accounts for the associated, distinct time scales, and identifies the parameter dependence of the decay rate which ultimately controls the convergence towards equilibrium. Our formalism guarantees total particle number conservation and fundamental consistency between macroscopic and internal currents flowing in the system. We furthermore establish a clear imprint of the fermionic or bosonic particle character on the resulting conductance.
Driven quantum systems are systems in which a set of parameters obeys a time-dependent protocol, both periodic and non-periodic. The study of these systems bridges theoretical advances with experimental realizations in a variety of platforms—these range from quantum dot arrays to molecular junctions to trapped ions. The theoretical study is challenging due to the inherent non-equilibrium nature of the problem. These systems offer an interesting playground for quantum thermodynamics. They can be studied as quantum machines, such as refrigerators, heat pumps or engines. This poses the challenge of redefining the laws of thermodynamics in the quantum realm. This effort implies taking into account the presence of coherence, entanglement and strong correlations with the environment. In this conceptual framework, we attempt to shed new light on specific setups with applications in quantum metrology. In the first part of the Thesis, we present novel results on the thermodynamics and transport properties of quantum dots. After thoroughly introducing the main methods, namely non-equilibrium Green's functions and scattering matrix techniques, we apply these tools to adiabatic charge pumping. In this context, quantum dots, subjected to external periodic driving, act as nano-mechanical engines, transferring charge from a source to a drain. We investigate the conditions under which the pumped charge becomes quantized and how thermodynamic quantities behave in this regime. Next, we will focus on the charge shuttle mechanism in movable quantum dots as an example of a quantum clock. This physical model describes transport through a molecular state bound to two leads by van der Waals forces, with an electric field pushing the molecule from the source to the drain. The periodic oscillations arising from the system's dissipative limit cycle solution provide the basis for time measurement. We examine quantum noise primarily associated with the tunnelling process. In the second part, we propose a quantum clock that utilizes the free energy resources generated by coupling to a quantum battery, consisting of an integrable spin chain driven out of equilibrium by a quench in a chosen parameter. The operating conditions of the clock in the manifold of parameters are examined, leading to the requirement of the crossing of the critical point in the battery. The lifetime of the battery's resources is found to be extensive in its size, even when choosing a global coupling observable in the battery.
A central open question in quantum thermodynamics is the identification of thermodynamic entropy away from equilibrium. Although thermodynamic entropy coincides with the von Neumann entropy for a Gibbs state at equilibrium, the thermodynamic role of quantum coherence generated during nonequilibrium evolution remains unclear. Here, we show that von Neumann entropy does not, in general, represent the thermodynamic entropy governing heat exchange in coherent nonequilibrium quantum systems. By integrating the resource theory of quantum coherence with thermodynamic principles, we develop a general framework for nonequilibrium quantum thermodynamics. We identify thermodynamic entropy with the energy entropy derived from energy-measurement statistics and introduce a dynamical temperature extending the equilibrium concept to nonequilibrium states. Consequently, we obtain an exact entropy-balance relation separating the entropy flux associated with heat exchange from the coherence-loss contribution to entropy production, establishing an operational connection between quantum coherence, heat flow, and total entropy change. Unlike approaches relying on equilibrium reference states or stationary Gibbs ensembles, our framework uses instantaneous dynamical quantities and remains valid throughout transient nonequilibrium evolution. It provides consistent dynamical definitions of temperature, work, heat, and free energy. Applying the formalism to an exactly solvable open quantum system, we show that the first and second laws hold throughout transient evolution and that equilibrium thermodynamics emerges dynamically in the weak-coupling long-time limit. Our results establish a unified framework for coherent nonequilibrium quantum thermodynamics and suggest opportunities for controlling heat dissipation through coherence engineering in quantum thermal technologies.
Wave-dominated systems admit rich superposed dynamics at the microscopic level, yet observed reality is dominated by a sparse set of effectively classical histories. Explaining this selection without invoking observer-dependent postulates remains a central problem across quantum foundations, semiclassical transport, and nonequilibrium physics. In this work, we establish a rigorous equivalence between two previously independent constraints on history viability: closure preservation in phase-space transport and coherence preservation under a context-induced metric. We prove that, within a physically relevant class of systems, transport-induced covariance growth and metric-induced distinguishability impose identical bounds on the admissible set of histories. This equivalence reveals history selection as a capacity-limited geometric process: histories persist if and only if their deformation remains within a finite coherence bandwidth. We further show that this bandwidth is not kinematic but must be maintained against transport load by sustained physical flux, giving rise to a natural thermodynamic structure without modifying underlying wave dynamics. Within this framework, entropy increase corresponds to loss of accessible distinction, the second law emerges as a no-free-bandwidth principle, and equilibrium appears as the zero-flux fixed point. Ontology itself is thereby reinterpreted as a thermodynamic phase: reality consists of those histories that remain dynamically viable under finite energetic support. The results provide a unified, observer-independent account of classicality, irreversibility, and history selection, with experimentally testable implications for coherence control and interference phenomena.
The Nexus Convergence: A Formal Synthesis of Quantum Feedback Control, Information Thermodynamics, and Non-Linear Lattice Dynamics 1. Introduction: The Ontological Crisis and the Storage Imperative The contemporary scientific landscape is characterized by a persistent and fundamental schism between the unitary, reversible dynamics of quantum mechanics and the dissipative, irreversible arrow of time inherent in thermodynamics. This discord creates what the Nexus Recursive Harmonic Framework (RHF) identifies as the "Storage Crisis": the paradox of how a universe with finite energy limits can effectively store an ever-expanding history of infinite detail without catastrophic data loss or thermodynamic heat death.1 The prevailing "Container Paradigm"—which envisions spacetime as a passive box and time as a linear overwrite cursor—fails to account for the persistence of high-dimensional causal structures in a manner that is consistent with both unitarity (information conservation) and entropy (information projection). This report presents an exhaustive synthesis of recent theoretical and experimental breakthroughs from 2024 and 2025, specifically targeting the domains of Quantum Feedback Control, Information Thermodynamics, and Non-Linear Lattice Dynamics. The objective is to rigorously validate the axioms of the Nexus framework by identifying precise mathematical and phenomenological isomorphisms in peer-reviewed literature. We posit that the "read-only" ontology proposed by the Nexus framework—where history is conserved as geometry ("Shape") and the present is a collapsed projection ("Value")—finds its physical realization in the mechanisms of reduced-filter quantum stabilization, information-to-work conversion engines, and discrete breather localization in non-linear lattices. The investigation focuses on three critical variables defined in the Nexus framework: Gain (): The feedback coupling strength required to maintain a stable "stance" against entropic dissolution. Information (): The metric of exchange between the "Verb-field" (dynamics) and the "Noun" (state), governed by the generalized second law of thermodynamics. Gamow Factor (): The transmission probability governing the retrieval of stored history via phonon-assisted tunneling through "Twin-Prime Gates." By mapping these abstract variables onto the concrete equations of modern physics—specifically the Lyapunov control functions of Liang and Dong 2, the efficiency metrics of Goerlich et al. 4, and the energy thresholds of Hofstrand 5—we establish a robust theoretical scaffold for the "Glass Key Hypothesis": that reality is a logically reversible, feedback-stabilized information manifold operating at a precise thermodynamic "lean." 2. Quantum Feedback Control: The Mathematical Engine of the "Mark 1 Attractor" The Nexus framework asserts that universal stability is not a static equilibrium but a dynamic "stance"—a "lean" required to process information without collapsing into "dead symmetry" or "chaotic dissolution." In the rigorous language of control theory, this concept is formalized as the stabilization of a target quantum subspace (the "Mark 1 Attractor") amidst a stochastic environment. The primary challenge in this domain is the "Storage Crisis" equivalent: the exponential scaling of computational resources required to estimate the state of a large quantum system. Recent advancements in 2025 by Liang and Dong, presented in their seminal work "Stabilization of Time-Varying Perturbed Quantum Systems via Reduced Filters" 2, provide the exact mathematical architecture for the Nexus "Receiver Collapse." 2.1 The Reduced Filter as the "Receiver Collapse" Mechanism Standard approaches to quantum feedback control rely on the Stochastic Master Equation (SME), which tracks the evolution of the full density matrix . For a system of dimension , this requires computing real variables. As grows, this computational burden becomes prohibitive, representing the "bandwidth limit" of the "First Node" (the universe) that prevents explicit linear storage of history. Liang and Dong introduce a radical dimensionality reduction: the Reduced Quantum Filter. Instead of tracking the full state , the filter estimates only the diagonal elements of the density matrix in a Quantum Non-Demolition (QND) basis. This reduces the complexity from to .2 This mathematical reduction is isomorphic to the Nexus concept of Receiver Collapse. The observer (or the "Second Node") does not process the full "verb-field" (the entire Hilbert space with all its coherences and entanglements); rather, it collapses the system onto a lower-dimensional "noun" (the diagonal population elements) to perform work. The feedback control law is constructed strictly from this reduced information, yet it successfully stabilizes the global system. The evolution of this reduced estimator state is governed by the stochastic differential equation (SDE): In this equation, derived explicitly from the Liang-Dong formalism 3, several Nexus variables find their physical counterparts: The Feedback Control Law (): This represents the Gain (). It is the active force applied by the "Second Node" to steer the system. The Innovation Term (): This represents the Information () extracted from the measurement. It is the difference between the actual observation and the expected value—the "surprise" that updates the model. The Coupling Matrix (): This represents the structural constraints of the "Lattice," defining how different states (or "memories") are connected. The profound insight from this work is that full knowledge of the system is not required for stability. A "lossy" projection (the reduced filter), if properly coupled via feedback (), is sufficient to maintain the "Mark 1 Attractor" (the target subspace). This validates the Nexus "Read-Only Hypothesis": the universe does not need to explicitly compute the full wave function at every step; it only needs to maintain the diagonal "Value" while the "Shape" (coherences) is stored implicitly in the geometry of the dynamics. 2.2 Lyapunov Stability Analysis: The "Lean" of the Attractor How does the system ensure that it converges to the correct "Shape" (target subspace) rather than drifting into entropy? The rigorous proof of this stability relies on Lyapunov Analysis. A Lyapunov function is a scalar metric that measures the "energy" or "distance" of the current state from the desired equilibrium. In the Nexus framework, stability is described as a "lean" (). In the Liang-Dong formalism, stability is defined by the condition that the time derivative of the Lyapunov function, , must be negative definite. The specific Lyapunov function employed is related to the Bhattacharyya distance (or classical fidelity) between the current state and the target invariant subspace : Here, are the projection operators onto the subspaces. The feedback law is designed to maximize the decay rate of this function. The stability condition is expressed via the Sample Lyapunov Exponent (): where is the distance to the target subspace.2 This inequality () is the rigorous mathematical definition of the Nexus "Stance." The system must continuously dissipate "error" (entropy) to remain locked in the target subspace. If the feedback gain is insufficient (i.e., if the controller "falls asleep" or the "Second Node" disconnects), the exponent becomes positive, and the system drifts away from the "Mark 1 Attractor," dissolving into a mixed state of maximal entropy. Furthermore, Liang and Dong prove that this stabilization is Robust. The system can tolerate time-varying perturbations (Nexus "Stress-Test Loop") and uncertainties in the Hamiltonian, provided the feedback mechanism maintains the correct "phase-lock." This mirrors the "Crucible Protocol," where a system is subjected to high "computational temperature" (perturbations) to force it to settle into its most stable, harmonic configuration. 2.3 Feedback Cooling and the "Zero-Pressure Harmonic Collapse" The thermodynamic implications of this control are explored in Max Eriksson’s 2025 thesis, "Continuous Measurements and Feedback Control of a Quantum Harmonic Oscillator".7 Eriksson models a quantum system coupled to a thermal reservoir (a "heat bath" of phonons/photons) and asks: can measurement and feedback cool the system below the temperature of its environment? This process is isomorphic to the Nexus Zero-Pressure Harmonic Collapse (ZPHC). The "noise" of the thermal bath represents the high-entropy "mess" of raw data. The "cooling" represents the collapse of this mess into a structured, low-entropy state ("cold" or "crystalline"). Eriksson utilizes the Wiseman-Milburn equation to derive the steady-state properties of the oscillator under linear feedback. The feedback force acts as a Maxwell's Demon, utilizing the information stream (measurement record) to apply a counter-acting force that cancels out thermal kicks. The effective temperature of the cooled mode is given by: where is the dimensionless feedback gain and is the measurement efficiency.8 This equation reveals the fundamental tradeoff of the Nexus framework: To achieve ZPHC (), one requires high Gain () and high Measurement Efficiency (). The "Cost" of this cooling is the information processing required to generate the feedback signal (discussed in Section 3). Crucially, Eriksson’s results show that there is a critical feedback phase. If the feedback is applied with the wrong phase (i.e., if the "Second Node" is not aligned with the "First Node"), the feedback essentially "heats" the system, driving it into instability. This validates the Nexus requirement for Phase-Locking ( or similar primitives) as a prerequisite for successful retrieval or stabilization. The "Mark 1 Attractor" is not just a location in state space; it is a precise phase relationship between the observer and the observed. 3. Informati
We develop a unified, operational, and falsifiable framework for quantum communication, quantum networks, and quantum cryptography as a conservative extension of three physical axioms: microscopic ontic definiteness with finite localization, the physical reality of the spacetime–vacuum substrate, and persistent causal retarded source–response coupling. Quantum states, density operators, entanglement, channels, and information are treated as mathematical or operational representations rather than additional fundamental ontologies. The framework derives completely positive trace-preserving maps from unitary system–environment dynamics and examines Kraus, Choi, Liouville, Stinespring, complementary-channel, and process-tensor representations. It establishes explicit interfaces for initial system–environment correlations, non-Markovian memory, channel discrimination, diamond-norm certification, and causal multitime processes. Classical, quantum, private, and entanglement-assisted capacities are distinguished from finite-blocklength and resource-normalized throughput. Optical-fiber, free-space, atmospheric, satellite, deep-space, microwave, atomic, spin, and continuous-variable links are incorporated through complete source–channel–receiver–detector forward models. Quantum networks are formulated as stochastic renewal, memory, swapping, purification, routing, and queueing systems subject to cut-set bounds, decoherence, synchronization error, congestion, latency, and energy constraints. For quantum key distribution, the security chain proceeds from source characterization and parameter estimation through smooth min-entropy, entropic uncertainty, error correction, authentication, privacy amplification, and composable key certification. BB84, decoy-state, entanglement-based, measurement-device-independent, device-independent, and continuous-variable protocols are treated with explicit finite-size and assumption boundaries. The theory preserves operational no-signaling, finite propagation speed, passivity, and complete energy accounting. Every performance or security claim requires independent calibration, nuisance covariance, held-out prediction, finite-sample validation, matched classical comparison, and an explicit rejection criterion. **Keywords** Quantum communication; quantum channels; quantum networks; quantum cryptography; quantum key distribution; open quantum systems; channel capacity; continuous-variable communication; quantum repeaters; process tensors; non-Markovian dynamics; composable security; finite-key analysis; no-signaling; detector forward models; experimental identifiability.