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Physics, Volume 7, Issue 4 (December 2025) – 6 articles

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29 pages, 970 KB

Open AccessReview

Molecular Quantum Electrodynamics: Developments of Principle and Progress in Applications

by David L. Andrews

Physics 2025, 7(4), 49; https://doi.org/10.3390/physics7040049 - 15 Oct 2025

Abstract

Molecular quantum electrodynamics is a powerful and effective tool for the representation and elucidation of optical interactions with matter. Its history spans nearly a century of significant advances in its detailed theory and applications, and in its wider appreciation. To fully appreciate the development of the subject into its modern form invites a perspective on progressive technical progress in the theory, noting a growth in applications that closely mirrors advances in optical experimentation. The challenges and deficiencies of alternative approaches to theory are also taken into consideration. Full article

(This article belongs to the Special Issue Quantum Theory 100 Years Later: Advances on Foundations and Applications)

8 pages, 767 KB

Open AccessCommunication

Exact Solutions, Critical Parameters and Accidental Degeneracy for the Hydrogen Atom in a Spherical Box

by Francisco M. Fernández

Physics 2025, 7(4), 48; https://doi.org/10.3390/physics7040048 - 15 Oct 2025

Abstract

This paper for the first time derives some properties of the hydrogen atom inside a box with an impenetrable wall. Scaling of the Hamiltonian operator proves to be practical for the derivation of some general properties of the eigenvalues. The radial part of the Schrödinger equation is conditionally solvable and the exact polynomial solutions provide helpful information. There are accidental degeneracies that take place at particular values of the box radius, some of which can be determined from the conditionally-solvable condition. Some of the roots stemming from the conditionally-solvable condition appear to converge towards the critical values of the model parameter. This analysis is facilitated by the Rayleigh–Ritz method that provides accurate eigenvalues. Full article

(This article belongs to the Section Quantum Mechanics and Quantum Systems)

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11 pages, 318 KB

Open AccessArticle

Non-Linear Quantum Dynamics in Coupled Double-Quantum- Dot-Cavity Systems

by Tatiana Mihaescu, Mihai A. Macovei and Aurelian Isar

Physics 2025, 7(4), 47; https://doi.org/10.3390/physics7040047 - 14 Oct 2025

Abstract

The steady-state quantum dynamics of a compound sample consisting of a semiconductor double-quantum-dot (DQD) system, non-linearly coupled with a leaking superconducting transmission line resonator, is theoretically investigated. Particularly, the transition frequency of the DQD is taken to be equal to the doubled resonator frequency, whereas the inter-dot Coulomb interaction is considered weak. As a consequence, the steady-state quantum dynamics of this complex non-linear system exhibit sudden changes in its features, occurring at a critical DQD-cavity coupling strength, suggesting perspectives for designing on-chip microwave quantum switches. Furthermore, we show that, above the threshold, the electrical current through the double-quantum dot follows the mean photon number into the microwave mode inside the resonator. This might not be the case any more below that critical coupling strength. Lastly, the photon quantum correlations vary from super-Poissonian to Poissonian photon statistics, i.e., towards single-qubit lasing phenomena at microwave frequencies. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Viktor Dodonov—55 Years in Quantum Physics)

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21 pages, 3119 KB

Open AccessArticle

Modelling Dynamic Parameter Effects in Designing Robust Stability Control Systems for Self-Balancing Electric Segway on Irregular Stochastic Terrains

by Desejo Filipeson Sozinando, Bernard Xavier Tchomeni and Alfayo Anyika Alugongo

Physics 2025, 7(4), 46; https://doi.org/10.3390/physics7040046 - 10 Oct 2025

Viewed by 271

Abstract

In this study, a nonlinear dynamic model is developed to examine the stability and vibration behavior of a self-balancing electric Segway operating over irregular stochastic terrains. The Segway is treated as a three-degrees-of-freedom cart–inverted pendulum system, incorporating elastic and damping effects at the wheel–ground interface. Road irregularities are generated in accordance with international standard using high-order filtered noise, allowing for representation of surface classes from smooth to highly degraded. The governing equations, formulated via Lagrange’s method, are transformed into a Lorenz-like state-space form for nonlinear analysis. Numerical simulations employ the fourth-order Runge–Kutta scheme to compute translational and angular responses under varying speeds and terrain conditions. Frequency-domain analysis using Fast Fourier Transform (FFT) identifies resonant excitation bands linked to road spectral content, while Kernel Density Estimation (KDE) maps the probability distribution of displacement states to distinguish stable from variable regimes. The Lyapunov stability assessment and bifurcation analysis reveal critical velocity thresholds and parameter regions marking transitions from stable operation to chaotic motion. The study quantifies the influence of the gravity–damping ratio, mass–damping coupling, control torque ratio, and vertical excitation on dynamic stability. The results provide a methodology for designing stability control systems that ensure safe and comfortable Segway operation across diverse terrains. Full article

(This article belongs to the Section Applied Physics)

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17 pages, 692 KB

Open AccessArticle

Disentanglement of a Bipartite System Portrayed in a (3+1)D Compact Minkowski Manifold: Quadridistances and Quadrispeeds

by Salomon S. Mizrahi

Physics 2025, 7(4), 45; https://doi.org/10.3390/physics7040045 - 28 Sep 2025

Viewed by 295

Abstract

In special relativity, particle trajectories, whether mass-bearing or not, can be traced on the Minkowski spacetime manifold in (3+1)D. Meantime, in quantum mechanics, trajectories in the phase space are not strictly outlined because coordinate and linear momentum cannot be measured simultaneously with arbitrary precision since they do not commute within the Hilbert space formalism. However, from the density matrix representing a quantum system, the extracted information still produces an imperative description of its properties and, furthermore, by appropriately reordering the matrix entries, additional information can be obtained from the same content. Adhering to this line of work, the paper investigates the definition and the meaning of velocity and speed in a typical quantum phenomenon, the disentanglement for a bipartite system when dynamical evolution is displayed in a (3+1)D pseudo-spacetime whose coordinates are constructed from combinations of entries to the density matrix. The formalism is based on the definition of a Minkowski manifold with compact support, where trajectories are defined following the same reasoning and formalism present in the Minkowski manifold of special relativity. The space-like and time-like regions acquire different significations referred to entangled-like and separable-like, respectively. The definition and the sense of speed and velocities of disentanglement follow naturally from the formalism. Depending on the dynamics of the physical state of the system, trajectories may meander between regions of entanglement and separability in the space of new coordinates defined on the Minkowski manifold. Full article

(This article belongs to the Special Issue A Themed Issue in Honor of Professor Viktor Dodonov—55 Years in Quantum Physics)

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27 pages, 608 KB

Open AccessReview

Temperature Dependence of the Response Functions of Graphene: Impact on Casimir and Casimir–Polder Forces in and out of Thermal Equilibrium

by Galina L. Klimchitskaya and Vladimir M. Mostepanenko

Physics 2025, 7(4), 44; https://doi.org/10.3390/physics7040044 - 26 Sep 2025

Viewed by 259

Abstract

We review and as well obtain some new results on the temperature dependence of spatially nonlocal response functions of graphene and their applications to the calculation of both the equilibrium and nonequilibrium Casimir and Casimir–Polder forces. After a brief summary of the properties of the polarization tensor of graphene obtained within the Dirac model in the framework of quantum field theory, we derive the expressions for the longitudinal and transverse dielectric functions. The behavior of these functions at different temperatures is investigated in the regions below and above the threshold. Special attention is paid to the double pole at zero frequency, which is present in the transverse response function of graphene. An application of the response functions of graphene to the calculation of the equilibrium Casimir force between two graphene sheets and the Casimir–Polder forces between an atom (nanoparticle) and a graphene sheet is considered with due attention to the role of a nonzero energy gap, chemical potential and a material substrate underlying the graphene sheet. The same subject is discussed for out-of-thermal-equilibrium Casimir and Casimir–Polder forces. The role of the obtained and presented results for fundamental science and nanotechnology is outlined. Full article

(This article belongs to the Section Condensed Matter Physics)

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