Physics, Volume 6, Issue 3 (September 2024) – 16 articles

Two-level quantum systems are building blocks of quantum technologies, where the qubit is the basic unit of quantum information. The ability to design driving fields that produce prespecified evolutions of relevant physical observables is crucial to the development of such technologies. Using vector algebra and recently developed strategies for generating solvable two-level Hamiltonians, we construct the general solution to the inverse problem for a spin in a time-dependent magnetic field and its extension to any two-level system associated with fictitious spin and field. We provide a general expression for the field that drives the dynamics of the system so as to realize prescribed time evolutions of the expectation values of the Pauli operators and the autocorrelation of the Pauli vector. The analysis is applied to two-state charge transfer systems, showing that the charge transfer process can be seen as a motion of the state of the associated fictitious qubit on the Bloch sphere, and that the expectation values of the related Pauli operators describe the interference between the two differently localized electronic states and their population difference. Our formulation is proposed as a basic step towards potential uses of charge transfer in quantum computing and quantum information transfer. Full article

The latest results of Higgs boson searches beyond the Standard Model from the ATLAS and CMS experiments are reviewed. This includes searches for additional neutral, charged, and double charged Higgs-like bosons, searches for dark matter produced in association with a Higgs boson, and searches for new physics in Higgs boson production and decay processes. Interpretations are given within the hMSSM, a special parameterization of the Minimal Supersymmetric extension of the Standard Model, in which the mass of the lightest Higgs boson is set to the value of 125 GeV measured at the LHC. Full article

The Casimir effect has been exploited in various MEMS (micro-electro-mechanical system) devices, especially to make sensitive force sensors and accelerometers. It has also been used to provide forces for a variety of purposes, for example, for the assembly of considerably small parts. Repulsive forces and torques have been produced using various configurations of media and materials. Just a few electronic devices have been explored that utilize the electrical properties of the Casimir effect. Recently, experimental results were presented that described the operation of an electronic device that employed a Casimir cavity attached to a standard MIM (metal–insulator–metal) structure. The DC (direct current) conductance of the novel MIM device was enhanced by the attached cavity and found to be directly proportional to the capacitance of the attached cavity. The phenomenological model proposed assumed that the cavity reduced the vacuum fluctuations, which resulted in a reduced injection of carriers. The analysis presented here indicates that the optical cavity actually enhances vacuum fluctuations, which would predict a current in the opposite direction from that observed. Further, the vacuum fluctuations near the electrode are shown to be approximately independent of the size of the optical cavity, in disagreement with the experimental data which show a dependence on the size. Thus, the proposed mechanism of operation does not appear correct. A more detailed theoretical analysis of these devices is needed, in particular, one that uses real material parameters and computes the vacuum fluctuations for the entire device. Such an analysis would reveal how these devices operate and might suggest design principles for a new genre of electronic devices that make use of vacuum fluctuations. Full article

This paper presents a two-qubit model derived from an SU(2)-symmetric 4×4 Hamiltonian. The resulting model is physically significant and, due to the SU(2) symmetry, is exactly solvable in both time-independent and time-dependent cases. Using the formal, general form of the related time evolution operator, the time dependence of the entanglement level for certain initial conditions is examined within the Rabi and Landau–Majorana–Stückelberg–Zener scenarios. The potential for applying this approach to higher-dimensional Hamiltonians to develop more complex exactly solvable models of interacting qubits is also highlighted. Full article

The elastic and inelastic scattering of deuterons on ¹⁰B nuclei and the ¹⁰B(d, t)⁹B reaction were studied at a deuteron energy of 14.5 MeV. In inelastic scattering, differential cross-sections for transitions to ¹⁰B states at excitation energies, E_x, of 0.718 MeV (1⁺), 2.154 MeV (1⁺), and 3.59 MeV (2⁺) were measured. The cross-sections of the (d, t) reaction were measured for the ground (3/2⁻) and excited states of the ⁹B nucleus at E_x = 2.361 MeV (5/2⁻) and 2.79 MeV (5/2⁺). An analysis of the corresponding angular distributions was carried out using the coupled channel method. As a result of the calculations, the values of the quadrupole deformation parameters (β₂ ≈ 0.7 ± 0.1) for various transitions in the ¹⁰B nucleus in inelastic scattering were extracted. From the analysis of the (d, t) reaction, the values of spectroscopic amplitudes (SA = 0.67 and SA = 0.94) for transitions to the states of the ⁹B nucleus were extracted. The results obtained here, taking into account possible measurement errors, are in good agreement with the previously obtained data and the theoretical predictions. Full article

Ferroelectric charged domain walls offer a revolutionary path for next-generation ferroelectric devices due to their exceptional conductivity within an otherwise insulating matrix. However, quantitative understanding of this “giant conductivity” has remained elusive due to the lack of robust models describing carrier behavior within CDWs. The current paper bridges this critical knowledge gap by employing a first-principles approach that incorporates Boltzmann transport theory and the relaxation time approximation. This strategy enables the calculation of carrier concentration, mobility, and conductivity for both head-to-head and tail-to-tail domain wall configurations within a stabilized periodic structure. The comprehensive transport analysis given here reveals that the accumulation of charge carriers, particularly their concentration, is the dominant factor governing domain wall conductance. Interestingly, observed conductance differences between head-to-head and tail-to-tail walls primarily arise from variations in carrier mobility. Additionally, this study demonstrates a significantly reduced domain wall width compared to previous reports. This miniaturization is attributed to the presence of compressive strain, which lowers the energy barrier for electron–hole pair generation. Furthermore, the findings here suggest that reducing the band gap presents a viable strategy for stabilizing charged domain walls. These results pave the way for the optimization and development of domain wall devices across a spectrum of ferroelectric materials. Full article

In the present introductory to the Special Issue “75 Years of the Casimir Effect: Advances and Prospects”, we summarize the state of the art in this field of physics, briefly describe the topics of the contributing papers, formulate several unresolved problems, and outline possible pathways towards their resolution. Special attention is given to experiments on measuring the Casimir force, to the known problem of the dissipation of conduction electrons when one compares experiment with theory, and to the Casimir effect in novel materials and non-traditional situations. We conclude that in the future, this multidisciplinary quantum effect will continue to play a crucial role in both fundamental physics and its applications. Full article

In this paper, we review some recent findings related to the Casimir effect. Initially, the thermal corrections to the vacuum Casimir energy density are calculated, for a quantum scalar field, whose modes propagate in the (3+1)-dimensional Euclidean spacetime, subject to a nontrivial compact boundary condition. Next, we analyze the Casimir effect induced by two parallel plates placed in a weak gravitational field background. Finally, we review the three-dimensional wormhole solutions sourced by the Casimir density and pressures associated with the quantum vacuum fluctuations of the Yang-Mills field. Full article

We review the formalism of center-of-mass tomograms, which allows us to describe quantum states in terms of probability distribution functions. We introduce the concept of separable and entangled probability distributions for center-of-mass tomography. We obtain the time evolution of center-of-mass tomograms of entangled states of the inverted oscillator. Full article

Traditional models of opinion dynamics provide a simplified approach to understanding human behavior in basic social scenarios. However, when it comes to issues such as polarization and extremism, a more nuanced understanding of human biases and cognitive tendencies are required. This paper proposes an approach to modeling opinion dynamics by integrating mental models and assumptions of individuals agents using Bayesian-inspired methods. By exploring the relationship between human rationality and Bayesian theory, this paper demonstrates the usefulness of these methods in describing how opinions evolve. The analysis here builds upon the basic idea in the Continuous Opinions and Discrete Actions (CODA) model, by applying Bayesian-inspired rules to account for key human behaviors such as confirmation bias, motivated reasoning, and human reluctance to change opinions. Through this, This paper updates rules that are compatible with known human biases. The current work sheds light on the role of human biases in shaping opinion dynamics. I hope that by making the model more realistic this might lead to more accurate predictions of real-world scenarios. Full article

Magnetic quantum oscillations (MQOs) are traditionally applied to investigate the electronic structure of metals. In layered quasi-two-dimensional (Q2D) materials, the MQOs have several qualitative features, offering additional helpful information, provided their theoretical description is developed. Within the framework of the Kubo formula and the self-consistent Born approximation, we reconsider the phase of the beats in the amplitude of the Shubnikov oscillations of the interlayer conductivity in Q2D metals. We show that the phase shift of the beats of the Shubnikov (conductivity) oscillations relative to the de Haas–van Alphen (magnetization) oscillations is larger than woud be expected and, under certain conditions, can reach the value of $\pi /2$, as observed experimentally. We explain the phase inversion of the MQOs during the 3D–2D crossover and predict the decrease in the relative MQO amplitude of the interlayer magnetoresistance in a strong magnetic field, larger than the beat frequency. Full article

An approach to derive the below-bandgap absorption in GaSb/GaAs self-assembled quantum dot devices using room-temperature external quantum efficiency measurement results is presented. Devices with five layers of delta-doped quantum dots placed in the intrinsic, n- and p-regions of a GaAs solar cell are studied. The importance of incorporating an extended Urbach tail absorption in analyzing the absorption strength of quantum dots and the transition states is demonstrated. The theoretically integrated absorbance via quantum dot ground states is calculated as 1.04 × 10¹⁵ cm⁻¹s⁻¹, which is in reasonable agreement with the experimentally derived value 8.1 × 10¹⁵ cm⁻¹s⁻¹. The wetting layer and quantum dot absorption contributions are separated from the tail absorption and their transition energies are calculated. Using these transition energies and the GaAs energy gap of 1.42 eV, the heavy hole confinement energies for the quantum dots (320 meV) and for the wetting layer (120 meV) are estimated. Full article

On 17 October 1947, Niels Bohr was made a knight of the Order of the Elephant by the King of Denmark in view of his outstanding achievements and contributions to science. Bohr designed his own coat of arms that featured a pattern of Yin and Yang (Tai Chi symbol) to symbolize the wave–particle complementarity. However, Bohr’s Yin-Yang diagram (YYD) was neither drawn based on the principles of quantum mechanics, nor did it originate from the traditional Taoist YYD. Scientists still have doubts about the legitimacy of using YYD as the icon of the wave–particle complementarity, because the YYD belonging to quantum mechanics itself is unknown so far. This paper reports the YYDs existing in quantum mechanics and justifies the role of YYD in the wave–particle duality by showing that any system, whether classical or quantum, has an ideal YYD as long as it satisfies Bohr’s principle of complementarity (BPC). The deviation of a deformed YYD from the ideal YYD indicates the extent to which a real system satisfies BPC. This paper constructs the quantum YYD by the complex quantum trajectory of a particle tunneling via a step barrier, which displays the continuous transition between the wave behavior and the particle behavior. It appears that the YYD designed by Bohr in his coat of arms resembles the YYD generated by tunneling motion, not only in appearance but also in the governing equation. Full article

Based on the position and momentum of noncommutative relations with a noncanonical map, we study the Dirac equation and analyze its parity and time reversal symmetries in a noncommutative phase space. Noncommutative parameters can be endowed with the Planck length and cosmological constant such that the noncommutative effect can be interpreted as an effective gauge potential or a $(0,2)$-type curvature associated with the Plank constant and cosmological constant. This provides a natural coupling between dynamics and spacetime geometry. We find that a free Dirac particle carries an intrinsic velocity and force induced by the noncommutative algebra. These properties provide a novel insight into the Zitterbewegung oscillation and the physical scenario of dark energy. Using perturbation theory, we derive the perturbed and nonrelativistic solutions of the Dirac equation. The asymmetric vacuum gaps of particles and antiparticles reveal the particle–antiparticle symmetry breaking in the noncommutative phase space, which provides a clue to understanding the physical mechanisms of particle–antiparticle asymmetry and quantum decoherence through quantum spacetime fluctuation. Full article

There are numerous instabilities present in charged particle beams that undergo exponential growth and reach saturation. In various applications, such as free-electron lasers or micro-bunching light sources, achieving saturation is desirable. Conversely, there are applications where these instabilities are utilized as linear broad-band amplifiers for signals embedded in the charged beam. In the latter scenario, the saturation of an instability induces non-linear distortions in the imprinted signal, thereby limiting the useful range of such amplifiers. Accurate evaluation of these instabilities necessitates a complete and comprehensive modeling approach that includes shot noise within the beam. Unfortunately, such modeling is not always feasible or practical. In this paper, we introduce a methodology utilizing the frequency and bandwidth of the instability as key parameters. Through this, we derive an estimation for the range of linear instability growth. Our derivation is conducted in a model-independent manner, making it applicable to a broad spectrum of instabilities. To validate our approach, we employ established and thoroughly benchmarked simulations with a free electron laser (FEL) code as well as self-consistent 3-dimensional simulation of plasma-cascade instability using code SPACE. Full article