Tunneling Dynamics Research Articles

Statistical Engineering for Wind Tunnel Testing of Mars Parachute Designs

Formal experiment design and analysis techniques featuring design of experiments were applied to a Mars parachute wind tunnel test. The experiments were part of an extensive study conducted in 2014 by the NASA Engineering and Safety Center to evaluate the performance of two candidate parachute systems. Reduced scale models of the legacy Disk-Gap-Band (DGB) parachute and new Supersonic Ringsail (SSRS) were compared for both model fidelity and performance measures. Two tests, conducted in the NASA Langley Transonic Dynamics Tunnel, are described with emphasis on statistical engineering principles. The first test compared drag coefficients between models of the same canopy at Earth atmospheric pressure but at mission dynamic pressures. The experiment proved that any manufacturing differences between models were not detectable in drag coefficient measurements. The second test focused on parachute drag coefficient modeling for both canopy designs and two fabric permeabilities over a representative range of mission dynamic pressures and Mach numbers. The SSRS was found to have higher drag coefficients in comparison to the DGB for both canopy material permeabilities over the operational range of Mach numbers and dynamic pressures.

Quantum tunneling time: Insights from an exactly solvable model

Quantum particles interacting with potential barriers are ubiquitous in physics, and the question of how much time they spend inside classically forbidden regions has attracted interest for many decades. Recent developments of new experimental techniques revived the issue and ignited a debate with often contradictory results. This motivates the present study of an exactly solvable model for quantum tunneling induced by a strong field. We show that the tunneling dynamics can depart significantly from the scenario in which the barrier-traversal time is zero or very small. However, our findings do not support the idea of a well-defined tunneling time either. Our numerically exact results should help in finding a consensus about this fundamental problem.

Visual exploration of large normal mode spaces to study protein flexibility

When studying the function of proteins, biochemists utilize normal mode decomposition to enable the analysis of structural changes on time scales that are too long for molecular dynamics simulation. Such a decomposition yields a high-dimensional parameter space that is too large to be analyzed exhaustively. We present a novel approach to reducing and exploring this vast space through the means of interactive visualization. Our approach enables the inference of relevant protein function from single structure dynamics through protein tunnel analysis while considering normal mode combinations spanning the whole normal modes space. Our solution, based on multiple linked 2D and 3D views, enables the quick and flexible exploration of individual modes and their effect on the dynamics of tunnels with relevance for the protein function. Once an interesting motion is identified, the exploration of possible normal mode combinations is steered via a visualization-based recommendation system. This helps to quickly identify a narrow, yet relevant set of normal modes that can be investigated in detail. Our solution is the result of close cooperation between visualization and the domain. The versatility and efficiency of our approach are demonstrated in several case studies.

Tunneling Dynamics of a Dipolar Bose–Einstein Condensate in a Double-Well Potential

We investigate the tunneling dynamics of a three-dimensional (3D) cigar-shaped dipolar Bose–Einstein condensate (BEC) of 52Cr atoms in an axially-symmetric double-well potential (DWP). By introduci...

Effect of countermeasures on the galloping instability of a long-span suspension footbridge

The aeroelastic stability of a long-span suspension footbridge with a bluff deck (prototype section) was examined through static and dynamic wind tunnel tests using a 1:10 scale sectional model of the main girder, and the corresponding aerodynamic countermeasures were proposed in order to improve the stability. First, dynamic tests of the prototype sectional model in vertical and torsional motions were carried out at three attack angles (α= 3°, 0°, -3°). The results show that the galloping instability of the sectional model occurs at α = 3° and 0°, an observation that has never been made before. Then, the various aerodynamic countermeasures were examined through the dynamic model tests. It was found that the openings set on the vertical web of the prototype section (web-opening section) mitigate the galloping completely for all three attack angles. Finally, static tests of both the prototype and web-opening sectional models were performed to obtain the aerodynamic coefficients, which were further used to investigate the galloping mechanism by applying the Den Hartog criterion. The total damping of the prototype and web-opening models were obtained with consideration of the structural and aerodynamic damping. The total damping of the prototype model was negative for α = 0° to 7°, with the minimum value being -1.07%, suggesting the occurrence of galloping, while that of the web-opening model was positive for all investigated attack angles of α = -12° to 12°.

Non-across-wind galloping of a square-section cylinder

This paper presents new insights on the galloping instability phenomenon of square-section prisms. The role of the orientation of the structural axes on the galloping response is studied through wind tunnel tests and quasi-steady theory. A new series of dynamic wind tunnel tests on a square section model were conducted to evaluate non-across-wind galloping vibrations, as well as conventional across-wind galloping. The results are then compared with theoretical predictions to evaluate the reliability of quasi-steady theory in assessing the galloping phenomenon. It is found that for a given angle of attack, the structure has different aeroelastic behaviour for different orientations of the principal axis. At an angle of attack close to the critical angle of attack of square prisms, the quasi-steady theory well predicts the critical wind velocity for the onset of non-across-wind galloping but it is not successful for the case of across-wind galloping.

LetB Structure Reveals a Tunnel for Lipid Transport across the Bacterial Envelope

Gram-negative bacteria are surrounded by an outer membrane composed of phospholipids and lipopolysaccharide, which acts as a barrier and contributes to antibiotic resistance. The systems that mediate phospholipid trafficking across the periplasm, such as MCE (Mammalian Cell Entry) transporters, have not been well characterized. Our ~3.5Å cryo-EM structure of the E.coli MCE protein LetB reveals an ~0.6 megadalton complex that consists of seven stacked rings, with a central hydrophobic tunnel sufficiently long to span the periplasm. Lipids bind inside the tunnel, suggesting that it functions as a pathway for lipid transport. Cryo-EM structures in the open and closed states reveal a dynamic tunnel lining, with implications for gating or substrate translocation. Our results support a model in which LetB establishes a physical link between the two membranes and creates a hydrophobic pathway for the translocation of lipids across the periplasm.

Many-body tunneling and nonequilibrium dynamics in double quantum dots with capacitive coupling

Double quantum dots (DQDs) systems may be the minimal setups for realization of QD-based qubits and quantum computation. Pauli spin blockade (PSB) and a kind of novel many-body tunneling (MBT) are identified to play important roles in these systems, and dominate the quantum tunneling at moderate and weak interdot coupling t, respectively. On the other hand, inter-dot Coulomb interaction U′ and related inter-dot Coulomb blockade (IDCB) is inevitable in DQDs. However, what would happen on the effect of U′ in DQDs has not been touched, in particular for PSB and MBT. Here, we study the tunneling processes and transport properties with various U′ in series-coupled DQDs, and find MBT process is rather robust against U′ within U′/U < 0.1, where U is the intra-dot Coulomb interaction. Meanwhile, the linearity relationship between the carrier doublon number and MBT current remains valid. These findings enrich the understanding of the many-body tunneling in the DQDs and may shed light on the manipulation of the QD-based qubits.

Breaking rotational symmetry in a trapped-ion quantum tunneling rotor

A trapped-ion quantum tunneling rotor (QTR) is in a quantum superposition of two different Wigner crystal orientations. In a QTR system, quantum tunneling drives the coherent transition between the two different Wigner crystal orientations. We theoretically study the quantum dynamics of a QTR, particularly when the spin state of one of the ions is flipped. We show that the quantum dynamics of an $\it{N}$-ion QTR can be described by continuous-time cyclic quantum walks. We also investigate the quantum dynamics of the QTR in a magnetic field. Flipping the spin state breaks the rotational symmetry of the QTR, making the quantum-tunneling-induced rotation distinguishable. This symmetry breaking creates coupling between the spin state of the ions and the rotational motion of the QTR, resulting in different quantum tunneling dynamics.

Entanglement-assisted tunneling dynamics of impurities in a double well immersed in a bath of lattice trapped bosons

We unravel the correlated tunneling dynamics of an impurity trapped in a double well and interacting repulsively with a majority species of lattice trapped bosons. Upon quenching the tilt of the double well it is found that the quench-induced tunneling dynamics depends crucially on the interspecies interaction strength and the presence of entanglement inherent in the system. In particular, for weak couplings the impurity performs a rather irregular tunneling process in the double well. Increasing the interspecies coupling it is possible to control the response of the impurity which undergoes a delayed tunneling while the majority species effectively acts as a material barrier. For very strong interspecies interaction strengths the impurity exhibits a self-trapping behavior. We showcase that a similar tunneling dynamics takes place for two weakly interacting impurities and identify its underlying transport mechanisms in terms of pair and single-particle tunneling processes.

Theory of valley-resolved spectroscopy of a Si triple quantum dot coupled to a microwave resonator

We theoretically study a silicon triple quantum dot (TQD) system coupled to a superconducting microwave resonator. The response signal of an injected probe signal can be used to extract information about the level structure by measuring the transmission and phase shift of the output field. This information can further be used to gain knowledge about the valley splittings and valley phases in the individual dots. Since relevant valley states are typically split by several , a finite temperature or an applied external bias voltage is required to populate energetically excited states. The theoretical methods in this paper include a capacitor model to fit experimental charging energies, an extended Hubbard model to describe the tunneling dynamics, a rate equation model to find the occupation probabilities, and an input–output model to determine the response signal of the resonator.

The attoclock and tunnelling time

Ultrafast laser pulses have enabled experiments probing the temporal dynamics of an electron tunnelling out of an atom. A quintessential example of such is known as the ‘attoclock’. While the investigation is ongoing, the window for non-zero ‘tunnelling times’ in this context appears to have largely closed.

VTST and RPMD kinetics study of the nine-body X + C2H6 (X ≡ H, Cl, F) reactions based on analytical potential energy surfaces.

Thermal rate constants of nine-atom hydrogen abstraction reactions, X + C2H6 → HX + C2H5 (X ≡ H, Cl, F) with qualitatively different reaction paths, have been investigated using two kinetics approaches - variational transition state theory with multidimensional tunnelling (VTST/MT) and ring polymer molecular dynamics (RPMD) - and full dimensional analytical potential energy surfaces. For the H + C2H6 reaction, which proceeds through a noticeable barrier height of 11.62 kcal mol-1, kinetics approaches showed excellent agreement between them (with differences less than 30%) and with the experiment (with differences less than 60%) in the wide temperature range of 200-2000 K. For X = Cl and F, however, the situation is very different. The barrier height is either low or very low, 2.44 and 0.23 kcal mol-1, respectively, and the presence of van der Waals complexes in the entrance channel leads to a very flat topography and, consequently, imposes theoretical challenges. For the Cl(2P) reaction, VTST/MT underestimates the experimental rate constants (with differences less than 86%), and RPMD demonstrates better agreement (with differences less than 47%), although the temperature dependence is opposite to the experiment at low temperatures. Finally, for the F(2P) reaction, available experimental information shows discrepancies, both in the absolute values of the rate constants and also in the temperature dependence. Unfortunately, kinetics theories did not resolve this discrepancy. Different possible causes of these theory/experiment discrepancies were analyzed.

A New Reliability Rock Mass Classification Method Based on Least Squares Support Vector Machine Optimized by Bacterial Foraging Optimization Algorithm

Classification of the surrounding rock is the basis of tunnel design and construction. However, conventional classification methods do not allow dynamic tunnel construction adjustments because they are time‐consuming and do not consider the randomness of rock mass. This paper presents a new reliability rock mass classification method based on a least squares support vector machine (LSSVM) optimized by a bacterial foraging optimization algorithm (BFOA). The LSSVM is adopted to express the implicit relationship between classification indicators and rock mass grades, which is a response surface function for reliability evaluation. LSSVM parameters were optimized by the BFOA to form a hybrid BFOA‐LSSVM algorithm. Using geological prediction and rock strength resilience results as classification indicators, samples were developed to train the LSSVM model using the hybrid algorithm. The Monte Carlo sampling method of reliability classification was implemented and applied to the Suqiao tunnel at the Puyan highway in the Fujian province of China; the influence of parameters on the performance of the algorithm is discussed. The results indicate that the new method is feasible for tunnel engineering; it can improve the classification accuracy of surrounding rock exhibiting randomness, to provide an effective means of classifying surrounding rock in the dynamic design of tunnel construction.

Dewetting: From Physics to the Biology of Intoxicated Cells.

Pathogenic bacteria colonize or disseminate into cells and tissues by inducing large-scale remodeling of host membranes. The physical phenomena underpinning these massive membrane extension and deformation are poorly understood. Invasive strategies of pathogens have been recently enriched by the description of a spectacular mode of opening of large transendothelial cell macroaperture (TEM) tunnels correlated to the dissemination of EDIN-producing strains of Staphylococcus aureus via a hematogenous route or to the induction of gelatinous edema triggered by the edema toxin from Bacillus anthracis. Remarkably, these highly dynamic tunnels close rapidly after they reach a maximal size. Opening and closure of TEMs in cells lasts for hours without inducing endothelial cell death. Multidisciplinary studies have started to provide a broader perspective of both the molecular determinants controlling cytoskeleton organization at newly curved membranes generated by the opening of TEMs and the physical processes controlling the dynamics of these tunnels. Here we discuss the analogy between the opening of TEM tunnels and the physical principles of dewetting, stemming from a parallel between membrane tension and surface tension. This analogy provides a broad framework to investigate biophysical constraints in cell membrane dynamics and their diversion by certain invasive microbial agents.

High-resolution spectroscopic study of the H2O–CO2 van der Waals complex in the 2OH overtone range

The jet-cooled spectrum (K) of the HO–CO van der Waals complex has been recorded in the 1.4 μm region by cavity ring-down spectroscopy. Two b-type vibrational bands have been observed and analysed. The rotational assignment has been achieved using a different asymmetric rotor Hamiltonian for each nuclear spin species, accounting for the internal rotation of the HO and CO units. The band at 7247 cm is assigned to (101) in terms of the () vibrational quantum number of the HO monomer. The band at 7238 cm is assigned to (200) + an intermolecular mode () excited in the complex. Vibration-rotation constants are provided for the excited states. The symmetry of the wavefunction, the effect of vibrational excitation on the tunnelling dynamics and the vibrational assignment are discussed.

Quenched tunneling in ammonia

Quantum Mechanics Inversion between two energetically equivalent umbrella-spaced configurations in ammonia (NH3) is a classic textbook example of tunneling in a symmetric double-minimum potential. Park et al. report on how a very strong direct-current electric field, as powerful as intermolecular interactions, can suppress this quantum mechanical phenomenon in the system. They observed that NH3 molecules isolated in a solid argon matrix at 10 kelvin strongly orient along the external field direction and that, as the field is increased up to 200 million volts per meter, their energy states shift, eventually quenching the tunneling from the normal state to the inverted one. The observed changes are reversible and open up opportunities for manipulating tunneling dynamics in many other molecules. Proc. Natl. Acad. Sci. U.S.A. 116 , 23444 (2019).

Generalized escape paths for dynamical tunneling in QFT

We present a formalism based on the functional Schr\"odinger equation to analyse time-dependent tunneling in quantum field theory at the semi-classical level. The full problem is reduced step by step to a finite dimensional quantum mechanical setup and solved using the WKB approximation. As an example, we consider tunneling from a homogeneous oscillating initial state in scalar quantum field theory.

The ground state and the tunnelling dynamics of the Bose-Einstein condensate in a tilted shallow trap

The ground state and the tunnelling dynamics of the Bose-Einstein condensate (BEC) loaded in a tilted shallow trap is studied analytically and numerically. The stable bound state, the quasi-bound state and the diffusion state are predicted. The thresholds for transition between the different states are obtained and the stability diagram in parameter space is presented. The tunnelling dynamics of the system in different states is revealed. The shape of the potential well and the atomic interaction play important role and have coupled effect on the tunnelling dynamics of the system. Furthermore, the resonant tunnelling phenomenon in the parametrically modulated shallow trap is observed. The results show that when the modulating frequency approaches the dipolar mode of the system, resonant tunnelling occurs and the whole system is unstable. Our results provide a theoretical evidence for studying the tunnelling dynamics of the ultracold atomic system.

Generalized Bloch oscillations of ultracold lattice atoms subject to higher-order gradients

The standard Bloch oscillation normally refers to the oscillatory tunneling dynamics of quantum particles in a periodic lattice plus a linear gradient. In this work we theoretically investigate the generalized form of the Bloch oscillation in the presence of additional higher order gradients, and demonstrate that the higher order gradients can significantly modify the tunneling dynamics, particularly in the spectrum of the density oscillation. The spectrum of the standard Bloch oscillation is composed of a single prime frequency and its higher harmonics, while the higher-order gradients in the external potential give rise to fine structures in the spectrum around each of these Bloch frequencies, which are composed of serieses of frequency peaks. Our investigation leads to a twofold consequence to the applications of Bloch oscillations for measuring external forces: For one thing, under a limited resolution of the measured spectrum, the fine structures would manifest as a blur to the spectrum, and leads to intrinsic errors to the measurement. For another, given that the fine structures could be experimentally resolved, they can supply more information of the external force than the strength of the linear gradient, and be used to measure more complicated forces.