Two-level System Research Articles

Shot-noise-suppressing quantum nonlinearity in the vacuum-field-induced photon-photon interaction

The shot noise in coherent control of a quantum system sets an ultimate precision limit in measurement and information processing. It is caused by the fundamental quantum fluctuation of the photon number of a coherent control field and thus is inevitable. By strongly coupling a two-level system to a quantum vacuum field, we propose an effective $N$-type four-level system. In such a configuration, the coupling of a conventional $N$-type system to a coherent control field is replaced by the quantum vacuum field. As a result, we can achieve a shot-noise-suppressing quantum nonlinearity, due to the vanishing fluctuation of photon number of the quantum vacuum field. The resulting photon blockade effect is only subject to the contribution from the probe field. Thus, it becomes shot-noise suppressing. Our work shows an advantage of a quantum vacuum protocol over a coherent-field protocol in manipulation of quantum systems and presents an opportunity to exploit the quantum vacuum field as a powerful tool in quantum information processing.

TASKS OF RISKS ASSESSMENT AND FAILURES PREVENTION OF THE SHIP’S MECHANICAL SYSTEMS

The subject of the research is the tasks of assessing risks and preventing failures of ship mechanical systems, since at present reliability indicators are of great importance at the design stages and during operation. The various types of maintenance, namely, corrective, scheduled, by “state” are compared in the paper. The performance of technical monitoring can provide valid information about the actual state of the ship’s mechanical systems, which can reduce the technical and economic costs of their maintenance. A set of risks assessment methods is considered according the following stages - hazards identification, risks analysis, control options, cost-benefit assessment, decision making. To improve current methods and make them more practical, risks assessment and failures prevention are considered using the example of marine mechanical systems. The concept of risks matrix priority, taken from the analysis of the probability and consequences of failures, is used. The assessment identifies five levels of probability and five levels of consequences that can consider each type of undesirable accident. If statistical data are not available for each stage of the assessment, it is proposed to apply expert review. Possible connections between the design calculation of the reliability of ship’s mechanical systems and risk assessment based on actual data on accidents are discussed, i. e. a two-level system that considers various aspects of ensuring the reliability of ship equipment at all stages of its life cycle. The proposed approach for the joint use of risks assessment methods and traditional reliability calculations allows choosing the safest operating mode for the selected risk level, as well as predicting the necessary corrective actions, choosing options for diagnosing and maintaining ship’s mechanical systems. The conclusions received can be used to prepare specifications for the design of ship’s systems in order to assess reliability indicators, and also to extend the life of such equipment.

Certification of a non-projective qudit measurement using multiport beamsplitters

The most common form of measurement in quantum mechanics projects a wavefunction onto orthogonal states that correspond to definite outcomes. However, generalized quantum measurements that do not fully project quantum states are possible and have an important role in quantum information tasks. Unfortunately, it is difficult to certify that an experiment harvests the advantages made possible by generalized measurements, especially beyond the simplest two-level qubit system. Here we show that multiport beamsplitters allow for the robust realization of high-quality generalized measurements in higher-dimensional systems with more than two levels. Using multicore optical fibre technology, we implement a seven-outcome generalized measurement in a four-dimensional Hilbert space with a fidelity of 99.7%. We present a practical quantum communication task and demonstrate a success rate that cannot be simulated in any conceivable quantum protocol based on standard projective measurements on quantum messages of the same dimension. Our approach, which is compatible with modern photonic platforms, showcases an avenue for faithful and high-quality implementation of genuinely non-projective quantum measurements beyond qubit systems. Generalized measurements that do not correspond to conventional basis projections of the quantum wavefunction are a part of several important protocols in quantum information. These measurements can be certifiably performed on higher-dimensional systems using optical fibre technology.

Integration model of control of lifelong education system in educational clusters and mechanisms of network interaction

Today, in conditions of uncertainty and a high level of competitiveness in the labor market, it is necessary to conduct high-quality training of professional personnel in secondary vocational and higher education in a shorter time. A two-level system of training on one educational platform allows you to maintain the continuity of the content of educational programs. The integration process in the field of education makes it possible to achieve high positive results through the diffusion of differentiated elements and properties into a single professional educational space. The paper considers the main directions of integration processes in the framework of continuing education in the format of educational clusters on the example of a University. This model makes it possible to improve the quality and efficiency of training highly qualified personnel for an intensively developing labor market. The implementation of the integration model of management of the system of continuing education through the mechanism of interaction of educational programs of secondary vocational education and higher education creates a collaborative educational environment, which allows to provide the labor market with qualified personnel in the shortest possible time. A variant of the interaction of the integration model of the functioning of the training of middle-level and higher specialists on the platform of the federal state budgetary educational institution of higher education Samara State Technical University is considered.

Time-resolved temperature-dependent photoluminescence spectroscopy of InGaAs/GaAs quantum well-dots

The paper presents the results of a study of dynamic properties of InGaAs quantum well-dots (QWD) embedded in a GaAs matrix carried out by time-resolved photoluminescence (TRPL) in the temperature range of 10–300 K. The time dependence of the PL shows a long region of constant signal intensity. It was attributed to the filling of all available QWD states due to the fast exchange with the matrix and the relatively long time of radiative recombination. The experimental results are consistent with the calculations performed in the rate equation model for a two-level system. It turned out that there is no need for the residual 2D quantum well to be taken into account, which is not typical for Stranski-Krastanow quantum dots, where the wetting layer plays an important role in carrier exchange processes. Nonradiative recombination can also be ignored, which indicates a high quality of the structure. By fitting the simulation results, the microscopic properties of the electron and hole states in QWDs were obtained for the first time: the radiative lifetime is 700 ps, the barriers for electron and hole in QWDs are 50 and 150 meV, respectively. Carrier capture time was found to be in the range of 10–30 ps.

Adiabaticity in nonreciprocal Landau-Zener tunneling

We investigate the Landau-Zener tunneling (LZT) of a self-interacting two-level system in which the coupling between the levels is nonreciprocal. In such a non-Hermitian system, when the energy bias between two levels is adjusted very slowly, i.e., in the adiabatic limit, we find that a quantum state can still closely follow the eigenstate solution until it encounters the exceptional points (EPs) at which two eigenvalues and their corresponding eigenvectors coalesce. In the absence of the nonlinear self-interaction, we can obtain explicit expressions for the eigenvectors and eigenvalues and analytically derive the adiabatic LZT probability from invariants at EPs. In the presence of the nonlinear interaction, the dynamics of the adiabatic evolutions are explicitly demonstrated with the help of classical trajectories in the plane of the two canonical variables of the corresponding classical Josephson Hamiltonian. We show that the adiabatic tunneling probabilities can be precisely predicted by the classical action at EPs in the weak nonreciprocal regime. In a certain region of strong nonreciprocity, we find that interestingly, the nonlinear interaction effects can be completely suppressed so that the adiabatic tunneling probabilities are identical to their linear counterparts. We also obtain a phase diagram for large ranges of nonreciprocity and nonlinear interaction parameters to explicitly demonstrate where the adiabaticity can break down, i.e., the emergence of the nonzero tunneling probabilities even in adiabatic limit.

Effect of Duschinskii Rotations on Spin-Dependent Electron Transfer Dynamics

We investigate spin-dependent electron transfer in the presence of a Duschinskii rotation. In particular, we propagate dynamics for a two-level model system for which spin-orbit coupling introduces an interstate coupling of the form eiWx, which is both position(x)-dependent and complex-valued. We demonstrate that two-level systems coupled to Brownian oscillators with Duschinskii rotations (and thus entangled normal modes) can produce marked increases in transient spin polarization relative to two-level systems coupled to simple shifted harmonic oscillators. These conclusions should have significant relevance for modeling the effect of nuclear motion on chiral induced spin selectivity.

Microscopic dynamics and an effective Landau-Zener transition in the quasiadiabatic preparation of spatially ordered states of Rydberg excitations

We examine the adiabatic preparation of spatially-ordered Rydberg excitations of atoms in finite one-dimensional lattices by frequency-chirped laser pulses, as realized in a number of recent experiments simulating quantum Ising model. Our aims are to unravel the microscopic mechanism of the phase transition from the unexcited state of atoms to the antiferromagnetic-like state of Rydberg excitations by traversing an extended gapless phase, and to estimate the preparation fidelity of the target state in a moderately sized system amenable to detailed numerical analysis. We find that, in the basis of the bare atomic states, the system climbs the ladder of Rydberg excitations predominantly along the strongest-amplitude paths towards the final ordered state. We show that, despite its complexity, the interacting many-body system can be described as an effective two-level system involving a pair of lowest-energy instantaneous collective eigenstates of the time-dependent Hamiltonian. The final preparation fidelity of the target state can then be well approximated by the Landau-Zener formula, while the nonadiabatic population leakage during the passage can be estimated using a perturbative approach applied to the instantaneous collective eigenstates.

Method of Changing Running Direction of Cheetah-Inspired Quadruped Robot.

The rapid change of motion direction during running is beneficial to improving the movement flexibility of the quadruped robot, which is of great relevance to its research. How to make the robot change its motion direction during running and achieve good dynamic stability is a problem to be solved. In this paper, a method to change the running direction of the cheetah-inspired quadruped robot is proposed. Based on the analysis of the running of the cheetah, a dynamic model of the quadruped robot is established, and a two-level stability index system, including a minimum index system and a range index system, is proposed. On this basis, the objective function based on the stability index system and optimization variables, including leg landing points, trunk movement trajectory, and posture change rule, are determined. Through these constraints, the direction changes with good dynamic stability of the cheetah-inspired quadruped robot during running is realized by controlling the leg parameters. The robot will not roll over during high-speed movement. Finally, the correctness of the proposed method is proven by simulation. This paper provides a theoretical basis for the quadruped robot's rapid change of direction in running.

Development of top-level software for the process control system for bentonite-containing bipolar membranes

An approach to automating the technological process of obtaining bentonite-containing ion-exchange bipolar membranes is considered. The structures of automated systems are given, both with the use of local automation tools, and with the use of industrial controllers and workstations. The advantages and disadvantages of each of the structures are described. A two-level control system with the use of sensors, actuators, and an industrial controller at the lower level and a workstation at the upper level is suggested. The scheme of the complex of technical means is implemented. The information model of the upper level software of the control system taking into account the peculiarities of the technological process is developed. The data flow diagram is described and functioning principles of the upper level software are proposed. A software module, that is designed both to calculate control actions for the lower control level and to simulate periodic laboratory bipolar electrodialysis of a sodium sulfate solution with experimental bentonite-containing bipolar membranes to produce acid and alkali, has been developed in C++. During the operation of the software module, it is possible to trace the dependence of technological parameters of electrodialysis (ion flux, current efficiency, energy costs for the production of the target product) on the type of bentonite modifier used (alkyldimethylbenzylammonium chloride (surfactant) and stearic acid) and its quantity in membranes (1, 2, 3% wt.) at different current densities. The software module can be used in existing industries within the framework of functioning process control systems in order to optimize operating parameters during control of electrodialysis processe.

Dynamics of quantum correlations in two 2-level atoms coupled to thermal reservoirs

In this paper, we examine the dynamics of quantum correlations in two noninteractive two-level atoms coupled to two separate identical thermal reservoirs. The two atoms are initially produced in a Gisin state, which is a blend of a maximally entangled two-qubit state and a separable mixed state. Quantum entanglement is measured by logarithmic negativity, while the nonclassical correlations are characterized by trace distance discord and local quantum uncertainty. Using the mean photon number of reservoirs and spontaneous emission rates of atoms as inputs, we explore how these quantum resources behave. Consequently, we demonstrate that the dynamics of quantum entanglement and quantum correlations depend upon the parameters driving the system. Significantly, we further demonstrate that specific parameters may be tweaked to preserve the quantum resources in the system. The results give a full grasp of the quantum features of such a two-level atomic system, showing capabilities to construct quantum technology.

Stochastic entropy production for continuous measurements of an open quantum system

We investigate the total stochastic entropy production of a two-level bosonic open quantum system under protocols of time dependent coupling to a harmonic environment. These processes are intended to represent the measurement of a system observable, and consequent selection of an eigenstate, whilst the system is also subjected to thermalising environmental noise. The entropy production depends on the evolution of the system variables and their probability density function, and is expressed through system and environmental contributions. The continuous stochastic dynamics of the open system is based on the Markovian approximation to the exact, noise-averaged stochastic Liouville-von Neumann equation, unravelled through the addition of stochastic environmental disturbance mimicking a measuring device. Under the thermalising influence of time independent coupling to the environment, the mean rate of entropy production vanishes asymptotically, indicating equilibrium. In contrast, a positive mean production of entropy as the system responds to time dependent coupling characterises the irreversibility of quantum measurement, and a comparison of its production for two coupling protocols, representing connection to and disconnection from the external measuring device, satisfies a detailed fluctuation theorem.

A selection scheme for energy storage to participate in primary frequency regulation of new energy

The choice of energy storage is the key content of the energy storage plan and design to participate in the primary frequency regulation of new energy. It involves many indicators such as safety, primary frequency modulation adaptability, economy, and environmental performance, and is a complex multi-objective decision-making problem. This paper analyses the characteristics of each index and constructs a two-level decision-making system. According to expert scores and practical application requirements, the weight of each decision-making index is obtained by the AHP method. The TOPSIS method is further used to select energy storage according to the weight of each index and related parameters.

Lyapunov-based fixed-time stabilization control of quantum systems

In this paper, we consider the fixed-time stabilization control problem of quantum systems modeled by Schrödinger equations. Firstly, the Lyapunov-based fixed-time stability criterion is extended to finite-dimensional closed quantum systems in the form of coherence vectors. Then for a two-level quantum system with single control input, a non-smooth fractional-order control law is designed using the relative state distance. By integrating the fixed-time Lyapunov control technique and the bi-limit homogeneity theory, the quantum system is proved to be stabilized to an eigenstate of the inherent Hamiltonian in a fixed time. Comparing with existing methods in quantum system control, the proposed approach can guarantee stabilization in a fixed time without depending on the initial states.

Driven Markovian master equation based on the Lewis-Riesenfeld-invariant theory

We derive a Markovian master equation for driven open quantum systems based on the Lewis-Riesenfeld invariants theory, which is available for arbitrary driving protocols.The role of the Lewis-Riesenfeld invariants is to help us bypass the time-ordering obstacle in expanding the propagator of the free dynamics, such that the Lindblad operators in our driven-Markovian master equation can be determined easily. We also illustrate that, for the driven open quantum systems, the spontaneous emission and the thermal excitation induce the transitions between eigenstates of the Lewis-Riesenfeld invariant, but not the system Hamiltonian's. As an example, we present the driven-Markovian master equation for a driven two-level system coupled to a heat reservoir. By comparing to the exactly solvable models, the availability of the driven-Markovian master equation is verified. Meanwhile, the adiabatic limit and inertial limit of the driven-Markovian master equation are also discussed, which result in the same Markovian master equations as those presented before in the corresponding limits.

Heat transport through a two-level system under continuous quantum measurement

We study the backaction of quantum measurements on heat transport through a two-level system by considering the continuous quantum measurement onto an eigenstate of the two-level system. For the nonselective measurement, the backaction appears as a dephasing effect on the two-level system. We formulate the heat current under the selective measurement with a stochastic master equation and show that the cross-correlation between the measurement outcomes and the heat current contains information on the backaction. We expect that our findings can be verified by using a platform of superconducting circuits.

Semi-Markov processes in open quantum systems: Connections and applications in counting statistics.

Using the age-structure formalism, we definitely establish connections between semi-Markov processes and the dynamics of open quantum systems that satisfy the Markov quantum master equations. A generalized Feynman-Kac formula of the semi-Markov processes is also proposed. In addition to inheriting all statistical properties possessed by the piecewise deterministic processes of wave functions, the semi-Markov processes show their unique advantages in quantum counting statistics. Compared with the conventional method of the tilted quantum master equation, they can be applied to more general counting quantities. In particular, the terms involved in the method have precise probability meanings. We use a driven two-level quantum system to exemplify these results.

A comparative study of different machine learning methods for dissipative quantum dynamics

It has been recently shown that supervised machine learning (ML) algorithms can accurately and efficiently predict long-time population dynamics of dissipative quantum systems given only short-time population dynamics. In the present article we benchmarked 22 ML models on their ability to predict long-time dynamics of a two-level quantum system linearly coupled to harmonic bath. The models include uni- and bidirectional recurrent, convolutional, and fully-connected feedforward artificial neural networks (ANNs) and kernel ridge regression (KRR) with linear and most commonly used nonlinear kernels. Our results suggest that KRR with nonlinear kernels can serve as inexpensive yet accurate way to simulate long-time dynamics in cases where the constant length of input trajectories is appropriate. Convolutional gated recurrent unit model is found to be the most efficient ANN model.

Unitary evolution for a two-level quantum system in fractional-time scenario.

The time-evolution operator obtained from the fractional-time Schrödinger equation(FTSE) is said to be nonunitary since it does not preserve the norm of the vector state in time. As done in the time-dependent non-Hermitian quantum formalism, for a traceless non-Hermitian two-level quantum system, we demonstrate that it is possible to map the nonunitary time-evolution operator in a unitary one. It is done by considering a dynamical Hilbert space with a time-dependent metric operator, constructed from a Hermitian time-dependent Dyson map, in respect to which the system evolves in a unitary way, and the standard quantum mechanics interpretation can be made properly. To elucidate our approach, we consider three examples of Hamiltonian operators and their corresponding unitary dynamics obtained from the solutions of FTSE, and the respective Dyson maps.

Coherent dynamics in a five-level atomic system

The coherent control of multi-partite quantum systems presents one of the central prerequisites in state-of-the-art quantum information processing. With the added benefit of inherent high-fidelity detection capability, atomic quantum systems in high-energy internal states, such as metastable noble gas atoms, promote themselves as ideal candidates for advancing quantum science in fundamental aspects and technological applications. Using laser-cooled neon atoms in the metastable 3P2 state of state 1s 22s 22p 53s (LS-coupling notation) (Racah notation: 2 P 3/23s[3/2]2) with five m J -sublevels, experimental methods for the preparation of all Zeeman sublevels |m J ⟩ = |+2⟩, |+1⟩, |0⟩, |−1⟩, |−2⟩ as well as the coherent control of superposition states in the five-level system |+2⟩, …, |−2⟩, in the three-level system |+2⟩, |+1⟩, |0⟩, and in the two-level system |+2⟩, |+1⟩ are presented. The methods are based on optimized radio frequency and laser pulse sequences. The state evolution is described with a simple, semiclassical model. The coherence properties of the prepared states are studied using Ramsey and spin echo measurements.