Strong Coupling Conditions Research Articles

The phonon effect of qubit in quantum ring

Under the condition of electron-LO photon strong coupling, the eigenfunction and the eigenenergy of the ground state and the first excited state of the electron in quantum ring, are obtained by solving precisely the Schrdinger equation. A qubit is formed by overlaying both the ground state and the first excited state of the electron-LO phonon system.Numerical calculations indicate that the distribution of the probability density of electrons in quantum oscillates periodically with time and angle coordinate. The oscillating period decreases with the increase of the electron-LO phonon coupling strength, which shows that the existence of phonon can reduce the coherence of qubit, and that the oscillating period increases with the increasing inner（or outer） radius of quantum ring. Therefore, the coherence of qubit can be improved by choosing proper size of quantum ring.

Properties of Unsymmetrical Parabolic Confinement Potential Quantum Dot Qubit

On the condition of electric-LO phonon strong coupling in unsymmetrical parabolic confinement potential quantum dot (QD), we obtain the eigenenergies of the ground state and the first-excited state, the eigenfunctions of the ground state, and the first-excited state by using variational method of Pekar type. This system in QD may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first-excited state, we obtain the time evolution of the electron density. The relations both the probability density of electron and the period of oscillation with the electron-LO-phonon coupling strength, the confinement strengths in the xy-plane and the z-direction are discussed.

Modeling of Rabi splitting in quantum well microcavities using time-dependent transfer matrix method

Rabi splitting in quantum well (QW) embedded in microcavities under strong coupling condition is modeled by a time-dependent transfer matrix model. The spectral response of QW under the influence of excitonic effects is simulated by infinite impulse digital filters. It is shown that the splitting energy obtained from the proposed model match well with that deduced from the reflection spectrum analysis. The lasing spectra observed from different transmission angles of the QW microcavity can also be calculated. Hence, it is proved that the proposed model can be used to design and analyze the lasing characteristics of QW microcavities under strong coupling condition.

Reentrant Condensation of Proteins in Solution Induced by Multivalent Counterions

Negatively charged globular proteins in solution undergo a condensation upon adding trivalent counterions between two critical concentrations C and C, C <C. This reentrant condensation behavior above C is caused by short-ranged electrostatic interactions between multivalent cations and acidic residues, mechanistically different from the case of DNA. Small-angle x-ray scattering indicates a short-ranged attraction between counterion-bound proteins near C and C. Monte Carlo simulations (under these strong electrostatic coupling conditions) support an effective inversion of charge on surface side chains through binding of the multivalent counterions.

Field-Cycling Effects on Dynamic Nuclear Polarization

Effects observed when nonthermal spin polarization is formed at a magnetic field that differs from the field of nuclear magnetic resonance detection are analyzed in detail. Particular attention is paid to the case of field variation faster than relaxation. At sufficiently low field when the scalar interaction among spins fulfills the condition of strong coupling, relaxation and polarization become collective properties of spin groups. As a result, the magnetic relaxation dispersion exhibits characteristic features and the polarization is transferred among spins. In regions of level-crossings these effects become particularly strong. The manipulation and possible utilization of such a polarization transfer in field-cycling experiments are discussed. The effects as demonstrated here by the example of hyperpolarized tryptophan are of a general nature and, therefore, not limited to special methods of dynamic polarization but are typical for field-cycling experiments.

Top grating index-coupled distributed feedback quantum cascade lasers

We report on the modeling and design of top grating distributed feedback (DFB) quantum cascade lasers (QCLs). A low loss, index-coupled, DFB design that is very robust against technological spreads is proposed. Strong DFB coupling conditions are obtained while maintaining a high laser output power. Following this design, DFB QCL lasers with InP cladding layers and InGaAs∕AlInAs active regions have been fabricated. Room temperature monomode QCLs with 30dB side mode suppression ratios are demonstrated over a 4–8μm wavelength range.

The effect of static loading and imperfections on the nonlinear vibrations of laminated cylindrical shells

The effect of static loading and imperfections on the nonlinear vibration behaviour of cylindrical shells is studied. The two analytical–numerical models (denoted as Level-1 and Level-2 Analysis) that are used for this purpose are based on Donnell-type governing equations and have different levels of complexity. Parametric studies have been performed for a specific laminated shell. The axial compressive loading and different imperfection shapes can significantly influence the linearized vibration behaviour and also affect the nonlinear vibration behaviour. Results show that certain axisymmetric imperfections, satisfying a strong-coupling condition with the asymmetric vibration mode, reduce the linearized vibration frequencies and make the nonlinearity less softening, while the static compressive axial loading makes the nonlinearity more softening.

Electron dynamics at a positive ion

The dynamics of electrons in the presence of a positive ion is considered for conditions of weak electron-electron coupling but strong electron-ion coupling. The equilibrium electron density and the electric field time correlation functions are evaluated for semiclassical conditions using a classical statistical mechanics with a regularized electron-ion interaction for molecular dynamics simulation (MD). Results are reported for the autocorrelation function of the electron electric field at the ion for 0< or =Z< or =40 , including conditions of strong electron-ion coupling. The electron stopping power and self-diffusion coefficient are determined from these results. Interpretation is provided by a theoretical analysis using the nonlinear Vlasov equation for the equilibrium structure, and a corresponding linear Vlasov equation for time correlation functions. The agreement of a simple mean field model with the semiclassical MD simulation is found to be quite good except for one state condition.

High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: Effects of spin–spin coupling

By combining magnetic field cycling in the range from 0.1 mT to 7 T with high-resolution NMR detection the T 1 relaxation dispersion (nuclear magnetic relaxation dispersion (NMRD)) of protons in the nucleotides adenosine mono-phosphate and guanosine mono-phosphate was measured in a site-specific way. While at high field the individual spins have distinctly different T 1 times, their scalar spin–spin interaction fulfills at low field the condition of strong coupling and leads to convergence of their T 1 dispersion curves. In addition, the spin–spin coupling can lead to oscillatory components in the relaxation kinetics traceable to a coupling between spin polarization and coherence in the relaxation process. As a consequence the NMRD curves do not directly reflect the spectral density function of the motional processes, but the effects of motion and spin coupling must be separated for a reliable evaluation. A theoretical approach is described allowing such an analysis.

Properties of Parabolic Linear Bound Potential and Coulomb Bound Potential Quantum Dot Qubit

On the condition of electric-LO phonon strong-coupling in a parabolic quantum dot, we obtain the eigenenergy of the ground-state and the first-excited state, the eigenfunctions of the ground-state and the first- excited state by using variational method of Pekar type. This system in quantum dot may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground- and the first-excited state, we obtain the time evolution of the electron density. The relation of the probability density of electron on the Coulomb binding parameter and the relations of the period of oscillation on the Coulomb binding parameter, the electron-LO-phonon coupling constant and the confinement length are derived.

The temperature effect of the parabolic linear bound potential quantum dot qubit

On the condition of electric-LO phonon strong coupling in parabolic quantum dot，we obtain the eigenenergy and the eigenfuctions of the ground state and the first-excited state by using variational method of Pekar type. This system in quantum dot may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first-excited state，we obtain the space-time evolution of the electron density. It is shown that the electron density increases with the temperature when the temperature is lower，and the electron density decreases with the temperature when the temperature is higher. At the same time，we found that sometimes the electron density vibrates with the temperature，and its change is slowed down with the strength of coupling increasing.

The effect of Coulomb potential to the decoherence of the parabolic quantum dot qubit

On the condition of electric-LO phonon strong coupling in parabolic quantum dot, we obtained the eigenenergy of the ground state and the first excited state, the eigenfuctions of the ground state and the first excited state by using variational method of Pekar type. This system of quantum dot may be employed as a two-level quantum system-qubit. The phonon spontaneous emission causes the decoherence of the qubit. We discussed the relatoion between the decoherence time and the Coulomb binding parameter，the couping strength, the confinement length, and the conefficient dispersion.

Method for nonlinear characterization of radio frequency coils made of high temperature superconducting material in view of magnetic resonance imaging applications

A contactless method based on reflectometry to accurately characterize an inductive radio frequency (rf) resonator even in the occurrence of a strong electrical nonlinearity is presented. Nonlinear extraction of the unloaded quality factor and resonance frequency is possible by combining an initial low-level swept-frequency calibration with high-level single-frequency measurements. The extraction protocol relies on a simple intrinsic R, L, C model and does not involve a fitting procedure according to a particular nonlinearity model. It includes a correction for strong coupling conditions between the probe and the rf coil, which allows extending the analysis over a wide range of transmitted power. Electrical modeling based on the extracted intrinsic data allows predicting the coil behavior when loaded by any kind of matching network. The method will have implications in different domains such as Magnetic Resonance (MR) applications with superconducting probe heads or analysis of rf properties in nonlinear materials. The method is demonstrated here by characterizing a high temperature superconducting (HTS) coil dedicated to MR imaging at 64 MHz. The coil consists in a multiturn spiral design that is self-resonant close to the MR frequency of interest. The Q factor and the resonance frequency are determined as a function of the actual power dissipated in the HTS coil accounting for losses occurring in the measurement system. Further characteristics of the HTS coil are considered in the present paper. The relation between the transmitted power and the magnetic field generated by the coil, which is the most relevant characteristics for MR applications, is directly accessible. The equivalent impedance of the coil under test is also expressed as a function of the total current flowing in the windings. The method could be extended to assess the fundamental properties of the nonlinear material (e.g., the London penetration depth or the critical current density) by including any pertinent model.

Ultralong-range plasmonic waveguides using quasi-two-dimensional metallic layers

We calculate the bound plasmonic modes of a quantum metamaterial slab, comprised of multiple quasi two dimensional electron gas (Q2DEG) layers, whose thickness is much smaller than the optical wavelength. For the first order transverse magnetic (TM) optical and the surface plasmonic modes we find propagation constants which are independent of both the electron density and of the scattering rates in the Q2DEGs. This leads to extremely long propagation distances. In a detailed case study of a structure comprising a slab of GaAs/AlGaAs multiple quantum well (MQW) material, we find propagation lengths of 100s of mm. In addition, the electric field enhancement associated with the plasmonic resonance is found to be sufficient to induce the condition of strong coupling between the slab modes and the intersubband transitions in the MQWs.

Parabolic linear bound potential quantum dot qubit and its optical phonon effect

On the condition of electric-LO phonon strong coupling in parabolic quantum dot, we obtain the eigenenergy and the eigenfuctions of the ground state and the first excited state by using variational method of Peakar type. This system in quantum dot may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first excited state, we obtain the time evolution of the electron density. It is shown that the period of oscillation decreases with the strength of coupling increasing when the confined length is constant. And the period of oscillation increases with the confined length increasing when the strength coupling is constant.

New Techniques for the Analysis and Design of Coupled-Oscillator Systems

An in-depth analysis of the nonlinear dynamics of coupled-oscillator arrays is presented for a better understanding of their complex autonomous behavior. In one-dimensional arrays, the constant inter-stage phase shift is varied by simultaneously detuning the two outermost oscillators in opposite directions. Thus, the array can be considered as a two-parameter system. Here, a two-parameter stability analysis of the oscillator array is carried out, investigating the phenomena that limit the achievable values of constant inter-stage phase shift under both weak and strong coupling conditions. The gradual evolution of the system behavior with increasing coupling strength is studied. A semianalytical approach is presented for an efficient design of the oscillator array, avoiding the computational expensiveness of harmonic balance (HB) in systems with a high number of oscillator elements. The proposed method, valid for weak coupling, uses a perturbation model of the elementary oscillator obtained with HB so it is of general application to any oscillator topology with accurate descriptions of its linear and nonlinear components. Approaches for the stability and phase-noise analyses based on this formulation are also presented. The new techniques have been applied to the design of a coupled system of three oscillators at 6 GHz. The results have been successfully compared with full HB simulations and with measurements

Nonequilibrium conductance of a three-terminal quantum dot in the Kondo regime: Perturbative renormalization group study

Motivated by recent experiments, we consider a single-electron transistor in the Kondo regime which is coupled to three leads in the presence of large bias voltages. Such a steady-state non-equilibrium system is to a large extent governed by a decoherence rate induced by the current through the dot. As the two-terminal conductance turns out to be rather insensitive to the decoherence rate, we study the conductance in a three-terminal device using perturbative renormalization group and calculate the characteristic splitting of the Kondo resonance. The interplay between potential biases and anisotropy in coupling to the three leads determines the decoherence rate and the conditions for strong coupling.

Approach to wetting-layer-assisted lateral coupling ofInAs∕InPquantum dots

We present new results on the simulation of two-dimensional (2D) quantum dot(s) (QDs) $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ superlattices, emitting at $1.55\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the optical telecommunication wavelength. QDs and wetting layer (WL) electronic energy states are close in such a system. Using the Fourier-transformed Schr\odinger equation developed on a mixed basis, we describe the wetting layer-assisted inter-QDs lateral (WLaiQD) coupling by studying the influence of WL states on QDs states and vice versa. The results show that WL and QDs have to be considered as a unique system, in strong coupling conditions. The increase of QDs density on the WL leads to enhanced splitting and miniband effects on QDs states. It induces a fragmentation of WL density of states interpreted as a 0D-like confinement of WL states. A comparison is made with a real high QDs density sample. It is expected to have an impact on carrier-capture phenomena in optoelectronic devices using high-density QDs in the active region.

Resonant transparency regimes under conditions of long/short-wave coupling

Nonlinear two-component electromagnetic pulse propagation through a resonant axially symmetric anisotropic medium having a permanent dipole moment is analyzed under conditions of strong coupling between the ordinary (short-wavelength) and extraordinary (long-wavelength) pulse components. It is shown that a pulse can propagate through the medium in regimes different from self-induced transparency if its ordinary component is detuned off resonance. In particular, a pulse propagating in the regime of self-induced super-transparency substantially changes quantum-level populations, but its group velocity remains almost equal to the linear velocity. If a pulse propagates in the extraordinary transparency regime and the carrier-frequency detuning from resonance is small, then its group velocity is substantially lower, while the level populations remain virtually invariant. Regimes of propagation through weakly excited media under quasi-resonance conditions are also identified.

Strong coupling in a single quantum dot–semiconductor microcavity system

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.