Coupled Quantum Research Articles

Exciton-polariton topological insulator.

Topological insulators-materials that are insulating in the bulk but allow electrons to flow on their surface-are striking examples of materials in which topological invariants are manifested in robustness against perturbations such as defects and disorder1. Their most prominent feature is the emergence of edge states at the boundary between areas with different topological properties. The observable physical effect is unidirectional robust transport of these edge states. Topological insulators were originally observed in the integer quantum Hall effect2 (in which conductance is quantized in a strong magnetic field) and subsequently suggested3-5 and observed6 to exist without a magnetic field, by virtue of other effects such as strong spin-orbit interaction. These were systems of correlated electrons. During the past decade, the concepts of topological physics have been introduced into other fields, including microwaves7,8, photonic systems9,10, cold atoms11,12, acoustics13,14 and even mechanics15. Recently, topological insulators were suggested to be possible in exciton-polariton systems16-18 organized as honeycomb (graphene-like) lattices, under the influence of a magnetic field. Exciton-polaritons are part-light, part-matter quasiparticles thatemerge from strong coupling of quantum-well excitons and cavity photons19. Accordingly, the predicted topological effects differ fromall those demonstrated thus far. Here we demonstrate experimentally an exciton-polariton topological insulator. Our lattice of coupled semiconductor microcavities is excited non-resonantly by a laser, and an applied magnetic field leads to the unidirectional flow of a polariton wavepacket around the edge of the array. This chiral edge mode is populated by a polariton condensation mechanism. We use scanning imaging techniques in real space and Fourier space to measure photoluminescence and thus visualize the mode as it propagates. We demonstrate that the topological edge mode goes around defects, and that its propagation direction can be reversed by inverting the applied magnetic field. Our exciton-polariton topological insulator paves the way for topological phenomena that involve light-matter interaction, amplification and the interaction of exciton-polaritons as a nonlinear many-body system.

Electron transport through two one-dimensional multi-quantum dot arrays coupled to four leads

Employing the non-equilibrium Green’s function technique, we have obtained the formula for dc current of two one-dimensional multi-quantum dot arrays, which couple to each other via tunneling coupling between two quantum dots connected to four leads, respectively. The retarded Green’s function is a staircase type, terminating at the four leads. Furthermore, the four quantum dots case is demonstrated. The influence of inter-dot coupling strength and quantum dot energy level on the transmission probability for TL, TM and TN branches is investigated. A non-resonant band is observed. By adjusting energy levels of quantum dots, a resonance emerges in the region of the non-resonance band. The system can be used as a quantum switch.

Excitation energy transport with noise and disorder in a model of the selectivity filter of an ion channel

A selectivity filter is a gate in ion channels that is responsible for the selection and fast conduction of particular ions across the membrane (with high throughput rates of 108 ions s−1 and a high 1:104 discrimination rate between ions). It is made of four strands as the backbone, and each strand is composed of sequences of five amino acids connected by peptide units H–N–C=O in which the main molecules in the backbone that interact with ions in the filter are carbonyl (C=O) groups that mimic the transient interactions of ion with binding sites during ion conduction. It has been suggested that quantum coherence and possible emergence of resonances in the backbone carbonyl groups may play a role in mediating ion conduction and selectivity in the filter. Here, we investigate the influence of noise and disorder on the efficiency of excitation energy transfer (EET) in a linear harmonic chain of N = 5 sites with dipole–dipole couplings as a simple model for one P-loop strand of the selectivity filter backbone in biological ion channels. We include noise and disorder inherent in real biological systems by including spatial disorder in the chain, and random noise within a weak coupling quantum master equation approach. Our results show that disorder in the backbone considerably reduces EET, but the addition of noise helps to recover high EET for a wide range of system parameters. Our analysis may help for better understanding of the coordination of ions in the filter as well as the fast and efficient functioning of the selectivity filters in ion channels.

Coherent Energy Transfer Dynamics and Photoluminescence of Coupled Quantum Dots: Fully Analytical Approach

We theoretically studied coherent excitation energy transfer between self-growth semiconductor quantum dots (QDs) by solving Heisenberg’s equations of motion for density matrix elements in second quantization regime. In a local excitation condition where only one QD electron is optically excited by the pump laser field, coherent excitation energy transfer to the other QD electron can be achieved through Coulomb (Forster) and electron-photon (radiation field) interactions. We calculated three diagonal and one off-diagonal Coulomb coupling constants, which are responsible for the biexcitonic frequency renormalization and the coherent energy transfer between QDs, respectively, and radiation field coupling coefficients by using electron and hole wave functions derived from eight-band kp-theorem, whose validity has already been tested by comparison with experiment. In linear optical regime where the occupation densities of electrons at higher energy level are negligibly small, we could successfully derive fully analytical behaviors of temporal dynamics of the interband polarizations and level occupation densities of both QDs by using Hartree-Fock approximation (HFA), in eventual, the stationary photoluminescence of the coupled QDs in an analytical form. Additionally, the validity of the HFA was examined by comparing the numerical results with those obtained from the exact correlation expansion model for different values of the pump field intensity.

Solutions to the Dirac Equation for Manning-Rosen Plus Shifted Deng-Fan Potential and Coulomb-Like Tensor Interaction Using Nikiforov-Uvarov Method

We solve the Dirac equation for the Manning-Rosen plus shifted Deng-Fan potential including a Coulomb-like tensor potential with arbitrary spin–orbit coupling quantum number κ. In the framework of the spin and pseudospin (pspin) symmetry, we obtain the energy eigenvalue equation and the corresponding eigenfunctions in closed form by using the Nikiforov–Uvarov method. Also Special cases of the potential as been considered and their energy eigen values as well as their corresponding eigen functions are obtained for both relativistic and non-relativistic scope.

Room temperature optical mass sensor with an artificial molecular structure based on surface plasmon optomechanics

We propose an optical weighing technique with a sensitivity down to a single atom through the coupling between a surface plasmon and a suspended graphene nanoribbon resonator. The mass is determined via the vibrational frequency shift on the probe absorption spectrum while the atom attaches to the nanoribbon surface. We provide methods to separate out the signals of the ultralow frequency vibrational modes from the strong Rayleigh background, first based on the quantum coupling with a pump-probe scheme. Owing to the spectral enhancement in the surface plasmon and the ultralight mass of the nanoribbon, this scheme results in a narrow linewidth (∼GHz) and ultrahigh mass sensitivity (∼30 yg). Benefitting from the low noises, our optical mass sensor can be achieved at room temperature and reach ultrahigh time resolution.

Nonreciprocal Transmission and Reflection of a Chirally Coupled Quantum Dot

We report strongly nonreciprocal behavior for quantum dot exciton spins coupled to nanophotonic waveguides under resonant laser excitation. A clear dependence of the transmission spectrum on the propagation direction is found for a chirally coupled quantum dot, with spin up and spin down exciton spins coupling to the left and right propagation directions, respectively. The reflection signal shows an opposite trend to the transmission, which a numerical model indicates is due to direction-selective saturation of the quantum dot. The chiral spin-photon interface we demonstrate breaks reciprocity of the system and opens the way to spin-based quantum optical components such as optical diodes and circulators in a chip-based solid-state environment.

Model discrimination in gravitational wave spectra from dark phase transitions

In anticipation of upcoming gravitational wave experiments, we provide a comprehensive overview of the spectra predicted by phase transitions triggered by states from a large variety of dark sector models. Such spectra are functions of the quantum numbers and (self-) couplings of the scalar that triggers the dark phase transition. We classify dark sectors that give rise to a first order phase transition and perform a numerical scan over the thermal parameter space. We then characterize scenarios in which a measurement of a new source of gravitational waves could allow us to discriminate between models with differing particle content.

Coupled Quantum Dots in Bilayer Graphene.

Electrostatic confinement of charge carriers in bilayer graphene provides a unique platform for carbon-based spin, charge, or exchange qubits. By exploiting the possibility to induce a band gap with electrostatic gating, we form a versatile and widely tunable multiquantum dot system. We demonstrate the formation of single, double and triple quantum dots that are free of any sign of disorder. In bilayer graphene, we have the possibility to form tunnel barriers using different mechanisms. We can exploit the ambipolar nature of bilayer graphene where pn-junctions form natural tunnel barriers. Alternatively, we can use gates to form tunnel barriers, where we can vary the tunnel coupling by more than 2 orders of magnitude tuning between a deeply Coulomb blockaded system and a Fabry-Pérot-like cavity. Demonstrating such tunability is an important step toward graphene-based quantum computation.

Continuous Frequency Tuning with near Constant Output Power in Coupled Y-Branched Terahertz Quantum Cascade Lasers with Photonic Lattice

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modeling and the spectral characteristics are modeled using a transfer matrix model. Continuous frequency tuning of ∼19 GHz is demonstrated while maintaining an output power of ∼4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically control...

Analytic treatment of the thermoelectric properties for two coupled quantum dots threaded by magnetic fields

Coupled double quantum dots (c-2QD) connected to leads have been widely adopted as prototype model systems to verify interference effects on quantum transport at the nanoscale. We provide here an analytic study of the thermoelectric properties of c-2QD systems pierced by a uniform magnetic field. Fully analytic and easy-to-use expressions are derived for all the kinetic functionals of interest. Within the Green’s function formalism, our results allow a simple inexpensive procedure for the theoretical description of the thermoelectric phenomena for different chemical potentials and temperatures of the reservoirs, different threading magnetic fluxes, dot energies and interdot interactions; moreover they provide an intuitive guide to parametrize the system Hamiltonian for the design of best performing realistic devices. We have found that the thermopower S can be enhanced by more than ten times and the figure of merit ZT by more than hundred times by the presence of a threading magnetic field. Most important, we show that the magnetic flux increases also the performance of the device under maximum power output conditions.

Designing Coupled Quantum Dot with ZnS-CdSe Hybrid Structure for Enhancing Exciton Lifetime

Fabrication of coupled quantum dot (CQD) may provide an excellent platform for the realization of high end opto-electronic applications as well as quantum information processing. CQD can be synthesized by well-known cation exchange method and less explored ‘nanoparticle fusion’ method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QD) with an emphasis on the formation of CQD comprised of bicompartmental Janus structure (i.e. Janus structure consisting of two compartments with the two QDs coupled) via ‘nanoparticle fusion’. With increase in the ratio of Cd/Zn from 0.9:1 to 1.3:1 to 2.5:1 to 4:1 to 12:1, we observe the evolution of structure from CdZnSeS alloy to Acorn Janus to Bicompartmental JanusA with ZnS Zinc Blende (ZB) – CdSe Wurtzite (Wz) to Bicompartmental JanusB (ZnS/CdSe-both ZB) and eventually to CdZnSeS alloy core - CdSe thick shell. Interestingly, the CQD possess two distinct emission bands (570/630 nm) in which 570 nm emission arises from the formation of new electronic states due the strong coupling between the two QDs whereas 630 nm is the characteristic CdSe emission. Further, the coupling enhances the exciton lifetimes of 570/630 nm emission (Bicompartmental JanusA - 36/31 ns and Bicompartmental JanusB : 41/94.8 ns) which can be exploited in various applications. Further, the DFT simulations provide the heuristics behind the formation of certain heterostructure with strain and interfacial energies of particular facets dictating the morphology during coupling of QDs.

Designing Coupled Quantum Dots with ZnS–CdSe Hybrid Structures for Enhancing Exciton Lifetime

The fabrication of coupled quantum dots (CQDs) may provide an excellent platform for the realization of high-end optoelectronic applications as well as quantum information processing. CQDs can be synthesized by the well-known cation-exchange method and less explored “nanoparticle fusion” method, in which the latter involves the coupling between constituting facets of two different semiconductors. Herein, we elucidate the mechanistic formation pathway of different heterostructures with ZnS and CdSe quantum dots (QDs) with an emphasis on the formation of CQDs comprised of bicompartmental Janus structures (i.e., Janus structure consisting of two compartments with the two QDs coupled) via nanoparticle fusion. With the increase in the ratio of Cd/Zn from 0.9:1 → 1.3:1 → 2.5:1 → 4:1 → 12:1, we observe the evolution of the structure from CdZnSeS alloy → Acorn Janus → bicompartmental JanusA with ZnS zinc blende (ZB)–CdSe wurtzite (Wz) → bicompartmental JanusB (ZnS–CdSe-both ZB) and eventually to CdZnSeS alloy cor...

Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling

Current crowding close to electrical contacts is a common challenge in all optoelectronic devices containing thin current spreading layers (CSLs). We analyze the effects of current spreading on the operation of the so-called double diode structure (DDS), consisting of a light emitting diode (LED) and a photodiode (PD) fabricated within the same epitaxial growth process, and providing an attractive platform for studying electroluminescent (EL) cooling under high bias conditions. We show that current spreading in the common n-type layer between the LED and the PD can be dramatically improved by the strong optical coupling between the diodes, as the coupling enables a photo-generated current through the PD. This reduces the current in the DDS CSL and enables the study of EL cooling using structures that are not limited by the conventional light extraction challenges encountered in normal LEDs. The current spreading in the structures is studied using optical imaging techniques, electrical measurements, simulations, as well as simple equivalent circuit models developed for this purpose. The improved current spreading leads further to a mutual dependence with the coupling efficiency, which is expected to facilitate the process of optimizing the DDS. We also report a new improved value of 63% for the DDS coupling quantum efficiency.

Enhanced Absorption Due to Formation of Quasi-Bound States in Type-II Coupled Quantum Rings

The absorption of type-II nanostructures is often weaker than type-I counterpart due to spatially indirect optical transitions. This phenomenon limits the application of type-II nanostructures in photon detections. In this paper, we show that with proper arrangements of conduction barriers, the formation of quasi-bound states can significantly boost up the absorption of type-II coupled quantum rings. Meanwhile, the rapid tunneling of electrons in these leaky states should make the corresponding external quantum efficiency comparable to that of single (uncoupled) rings. These features may improve the performance of photon detectors based on type-II semiconductor nanostructures.

Current Reduction Caused by the Quantum Coupling of Hot Electrons in AlGaN/GaN Transistors

Barrier height of AlGaN/GaN transistor is lowered by the quantum coupling of hot electrons with large transverse energy via electro‐thermal effect. The coupling effectively raises the quantized levels of hot electrons, changes the electron density of the two‐dimensional electron gas, and consequently decreases the source drain current of the transistor. Based on the theory of quantum coupling of hot electrons, the current reduction is successfully modeled. The model is able to explain the effect of gate voltage, device temperature, gate length, gate metals, passivation, field‐plate structure, traps, and electron‐phonon interaction on current reduction. It also predicts a relative decrease of 4.6% in the source drain current caused by the quantum coupling at moderately low electron temperature of 1771 K.

New Decoupling Scheme for Investigation of Transmission Coefficient Peaks for Asymmetrically Coupled Quantum Dot System

New Decoupling Scheme for Investigation of Transmission Coefficient Peaks for Asymmetrically Coupled Quantum Dot System

Coupled quantum mechanics/molecular mechanics modeling of metallic materials: Theory and applications

Abstract

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells.

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, T_{C}=4.8 K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density-temperature phase diagram over the temperature range 0.1-4.8K. We find that the latent heat increases linearly with temperature at T≲1.1 K, similarly to a Bose-Einstein condensate transition, and becomes constant at 1.1≲T<4.8 K. Resonant Rayleigh scattering measurements reveal that the disorder in the sample is strongly suppressed and the diffusion coefficient sharply increases with decreasing temperature at T<T_{C}, allowing the liquid to spread over large distances away from the excitation region. We suggest that our findings are manifestations of a quantum liquid behavior.

Lindblad dynamics of the quantum spherical model

The purely relaxational non-equilibrium dynamics of the quantum spherical model as described through a Lindblad equation is analysed. It is shown that the phenomenological requirements of reproducing the exact quantum equilibrium state as stationary solution and the associated classical Langevin equation in the classical limit fix the form of the Lindblad dissipators, up to an overall time-scale. In the semi-classical limit, the models’ behaviour becomes effectively the one of the classical analogue, with a dynamical exponent z = 2 indicating diffusive transport, and an effective temperature Teff, renormalised by the quantum coupling g. A different behaviour is found for a quantum quench, at zero temperature, deep into the ordered phase , for d > 1 dimensions. Only for d = 2 dimensions, a simple scaling behaviour holds true, with a dynamical exponent z = 1 indicating ballistic transport, while for dimensions , logarithmic corrections to scaling arise. The spin–spin correlator, the growing length scale and the time-dependent susceptibility show the existence of several logarithmically different length scales.