Stability Of Quantum Research Articles

Approaches to Enhance the Stability of Colloidal Quantum Well Light- Emitting Diodes.

Colloidal quantum well light-emitting diodes (CQW-LEDs) show great potential for applications in displays and lighting due to their advantages, such as high color purity, spectral tunability and compatibility with flexible electronics. So far, attention has been mainly devoted to pursuing device efficiencies rather than achieving device stability, leading to the fact that the lifetime of CQW-LEDs is far from the demand for practical applications. In this perspective, various approaches to enhance the stability of CQW-LEDs have been discussed, including the synthesis of stable CQW materials, the selection of stable transport layers, the improvement of charge balance, and the introduction of advanced encapsulation techniques.

Asynchronous quantum Kármán vortex street in two-component Bose-Einstein condensate with PT symmetric potential

The dynamics of a miscible two-component Bose-Einstein condensate (BEC) with PT (parity-time) symmetric potential are investigated numerically. The dynamical behaviors of the system is described by Gross-Pitaevskii (GP) equations under the mean-field theory. Firstly, the ground state of the system is obtained by the imaginary-time propagation method. Then dynamical behaviors are numerically simulated by the time-splitting Fourier pseudo-spectral approach under periodic boundary conditions. By adjusting the width and velocity of the obstacle potential, various patterns such as no vortex, oblique drifting vortex dipole, V-shaped vortex pairs, irregular quantum turbulence and combined modes are studied. It is noted that the shedding vortex pairs in components 1 and 2 are staggered, which is called “the asynchronous quantum Kármán vortex street”. Here, the ratio of the distance between two vortex pairs in one row to the distance between vortex rows is approximately 0.18, which is less than the stability criterion 0.28 of classical fluid. We calculated the drag force acting on the obstacle potential during generation of the asynchronous quantum Kármán vortex street. It is found that periodical oscillation of the drag force is generated via drifting up or down of the vortex pairs. Meanwhile, we analyzed the influence of the imaginary part of the PT symmetric potential with gain-loss for wake. The trajectory and frequency of the vortex are changed, due to the imaginary part breaks the local symmetry of the system. In addition, the imaginary part affects the stability of the asynchronous quantum Kármán vortex street. Lots of numerical simulations are carried out to determine the parameter regions of various vortex shedding modes. We also proposed an experimental protocol to realize the asynchronous quantum Kármán vortex street in the miscible two-component BEC with PT symmetric potential.

Luminescence, stability, and applications of CsPbBr3 quantum dot/polymethyl methacrylate composites prepared by a solvent- and ligand-free ball milling method

CsPbBr3 perovskite quantum dots/polymethyl methacrylate composites were prepared by a ball milling method, using PMMA as polymer matrix and cesium stearate, lead stearate, and KBr as perovskite sources. We investigated the stability of the composites against UV light, heat, ethanol, and water vapor and evaluated the luminescence performance of the products. The results show that the 1.5% CsPbBr3 PQDs/PMMA composite prepared by the present method exhibited excellent performance, with a PLQY up to ∼78% and a high stability. The luminous intensity remained at 80%, 87%, 53%, 59%, and 64% of its initial value under ultraviolet light irradiation for 12 days, 467.5 nm LED irradiation for 10 days, heating at 60 °C for 10 days, erosion in water vapor at 95% relative humidity and 25 °C for 90 h, and soaking in ethanol for 3 h, respectively. The luminous efficiency of a white light-emitting diode fabricated with the CsPbBr3 PQDs/PMMA composite was 76.73 lm·W−1, and the RhB degradation percentage under visible light reached 92.2% within 60 min. The preparation method has the advantages of simple processing, solvent- and ligand-free nature, low synthesis temperature, and easy scalability; the as-prepared PQDs composites have high quantum yields and good environmental stability, which highlights the great application prospects of the present method.

Improving stability and photoluminescence of CsPbBr3 quantum dots/CMC polymer composite for optoelectronics application

Cesium lead halide perovskite quantum dots (PQDs) are the most promising materials for optoelectronic application due to their high photoluminescence quantum yield. However, the poor stability of these PQDs is the major bottleneck towards commercial applications. There have been several strategies to improve the stability of these PQDs which includes PQDs-polymer composite with limited success. In this article we have synthesized CsPbBr3 quantum dots- Carboxymethylcellulose (CMC) composite for the first time to improve the optoelectronic properties as well as stability of these PQDs compared to pure PQDs. Photoluminescence (PL) is significantly improved while the degradation becomes slower than pure PQDs. There is a slight blue shift of ∼8 nm in the absorption spectrum of PQDs/CMC composite indicating almost negligible bandgap alteration. This is also confirmed by the high-resolution transmission electron microscopy analysis. However, 78 meV increase in quasi-fermi level splitting (QFLS) is observed in PQD/CMC optimum composite indicating reduced non-radiative recombination due to defect passivation.However, excess CMC beyond 0.3 mol% in PQDs reduces the PL intensity possibly due to increased defect states or the structural changes at higher CMC concentrations. This can be also confirmed by analysing the x-ray diffraction peak at (112) which disappears at higher concentration of CMC.

Improved photocatalytic activity and stability of InGaN quantum dots/C3N4 heterojunction photoelectrode for CO2 reduction and hydrogen production

Photocatalytic conversion of CO2 to produce fuel is considered a promising approach to reduce CO2 emissions and tackle energy crisis. GaN-based materials have been studied for CO2 reduction because of their excellent optical properties and band structure. However, low photocatalytic activity and severe photocorrosion of GaN-based photoelectrode greatly limit their applications. In this work, photocatalytic activity was improved by adopting InGaN quantum dots (QDs) combined with C3N4 nano-sheets as photoanode, and thus the efficiency of CO2 reduction and the selectivity of hydrogen production were increased significantly. In addition, the photoelectron-chemical corrosion of photoelectrodes has been apparently controlled. InGaN QDs/C3N4 has the highest CO and H2 productions rates of 14.69 μmol mol−1 h−1 and 140 μmol mol−1 h−1 which were 2.2 times and 14.5 times than that of InGaN film photoelectrode, respectively. The enhancement of photocatalytic activity is attributed to C3N4 modification and a large electric dipole forming on the surface of InGaN QDs, which facilitate the separation and transfer of photo-generated carriers and thus promote CO2 reduction reaction. This work provides a promising strategy for the development of GaN-based photoanodes with superior stability and efficiency.

Graphitic Carbon Nitride/CdSe Quantum Dot/Iron Carbonyl Cluster Composite for Enhanced Photocatalytic Hydrogen Evolution

A g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite has been successfully constructed. The structure and chemical composition of the composite were investigated via, inter alia, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The ability of the assembly to act as a photocatalyst for proton reduction to form hydrogen gas was studied. With visible light irradiation for 4 h, the total H2 production catalyzed by the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite was found to be 9 times as high as a corresponding CdSe/[Fe2S2(CO)6] assembly and significantly higher than either the CdSe quantum dots or g-C3N4 alone. The g-C3N4 support/matrix was found to enhance the stability and efficiency of the CdSe quantum dot/iron carbonyl cluster assembly in the photocatalytic hydrogen evolution process. Results from recycling tests showed that the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite is a sustainable and robust photocatalyst, maintaining the same activity after three cycles. The photoinduced charge carrier transfer dynamics in the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite system has been investigated by transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopies. The spectroscopic results indicate efficient hole transfer from the valence band of the excited CdSe quantum dots to the molecular iron carbonyl clusters and from the defect state of the quantum dots to g-C3N4 in the g-C3N4/CdSe quantum dot/[Fe2S2(CO)6] composite, which significantly inhibits the recombination of photogenerated charge carriers in CdSe quantum dots and boosts the photocatalytic activity and stability for hydrogen evolution. Energy transfer from g-C3N4 to the CdSe quantum dot/[Fe2S2(CO)6] assembly with a time constant of 0.7 ns also contributed to the charge transfer process.

Scalar perturbations and stability of a loop quantum corrected Kruskal black hole

We investigate the massless scalar field perturbations of a new loop quantum gravity motivated regular black hole proposed by Ashtekar {\it et al.} in [Phys.Rev.Lett. 121, 241301 (2018), Phys.Rev.D 98, 126003 (2018)]. The spacetime of this black hole is distinguished by its asymptotic properties: in Schwarzschild coordinates one of the metric functions diverges as $r\to \infty$ even though the spacetime is asymptotically flat. We show that despite this unusual asymptotic behavior, the quasinormal mode potential is well defined everywhere when Schwarzschild coordinates are used. We propose a useful approximate form of the metric, which allows us to produce quasinormal mode frequencies and ringdown waveforms to high accuracy with manageable computation times. Our results indicate that this black hole model is stable against massless scalar field perturbations. We show that, compared to the Schwarzschild black hole, this black hole oscillates with higher frequency and less damping. We also observe a qualitative difference in the power-law tail of the ringdown waveform between this black hole model and the Schwarzschild black hole. This suggests the quantum corrections affect the behavior of the waves at large distances from the black hole.

Thermodynamics of General Heisenberg Spin Tetramers Composed of Coupled Quantum Dimers

Advances in quantum computing technology have been made in recent years due to the evolution of spin clusters. Recent studies have tended towards spin cluster subgeometries to understand more complex structures better. These molecular magnets provide a multitude of phenomena via exchange interactions that allow for advancements in spintronics and other magnetic system applications due to the possibility of increasing speed, data storage, memory, and stability of quantum computing systems. Using the Heisenberg spin–spin exchange Hamiltonian and exact diagonalization, we examine the evolution of quantum energy levels and thermodynamic properties for various spin configurations and exchange interactions. The XXYY quantum spin tetramer considered in this study consists of two coupled dimers with exchange interactions α1J and α1′J and a dimer–dimer exchange interaction α2J. By varying spin values and interaction strengths, we determine the exact energy eigenstates that are used to determine closed-form analytic solutions for the heat capacity and magnetic susceptibility of the system and further analyze the evolution of the properties of the system based on the parameter values chosen. Furthermore, this study shows that the Schottky anomaly shifts towards zero as the ground-state of the system approaches a quantum phase transition between spin states. Additionally, we investigate the development of phase transitions produced by the convergence of the Schottky anomaly with both variable exchange interactions and external magnetic field.

Impact of Lattice Vibrations on the Dynamics of a Spinor Atom-Optics Kicked Rotor

We investigate the effect of amplitude and phase noise on the dynamics of a discrete-time quantum walk and its related evolution. Our findings underline the robustness of the motion with respect to these noise sources, and can explain the stability of quantum walks that has recently been observed experimentally. This opens the road to measure topological properties of an atom-optics double kicked rotor with an additional internal spin degree of freedom.

In Situ Microwave‐Assisted Fabrication of Hierarchically Arranged Metal Sulfide Counter Electrodes to Boost Stability and Efficiency of Quantum Dot‐Sensitized Solar Cells

AbstractThis study describes preparation of metal sulfide counter electrodes (CEs) through one‐pot microwave‐assisted route to improve power conversion efficiency (PCE) of quantum dot‐sensitized solar cells at a lower cost. The CuS nanorods, Ni0.96S nanoparticles, and PbS nanocubes are synthesized and deposited in situ on fluorine‐doped tin oxide substrate to serve as CEs without further post‐treatment. Effects of several reaction parameters including sulfur precursor (Na2S, C2H5NS, CH4N2S), Cu concentration, reaction time, and choice of cation (Cu, Ni, Pb) on the CEs morphology, electrochemical characteristics, and PCE are studied. Furthermore, nanostructure formation and thin film growth are studied and correlated with PCE, from which morphology‐ and composition‐performance relationships can be inferred. Hierarchically assembled nanorod CuS CEs exhibit higher electrochemical stability in the S2–/Sn2– redox reaction. Together with the efficient charge transfer and higher diffusion coefficient of polysulfide redox at the electrode/electrolyte interface, deduced from electrochemical impedance spectroscopy and Tafel analyses, a PCE of 8.32% is achieved for the CuS CE. The enhanced photovoltaic performance is ascribed to the 1D CuS nanorods forming a diffusive structure which decreases charge transfer impedance and facilitates regeneration of polysulfide redox leading to a higher short‐circuit current density and fill factor.

Improvement in efficiency and stability of quantum dot/polymer nanocomposite film for light-emitting diodes using refractive index-controlled quantum dot–silica hybrid particles

Controlling refractive index of hybrid particles suppressed light scattering and enhanced efficiency and stability of nanocomposite film.

Stability of quantum corrected thin shell wormholes with a different equation of state

The general dynamic equations of quantum corrected thin shell wormholes by applying Darmois–Israel formalism are derived. The stability analyses of wormhole with a generalized cosmic Chaplygin gas, a modified generalized Chaplygin gas and a polytropic equation of state are studied. The presence of unstable and stable solutions depends on the appropriate values of different parameters.

On the stability of fiber-optic quantum cryptography at arbitrary losses in a communication channel: Exclusion of unambiguous measurements

Since a source of quantum states is not strictly single-photon and losses exist in a communication channel, an attack with unambiguous measurements is possible, leading to loss of security. The problem of the stability of quantum key distribution protocols in a channel with large losses is still unsolved. A radical solution of this problem has been proposed by completely excluding unambiguous measurements by an eavesdropper. The problem is solved by means of counting of classical reference pulses. The conservation of the number of classical sync pulses results in the impossibility of unambiguous measurements. In this case, losses in the communication channel are considered as a priori unknown and can vary during a series of messages.

Stability of topologically-protected quantum computing proposals as seen through spin glasses

Sensitivity to noise makes most of the current quantum computing schemes prone to error and nonscalable, allowing only for small proof-of-principle devices. Topologically-protected quantum computing aims at solving this problem by encoding quantum bits and gates in topological properties of the hardware medium that are immune to noise that does not impact the entire system at once. There are different approaches to achieve topological stability or active error correction, ranging from quasiparticle braidings to spin models and topological colour codes. The stability of these proposals against noise can be quantified by their error threshold. This figure of merit can be computed by mapping the problem onto complex statistical-mechanical spin-glass models with local disorder on nontrival lattices that can have many-body interactions and are sometimes described by lattice gauge theories. The error threshold for a given source of error then represents the point in the temperature-disorder phase diagram where a stable symmetry-broken phase vanishes. An overview of the techniques used to estimate the error thresholds is given, as well as a summary of recent results on the stability of different topologically-protected quantum computing schemes to different error sources.

Stability of quantum isolated horizon: a local observer's view

It is shown that a quantum isolated horizon (QIH), as observed by a local observer, is locally unstable as a thermodynamic system. The result is derived in two different ways. Firstly, the specific heat of the QIH is shown to be negative definite through a quantum statistical analysis. Then, it is shown, in the thermal holographic approach, that the canonical partition function of the QIH diverges under Gaussian thermal fluctuations of such energy spectrum, implying local instability of such a QIH as a thermodynamic system.

Stability of quantum isolated horizon: a local observer's view

It is shown that a quantum isolated horizon (QIH), as observed by a local observer, is locally unstable as a thermodynamic system. The result is derived in two different ways. Firstly, the specific heat of the QIH is shown to be negative definite through a quantum statistical analysis. Then, it is shown, in the thermal holographic approach, that the canonical partition function of the QIH diverges under Gaussian thermal fluctuations of such an energy spectrum, implying local instability of such a QIH as a thermodynamic system.

Topological order, entanglement, and quantum memory at finite temperature

We compute the topological entropy of the toric code models in arbitrary dimension at finite temperature. We find that the critical temperatures for the existence of full quantum (classical) topological entropy correspond to the confinement–deconfinement transitions in the corresponding Z2 gauge theories. This implies that the thermal stability of topological entropy corresponds to the stability of quantum (classical) memory. The implications for the understanding of ergodicity breaking in topological phases are discussed.

Propagation and stability of quantum dust-ion-acoustic shock waves in planar and nonplanar geometry

Dust-ion-acoustic (DIA) shock waves are studied in an unmagnetized quantum plasma consisting of electrons, ions, and dust by employing the quantum hydrodynamic (QHD) model. In this context, a Korteweg–deVries–Burger (KdVB) equation is derived by employing the small amplitude perturbation expansion method. The dissipation is introduced by taking into account the kinematic viscosity among the plasma constituents. It is found that the strength of the quantum DIA shock wave is maximum for spherical, intermediate for cylindrical, and minimum for the planar geometry. The effects of quantum Bohm potential, dust concentration, and kinematic viscosity on the quantum DIA shock structure are also investigated. The temporal evolution of DIA KdV solitons and Burger shocks are also studied by putting the dissipative and dispersive coefficients equal to zero, respectively. The effects of the quantum Bohm potential on the stability of the DIA shock is also investigated. The present investigation may be beneficial to understand the dissipative and dispersive processes that may occur in the quantum dusty plasmas found in microelectronic devices as well as in astrophysical plasmas.

Mini-Workshop: Attraction to Solitary Waves and Related Aspects of Physics

Mini-Workshop: Attraction to Solitary Waves and Related Aspects of Physics

Dynamical stability of quantum "chaotic" motion in a hydrogen atom.

A simple numerical reversibility test which proves useful in exposing the chaotic nature of classical dynamical systems is applied to the quantum model of a hydrogen atom in a microwave field. The remarkable result is that, in spite of some apparent chaotic features, the quantum motion proves to be perfectly stable in contrast to the high instability of the classical chaotic motion.