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A simplified analytical model of the effect of high pressure on the critical temperature and other thermodynamic properties of superconducting systems is developed using the general conformal transformation method and group-theoretical arguments. Relationships between the characteristic ratios $${ {\mathcal R} }_{1}\equiv 2{\rm{\Delta }}(0)/{T}_{{\rm{c}}}$$ ℛ 1 ≡ 2 Δ ( 0 ) / T c and $${ {\mathcal R} }_{2}\equiv {\rm{\Delta }}C({T}_{{\rm{c}}})/{C}_{{\rm{N}}}({T}_{{\rm{c}}})$$ ℛ 2 ≡ Δ C ( T c ) / C N ( T c ) and the stability of the superconducting state is discussed. Including a single two-parameter fluctuation in the density of states, placed away from the Fermi level, stable solutions determined by $${ {\mathcal R} }_{1}$$ ℛ 1 are found. It is shown that the critical temperature Tc(p), as a function of high external pressure, can be predicted from experimental data, based on the values of the two characteristic ratios, the critical temperature, and a pressure coefficient measured at zero pressure. The model can be applied to s-wave low-temperature and high-temperature superconductors, as well as to some novel superconducting systems of the new generation. The problem of emergence of superconductivity under high pressure is explained as well. The discussion is illustrated by using experimental data for superconducting elements available in the literature. A criterion for compatibility of experimental data is formulated, allowing one to identify incompatible measurement data for superconducting systems for which the maximum or the minimum critical temperature is achieved under high pressure.

It is shown that in a germanium/silicon nanosystem with germanium quantum dots, the hole leaving the germanium quantum dot causes the appearance of the hole energy level in the bandgap energy in a silicon matrix. The dependences of the energies of the ground state of a hole and an electron are obtained as well as spatially indirect excitons on the radius of the germanium quantum dot and on the depth of the potential well for holes in the germanium quantum dot. It is found that as a result of a direct electron transition in real space between the electron level that is located in the conduction band of the silicon matrix and the hole level located in the bandgap of the silicon matrix, the radiative recombination intensity in the germanium/silicon nanosystem with germanium quantum dots increases significantly.

We discuss a simple idealistic quantum entanglement based protocol for quantum random number generation allowing a trusted third party to publicly perform arbitrarily complex tests of randomness without any violation of the secrecy of the generated bit sequences. The protocol diminishes also an average time of the randomness testing (thus enabling arbitrary shortening of this time with increasing number of entangled qubits).

Within the general conformal transformation method a simplified analytical model is proposed to study the effect of external hydrostatic pressure on low- and high-temperature superconducting systems. A single fluctuation in the density of states, placed away from the Fermi level, as well as external pressure are included in the model to derive equations for the superconducting gap, free energy difference, and specific heat difference. The zero- and sub-critical temperature limits are discussed by the method of successive approximations. The critical temperature is found as a function of high external pressure. It is shown that there are four universal types of the response of the system, in terms of dependence of the critical temperature on increasing external pressure. Some effects, which should be possible to be observed experimentally in s-wave superconductors, the cuprates (i.e. high-Tc superconductors) and other superconducting materials of the new generation such as two-gap superconductors, are revealed and discussed. An equation for the ratio $${{\boldsymbol{ {\mathcal R} }}}_{{\bf{1}}}$$ ℛ 1 ≡ 2Δ(0)/Tc, as a function of the introduced parameters, is derived and solved numerically. Analysis of other thermodynamic quantities and the characteristic ratio $${{\boldsymbol{ {\mathcal R} }}}_{{\bf{2}}}$$ ℛ 2 ≡ ΔC(Tc)/CN(Tc) is performed numerically, and mutual relations between the discussed quantities are investigated. The simple analytical model presented in the paper may turn out to be helpful in searching for novel superconducting components with higher critical temperatures induced by pressure effects.

We study strong optical coupling of metal nanoparticle arrays with dielectric substrates. Based on the Fermi Golden Rule, the particle&ndash, substrate coupling is derived in terms of the photon absorption probability assuming a local dipole field. An increase in photocurrent gain is achieved through the optical coupling. In addition, we describe light-induced, mesoscopic electron dynamics via the nonlocal hydrodynamic theory of charges. At small nanoparticle size (&lt, 20 nm), the impact of this type of spatial dispersion becomes sizable. Both absorption and scattering cross sections of the nanoparticle are significantly increased through the contribution of additional nonlocal modes. We observe a splitting of local optical modes spanning several tenths of nanometers. This is a signature of semi-classical, strong optical coupling via the dynamic Stark effect, known as Autler&ndash, Townes splitting. The photocurrent generated in this description is increased by up to 2%, which agrees better with recent experiments than compared to identical classical setups with up to 6%. Both, the expressions derived for the particle&ndash, substrate coupling and the additional hydrodynamic equation for electrons are integrated into COMSOL for our simulations.

This paper presents a new type of solar cellwith enhanced optical-current characteristics using an ultra-thin CuIn1&minus, xGaxSe2 hole-transporting material (HTM) layer (&lt, 400 nm). The HTM layer was between a bi-layer Mo metal-electrode and a CH3NH3PbI3 (MAPbI3) perovskite active absorbing material. It promoted carrier transportand led to an improved device with good ohmic-contacts. The solar cell was prepared as a bi-layer Mo/CuIn1&minus, xGaxSe2/perovskite/C60/Ag multilayer of nano-structures on an FTO (fluorine-doped tin oxide) glass substrate. The ultra-thin CuIn1&minus, xGaxSe2 HTM layers were annealed at various temperatures of 400, 500, and 600 &deg, C. Scanning electron microscopy studies revealed that the nano-crystal grain size of CuIn1&minus, xGaxSe2 increased with the annealing temperature. The solar cell results show an improved optical power conversion efficiency at ~14.2%. The application of the CuIn1&minus, xGaxSe2 layer with the perovskite absorbing material could be used for designing solar cells with a reduced HTM thickness. The CuIn1&minus, xGaxSe2 HTM has been evidenced to maintain a properopen circuit voltage, short-circuit current density and photovoltaic stability.

This paper presents a novel solar cell with FTO/MoSe2/perovskite/PMMA/C60/AZO/silver nanostructures. The hole-transporting material (HTM), active absorber layer, electron-transporting material (ETM), electron buffer layer, and transparent-conducting oxide layer were made of MoSe2, MAPbI3, PMMA, fullerene, and Al-doped ZnO, respectively. The SEM analysis confirmed that the MAPbI3 perovskite crystal size increased with the annealing temperature of the MoSe2 HTM films. The unique modified PMMA ETM films have been prepared and characterized at various thicknesses. At the optimal thickness of 25 nm, the experimental results showed that the multilayer solar cell's open-circuit voltage increased from 0.99 V to 1.11 V, and the short-circuit current value was elevated from 16.98 mA/cm2 to 21.11 mA/cm2. The device fill factor value was also improved from 60.67% to 76.12%. Thus, the device power-conversion efficiency exhibited significant improvement from 10.2% to 18.0%. In addition, the output power Pmax was as good as 1.79 mW. The PMMA, in conjunction with fullerene C60, has become an excellent candidate for the role of charge transporting for the multilayer perovskite solar cells.

We discuss a simple idealistic quantum entanglement based protocol for quantum random number generation allowing a trusted third party to publicly perform arbitrarily complex tests of randomness without any violation of the secrecy of the generated bit sequences. The protocol diminishes also an average time of the randomness testing (thus enabling arbitrary shortening of this time with increasing number of entangled qubits).

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

A simplified analytical model of the effect of high pressure on the critical temperature and other thermodynamic properties of superconducting systems is developed using the general conformal transformation method and group-theoretical arguments. Relationships between the characteristic ratios \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${ {\mathcal R} }_{1}\equiv 2{\rm{\Delta }}(0)/{T}_{{\rm{c}}}$$\end{document}ℛ1≡2Δ(0)/Tc and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${ {\mathcal R} }_{2}\equiv {\rm{\Delta }}C({T}_{{\rm{c}}})/{C}_{{\rm{N}}}({T}_{{\rm{c}}})$$\end{document}ℛ2≡ΔC(Tc)/CN(Tc) and the stability of the superconducting state is discussed. Including a single two-parameter fluctuation in the density of states, placed away from the Fermi level, stable solutions determined by \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${ {\mathcal R} }_{1}$$\end{document}ℛ1 are found. It is shown that the critical temperature Tc(p), as a function of high external pressure, can be predicted from experimental data, based on the values of the two characteristic ratios, the critical temperature, and a pressure coefficient measured at zero pressure. The model can be applied to s-wave low-temperature and high-temperature superconductors, as well as to some novel superconducting systems of the new generation. The problem of emergence of superconductivity under high pressure is explained as well. The discussion is illustrated by using experimental data for superconducting elements available in the literature. A criterion for compatibility of experimental data is formulated, allowing one to identify incompatible measurement data for superconducting systems for which the maximum or the minimum critical temperature is achieved under high pressure.

Nanoparticles&mdash, regularly patterned or randomly dispersed&mdash, are a key ingredient for emerging technologies in photonics. Of particular interest are scattering and field enhancement effects of metal nanoparticles for energy harvesting and converting systems. An often neglected aspect in the modeling of nanoparticles are light interaction effects at the ultimate nanoscale beyond classical electrodynamics. Those arise from microscopic electron dynamics in confined systems, the accelerated motion in the plasmon oscillation and the quantum nature of the free electron gas in metals, such as Coulomb repulsion and electron diffusion. We give a detailed account on free electron phenomena in metal nanoparticles and discuss analytic expressions stemming from microscopic (Random Phase Approximation&mdash, RPA) and semi-classical (hydrodynamic) theories. These can be incorporated into standard computational schemes to produce more reliable results on the optical properties of metal nanoparticles. We combine these solutions into a single framework and study systematically their joint impact on isolated Au, Ag, and Al nanoparticles as well as dimer structures. The spectral position of the plasmon resonance and its broadening as well as local field enhancement show an intriguing dependence on the particle size due to the relevance of additional damping channels.

The topology-based explanation of the fractional quantum Hall effect (FQHE) is summarized. The cyclotron braid subgroups crucial for this approach are introduced in order to identify the origin of the Laughlin correlations in 2D (two-dimensional) Hall systems. Flux-tubes and vortices for composite fermions in their standard constructions are explained in terms of cyclotron braids. The derivation of the hierarchy of the FQHE is proposed by mapping onto the integer effect within the topology-based approach. The experimental observations of the FQHE supporting the cyclotron braid picture are reviewed with a special attention paid to recent experiments with a suspended graphene. The triggering role of a carrier mobility for organization of the fractional state in Hall configuration is emphasized. The prerequisites for the FQHE are indicated including topological conditions substantially increasing the previously accepted set of physical necessities. The explanation of numerical studies by exact diagonalizations of the fractional Chern insulator states is formulated in terms of the topology condition applied to the Berry field flux quantization. Some new ideas withz regard to the synthetic fractional states in the optical lattices are also formulated.

The commensurability condition is applied to determine the hierarchy of fractional filling of Landau levels for fractional quantum Hall effect (FQHE) in monolayer and bilayer graphene. The difference in FQHE hierarchy for bilayer and monolayer graphene is outlined and explained. Good agreement with experimental data is achieved. The presence of even-denominator filling fractions in the hierarchy of the FQHE in bilayer graphene is explained, including the state at ν = − 1 / 2 .

The odd structure of the fractional filling hierarchy, which is referred to as the fractional quantum Hall effect, is studied in higher Landau levels using the commensurability condition. The hierarchy of fillings that are derived in this manner is consistent with the experimental observations in the first three Landau levels. The relative poverty of the fractional structure in higher Landau levels compared with the lowest Landau level is explained using commensurability topological arguments. The commensurability criterion for correlated states specific for higher Landau levels (with n ≥ 1), including also the paired states at half fillings of the spinsubbands of these levels, is formulated.

We show that, by applying the conformal transformation method, strongly correlated superconducting systems can be discussed in terms of the Fermi liquid with a variable density of states function. Within this approach, it is possible to formulate and carry out purely analytical study based on a set of fundamental equations. After presenting the mathematical structure of thes-wave superconducting gap and other quantitative characteristics of superconductors, we evaluate and discuss integrals inherent in fundamental equations describing superconducting systems. The results presented here extend the approach formulated by Abrikosov and Maki, which was restricted to the first-order expansion. A few infinite families of integrals are derived and allow us to express the fundamental equations by means of analytical formulas. They can be then exploited in order to find quantitative characteristics of superconducting systems by the method of successive approximations. We show that the results can be applied in studies of high-Tcsuperconductors and other superconducting materials of the new generation.

The topology-based explanation of the origin of the fractional quantum Hall effect is summarized. The cyclotron braid subgroups crucial for this approach are introduced in order to identify the origin of Laughlin correlations in 2D Hall systems. The so-called composite fermions are explained in terms of the homotopy cyclotron braids. Some new concept for fractional Chern insulator states is formulated in terms of the homotopy condition applied to the Berry field flux quantization.

We evaluate some thermodynamic quantities and characteristic ratios that describe low- and high-temperature s-wave superconducting systems. Based on a set of fundamental equations derived within the conformal transformation method, a simple model is proposed and studied analytically. After including a one-parameter class of fluctuations in the density of states, the mathematical structure of the s-wave superconducting gap, the free energy difference, and the specific heat difference is found and discussed in an analytic manner. Both the zero-temperature limit T = 0 and the subcritical temperature range T ≲ Tc are discussed using the method of successive approximations. The equation for the ratio R1, relating the zero-temperature energy gap and the critical temperature, is formulated and solved numerically for various values of the model parameter. Other thermodynamic quantities are analyzed, including a characteristic ratio R2, quantifying the dynamics of the specific heat jump at the critical temperature. It is shown that the obtained model results coincide with experimental data for low-Tc superconductors. The prospect of application of the presented model in studies of high-Tc superconductors and other superconducting systems of the new generation is also discussed.

In this paper the topological approach to quantum Hall effects is carefully described. Commensurability conditions together with proposed generators of a system braid group are employed to establish the fractional quantum Hall effect hierarchies of conventional semiconductors, monolayer and bilayer graphene structures. Obtained filling factors are compared with experimental data and a very good agreement is achieved. Preliminary constructions of ground-state wave functions in the lowest Landau level are put forward. Furthermore, this work explains why pyramids of fillings from higher bands are not counterparts of the well-known composite-fermion hierarchy – it provides with the cause for an intriguing robustness of ν = 7/3 , 8/3 and 5/2 states (also in graphene). The argumentation why paired states can be developed in two-subband systems (wide quantum wells) only when the Fermi energy lies in the first Landau level is specified. Finally, the paper also clarifies how an additional surface in bilayer systems contributes to an observation of the fractional quantum Hall effect near half-filling, ν = 1/2 .

Recent experiments with suspended graphene have corroborated an important role of carrier mobility in the competition between Laughlin state and insulating state, presumably of Wigner-type electron crystal. Moreover, the fractional quantum Hall effect (FQHE) in graphene has been observed at low carrier densities when the interaction was reduced due to carrier dilution. This suggests that not solely interaction and the flat band with quenched kinetic energy may be important for formation of FQHE. Here, some exclusive for 2D topological arguments are supposed to explain the triggering role of carrier mobility in formation of the collective FQHE state, when conditions of sufficient flattening of a band and interaction presence are fulfilled.

Recent experiments with suspended graphene have indicated the crucial role of carrier mobility in the competition between Laughlin collective state and insulating state, probably of Wigner-crystal-type. Moreover, the fractional quantum Hall effect (FQHE) in graphene has been observed at a low carrier density where the interaction is reduced as a result of particles dilution. This suggests that the interaction may not be a sole factor in the triggering of FQHE as it was expected basing on the standard formulation of the composite fermion model. Here, the topological arguments are presented to explain the observed features of the FQHE in graphene and the triggering role of carrier mobility in formation of the Laughlin state.

A framework for analytical studies of superconducting systems is presented and illustrated. The formalism, based on the conformal transformation of momentum space, allows one to study the effects of both the dispersion relation and the structure of the pairing interaction in two-dimensional anisotropic high-Tc superconductors. In this method, the number of employed degrees of freedom coincides with the dimension of the momentum space, which is different compared to that in the standard Van Hove scenario with a single degree of freedom. A new function, the kernel of the density of states, is defined and its relation to the standard density of states is explained. The versatility of the method is illustrated by analyzing coexistence and competition between spin-singlet and spin-triplet order parameters in superconducting systems with a tight-binding-type dispersion relation and an anisotropic pairing potential. Phase diagrams of stable superconducting states in the coordinates I· (the ratio of hopping parameters) and n (the carrier concentration) are presented and discussed. Moreover, the role of attractive and repulsive on-site interactions for the stability of the s-wave order parameter is explained.

An explanation of a large plasmon-induced PV efficiency enhancement of metallically surface-modified photo-cell is given by inclusion of all indirect inter-band electron transitions in semiconductor due to near-field coupling with plasmon radiation of a nano-scale metallic components. The model of nanosphere plasmon is formulated (of RPA-type, adjusted to large clusters) for both surface and volume modes. Damping of plasmons is analyzed including irradiation losses due to the Lorentz friction. Probability of the interband transition in substrate semiconductor caused by the coupling with plasmons in near-field regime turns out to be larger by 4-orders (in an idealised atomic regime) than for coupling of electrons with planar-wave photons. Inclusion of proximity and interference effects allows for explanation of photo-current growth measured in experimental metallically modified photo-diode systems.

We analyze theoretically a hybridization type dressing of orbital (i.e., charge) and spin degrees of freedom of excitations captured in semiconductor quantum dot with band phonons and magnons (the latter for magnetic semiconductor surroundings), as a mechanism of dephasing of rapidly excited dot exciton state. Within the Green function approach we derive our previously formulated heuristic general rule for estimation of corresponding dephasing time-rate. The pure dephasing (off-diagonal decoherence) of quantum dot exciton resulting due to this dressing is studied, for both orbital and spin states of the exciton. A significant difference between phonon-induced (for charge) and magnon-assisted (for spin) quantum dot exciton dephasing is indicated as the result of the spin conservation, which leads to the disappearance of the exciton spin pure-dephasing at T = 0 (freezing out of spin dephasing), in contrary to the exciton charge dephasing caused by phonons, which maintains strong even at T = 0.

Electromagnetic response of cuprate superconductors is studied within the model of kinetic energy driven d-wave superconductivity by analyzing the Meissner eect. The kernel of the linear response function is found and employed to calculate the magnetic field penetration depth and the superfluid density of cuprate superconductors within the specular reflection model for a purely transverse vector potential. It is shown that the magnetic field penetration depth and the superfluid density depend linearly on temperature, except for a strong deviation from the linear characteristics at extremely low temperatures, which is attributed to nonlocal eects. The zero-temperature superfluid density is found to decrease linearly with decreasing doping concentration in the underdoped regime. The problem of gauge invariance in the theoretical description of the electromagnetic response is addressed, and an approximation which does not violate local charge conservation is proposed.

Equations describing the behavior of plasma oscillations that can arise on the surface as well as inside metallic spherical nanoparticles are obtained in the random-phase approximation. The frequencies of all characteristic plasma oscillations arising on the surface and in the volume of a particle are determined. It is shown that the volume plasma oscillations can give rise to forced oscillations on the surface of a nanoparticle. The effect of plasma oscillations on the optical properties of a dielectric medium containing metallic nanoparticles is determined.

We demonstrate disappearance at 0 T = 0, and strong reduction at low temperatures, of pure-dephasing of exciton spin located in a quantum dot (QD) embedded in a magnetic medium, e.g. in a diluted magnetic semiconductor. The off-diagonal spin decoherence results due to hybridization of local QD exciton spin with magnons from surrounding crystal–we estimate the corresponding time-scale and formulate a general rule for the scaling of dephasing time versus QD diameter derived within the Green function approach (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A two-dimensional model of an anisotropic superconductor with a relatively wide partially filled conduction band and a dispersion relation implying singularities in the density of states is considered. Employing the conformal transformation method and choosing the Fourier harmonics as the complete and orthogonal basis for C4v irreducible representations the symmetry of the superconducting order parameter is identified. Detailed numerical calculations are carried out for the two-dimensional tight-binding band model and compared with the results obtained within the BCS-type approximation. Diagrams of stable superconducting states in the coordinates η = 2t1/t0 and n (carrier concentration) are outlined. Symmetry relations with respect to replacing η with −η are found and discussed. The problem of coexistence of s-, p- and d-wave order parameters is addressed. The triple points are localized and supreme transition temperatures are evaluated. It is shown that the results obtained for various values of the pairing amplitude V1 coincide, whereas the tight-binding results and some obtained within the BCS-type approximation are dissimilar. The influence of the attractive or repulsive onsite interaction on the stability of the s-wave order parameter is explained. The developed and widely illustrated formalism can be easily applied to some more composed models. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, the commensurability conditions, which originated from the unique topology of two-dimensional systems, are applied to determine the quantum Hall effect hierarchy in the case of a monolayer graphene. The fundamental difference in a definition of a typical semiconductor and a monolayer graphene filling factor is pointed out. The calculations are undertaken for all spin-valley branches of two lowest Landau levels, since only they are currently experimentally accessible. The obtained filling factors are compared with the experimental data and a very good agreement is achieved. The work also introduces a concept of the single-loop fractional quantum Hall effect.

Phonon-induced decoherence of orbital degrees of freedom in quantum dots (QDs) (GaAs/InAs) is studied and the relevant time-scales are estimated versus dot dimension. Dephasing of excitons due to acoustic phonons and optical phonons, including enhancement of the effective Frohlich constant caused by localization, is assessed for the state-of-art QDs. Temporal inefficiency of Pauli blocking in QDs due to lattice inertia is additionally predicted. For QD placed in a diluted magnetic semiconductor medium a magnon induced dephasing of spin is estimated in accordance with experimental results for Zn(Mn)Se/CdSe. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The spin degrees of freedom in quantum dot (QD) embedded in a diluted magnetic semiconductor (DMS) medium are considered in a model of a qubit and a gate for quantum information processing (QIP). The qubit is defined as a singlet and triplet pair of states of two electrons in a He-type QD in the DMS medium with strongly enhanced gyromagnetic factor. Methods of qubit rotation (Rabi oscillations) as well as two-qubit operations are suggested and analyzed. Moreover, decoherence related to spin waves (magnon-induced dephasing) in this new system (QD in DMS) is studied, and the relevant time-scale is estimated in accordance with preliminary experimental results. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We show that the phonon-induced pure dephasing of excitons in quantum dots can be interpreted in terms of information leakage from the carrier subsystem to the lattice environment. We derive a quantitative relation between the coherence of the system, as manifested by the amplitude of the coherent optical polarization, and the amount of available which path information on the system state, quantifled by the distinguishability of states.

We show that phonon-induced pure dephasing of excitons in quantum dots, which leads only to partial decay of system coherence, may completely erase entanglement between two excitons. We also discuss the role of the distance between the subsystems for this disentanglement process.

We show that phonon-induced pure dephasing of an excitonic (charge) quantum bit in a quantum dot (QD) may be reduced by collective encoding of logical qubits in QD arrays. We define the logical qubit on an array of 2, 4 and 8 QDs, connecting the logical |0 state with the presence of excitons in the appropriately chosen half of dots and the logical |1 state with the other half of the dots occupied. We give quantitative estimates of the resulting total error of a single qubit operation for an InAs/GaAs system.

We describe the mechanism of phonon-induced dynamical dephasing of an exciton confined in a quantum dot and discuss methods of reducing the dephasing effect by appropriately tailored driving.

We show that phonon-induced dephasing of exciton states confined in a quantum dot may be interpreted in terms of which way information transferred from the carrier system to the lattice environment. Using distinguishability of quantum states as a measure for this information transfer we cast this interpretation in the form of a quantitative complementarity relation.

Based upon the tight-binding formalism a model of a high-Tc superconductor with isotropic and anisotropic attractive interactions is considered analytically. Symmetry facets of the group C4v are included within a method of successive transformations of the reciprocal space. Complete sets of basis functions of C4v irreducible representations are given. Plausible spin-singlet and spin-triplet superconducting states are classified with regard to the chosen basis functions. It is displayed that pairing interaction coefficients and the dispersion relation, which can be characterized by the parameter η= 2t1/t0, have a diverse and mutually competing influence on the value of the transition temperature. It is also shown that in the case of a nearly half-filled conduction band and an anisotropic pairing interaction the spin-singlet d-wave symmetry superconducting state is realized for small values of the parameter η, whereas in the opposite limit, for sufficiently large values, the spin-triplet p-wave symmetry superconducting state has to be formed. This result cannot be obtained within the Van Hove scenario or BCS-type approaches, where the p-wave symmetry superconducting state absolutely dominates. The specific heat jump and the isotope shift as functions of the parameter η are assessed and discussed for the d-wave symmetry singlet and the p-wave symmetry triplet states.

We define the qubit as a pair of singlet and triplet states of two electrons in a He-type quantum dot (QD) placed in a diluted magnetic semiconductor (DMS) medium. The molecular field is here essential as it removes the degeneracy of the triplet state and strongly enhances the Zeeman splitting. Methods of qubit rotation as well as two-qubit operations are suggested. The system of a QD in a DMS is described in a way which allows an analysis of the decoherence due to spin waves in the DMS subsystem.

The interaction of a exciton confined in a semiconductor quantum dot (QD) with bulk phonons (acoustical and optical) responsible for the exciton dephasing is studied. The decoherence of the exciton due to the creation of a polaron with long-living or decaying phonons is described. Characteristic dephasing times for an InAs / GaAs QD are estimated using Green function methods in order to determine fundamental time limitations for use of the QD exciton in quantum information processing.