Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly Pumped Quantum Dots.

Two-photon resonant excitation of the biexciton-exciton cascade in a quantum dot generates highly polarization-entangled photon pairs in a near-deterministic way. However, the ultimate level of achievable entanglement is still debated. Here, we observe the impact of the laser-induced ac-Stark effect on the quantum dot emission spectra and on entanglement. For increasing pulse-duration-to-lifetime ratios and pump powers, decreasing values of concurrence are recorded. Nonetheless, additional contributions are still required to fully account for the observed below-unity concurrence.

Two-Photon Excitation Sets Limit to Entangled Photon Pair Generation from Quantum Emitters.

Entangled photon pairs are key to many novel applications in quantum technologies. Semiconductor quantum dots can be used as sources of on-demand, highly entangled photons. The fidelity to a fixed maximally entangled state is limited by the excitonic fine-structure splitting. This work demonstrates that, even if this splitting is absent, the degree of entanglement cannot reach unity when the excitation pulse in a two-photon resonance scheme has a finite duration. The degradation of the entanglement has its origin in a dynamically induced splitting of the exciton states caused by the laser pulse itself. Hence, in the setting explored here, the excitation process limits the achievable concurrence for entangled photons generated in an optically excited four-level quantum emitter.

Entanglement Swapping with Photons Generated on Demand by a Quantum Dot.

Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of nonclassical light were used for seminal demonstration of entanglement swapping, but applications in quantum technologies demand push-button operation requiring single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent prerequisites on the efficiency and purity of the generation of entangled states. Here we show a proof-of-concept demonstration of all-photonic entanglement swapping with pairs of polarization-entangled photons generated on demand by a GaAs quantum dot without spectral and temporal filtering. Moreover, we develop a theoretical model that quantitatively reproduces the experimental data and provides insights on the critical figures of merit for the performance of the swapping operation. Our theoretical analysis also indicates how to improve state-of-the-art entangled-photon sources to meet the requirements needed for implementation of quantum dots in long-distance quantum communication protocols.

Quantum-Dot Single-Photon Sources for Entanglement Enhanced Interferometry.

Multiphoton entangled states such as "N00N states" have attracted a lot of attention because of their possible application in high-precision, quantum enhanced phase determination. So far, N00N states have been generated in spontaneous parametric down-conversion processes and by mixing quantum and classical light on a beam splitter. Here, in contrast, we demonstrate superresolving phase measurements based on two-photon N00N states generated by quantum dot single-photon sources making use of the Hong-Ou-Mandel effect on a beam splitter. By means of pulsed resonance fluorescence of a charged exciton state, we achieve, in postselection, a quantum enhanced improvement of the precision in phase uncertainty, higher than prescribed by the standard quantum limit. An analytical description of the measurement scheme is provided, reflecting requirements, capability, and restraints of single-photon emitters in optical quantum metrology. Our results point toward the realization of a real-world quantum sensor in the near future.

The application of antimicrobial photodynamic therapy (aPDT) in dentistry: a critical review.

In recent years there have been an increasing number of in vitro and in vivo studies that show positive results regarding antimicrobial photodynamic therapy (aPDT) used in dentistry. These include applications in periodontics, endodontics, and mucosal infections caused by bacteria present as biofilms. Antimicrobial photodynamic therapy is a therapy based on the combination of a non-toxic photosensitizer (PS) and appropriate wavelength visible light, which in the presence of oxygen is activated to produce reactive oxygen species (ROS). ROS induce a series of photochemical and biological events that cause irreversible damage leading to the death of microorganisms. Many light-absorbing dyes have been mentioned as potential PS for aPDT and different wavelengths have been tested. However, there is no consensus on a standard protocol yet. Thus, the goal of this review was to summarize the results of research on aPDT in dentistry using the PubMed database focusing on recent studies of the effectiveness aPDT in decreasing microorganisms and microbial biofilms, and also to describe aPDT effects, mechanisms of action and applications.

Role of surface-segregation-driven intermixing on the thermal transport through planar Si/Ge superlattices.

It has been highly debated whether the thermal conductivity κ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to κ values not only lower than the alloy's, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that κ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower κ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of ∼3 THz. Our calculated κ's quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.

Universal recovery of the energy-level degeneracy of bright excitons in InGaAs quantum dots without a structure symmetry.

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Monolithic growth of ultrathin Ge nanowires on Si(001).

Self-assembled Ge wires with a height of only 3 unit cells and a length of up to 2 micrometers were grown on Si(001) by means of a catalyst-free method based on molecular beam epitaxy. The wires grow horizontally along either the [100] or the [010] direction. On atomically flat surfaces, they exhibit a highly uniform, triangular cross section. A simple thermodynamic model accounts for the existence of a preferential base width for longitudinal expansion, in quantitative agreement with the experimental findings. Despite the absence of intentional doping, the first transistor-type devices made from single wires show low-resistive electrical contacts and single-hole transport at sub-Kelvin temperatures. In view of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization of hole systems with exotic properties and provide a new development route for silicon-based nanoelectronics.

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers.

The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Green's function simulations.

Dependence of the redshifted and blueshifted photoluminescence spectra of single In(x)Ga(1-x)As/GaAs quantum dots on the applied uniaxial stress.

We apply external uniaxial stress to tailor the optical properties of In(x)Ga(1-x)As/GaAs quantum dots. Unexpectedly, the emission energy of single quantum dots controllably shifts to both higher and lower energies under tensile strain. Theoretical calculations using a million atom empirical pseudopotential many-body method indicate that the shifting direction and magnitude depend on the lateral extension and more interestingly on the gallium content of the quantum dots. Our experimental results are in good agreement with the underlying theory.

Collective shape oscillations of SiGe islands on pit-patterned Si(001) substrates: a coherent-growth strategy enabled by self-regulated intermixing.

The shape of coherent SiGe islands epitaxially grown on pit-patterned Si(001) substrates displays very uniform collective oscillations with increasing Ge deposition, transforming cyclically between shallower "dome" and steeper "barn" morphologies. Correspondingly, the average Ge content in the alloyed islands also displays an oscillatory behavior, superimposed on a progressive Si enrichment with increasing size. We show that such a growth mode, remarkably different from the flat-substrate case, allows the islands to keep growing in size while avoiding plastic relaxation.

Tuning the exciton binding energies in single self-assembled InGaAs/GaAs quantum dots by piezoelectric-induced biaxial stress.

We study the effect of an external biaxial stress on the light emission of single InGaAs/GaAs(001) quantum dots placed onto piezoelectric actuators. With increasing compression, the emission blueshifts and the binding energies of the positive trion (X+) and biexciton (XX) relative to the neutral exciton (X) show a monotonic increase. This phenomenon is mainly ascribed to changes in electron and hole localization and it provides a robust method to achieve color coincidence in the emission of X and XX, which is a prerequisite for the possible generation of entangled photon pairs via the recently proposed "time reordering" scheme.

Microphotoluminescence spectroscopy of single CdTe/ZnTe quantum dots grown on Si001 substrates.

We have studied the emission properties of single CdTe/ZnTe quantum dots (QDs) grown on Si(001) substrates by using molecular beam epitaxy and atomic layer epitaxy. The good quality of the QDs is attested by the resolution-limited emission, negligible background and absence of measurable spectral jitter or blinking. Power-dependent, polarization-dependent, and temperature-dependent microphotoluminescence spectroscopy measurements were performed to identify the exciton, the biexciton, and two oppositely charged excitons in the emission spectra of single QDs.

Structure-activity relationships in dihydropteroate synthase inhibition by sulfanilamides. Comparison with the antibacterial activity.

A set of 12 acidic, 5 imidic, and 5 amidic sulfanilamides (SA) were tested for their inhibitory activity on dihydropteroate synthase of Escherichia coli. The enzyme inhibition indexes (EII50) were compared with the growth inhibition indexes (GII50), and electronic structures of SA and cell permeability effects were discussed as possible determinant factors of the observed variation of the activity in the SA set. The results strongly support the following conclusion: (a) permeability factors are highly effective in depressing the activity of SA in growth inhibition with respect to enzyme inhibition, but they do not appear to contribute significantly to the activity variation; (b) the activities of the different SA, both in growth and enzyme inhibition experiments, are well accounted for by the electronic features of these compounds.

Electronic aspects of the antibacterial action of sulfanilamides.

Intramolecular interactions in N1-substituted sulfanilamides (SA) can rationalize the trend of their antibacterial powers with the use of a resonance scheme, derived from d orbital symmetry and tested with an extensive spectroscopic investigation on amidic, imidic, and anionic SA. On quantitative grounds, a good relationship is presented between the antibacterial power and the proton chemical shift of the p-amino group. The electronic features for high activity are described.

Concerted vs stepwise mechanism in 1,3-dipolar cycloaddition of nitrone to ethene, cyclobutadiene, and benzocyclobutadiene. A computational study

The problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive alternative. The R(U)B3LYP/6-31G study was extended to the reaction of the same 1, 3-dipole with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has an electronic structure that is particularly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise process can compete with the concerted one on similar footing. Attenuation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus, our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.

Facial selectivity in epoxidation of 2-cyclohexen-1-ol with peroxy acids. A computational DFT study

We addressed the mechanism of epoxidation of 2-cyclohexen-1-ol by locating all the transition structures (TSs) for the reaction of peroxyformic acid (PFA) with both pseudoequatorial and pseudoaxial cyclohexenol conformers (five TSs for each conformer) and, for purpose of comparison, also those for the PFA epoxidation of cyclohexene. Geometry optimizations were performed at the B3LYP/6-31G level, energies refined with single point B3LYP/6-311+G// B3LYP/6-31G calculations and solvent effects introduced with the CPCM method. Our results can be summarized as follows: (i) all TSs exhibit a spiro-like structure, that is, the dihedral angle between the peroxy acid plane and the forming oxirane plane is closer to 90 degrees than to 0 degrees (or 180 degrees ); (ii) there is a stabilizing hydrogen bonding interaction in syn TSs that, however, is partly counteracted by unfavorable entropic effects; (iii) syn,exo TSs with hydrogen bonding at the PFA peroxy oxygens are definitely more stable than syn,endo TSs hydrogen bonded at the PFA carbonyl oxygen; (iv) facial selectivity of epoxidation of both cyclohexenol conformers is mostly the result of competition between only two TSs, namely, an anti,exo TS and its syn,exo counterpart. The latter TS is more stable than the former one, as stabilization by hydrogen bonding overrides the unfavorable entropic and solvent effects; (v) calculations correctly predict both the experimental dominance of attack leading to syn epoxide for both cyclohexenol conformers and the higher syn selectivity observed for the pseudoequatorial as compared to the pseudoaxial derivative. Moreover, also the experimental relative and absolute epoxidation rates for cyclohexene and cyclohexenol as well as for pseudoaxial and pseudoequatorial cyclohexenol derivatives are fairly well reproduced by computational data.

Epoxidation of acyclic chiral allylic alcohols with peroxy acids: spiro or planar butterfly transition structures? A computational DFT answer

The mechanism of the epoxidation of two chiral allylic alcohols, i.e., 3-methyl-3-buten-2-ol and (Z)-3-penten-2-ol, with peroxyformic acid has been investigated by locating 20 transition structures with the B3LYP/6-31G* method and by evaluating their electronic energy also at the B3LYP/6-311+G**@B3LYP/6-31G* theory level. Relative stability of TSs, as far as electronic energy is concerned, is basis set dependent; moreover, it also depends on entropy and solvent effects. Free enthalpies, calculated by using electronic energy at the higher theory level and with inclusion of solvent effects, indicates that syn, exo TSs, where the olefinic OH group hydrogen bonds the peroxy oxygens of the peroxy acid, outweigh syn, endo TSs, where the peroxy acid carbonyl oxygen is involved in hydrogen bonding. In the former TSs the peroxy acid moiety maintains its planar geometry while in the latter ones a strong out-of-plane distortion of peroxy acid is observed. This distortion makes it viable an unprecedented 1,2-H shift, as a possible alternative to the 1,4-H shift, for the peroxy acid hydrogen. In fact, for one syn, endo TS IRC analysis demonstrated that the 1,2-H shift mechanism is actually operative. The geometry of all TSs substantially conforms to a spiro (i.e., with the peroxy acid plane almost perpendicular to the C=C bond axis) butterfly orientation of the reactants while no TS resembles, even loosely, the planar butterfly structure. Theoretical threo/erythro epoxide ratios are in fair accord with experimental data. Calculations indicate that threo epoxides derive mostly from TSs in which the olefinic OH assumes an outside conformation while erythro epoxides originate from TSs with the OH group in an inside position. Computational findings do not support the qualitative TS models recently proposed for these reactions.

Influence of fasting on the effects of diazoxide in the ischemic-reperfused rat heart.

This investigation aimed to assess whether the mitochondrial ATP-sensitive potassium channel opener diazoxide could reproduce the protection conferred by ischemic preconditioning and to ascertain whether its effects are associated with changes in glycogen breakdown and glycolytic activity. Hearts of fed and 24-h fasted rats were perfused with 10 mM glucose containing medium and exposed to 25 min no-flow ischemia plus 30 min reperfusion. Diazoxide (10 microM) perfusion was begun 10 min before ischemia and continued throughout the experiment. Fasting accelerated reperfusion recovery of contraction, reduced the post-ischemic contracture and decreased lactate accumulation during ischemia but had no effects on glycogen levels and cellular viability. Diazoxide, did not affect glycogen catabolism but improved reperfusion recovery of contraction. Furthermore, diazoxide reduced ischemic lactate accumulation and contracture amplitude only in the fed group whereas it improved cell viability in the fed and fasted groups. These data indicate that: 1) reduced lactate production which may attenuate myocyte acidification might explain, at least in part, the beneficial effects of diazoxide on mechanical function, although data obtained with the fasted rat hearts indicate that other mechanisms must be involved as well; 2) the reduction of lactate production occurring in the fed group, does not seem to be related to glycogenolysis; and 3) since diazoxide improved cell viability in the fasted rat group where it did not reduce glycolytic activity, other mechanisms may be responsible for this cytoprotective effect.

[Arsenic-iron balneotherapy in anxiety syndromes. Controlled clinical study at the Levico thermal baths]. / La balneoterapia arsenicale-ferruginosa nelle sindromi ansiose. Studio clinico controllato alle Terme di Levico.

A comparative clinical study was conducted in order to assess the efficacy of arsenic-iron bath treatment in subjects suffering from endogenous reactive anxiety syndromes with somatisation. The study group of 76 subjects included 50 given the classic arsenic-iron bath treatment for two weeks and 26 untreated controls. Zung and Crown self-assessment Rating Scales amplified with items of socioepidemiological interest were completed by all subjects on entry, immediately after treatment and a mean 4 months later. Biometrical analysis of the data collected each time revealed a statistically significant (0.001 less than P less than 0.05) reduction in all parameters evaluated (anxiety, somatisation forms, consumption of drugs and number of medical consultations) among the treated Group. In contrast no significant difference was revealed among the untreated group. In order to verify the possible influence of climate the data on the control group, all local people, were then compared only with data on treated "locals". Here too there was a statistically significant reduction in anxiety and somatisation parameters only among treated subjects. It is therefore clear that the difference between the two groups is solely attributable to the arsenic-iron baths whose therapeutic efficacy is therefore confirmed.