In principle, quantum key distribution (QKD) can be used to make unconditionally secure private communication. However, the security of the existing real system for QKD needs to be carefully examined. Actually, the existing experiments based on weak coherent states are not secure under photon-number-splitting attack. Fortunately, the decoy-state method and the entanglement-distribution method can be used to realize the unconditionally secure QKD based on real-life systems with existing technology.

Feedback is a significant strategy for the control of quantum system. Information acquisition is the greatest difficulty in quantum feedback applications. After discussing several basic methods for information acquisition, we review three kinds of quantum feedback control strategies: quantum feedback control with measurement, coherent quantum feedback, and quantum feedback control based on cloning and recognition. The first feedback strategy can effectively acquire information, but it destroys the coherence in feedback loop. On the contrary, coherent quantum feedback does not destroy the coherence, but the capability of information acquisition is limited. However, the third feedback scheme gives a compromise between information acquisition and measurement disturbance.

This review has introduced a new near-field optical microscope (NOM) atomic force microscope combined with photon scanning tunneling microscope (AF/PSTM). During scanning, AF/PSTM could get two optical images of refractive index image and transmissivity image, and two AFM images of topography image and phase image. A reflected near-field optical microscope (AF/RSNOM ) has also been developed on AF/PSTM platform. The NOM has been reviewed in this paper and the comparison between AF/PSTM & RSNOM and the commercial A-SNOM & RNOM has also been discussed. The functions of AF/PSTM & RSNOM are much better than A-SNOM & RNOM.

The advanced experimental and theoretical techniques enable us to obtain information on the rearrangement of atoms or molecules in a reaction nowadays. As an example, we report on our research work on acetone isomerization and aggregation to give an insight into the reaction pathways, the products and their structures, and the growth regularity of aggregation. The evidences on the structural change of acetone and the stability of acetone clusters are found by a laser ionization mass spectrometer and the results are interpreted from theoretical analysis based on the DFT/B3LYP method. Various isomerization channels of acetone have been established and the optimal structures of the neutral clusters (CH₃COCH₃)_n and the protonated acetone clusters (CH₃COCH₃)_n H⁺ for n = 1-7 have been determined.

In this paper, we introduce the photo-induced ultrafast dynamics taking place in the peripheral light harvesting antenna LH2 from purple bacteria Rhodobacter sphaeroides by using absorption, fluorescence emission and ultrafast spectroscopic techniques. Three kinds of LH2 samples, pH treated LH2 (complete removal of B800 pigments), carotenoid mutated LH2 (GM 309) and electrochemical oxidation treated LH2 were used in comparison with native LH2 to investigate the mechanism of photo- induced ultrafast energy transfer within the LH2 complex.

The coherent superposition of atomic states leads to the characteristic change of interacting lights because of the coupling between the lights and atoms. In this paper, the noise spectrum of the quantified light interacting with the atoms is studied under the condition of electromagnetically induced transparency (EIT). It is shown that the noise spectrum displays a double M-shape noise profile resulted from the conversion of phase noise of probe beam. A squeezing of 0.3 dB can be observed at the detuning of probe light at the proper parameters of atoms and coupling beam.

A new type of HTc superconducting film comb-shape resonator for radio frequency superconducting quantum interference devices (RF SQUID) has been designed. This new type of superconducting film comb-shape resonator is formed by a foursquare microstrip line without a flux concentrator. The range of the center frequency of this type of resonator varies from 800 MHz to 1 300 MHz by changing the length of the teeth. In this paper, we report on simulating the relationship of the value of the center frequency and the length of the teeth, and testing the noise of HTc RF SQUID coupling this comb-shape resonator.

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm² ) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi -peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi- conductor transitions can be achieved by the doping of alkali atoms.

This paper summarizes our recent work on the study of quantum size effects (QSE) and novel physical properties of the Pb/Si (111) heterostructure. Two different types of samples were investigated. One is wedge-shaped Pb islands, and the other is atomically flat Pb thin films. With scanning tunneling microscopy (STM) manipulation, we observed an intriguing morphology dynamics of the islands that swings between two extreme energy states, like that in a classical pendulum. We show that the dynamics is a result of the competition between the QSE and the classical step free energy minimizing effect. For the second type of the samples, the QSE is studied in terms of thickness-dependent film stability, electronic structure and physical properties by using STM, angle-resolved photoemission spectroscopy (ARPES) and transport measurement. The results consistently reveal the formation of quantum well states (QWS) due to electron confinement in the films. This size effect could greatly modify the electronic structure near the Fermi level and lead to quantum oscillations in superconductivity, electron-phonon coupling and thermal expansion. The work unambiguously demonstrates the possibility of quantum engineering of physical properties of thin films by exploiting well-controlled and thickness-dependent QSE.

Quantum dots infrared photodetectors (QDIPs) theoretically have several advantages compared with quantum wells infrared photodetectors (QWIPs). In this paper, we discuss the theoretical advantages of QDIPs including the normal incidence response, lower dark current, higher responsivity and detectivity, etc. Recent device fabrication and experiment results in this field are also presented. Based on the analysis of existing problems, some approaches that would improve the capability of the device are pointed out.

The B3LYP hybrid density functional method, which is very successful in the study of thermochemistry of atoms and molecules, has been applied to some periodic systems recently. The applications to solids and surfaces show that the B3LYP hybrid functional reproduces the experimental energy gaps and magnetic moments for a variety of materials.

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T_c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T_c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T_c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T_c superconductors, and then review the results of Nernst effect for different high-T_c superconductors. Related theoretical models are also discussed.

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N "e312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting / freezing circle is derived in noble metal clusters.

In order to demonstrate the existence of the vortex pancake in high temperature superconductor experimentally, a configuration in which the current and voltage electrodes lies separately on the top and bottom surface is used. The E-j relation obtained with this electrodes spatial configuration is different from the expected E-j behavior of the stiff vortex line model. Thus, the current results support the existence of the vortex pancake in high temperature superconductor.

Earlier theoretical approaches to manganites mainly stem from magnetic framework in which the electronic transports are thought to be spin-dependent and the double exchange plays a vital role. However, quite a number of experimental observations cannot be explained in the magnetic framework, yet. For example, multiplicate insulator-metal transitions and resistivity reduction induced by perturbations other than magnetic field, such as electric current, are not well understood in this framework. Here we present a comprehensive analysis on the magnetic framework and give a Monte Carlo study on the resistivity of a charge ordered/disordered model without accounting for the spin degree of freedom. The result shows a colossal resistivity change as a resultant of the transition between two types of insulated states. This transition has intrinsic difference from the popular insulated-to-metallic transition in the magnetic framework. The present scenario can be used to explain some experimental facts for electronic transports in manganites, which are not accessible in the magnetic framework.

The oxygen adsorption-desorption properties of RBa₂Cu₃O_7-δ R = Gd, Er, Eu, Dy, Sm, Ho and Nd?nd Y_1-χLa_χBa₂Cu₃O_7-δ (χ = 0.1, 0.5 and 1.0) were investigated from room temperature to 950 oC by thermogravimetry (TG). The results show that all samples will release oxygen with the increasing of temperature and the released oxygen can be absorbed back into the sample when temperature decreases. However, dependent on the rare earth element, the amount of the released oxygen is different for these samples. Moreover, in the temperature increasing and decreasing circle the repetition of oxygen adsorption-desorption is also different.

The thermodynamic interaction at thermodynamic equilibrium in the free fermion gas is described in an alternative way by the coupling of particles with a scalar thermodynamic field that features self-interaction. This alternative coupling is similar to the Higgs coupling and is helpful in understanding the temperature transformation at thermodynamic equilibrium under the Lorentz boost. As this coupling is applied in the abelian interaction fermion gas, nothing nontrivial is obtained. However, an interesting thing happens in the nonabelian interaction fermion gas where the difference appears for the diagonal and off-diagonal intermediate bosons as the Higgs-like coupling is added.

We study the basic behaviors of buses using bus route models (BRM) by introducing a special kind of noise induced by impacts of other vehicles. The peak where the maximum velocity of buses exists shrinks in low noise conditions, which is worth the further study. Furthermore, we extend the model to take into consideration more realistic and important parameters, such as the number of passengers, the capacity of buses, and the possibility of overtaking, for performing simulations of the first loop. Suggestions on the choice of the number of buses and the maximum velocity are provided for the practical operation.