诱惑态量子密钥分配系统中统计涨落的研究

针对实用的量子密钥分配(QKD)系统是基于强衰减的弱激光脉冲作为单光子源, 光子数分束攻击极大限制了通信双方在非理想条件下QKD的传输距离和密钥生成率,采用大数定律对诱惑态协议中单光子的计数率、单光子增益和误码率分别进行统计涨落分析, 利用双诱惑态比较了1310 nm和1550 nm条件下,编码脉冲的长度为(N = 10⁶-N = 10¹²)实际QKD协议中密钥的生成率与安全传输距离之间的关系、安全传输距离随编码长度的变化的关系, 得出脉冲编码长度增大到N = 10¹²时,密钥的最大安全传输距离为135 km.     

Controlled teleportation of a multi-photon GHZ polarization-entangled state using linear optical elements

We propose a linear optical protocol for teleporting a multi(W)-photon Greenberger-Horne-Zeilinger (GHZ) polarization-entangled state from a sender to a receiver under control of a number of controllers in terms of optical elements. We show that the protocol can be successfully realized with a certain probability. This protocol has the advantage of transmitting much less particles and classical information for teleporting the W-photon GHZ polarization-state than others, and the protocol is based on linear optical elements and postselection strategy, which are feasible with existing experimental technology.     

Quantum key distribution in terms of the Greenberger-Horne-Zeilinger state: Multi-key generation

In this paper, we develop, a quantum key distribution protocol based on the Greenberger-Horne-Zeilinger states (GHZs). The particles are exchanged among the users in blocks through two steps. In this protocol, for three-particle GHZs three keys can be simultaneously generated. The advantage of this is that the users can select the most suitable key for communication. The protocol can be generalized to N users to provide N keys. The protocol has two levels for checking the eavesdroppers. Moreover, we discuss the security of the protocol against different attacks.     

Electron-positron scaling in bound state models

We investigate scaling assuming a generalized vector meson dominance picture. The vector mesons are described as relativistic quark-antiquark bound states by a Bethe-Salpeter equation which yields the mass spectrum and the coupling to e⁺e^? pairs. We discuss the spin structure and find that scaling can occur only for a γ_μ type amplitude. We solve the BS equation using a generalized WKB approximation and find scaling, independent of the detailed shape of the interaction. This means that scaling in e⁺e^? annihilation does not select a particular “confinement potential”. The scaling constant depends on the current renormalization constant and on the details of the relativistic spin structure.     

Hot QCD equations of state and RHIC

We show how hot QCD equations of states can be adapted to make definite predictions for quarkgluon plasma at RHIC.We consider equations of state up to O(g⁵) and O[g⁶(ln(1/g)+δ)]. Our method involves the extraction of equilibrium distribution functions for gluons and quarks from these equations of state by capturing the interaction effects entirely in the effective chemical potentials. We further utilize these distribution functions to study the screening length in hot QCD and dissociation phenomenon of heavy quarkonia states by combining this understanding with the semi-classical transport theory.     

Temperature dependence of bound state spectra in field theory

We analyze the behaviour of kinks and semiclassical bound states at finite temperatures by applying quantum statistics to the fluctuations which determine the quantum dynamics of these states. We consider two theories in one space dimension — the ?⁴ theory with a dynamical symmetry breaking and the Gross-Neveu model. For the ?⁴ theory, the one-loop temperature corrections are obtained by using temperature-dependent Green function techniques. We show that the same result can be obtained by applying quantum statistics to the fluctuations around the kink. For the Gross-Neveu model, the temperature dependence of the bound states, which correspond to time-independent field configurations, is obtained. We show that for every bound state there exists a critical temperature at which this state breaks up into its constituents. This critical temperature increases with the number of constituents of the bound state.     

Quantum computation and entangled state generation through a cavity output process

We propose a protocol to realize quantum phase gates and generate entangled states between two atoms trapped in one cavity. In Lamb-Dick limits, it is not necessary to require coincidence detections, which will relax the conditions for the experimental realization. The protocol can be generalized to generate N-atom entangled states.     

A model for initial state parton showers

We present a detailed model for exclusive properties of initial state parton showers. A numerically efficient algorithm is obtained by tracing the parton showers backwards, i.e. start with the hard scattering partons and then successively reconstruct preceding branchings in falling sequence of spacelike virtualities Q² and rising sequence of parton energies. We show how the Altarelli-Parisi equations can be recast in a form suitable for this, and also discuss the kinematics of the branchings. The complete model is implemented in a Monte Carlo program, and some first results are presented.     

Properties of the finite-mass helium ground state

ABSTRACT

We describe a simple method of incorporating the finite mass of the nucleus directly into atomic variational Monte Carlo calculations. To test this algorithm we computed the energy and 20 other properties of ⁴He. We then compared these values with those obtained from our earlier infinite nuclear mass algorithm. All of our expectation values are in excellent agreement with previous results on this system.     

Learning about the nuclear equation of state using flow

We present results for transverse flow, the balance energy, and elliptic flow for Ar+Sc, ⁵⁸Fe+⁵⁸Fe, ⁵⁸Ni+⁵⁸Ni, Kr+Nb, and Au+Au. We find that a soft equation of state is required to explain these results. We find that a momentum dependent formulation of the nuclear mean field is required. A 30% density dependent reduction of the in-medium nucleon-nucleon cross sections in BUU is required to reproduce the observed balance energies for light systems.     

Precise measurement of hyperfine structure in the 52P1/2 state of Rb

We have measured hyperfine structure in the 5 ²P_1/2 state of Rb using a frequency-stabilized diode laser, which is locked to one hyperfine transition, and an acousto-optic modulator, whose frequency is locked to the interval of interest. We check for optical-pumping errors by repeating the measurement at different values of pump power in the saturated-absorption spectrometer. We obtain precise values of the hyperfine constant: A=120.645(5) MHz for ⁸⁵Rb and A=406.119(7) MHz for ⁸⁷Rb. The values resolve a large discrepancy between two earlier high-accuracy measurements on this state.     

Study of non-radiative silicon deexcitation in an oxygen atmosphere using the differential measurement of the excited state lifetimes

Having developed a new method of differential lifetime measurement using atomic ionoluminescence as an excitation process, we applied this method to a monocrystaline silicon sample in an oxygen atmosphere of variable pressure. We found decreased values of the experimental lifetimes concerning levels 4s ³ P ⁰ and 4s ¹ P ⁰, which are more marked when the oxygen pressure increases. This tends then towards a saturation of the observed phenomenon. We propose, in agreement with other authors, an explanation which is based on the existence of non-radiative deexcitations. We present also a mathematical model for calculating this transition effect on the lifetime measurements. We consider that it is possible to take advantage of this experimental lifetime variation to determine a parameter of the model which characterises the non-radiative deexcitations. This measuring method appears to be a simple and original procedure for the study of certain nonradiative transitions.     

Generation of remote multi-photon entangled state from Einstein-Podolsky-Rosen photon pairs

We describe a protocol for generation of remote multi-photon entanglement using Einstein-Podolsky-Rosen (EPR) photon pairs via entanglement swapping. According to the requests of users, Quantum Switch (QS) can prepare three-photon W entangled states or Greenberger-Horne-Zeilinger (GHZ) states based on independent, spatially separated EPR pairs among three distant users. Only Bell states measurement (BSM) is needed utilizing a CPHASE gate and PAs. This protocol can also generate remote N-photon GHZ entangled states.     

Optimal Remote Preparation of a Four-Qubit Entangled Cluster-Type State Via Two Non-Maximally Entangled GHZ-Type States

We devise an highly efficient protocol for remotely preparing a four-qubit entangled cluster-type state. In this protocol, two non-maximally entangled GHZ-type states are employed to link the sender Alice and the receiver Bob, and the to-be-prepared state can be reconstructed successfully with the probability of (b₁b₂)² in general case. Then to achieve our concerns of constructing efficient remote preparation with higher success probability, some special ensembles of four-qubit states are minutely investigated. As a result, it is shown that the total probability of the RSP protocol, in these particular cases, can be improved to twice or even fourfold as that in general case.     

Spin-dependent recombination of the charge-transfer state in photovoltaic polymer/fullerene blends

ABSTRACT

Q-band electron spin echo (ESE) spectroscopy was applied for studying the spin-dependent recombination of charge transfer (CT) states in the benchmark organic photovoltaics (OPV) blend of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT/PC₆₀BM). Selective microwave excitation and a special protocol for ESE data treatment allowed to suppress the ESE signal of thermalised polarons and weakly coupled CT states and to address CT states with a relatively short distance between positive and negative polarons (1.5 nm < r?<?2.5 nm). Inversion of the in-phase ESE signal with increase of the delay after laser flash was observed for the regioregular P3HT^?+/PC₆₀BM^?? CT state at a temperature of 40 K. This effect is very similar to the inversion of the time resolved (TR) EPR spectrum of the same system obtained previously. Both effects can be explained by spin-dependent recombination of the CT state, with the recombination via the triplet channel proceeding much slower than via the singlet channel. For the regiorandom P3HT^?+/PC₆₀BM^?? CT state no ESE sign inversion was observed in an analogous experiment. The result suggests the importance of CT state formation via a triplet exciton, a process which was not considered previously for the P3HT/PC₆₀BM blend.     

Recent progress in simulation of the ground state of many Boson systems

We present two recent advances in the simulation of ⁴He in the condensed phase at zero temperature. Within the variational theory of strongly interacting bosons we have studied a cluster of ⁴He atoms with one alkali ion K⁺. For the wave function we have used a new shadow wave function (SWF) in which the coupling between one ⁴He atom and its shadow variable depends on its distance from the ion. This substantially improves the energy. The first shell around the ion contains 14 atoms which are spatially ordered. However the atoms of the first shell are not completely localized and frequent exchanges with atoms which are further from the ion take place. This also suggests that at least for the ion K⁺ the atoms of the first shell participate in the superfluidity. We have also introduced a new extension of the path integral ground state (PIGS) method which is able to compute exact ground state expectation values without extrapolations and with a SWF as the trial variational wave function to project on the ground state. This is an important extension which opens up the possibility of studying disorder phenomena in the solid phase by an exact method at zero temperature. We have applied this technique to compute the energy of formation of a vacancy at different densities in the solid phase of ⁴He. This computation confirms the variational result that a vacancy is a delocalized defect in the low density helium solid.     

Resonant and intrastate transitions to the 2+ state in8Be

We have studied resonant, intrastate and non-resonant bremsstrahlungγ-ray transitions leading to the 2⁺ state in⁸Be atE _x=2.94 MeV. We find aγ-width of Γ _γ =0.45 eV for the 4⁺ state in⁸Be, corresponding to anE 2 strength of 19 W.u. For the intrastateγ-ray transition within the 2⁺ resonance we predict a maximum cross section of 2.5 nb atE≈3.3 MeV. An experimental observation of this novel type ofγ-ray emission, however, appears to be difficult due to the dominance of competing resonant and direct capture processes.     

Precise measurement of hyperfine structure in the state of Rb

We demonstrate a technique to measure hyperfine structure using a frequency-stabilized diode laser and an acousto-optic modulator locked to the frequency difference between two hyperfine peaks. We use this technique to measure hyperfine intervals in the 5 P _3/2 state of ⁸⁵Rb and obtain a precision of 20 kHz. We extract values for the magnetic-dipole coupling constant A = 25.038(5) MHz and the electric-quadrupole coupling constant B = 26.011(22) MHz. These values are a significant improvement over previous results. Received 6 March 2003 Published online 15 April 2003     

Uniqueness of the translationally invariant ground state in quantum spin systems

We introduce a class of quantum spin systems on ^d. We show that the translationally invariant ground state is unique for this system if it is in a strong external field.Supported by the Science and Engineering Research Council     

Description of low-lying state structures with Skyrme interaction

We use the quasiparticle random phase approximation to study properties of the low-lying 2⁺ states in the even-even nuclei around ¹³²Sn. Starting from a Skyrme interaction in the particle-hole channel and a density-dependent zero-range interaction in the particle-particle channel, the calculation within the finite-rank separable approximation for the residual interaction is performed.