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Magnetism induced by nonmagnetic dopants in zinc-blende SiC: First-principle calculations

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Science china(Physics,Mechanics & Astronomy) 2010年第1期53卷 1-10页

作者： LIU ZhaoQing & NI Jun Department of Physics and key laboratory of atomic and molecular Nanoscience (Ministry of Education), tsinghua university, {5. 100084, china Department of Physics and key laboratory of atomic and molecular Nanoscience (Ministry of Education) Tsinghua University Beijing China

Magnetism induced by the nonmagnetic dopants in the zinc-blende SiC (3C-SiC) is investigated by first-principle calculations. The atoms of the first 20 elements in the periodic table except inert gas are used to replace either Si or C atoms as dopants. We find that some nonmagnetic substitutional dopants (mainly the Group IA, Group IIA, Group IIIB, and Group VIIB elements) prefer the spin-polarized ground states with local magnetic moments. In general, the condition for obtaining the local magnetic moments and the magnetic ground state requires that the dopants are p-type and have large electronegativity difference from the neighboring host atoms. The magnetic moments can be tuned over a range between 1 μ B and 3 μ B by doping with the nonmagnetic elements. The nearest-neighbor exchange couplings J 0 between the local magnetic moments are quite large and the codoping method is proposed to increase the dopant concentration. These imply that the nonmagnetic doping in SiC may exhibit collective magnetism. Moreover, the Group IIA Mg and Ca atoms substituting the preferred Si atoms favor the ferromagnetic ground states with the half-metallic electronic properties, which suggests that Mg or Ca substitutional doping on the Si sites in SiC could be a potential route to fabricating the diluted magnetic semiconductors.

关键词： nonmagnetic dopant magnetism SiC diluted magnetic semiconductor first-principle calculation

Measuring-basis-encrypted six-state quantum key distribution scheme

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Progress in Natural Science:Materials International 2006年第1期16卷 84-89页

作者： CHEN Pan1, DENG Fuguo1, WANG Pingxiao1 and LONG Guilu1,2 (key laboratory for Quantum Information and Measurements and Department of Physics, tsinghua university, beijing 100084, {5. 2. key laboratory for atomic and molecular nanosciences, tsinghua university, beijing 100084, {5.) key laboratory for Quantum Information and Measurements Department of Physics Tsinghua University Beijing 100084 China key laboratory for atomic and molecular nanosciences Tsinghua University Beijing 100084 China

A six-state quantum key distribution scheme with measuring-basis encryption technique, which is based on the six-state protocol and the MBE protocol, is proposed. In this modified six-state quantum key distribution protocol, Alice and Bob use a control key to synchronize the use of their measuring-basis so that they always use the same measuring-basis. This modified six-state quantum key distribution protocol retains the advantage of higher security and at the same time has a higher efficiency.

关键词： quantum key distribution measuring-basis encrypted six-state security.

Suppression of Phase and Amplitude Damping Decoherence in a Three-Level Atom in A-Configuration Using Bang-Bang Controls

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Communications in Theoretical Physics 2007年第7期48卷 71-74页

作者： WANG Yan-Hui LIU Xiao-Shu LONG Gui-Lu1 key laboratory for atomic and molecular nanosciences and Department of Physics,{5. university,beijing 100084.china2 {5. National laboratory for Information Science and Technology,{5. university,beijing 100084.china key laboratory for atomic and molecular nanosciences and Department of Physics Tsinghua University Beijing 100084 China tsinghua National laboratory for Information Science and Technology Tsinghua University Beijing 100084 China

In this paper,we present decoupling bang-bang operations for the suppression of general decoherence,bothamplitude and phase damping,in a three-level system in *** give a program to design a sequence ofperiodic twinbor... 详细信息

关键词： bang-bang operation general decoherence three-level system

Implementation of a multiple round quantum dense coding using nuclear magnetic resonance

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Science china(Physics,Mechanics & Astronomy) 2005年第6期48卷 706-715页

作者： ZHANG Jingfu1, XIE Jingyi2, WANG Chuan1, DENG Zhiwei3, LU Zhiheng4, & LONG Guilu1,5.1. key laboratory For Quantum Information and Measurements of MOE, Department of Physics, tsinghua university, beijing 100084.china 2. Department of Materials Science and Engineering, beijing Normal {5., beijing 100875. china 3. Testing and Analytical Center, beijing Normal {5., beijing 100875.china 4. Department of Physics, beijing Normal {5., beijing 100875. china 5. key laboratory for atomic and molecular nanosciences, tsinghua university, beijing 100084.china key laboratory For Quantum Information and Measurements Department of Physics Tsinghua University Beijing 100084 China Department of Materials Science and Engineering Beijing Normal University Beijing 100875 China Testing and Analytical Center Beijing Normal University Beijing 100875 China Department of Physics Beijing Normal University Beijing 100875 China key laboratory for atomic and molecular nanosciences Tsinghua University Beijing 100084 China

A multiple round quantum dense coding scheme based on the quantum phase estimation algorithm is proposed and implemented in a three qubit nuclear magnetic resonance (NMR) quantum computer. Using an m + 1 qubit system, Bob can transmit one of 2m+1 messages to Alice, through manipulating only one qubit and exchanging it between Alice and Bob for m rounds. The information capacity is enhanced to m + 1 bits as compared to m bits in a classical scheme. The scheme has been demonstrated in NMR system, and the experimental results show a good agreement between theory and experiment.

关键词： quantum dense coding, multiple round, nuclear magnetic resorance

Multiparty quantum secret sharing of classical and quantum messages

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Progress in Natural Science:Materials International 2007年第1期17卷 26-31页

作者： CHEN Pan, DENG Fuguo and LONG Guilu ( key laboratory for Quantum Information and Measurements, and Department of Physics, tsinghua university, beijing 100084, {5. key laboratory of Beam Technology and Material Modification, MOE, Institute of Low Energy Nuclear Physics, and Department of Material Science and Engineering, beijing Normal university, beijing 100875. {5. beijing Radiation Center, beijing 100875. {5. key laboratory for atomic and molecular Nano-Sciences, tsinghua university, beijing 100084, {5.) key laboratory for Quantum Information and Measurements Department of Physics Tsinghua University Beijing 100084 China key laboratory of Beam Technology and Material Modification Beijing Normal University Beijing 100875 China Institute of Low Energy Nuclear Physics Department of Material Science and Engineering Beijing Normal University Beijing 100875 China beijing Radiation Center Beijing 100875 China key laboratory for atomic and molecular Nano-Sciences Tsinghua University Beijing 100084 China

A scheme for multiparty quantum secret sharing of classical and quantum messages is proposed by using entanglement swapping. This scheme can distribute not only the classical information but also the quantum informati... 详细信息

关键词： quantum secret sharing classical message quantum message EPR pair.

Absence of State Collapse and Revival in a Superconducting Charge Qubit

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Communications in Theoretical Physics 2007年第12期48卷 1029-1032页

作者： HUO Wen-Yi~1 and LONG Gui-Lu~(1,2,3)~1 Department of Physics amd key laboratory for Quantum Information and Measurements,tsinghua university,beijing{5.~2 key laboratory for atomic and molecular nanosciences,tsinghua university,beijing {5.~3 National laboratory for Information Science and Technology,Hsinghua university,beijing {5. Department of Physics amd key laboratory for Quantum Information and Measurements Tsinghua University Beijing 100084 China key laboratory for atomic and molecular nanosciences Tsinghua University Beijing 100084 China National laboratory for Information Science and Technology Hsinghua University Beijing 100084 China

The Hamiltonian of a superconducting charge qubit with a configuration of dc SQUID contains an interac-tion between the LC oscillator part and charge *** interaction may leads to quantum state collapse and revivalwhich degrades the charge qubits and leads to serious *** analysis shows that the existing charge qubitparameters do not lead to this phenomenon,which is very good for the superconducting charge

关键词： quantum computer superconducting charge qubit absence and revival

Eavesdropping in a quantum secret sharing protocol based on Grover algorithm and its solution

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Science china(Physics,Mechanics & Astronomy) 2010年第3期53卷 491-495页

作者： HAO Liang1, LI JunLin1 & LONG GuiLu1,2 1key laboratory for atomic and molecular nanosciences and Department of Physics,{5., beijing 100084, china 2tsinghua National laboratory for Information Science and Technology, beijing 100084, china key laboratory for atomic and molecular nanosciences and Department of Physics Tsinghua University Beijing China tsinghua National laboratory for Information Science and Technology Beijing China

A detailed analysis has showed that the quantum secret sharing protocol based on the Grover algorithm (Phys Rev A, 2003, 68: 022306) is insecure. A dishonest receiver may obtain the full information without being detected. A quantum secret-sharing protocol is presents here, which mends the security loophole of the original secret-sharing protocol, and doubles the information capacity.

关键词： quantum secret sharing Grover algorithm quantum cryptography quantum search algorithm

Experimental realization of quantum cryptography communication in free space

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Science china(Physics,Mechanics & Astronomy) 2005年第2期48卷 237-246页

作者： WANG Chuan1, ZHANG Jingfu1, WANG Pingxiao1, DENG Fuguo1, AI Qing1 & LONG Guilu1,2 1. Department of Physics, key laboratory for Quantum Information and Measurements, tsinghua Univer- sity, beijing 100084, {5. 2. Center for atomic and molecular nanosciences, tsinghua university, beijing 100084, {5. Department of Physics Key Laboratory for Quantum Information and Measurements Tsinghua University Beijing China atomic and molecular nanosciences Tsinghua University Beijing China

Utilizing linear optical devices, the principle of B92 quantum key distribution (QKD) protocol is demonstrated in free space with a distance of transmission of 2.2 meters. The faint laser pulses with 65. nm wavelength are used as the single photon sources. The experimental results show that the eavesdropping behavior in the signal transmission can be detected. We also discuss the problems and solutions in using the quantum cryptography communication practically. It is pointed out that one of the approaches to increasing the distance of the quantum communication is to overcome the attenuation of the single photon in transmission. This could not be solved by the use of single photon source, and new quantum communication protocols are needed to solve these problems.

关键词： quantum key distribution, free space, linear optics, faint pulses.

Analysis micro-mechanism of burrs formation in the case of nanometric cutting process using numerical simulation method

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Science china(Technological Sciences) 2007年第2期50卷 129-137页

作者： HAN XueSong & HU YuanZhong State key laboratory of Tribology, tsinghua university, beijing 100084, {5. State key laboratory of Tribology Tsinghua University Beijing China

Burrs (exit failure), being one of the important factors influencing the final precision of workpiece, have been widely studied. Today, with the development of manufacturing technology, the depth of cut falls into the range of nanometer or subnanometer, there may be some different disciplines dominating the burrs generation process. molecular dynamics (MD) emthod, which is different from continuous mechanics, plays an important role in describing microscopic world. Take the example of single crystal copper, this paper carries out MD analysis micro-mechanism of burrs generation process. The results show that the burrs geometry depends on the type of workpiece (ductile or brittle). The depth of cut is decreased in the case of positive burrs generation process while the depth of cut is increased in case of negative burrs generation process. keywords burr - molecular dynamics - micro-mechanism Supported by china Postdoctor Science Foundation (Grant No. 2006039045.)

关键词： burr molecular dynamics micro-mechanism