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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,{3., 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,{3., Beijing 100084,china 1. key laboratory For Quantum Information and Measurements of MOE Department of Physics Tsinghua University 100084 Beijing China 2. Department of Materials science and Engineering Beijing Normal University 100875 Beijing China 3. Testing and Analytical Center Beijing Normal University 100875 Beijing China 4. Department of Physics Beijing Normal University 100875 Beijing China 5. key laboratory for atomic and molecular nanosciences Tsinghua University 100084 Beijing 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

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 {3. Nanoscience (Ministry of Education), tsinghua university, Beijing {5., china 1. Department of Physics and key laboratory of atomic and {3. Nanoscience (Ministry of Education) Tsinghua University Beijing 100084 China

Magnetism induced by the nonmagnetic dopants in the zinc-blende SiC (3.-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 {3., china 2. key laboratory for atomic and molecular nanosciences, tsinghua university, Beijing {3., china) [a]Department of Physics Tsinghua University Key Laboratory for Quantum Information and Measurements Beijing China [b]tsinghua university Key Laboratory for Atomic and Molecular NanoSciences Beijing 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 {3. 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 {3. 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

An N/4 fixed-point duality quantum search algorithm

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中国科学：物理学、力学、天文学英文版 2010年第9期53卷 1765-1768页

作者： HAO Liang1,LIU Dan2 & LONG GuiLu1,3.1key laboratory for atomic and {3. nanosciences and Department of Physics,{5. university,Beijing 100084,china 2School of Sciences,Dalian Nationalities university,Dalian 116600,china 3.5. National laboratory for Information Science and Technology,Beijing 100084,china 1. key laboratory for atomic and {3. nanosciences and Department of Physics Tsinghua University Beijing 100084 China2. School of Sciences Dalian Nationalities University Dalian 116600 China3. Tsinghua National Laboratory for Information Science and Technology Beijing 100084 China

Here a fixed-point duality quantum search algorithm is *** algorithm uses iteratively non-unitary operations and measurements to search an unsorted *** the marked item is found,the algorithm stops *** algorithm uses a constant non-unitary operator,and requires N/4 steps on average(N is the number of data from the database) to locate the marked *** implementation of this algorithm in a usual quantum computer is also demonstrated.

关键词： fixed-point search algorithm duality quantum computing the N4 duality search algorithm

Influence of magnetic current on the ground state entanglement in an isotropic transverse XY chain

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Progress in Natural Science:Materials International 2007年第10期17卷 1147-1151页

作者： Liu Dan1,2 , Zhang Yong3.and Long Guilu1,2(1. key laboratory for atomic and {3. nanosciences, and Department of Physics,{5., Beijing 100084, china 2. Center of Quantum Information Science and Technology, tsinghua National laboratory for information Science and Technology, Beijing 100084, china 3. Institute of Physics, School of Sciences, Beijing university of Posts and Telecommunications, Beijing 100876, china) key laboratory for atomic and {3. nanosciences and Department of Physics Tsinghua University Beijing 100084 China Center of Quantum Information Science and Technology Tsinghua National Laboratory for information Science and Technology Beijing 100084 China Institute of Physics School of Sciences Beijing University of Posts and Telecommunications Beijing 100876 China

We study the ground state entanglement in a three-qubit XY chain under the influence of a magnetization current. It is found that the ground state changes from a W state to a disentangled state as the magnetic field varies. When the Lagrange multiplier λ>3. namely under a strong magnetic current, the ground state holds onto a W state for a large range of the magnetic field strength. A phase transition to the magnetization-current phase occurs under a certain condition.

关键词： Heisenberg-XY chain magnetic-current phase transition.

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 {3. nanosciences and Department of Physics,{5., Beijing 100084, china 2tsinghua National laboratory for Information Science and Technology, Beijing 100084, china 1. key laboratory for atomic and {3. nanosciences and Department of Physics Tsinghua University Beijing 100084 China2. Tsinghua National Laboratory for Information Science and Technology Beijing 100084 China

A detailed analysis has showed that the quantum secret sharing protocol based on the Grover algorithm (Phys Rev A, 2003. 68: 0223.6) 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 {3., china 2. Center for {5. and molecular nanosciences, tsinghua university, Beijing {3., china Department of Physics Key Laboratory for Quantum Information and Measurements Tsinghua University Beijing 100084 China atomic and molecular nanosciences Tsinghua University Beijing 100084 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.

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 {3., china 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. china Beijing Radiation Center, Beijing 100875. china key laboratory for atomic and molecular Nano-Sciences, tsinghua university, Beijing {3., china) [a]key laboratory for Quantum Information and Measurements and Department of Physics Tsinghua University Beijing China [b]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 China [c]Beijing Radiation Center Beijing China [d]key laboratory for atomic and {3. Nano-Sciences Tsinghua University Beijing 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.