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Progress in Natural Science:Materials International 2006年第6期16卷 594-599页

作者： HUO Wenyi and LONG Guilu (key laboratory for quantum information and measurements, and Department of Physics, Tsinghua university, Beijing 100084, China key laboratory for Atomic and Molecular NanoSciences, Tsinghua university, Beijing 100084, China) key laboratory for quantum information and measurements and Department of Physics Tsinghua University Beijing 100084 China key laboratory for Atomic and Molecular NanoSciences Tsinghua University Beijing 100084 China

We have studied the implementation of the Deutsch-Josza quantum algorithm in a superconducting charge-qubit quantum computer. Different from previous studies, we have used the inductance coupled system of You et al. The detailed pulse sequences have been designed for the four possible functions in a 2-qubit system. The result is generalized to an arbitrary n-qubit system. This scheme will be useful for practical implementation of the algorithm.

关键词： superconducting quantum computer Deutsch-Josza algorithm.

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,Beijing100084,China~2 key laboratory for Atomic and Molecular Nanosciences,Tsinghua university,Beijing 100084,China~3 national laboratory for information Science and Technology,Hsinghua university,Beijing 100084,China Department of Physics amd key laboratory for quantum information and measurements Tsinghua University key laboratory for Atomic and Molecular Nanosciences Tsinghua University national laboratory for information Science and Technology Hsinghua Univers

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

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, 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 100084, China) key laboratory for quantum information and measurements and Department of Physics Tsinghua University Beijing 100084 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 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.

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, China 2. key laboratory for Atomic and Molecular NanoSciences, Tsinghua university, Beijing 100084, China) key laboratory for quantum information and measurements and Department of Physics 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.

Dense coding scheme using superpositions of Bell-states and its NMR implementation

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Science China(Physics,Mechanics & Astronomy) 2005年第1期48卷 57-67页

作者： ZHANG Jingfu, XIE Jingyi, DENG Zhiwei & LU Zhiheng key laboratory for quantum information and measurements, Department of Physics, Tsinghua university, Beijing 100084, China Center for quantum information, 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 quantum information and measurements Department of Physics Tsinghua University Beijing 100084 China Center for quantum information Tsinghua University Beijing 100084 China Department of Materials Science and Engineering Beijing 100875 China Testing and Analytical Center Beijing Normal University Beijing 100875 China Department of Physics Beijing Normal University Beijing 100875 China

Dense coding using superpositions of Bell-states is proposed. The generalized Grover's algorithm is used to prepare the initial entangled states, and the reverse process of the quantum algorithm is used to determine the entangled state in the decoding measurement. Compared with the previous schemes, the superpositions of two Bell-states are exploited. Our scheme is demonstrated using a nuclear magnetic resonance (NMR)quantum computer. The corresponding manipulations are obtained. Experimental results show a good agreement between theory and experiment. We also generalize the scheme to transmit eight messages by introducing an additional two-state system.

关键词： dense coding, superpositions of Bell-states, generalized Grover's algorithm, nuclear magnetic res-onance, quantum computer.

A new simulated annealing method of designing NMR biplanar shim coils

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Progress in Natural Science:Materials International 2006年第7期16卷 747-752页

作者： QI Feng, TANG Xin, WANG Weimin, SHEN Yifei, LIU Shaopo and JIANG Zhongde ( key laboratory for quantum information and measurements of Ministry of Education, School of Electronics Engineering & Computer Science, peking university, Beijing 100871, China Institute of Heavy Ion Physics, and Beijing key laboratory of Medical Physics and Engineering, Physical School, peking university, Beijing 100871, China) peking Univ Sch Elect Engn & Comp Sci Minist Educ Key Lab Quantum Informat & Measurements Beijing 100871 Peoples R China peking Univ Sch Phys Beijing Key Lab Med Phys & Engn Beijing 100871 Peoples R China peking Univ Inst Heavy Ion Phys Beijing 100871 Peoples R China

A new SA (simulated annealing) method is presented for the design of longitudinal shim coils for NMR (nuclear magnetic resonance) equipment. First, AGL (adaptive Gauss-Lobatto) quadrature used by the traditional SA method is analyzed. Two kinds of complete elliptic integrals are introduced to integrate the spatial magnetic field, which increases the computing speed by 13. 3 times. Then, the system energy calculation is extended to the field of the z-axis. The results illustrate that the coil-set efficiency designed by the new SA method is 15 % higher than that of the traditional one, and the field inhomogeneity is reduced from 1.20 % to 0.49 % . Finally, a prototype of coil assembly for NMR rock-core analyzer is built, and the experimental results are in good agreement with the theoretically calculated results.

关键词： NMR shim coil design simulated-annealing method complete elliptic integral.

Entropy product measure for multipartite pure states

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Science China(Physics,Mechanics & Astronomy) 2006年第5期49卷 606-615页

作者： CAO Wancang1,2, LIU Dan2, PAN Feng3 & LONG Guilu2,4 1. Department of Physics, Chifeng College, Chifeng 024001, China 2. key laboratory for quantum information and measurements, Department of Physics, Tsinghua university, Beijing 100084, China 3. Department of Physics, Liaoning Normal university, Dalian 116029, China 4. key laboratory for Atomic and Molecular Nano-Sciences, Tsinghua university, Beijing 100084, China Correspondence should be addressed to Long Guilu Department of Physics Chifeng College Chifeng China key laboratory for quantum information and measurements Department of Physics Tsinghua University Beijing China Department of Physics Liaoning Normal University Dalian China key laboratory for Atomic and Molecular Nano-Sciences Tsinghua University Beijing China

An entanglement measure for multipartite pure states is formulated using the product of the von Neumann entropy of the reduced density matrices of the constituents. Based on this new measure, all possible ways of the maximal entanglement of the triqubit pure states are studied in detail and all types of the maximal entanglement have been compared with the result of ‘the average entropy’. The new measure can be used to calculate the degree of entanglement, and an improvement is given in the area near the zero entropy.

关键词： multipartite pure state von Neumann entropy entanglement measure

SGC Switching Between Subluminal to Superluminal Propagation in Ⅴ-Type Atom

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Communications in Theoretical Physics 2006年第10期 731-734页

作者： HAN Ding-An, GUO Hong,~1 BAI Yan-Feng,~1 SUN Hui,~1 and ZENG Ya-Guang~2 1 CREAM Group, key laboratory for quantum information and measurements of Ministry of Education, School of Electronics Engineering and Computer Science, peking university, Beijing 100871, China 2 physics Department, Foshan university, Foshan 528000, China CREAM Group，key laboratory for quantum information and measurements of Ministry of Education，School of Electronics Engineering and Computer Science，peking university，Beijing 100871，China Physics Department，Foshan university，Foshan 528000，China CREAM Group，key laboratory for quantum information and measurements of Ministry of Education，School of Electronics Engineering and Computer Science，peking university，Beijing 100871，China Physics Department，Foshan university，Foshan 528000，China

For a V-type three-level atomic system with two closely spaced upper levels, we investigate the light pulse propagation properties with only one laser field. Due to spontaneously generated coherence, the group velocity of the light pulse can be changed from subluminal to superluminal. The effects of the field intensity and the two-upper level splitting on the group velocity are also shown. At last, an analytical expression for the group velocity is given in the case of a weak field.

关键词： spontaneously generated coherence superluminal subluminal

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, China 2. Center for Atomic and Molecular NanoSciences, Tsinghua university, Beijing 100084, China 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 650 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.