Stability of the positively charged manganese centre in GaAs heterostructures examined theoretically by the effective mass approximation calculation near the critical pointWang, Li-Guo; Shen, Chao; Zheng, Hou-Zhi; Zhu, Hui; Zhao, Jian-Hua
doi: 10.1088/1674-1056/20/10/100301pmid: N/A
This paper describes an nipin model heterostructure with a manganese (Mn)-doped p-type base region to check the stability of a positively charged manganese A+Mn centre with two holes weakly bound by a negatively charged 3d5(Mn) core of a local spin S = 5/2 in the framework of the effective mass approximation near the critical point (k 0). By including the carrier screening effect, the ground state energy and the binding energy of the second hole in the positively charged centre A+Mn are calculated within a hole concentration range from 1 × 1016 cm3 to 1 × 1017 cm3, which is achievable by biasing the structure under photo-excitation. For comparison, the ground-state energy of a single hole in the neutral A0Mn centre is calculated in the same concentration range. It turns out that the binding energy of the second hole in the A+Mn centre varies from 9.27 meV to 4.57 meV. We propose that the presence of the A+Mn centre can be examined by measuring the photoluminescence from recombination of electrons in the conduction band with the bound holes in the A+Mn centre since a high frequency dielectric constant of ∞ = 10.66 can be safely adopted in this case. The novel feature of the ability to tune the impurity level of the A+Mn centre makes it attractive for optically and electrically manipulating local magnetic spins in semiconductors.
Quantum and classical correlations for a two-qubit X structure density matrixDing, Bang-Fu; Wang, Xiao-Yun; Zhao, He-Ping
doi: 10.1088/1674-1056/20/10/100302pmid: N/A
We derive explicit expressions for quantum discord and classical correlation for an X structure density matrix. Based on the characteristics of the expressions, the quantum discord and the classical correlation are easily obtained and compared under different initial conditions using a novel analytical method. We explain the relationships among quantum discord, classical correlation, and entanglement, and further find that the quantum discord is not always larger than the entanglement measured by concurrence in a general two-qubit X state. The new method, which is different from previous approaches, has certain guiding significance for analysing quantum discord and classical correlation of a two-qubit X state, such as a mixed state.
Preparation of cluster states with trapped electrons on a liquid helium surfaceAi, Ling-Yan; Shi, Yan-Li; Zhang, Zhi-Ming
doi: 10.1088/1674-1056/20/10/100303pmid: N/A
We present a scheme for the preparation of one-dimensional (1D) and two-dimensional (2D) cluster states with electrons trapped on a liquid helium surface and driven by a classical laser beam. The two lowest levels of the vertical motion of the electron act as a two-level system, and the quantized vibration of the electron along one of the parallel directions (the x direction) serves as the bosonic mode. The degrees of freedom of the vertical and parallel motions of the trapped electron can be coupled together by a classical laser field. With the proper frequency of the laser field, the cluster states can be realized.
General description of discriminating quantum operationsZhang, Ke-Jia; Zhu, Ping; Gao, Fei; Guo, Fen-Zhuo; Qin, Su-Juan; Wen, Qiao-Yan
doi: 10.1088/1674-1056/20/10/100304pmid: N/A
The discrimination of quantum operations plays a key role in quantum information and computation. Unlike discriminating quantum states, it has some special properties which can be carried out in practice. In this paper, we provide a general description of discriminating quantum operations. Concretely speaking, we describe the distinguishability between quantum operations using a measure called operator fidelity. It is shown that, employing the theory of operator fidelity, we can not only verify some previous results to discriminate unitary operations, but also exhibit a more general discrimination condition. We further apply our results to analysing the security of some quantum cryptographic protocols and discuss the realization of our method using well-developed quantum algorithms.
A practical two-way system of quantum key distribution with untrusted sourceChen, Ming-Juan; Liu, Xiang
doi: 10.1088/1674-1056/20/10/100305pmid: N/A
The most severe problem of a two-way “plug-and-play" (p& p) quantum key distribution system is that the source can be controlled by the eavesdropper. This kind of source is defined as an “untrusted source". This paper discusses the effects of the fluctuation of internal transmittance on the final key generation rate and the transmission distance. The security of the standard BB84 protocol, one-decoy state protocol, and weak+vacuum decoy state protocol, with untrusted sources and the fluctuation of internal transmittance are studied. It is shown that the one-decoy state is sensitive to the statistical fluctuation but weak+vacuum decoy state is only slightly affected by the fluctuation. It is also shown that both the maximum secure transmission distance and final key generation rate are reduced when Alice's laboratory transmittance fluctuation is considered.
Lower bound for the security of differential phase shift quantum key distribution against a one-pulse-attackLi, Hong-Wei; Yin, Zhen-Qiang; Wang, Shuang; Bao, Wan-Su; Guo, Guang-Can; Han, Zheng-Fu
doi: 10.1088/1674-1056/20/10/100306pmid: N/A
Quantum key distribution is the art of sharing secret keys between two distant parties, and has attracted a lot of attention due to its unconditional security. Compared with other quantum key distribution protocols, the differential phase shift quantum key distribution protocol has higher efficiency and simpler apparatus. Unfortunately, the unconditional security of differential phase shift quantum key distribution has not been proved. Utilizing the sharp continuity of the von Neuman entropy and some basic inequalities, we estimate the upper bound for the eavesdropper Eve's information. We then prove the lower bound for the security of the differential phase shift quantum key distribution protocol against a one-pulse attack with DevatakWinter's secret key rate formula.
Using a quantum dot system to realize perfect state transferLi, Ji; Wu, Shi-Hai; Zhang, Wen-Wen; Xi, Xiao-Qiang
doi: 10.1088/1674-1056/20/10/100308pmid: N/A
There are some disadvantages to Nikolopoulos et al.'s protocol [Nikolopoulos G M, Petrosyan D and Lambropoulos P 2004 Europhys. Lett. 65 297] where a quantum dot system is used to realize quantum communication. To overcome these disadvantages, we propose a protocol that uses a quantum dot array to construct a four-qubit spin chain to realize perfect quantum state transfer (PQST). First, we calculate the interaction relation for PQST in the spin chain. Second, we review the interaction between the quantum dots in the HeitlerLondon approach. Third, we present a detailed program for designing the proper parameters of a quantum dot array to realize PQST.