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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Loss–DiVincenzo quantum computer ： ウィキペディア英語版 Loss–DiVincenzo quantum computer The Loss–DiVincenzo quantum computer (or spin-qubit quantum computer) is a scalable semiconductor-based quantum computer proposed by Daniel Loss and David P. DiVincenzo in 1997.〔D. Loss and D. P. DiVincenzo, "Quantum computation with quantum dots", ''Phys. Rev. A'' 57, p120 (1998); (on arXiv.org in Jan. 1997 )〕 The proposal was to use as qubits the intrinsic spin-1/2 degree of freedom of individual electrons confined to quantum dots. This was done in a way that fulfilled DiVincenzo Criteria for a scalable quantum computer,〔D. P. DiVincenzo, in Mesoscopic Electron Transport, Vol. 345 of NATO Advanced Study Institute, Series E: Applied Sciences, edited by L. Sohn, L. Kouwenhoven, and G. Schoen (Kluwer, Dordrecht, 1997); (on arXiv.org in Dec. 1996 )〕 namely: * identification of well-defined qubits; * reliable state preparation; * low decoherence; * accurate quantum gate operations and * strong quantum measurements. A candidate for such a quantum computer is a lateral quantum dot system. ==Implementation of the two-qubit gate== The Loss–DiVincenzo quantum computer operates, basically, using inter-dot gate voltage for implementing Swap (computer science) operations and local magnetic fields (or any other local spin manipulation) for implementing the Controlled NOT gate (CNOT gate). The Swap operation is achieved by applying a pulsed inter-dot gate voltage, so the exchange constant in the Heisenberg Hamiltonian becomes time-dependent: :$H\_s(t)\; =\; J(t)\backslash vec\_L\; \backslash cdot\; \backslash vec\_R\; .$ This description is only valid if: *the level spacing in the quantum-dot $\backslash Delta\; E$ is much greater than $\backslash ;\; kT$; *the pulse time scale $\backslash tau\_s$ is greater than $\backslash hbar\; /\; \backslash Delta\; E$, so there is no time for transitions to higher orbital levels to happen and *the decoherence time $\backslash Gamma\; ^$ is longer than $\backslash tau\_s$. From the pulsed Hamiltonian follows the time evolution operator :$U\_s(t)\; =\; \backslash mathbf\; \backslash exp\backslash .$ We can choose a specific duration of the pulse such that the integral in time over $J(t)$ gives $J\_0\; \backslash tau\_s\; =\; \backslash pi\; (\backslash text2\backslash pi),$ and $U\_s$ becomes the Swap operator $U\_s\; (J\_0\; \backslash tau\_s\; =\; \backslash pi)\; \backslash equiv\; U\_$. The XOR gate may be achieved by combining $\backslash sqrt\; =\; e^S\_L^z\}e^S\_R^z\}U\_^$ e^U_^. This operator gives a conditional phase for the state in the basis of $\backslash vec\_L\; +\; \backslash vec\_R$. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Loss–DiVincenzo quantum computer」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.