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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 Amplitude damping channel ： ウィキペディア英語版 Amplitude damping channel In the theory of quantum communication, an amplitude damping channel is a quantum channel that models physical processes such as spontaneous emission. A natural process by which this channel can occur is a spin chain through which a number of spin states, coupled by a time independent Hamiltonian, can be used to send a quantum state from one location to another. The resulting quantum channel ends up being identical to an amplitude damping channel, for which the quantum capacity, the classical capacity and the entanglement assisted classical capacity of the quantum channel can be evaluated. ==Model for a Spin Chain Quantum Channel== The main construct of the quantum channel based on spin chain correlations is to have a collection of N coupled spins. At either side of the quantum channel, there are two groups of spins and we refer to these as quantum registers, A and B. A message is sent by having the sender of the message encode some information on register A, and then, after letting it propagate over some time t, having the receiver later retrieve it from B. The state $\backslash rho\_$ is prepared on A by first decoupling the spins on A from those on the remainder of the chain. After preparation, $\backslash rho\_$ is allowed to interact with the state on the remainder of the chain, which initially has the state $\backslash sigma\_$. The state of the spin chain as time progresses can be described by $R(t)\; =\; U(t)(\backslash rho\_\; \backslash otimes\; \backslash sigma\_)U^(t)$. From this relationship we can obtain the state of the spins belonging to register B by tracing away all other states of the chain. $\backslash rho\_B(t)=\; \backslash mbox^\; (U(t)\; (\backslash rho\_A\; \backslash otimes\; \backslash sigma\_0)\; U^(t))$ This gives the mapping below, which describes how the state on A is transformed as a function of time as it is transmitted over the quantum channel to B. U(t) is just some unitary matrix which describes the evolution of the system as a function of time. $\backslash rho\_A\; \backslash rightarrow\; \backslash mathcal(\backslash rho\_A\; )\; \backslash equiv\; \backslash rho\_B(t)=$ \mbox^ (U(t) (\rho_A \otimes \sigma_0) U^(t) ) There are, however, a few issues with this description of the quantum channel. One of the assumptions involved with using such a channel is that we expect that the states of the chain are not disturbed. While it may be possible for a state to be encoded on A without disturbing the chain, a reading of the state from B will influence the states of the rest of the spin chain. Thus, any repeated manipulation of the registers A and B will have an unknown impact on the quantum channel. Given this fact, solving the capacities of this mapping would not be generally useful, since it will only apply when several copies of the chain are operating in parallel. In order to calculate meaningful values for these capacities, the simple model below allows for the capacities to be solved exactly. 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「Amplitude damping channel」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.