Fund Project:Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0305200, 2016YFA0301700) and the Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030312012).
Received Date:24 January 2019
Accepted Date:26 March 2019
Available Online:01 June 2019
Published Online:05 June 2019
Abstract:Quantum computation is a computing model based on quantum theory, which can outperform the classical computation in solving certain problems. With the increase of the complexity of quantum computing tasks, it becomes important to distribute quantum computing resources to multi-parties to cooperatively fulfill the complex tasks. Here in this paper a scheme based on the one-way quantum computing model is proposed to realize collaborative quantum computation. The standard one-way quantum computing model is based on graph states. With graph states used as resources, one can realize a universal quantum computer through using single-qubit measurements and feed-forward. In contrast to the standard one-way computation, the main resource for collaborative quantum computation is a redundant graph state (also a multi-particle highly entangled state). Unlike in the traditional graph state where each particle corresponds to a specific node, in a redundant graph state, several particles correspond to a single node, which means that each node of the graph has several redundant copies. With the help of a redundant graph state, several parties can share a graph state flexibly at will. A redundant graph state is prepared and then distributed to several parties where each of them obtains a full copy of all nodes. By communicating with each other and measuring the particles in different ways, a standard graph state is prepared and distributed among these parties. The collaborative computation then finishes through the common one-way quantum computing operations. Besides the general scheme, a concrete optical implementation of a two-party cooperative single-qubit quantum state preparation based on a six-photon redundant graph state is also put forward. Such a redundant graph state is proposed to be prepared by using the spontaneous parametric down-conversion entangled source and quantum interference. With this redundant graph state, a standard three-node graph state can be shared with the two parties in an arbitrary way. This scheme does not only make the collaborative quantum computation across several parties possible and flexible, but also guarantee the privacy of each party’s operations. This feature would be particularly useful in the case where the computing resource is obtained from an outside provider. This scheme paves the way for realizing quantum computation in more general and complicated applications. Keywords:quantum computation/ graph state/ quantum algorithm
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2.1.对图态进行局域泡利测量的重要性质
图态是一种特殊的纠缠态, 其中每个节点对应着一个量子比特, 连接两个节点的线段代表一个控制相位翻转门操作(controlled-phase gate). 若对图态的节点进行单比特泡利测量, 剩下的节点及连线所构成的状态经过经典的反馈操作后, 等价于只对原图态进行节点删除操作和局域互补操作[24]所产生的图态. 图态具有以下两个性质[25,26]: 第一, 如图1(a)所示, 对图态中的任何一个粒子进行σz测量, 当测量结果为+1时, 剩余的图态等价于从原图态中删除该粒子以及所有与它相连的线, 当测量结果是–1时, 则对剩下的图态进行一个σz操作, 便可获得与测量结果为+1时相同的图态; 第二, 如图1(b)所示, 对图态上相邻的两个粒子分别进行σx测量, 若测量结果均为+1, 则剩余的图态等价于从原图态中删除这两个粒子并将与这两个粒子相邻的粒子连接起来. 反之, 若测量结果中存在–1, 则需要对剩余的图态进行一个相应的局域幺正操作, 使之与测量结果均为+1时所产生的图态相同. 图 1 对图态进行局域泡利测量并进行相应的幺正变换后得到新图态 (a)对图态中的任何一个粒子进行σz测量; (b)对图态上相邻的两个粒子分别进行σx测量; (c)对粒子5a, 5b进行σx测量, 对n个粒子5c中的任意一个粒子5ci进行M测量, 其余的n – 1个粒子进行σz测量; (d)对粒子5做一个单比特测量M Figure1. Graph states after local measurements and the corresponding unitary operations: (a) σz measurement on any particle in the graph state; (b) two neighboring σx measurements on the graph state; (c) σx measurements on 5a, 5b, measurement M on 5ci and σz measurements on 5ck(k ≠ i); (d) measurement M on single-qubit 5.