1.Department of Physics, Taiyuan Normal University, Jinzhong 030619, China 2.State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China 3.Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11804246, 11805141, 11904218, 12004276, 11847111, 61775127, 11654002), the National Key R&D Program of China (Grant No. 2016YFA0301402), the Natural Science Foundation of Shanxi Province, China (Grant No. 201901D111293), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi, China (Grant Nos. 2019L0794, 2020L0516), the Program for Sanjin Scholars of Shanxi Province, the Fund for Shanxi “1331Project” Key Subjects Construction, China, the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, China and the “1331Program” of Taiyuan Normal University
Received Date:21 October 2020
Accepted Date:04 December 2020
Available Online:08 May 2021
Published Online:20 May 2021
Abstract:Quantum teleportation is one of the most basic quantum protocols, which transfers an unknown quantum state from one location to another through local operation and classical communication by using shared quantum entanglement without physical transfer of the information carrier. And it has been widely used in various quantum information protocols such as entanglement swapping, quantum repeaters, quantum gate teleportation, quantum computation based on measurement, and quantum teleportation networks, which have important application value in quantum computation and quantum information. Quantum teleportation is a naturally bipartite process, in which an unknown quantum state can only be transmitted from one node to another. With the further development of quantum information research, it is necessary to transfer quantum states or quantum information among more and more nodes. Multipartite quantum protocols are expected to form fundamental components for larger-scale quantum communication and computation. A bipartite quantum teleportation should be extended to a multipartite protocol known as a quantum teleportation network. In this paper, a multifunctional quantum teleportation network is proposed theoretically. We first propose a special method of constructing four-partite quantum resources in continuous variables (CVs), and based on this, construct two different types of CV quantum teleportation networks. One type of network contains just one quantum teleportation process consisting of a sender, a receiver and two controllers. In this type of network, the unknown quantum state can be recovered at any other node according to the requirement after the measurement in the input node, which enriches the transfer direction and transfer mode of the unknown quantum state. And meanwhile, the two controllers can control the transfer of a quantum state from the sender to the receiver by restricting the sender and receiver’s access to their information, which makes the quantum teleportation network controllable. The other type of network has two quantum teleportation processes, each containing only a sender, a receiver and no controllers, which increases the number of quantum states that can be transmitted. Then we analyze the dependence of the fidelity of each quantum teleportation network on different physical parameters, and compare the characteristics, advantages and disadvantages among different types of quantum teleportation networks. The scheme for constructing a multifunctional quantum teleportation network in this paper shows some advantages, such as the greater number of quantum nodes, diversity of types, simple operation procedure. And all these advantages provide a broader application prospect for establishing larger and more complex quantum information networks in the future and quicken the pace of the application of quantum information. Keywords:quantum teleportation network/ controllability/ multifunction/ fidelity
式(9)和式(10)表明, 输出4个子模正交振幅分量以及正交位相分量之间的量子关联噪声均小于相应的散粒噪声基准4, 他们之间的量子关联特性可以用于构建四用户的连续变量量子远程传态网络. 如图2所示为将一个未知量子态传送至Claire处的4组份量子远程传态网络的结构示意图. 该方案中以传送相干态为例来进行说明, 除了相干态以外, 压缩态、纠缠态等非经典态都可以作为传送的未知量子态. 图 2 将一个未知量子态传送至Claire处的四组份量子远程传态网络的结构示意图, 其中AM为振幅调制器, PM为位相调制器, BS为分束器, HD为平衡零拍探测器 Figure2. Schematic diagram of four-partite quantum teleportation network teleporting an unknown quantum state to Claire, where AM is Amplitude modulator, PM is Phase modulator, BS is Beam splitter, HD is Homodyne detector.
将式(1)—(8)、式(13)和(14)结合起来, 代入保真度的表达式, 可以理论计算出保真度的大小随增益因子的变化曲线, 如图3所示. 曲线1, 2, 3分别对应压缩度为0.5, 0.8和1.5的情况, 对于同一个增益因子, 显然曲线3对应的保真度大于曲线2对应的保真度, 曲线2对应的保真度又大于曲线1对应的保真度, 都大于经典边界值0.5, 成功实现了相干态的传送, 说明可以通过提高压缩参数来提高保真度的大小. 单条曲线来看, 保真度随着增益因子的增大而减小, 在g为0时, 保真度最大, 这里将使保真度最大的增益因子的取值称之为最佳增益因子. 也就是说在图2所示的量子远程传态方案中, 控制方的最佳增益因子为0, 正好对应仅有发送者和接收者参与的两用户量子远程传态的情况, 图3的曲线表明, 两用户的量子远程传态保真度大于四用户的量子远程传态保真度的最佳值. 但如果为了构建包含4个用户的较为复杂的量子远程传态网络系统, g因子不能取值为0, 较小的g因子可以得到较高的保真度. 图 3 四用户量子远程传态保真度随增益因子g的变化曲线, 曲线1—3分别对应压缩参数为0.5, 0.8和1.5时的保真度大小 Figure3. Dependences of the fidelity of quantum teleportation with four parties on gain factor g, the traces 1, 2 and 3 are the calculated fidelity when squeezing factor r is selected as 0.5, 0.8 and 1.5, respectively.
其中$\hat x_b^0$, $\hat p_b^0$分别表示平衡外差探测系统中真空光场的振幅分量和位相分量算符, 将式(1)—(8)、式(15)和(16)结合起来, 代入保真度的公式, 理论计算了当压缩参数r = 1.5时, 只有一个控制方以及两个控制方同时参与的量子远程传态网络的保真度随g的变化曲线对比图, 如图4所示. 曲线1对应两个控制方同时参与的情况, 曲线2对应只有一个控制方参与的情况, 显然, 当增益因子以及压缩参数取值相同时, 曲线2所示的保真度一直小于曲线1所示的保真度, 这是因为相较于四用户之间的量子关联, 三用户之间的量子关联较弱, 且平衡外差探测过程中真空光场噪声的引入也减小了远程传态的保真度. 曲线2与远程传态的经典极限值0.5相交于临界点, 当增益因子g大于该临界值时, 保真度小于0.5, 量子远程传态过程失败. 两条曲线的对比说明, 控制方可以控制自己是否参与量子远程传态的过程来影响该过程的成败, 使得该网络具有可控性. 图 4 控制方数量不同的量子远程传态保真度随增益因子g的变化曲线对比图, 曲线1表示有两个控制者参与时的保真度, 曲线2表示仅有一个控制者参与时的保真度, 曲线3表示远程传态保真度的经典极限值 Figure4. Dependences of the fidelity of quantum teleportation with different number of controllers on gain factor g, trace 1 is the calculated fidelity of quantum teleportation with two controllers, trace 2 is the calculated fidelity of quantum teleportation with only one controller, trace 3 is the classical limit of quantum teleportation.
该公式表明, 子模${\hat{b}}_{1(2)}$和$\hat{b}_{3(4)}$的正交振幅分量以及正交位相分量之间的量子关联噪声小于散粒噪声基准线2. 如果利用两个子模之间的量子关联来构建量子远程传态网络, 该网络可以同时传送两个不同的未知量子态, 增加了可传送量子态的数量及种类. 4个用户中, 只要二者所拥有的子模相互关联, 就可以作为发送方和接收方来传送量子态, 组合形式种类多样, 同样因为4组份量子资源制备方式的对称性, 不同组合形式下量子远程传态的效果相同. 图5所示为其中一种组合形式下, 可同时传送两个未知量子态的量子远程传态网络的结构示意图. 图中包括两个量子远程传态的过程, 一个是Alice作为发送方, Claire作为接收方, 另一个是Bob作为发送方, David作为接收方, 这两个过程的量子远程传态效果相同, 下面以Alice发送量子态给Claire为例进行理论分析. 该量子远程传态过程中, 最终Claire所重构的未知量子态的表达式与式(13)和(14)类似, 不同的是, 该过程仅包含了必要的发送方和接收方, 因为没有其他任何控制者的参与, 控制方的经典反馈增益因子$g_{2{{\rm{C}}}}^x$, $g_{2{{\rm{C}}}}^p$的取值为0, 结合保真度的计算公式, 理论计算得到了保真度的大小随压缩参数r的变化曲线, 如图6中的曲线1所示. 该曲线变化趋势表明, 当压缩参数r等于零时, 保真度的大小为量子远程传态的经典极限值0.5, 因为没有用到量子资源, 此时无法实现真正意义上的量子远程传态. 而当压缩参数r大于0后, 相应的保真度大于经典极限值0.5, 成功实现量子远程传态. 随着压缩参数r的持续增大, 对应的保真度也持续增大, 直至r趋近于无穷大时, 保真度趋近于理想值1, 说明更大的压缩参数, 即高质量的量子资源可以更高质量的传送未知量子态, 使得重构量子态的量子特性更接近于被传送量子态的量子特性. 为了方便对比四用户和两用户量子远程传态的效果, 同时理论计算了四用户量子远程传态过程中, 当控制方增益因子分别取值为0.5, 0.8和1时, 保真度随压缩参数r的变化曲线, 分别对应图6中的曲线2, 3和4. 图6中的曲线1同时也对应四用户量子远程传态过程中两个控制方增益因子为0的情况. 这四条曲线的变化趋势一致, 保真度都随着压缩参数的增大而增大, 且和量子远程传态的经典极限值0.5相交于不同的临界点, 当压缩参数大于相应的临界点时, 保真度大于0.5, 才能实现真正意义上的量子远程传态, 否则量子态的传送过程失败. 说明增益因子和压缩参数同时影响保真度的大小, 在压缩参数一定的情况下, 必须选择合适的增益因子, 才能真正实现成功的量子远程传态. 图 5 可同时传送两个未知量子态的量子远程传态网络结构示意图, 其中 AM为振幅调制器, PM为位相调制器, BS为分束器, HD为平衡零拍探测器 Figure5. Schematic diagram of four-partite quantum teleportation network that can simultaneously teleport two unknown quantum states, where AM is Amplitude modulator; PM is Phase modulator, BS is Beam splitter, HD is Homodyne detector.
图 6 量子远程传态保真度随压缩参数r的变化曲线, 曲线1—4分别对应增益因子为0, 0.5, 0.8和1时的保真度大小, 曲线5表示远程传态保真度的经典极限值 Figure6. Dependences of the fidelity of quantum teleportation on squeezing factor r, the traces 1, 2, 3 and 4 are the calculated fidelity when gain factor is selected as 0, 0.5, 0.8 and 1, respectively, trace 5 is the classical limit of quantum teleportation.