Measurement of qubits
Measurement of qubits
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DOI：
10.1103/physreva.64.052312
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发表时间：
2001-11-01
影响因子：
2.9
通讯作者：
White, AG
中科院分区：
文献类型：
--
作者：
James, DFV;Kwiat, PG;White, AG
我们详细描述的理论支撑的一对量子二能级系统（“量子比特”）的密度矩阵的测量。我们特别强调的是由下转换实验中产生的一对纠缠光子的两个偏振自由度实现的量子比特;然而，无论实际的物理实现如何，讨论都适用于一般情况。两种技术进行了讨论，即，层析重建（其中的密度矩阵是线性相关的一组测量的数量）和最大似然技术，需要数值优化（但具有的优势，生产密度矩阵，总是非负定）。此外，还进行了详细的误差分析，从而可以估计由密度矩阵导出的量（如地层熵或地层纠缠度）的误差。基于下转换实验的例子来说明我们的结果。
We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems ("qubits"). Our particular emphasis is on qubits realized by the two polarization degrees of freedom of a pair of entangled photons generated in a down-conversion experiment; however, the discussion applies in general, regardless of the actual physical realization. Two techniques are discussed, namely, a tomographic reconstruction (in which the density matrix is linearly related to a set of measured quantities) and a maximum likelihood technique which requires numerical optimization (but has the advantage of producing density matrices that are always non-negative definite). In addition, a detailed error analysis is presented, allowing errors in quantities derived from the density matrix, such as the entropy or entanglement of formation, to be estimated. Examples based on down-conversion experiments are used to illustrate our results.