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arXiv quant-ph/0303167quant-ph
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广义测量的纠缠成本

Entanglement cost of generalised measurements

Richard Jozsa, Masato Koashi, Noah Linden, Sandu Popescu, Stuart Presnell, Dan Shepherd, Andreas Winter

摘要:

两体纠缠是量子信息理论的基本可量化资源之一。我们提出了一个新的应用这个资源的理论量子测量。根据奈马克定理,任何秩为1的广义测量(POVM)M都可以表示为系统的扩展(张量积)空间中的冯诺依曼测量加上辅助。通过考虑这些测量方向上纠缠的适当平均值并在所有Naimark扩展上最小化,我们定义了M的纠缠代价Emin(M)的概念。我们给出了一个建设性的方法来刻画一个给定的POVM的所有Naimark扩展。我们确定了各种类别的POVM的零和非零成本,并明确表示所有POVM在2维零成本。我们证明了一个恒定的上界的纠缠成本的任何POVM在任何维度。因此,对于所有POVM,渐近纠缠成本(即M的n个应用的成本的大n极限除以n)为零。三角测量由三个秩为1的元素定义,其方向对称地放置在布洛赫球上的一个大圆周围。我们给出了它的纠缠代价的解析表达式。定义一个标准化成本的任何d维POVM E_min(M)/log(d),我们表明(使用分析和数值技术的组合),三分之一的测量是更昂贵的比任何其他POVM与d>2,或与d=2和辅助维2。这有力地表明,三分测量是所有POVM中成本最高的。

英文摘要:

Bipartite entanglement is one of the fundamental quantifiable resources of quantum information theory. We propose a new application of this resource to the theory of quantum measurements. According to Naimark's theorem any rank 1 generalised measurement (POVM) M may be represented as a von Neumann measurement in an extended (tensor product) space of the system plus ancilla. By considering a suitable average of the entanglements of these measurement directions and minimising over all Naimark extensions, we define a notion of entanglement cost E_min(M) of M. We give a constructive means of characterising all Naimark extensions of a given POVM. We identify various classes of POVMs with zero and non-zero cost and explicitly characterise all POVMs in 2 dimensions having zero cost. We prove a constant upper bound on the entanglement cost of any POVM in any dimension. Hence the asymptotic entanglement cost (i.e. the large n limit of the cost of n applications of M, divided by n) is zero for all POVMs. The trine measurement is defined by three rank 1 elements, with directions symmetrically placed around a great circle on the Bloch sphere. We give an analytic expression for its entanglement cost. Defining a normalised cost of any d-dimensional POVM by E_min(M)/log(d), we show (using a combination of analytic and numerical techniques) that the trine measurement is more costly than any other POVM with d>2, or with d=2 and ancilla dimension 2. This strongly suggests that the trine measurement is the most costly of all POVMs.

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