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arXiv 1509.05970quant-ph
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费米子系统中的纠缠

Entanglement in fermion systems

N. Gigena, R. Rossignoli

摘要:

本文分析了在宇称数固定但粒子数不一定固定的费米子系统的纯态和混合态中量子纠缠的量化问题。首先考虑了单粒子能级与其正交补能级之间的“模纠缠”,定义了单粒子希尔伯特空间$\mathcal H$的特定基的这种划分的纠缠熵.该熵的所有单粒子模式的总和被引入作为系统相对于所选基的总纠缠度的度量,并且表明其在$\mathcal H$的所有基上的最小值是单体密度矩阵的函数。此外,我们表明,如果最小化扩展到所有相关的基地通过Bogoliubov变换,那么纠缠熵是一个函数的广义单体密度矩阵。这些结果,然后被用来量化纠缠费米子系统与四个单粒子水平。对于这种系统的一般纯态,导出了费米子纠缠度的封闭表达式,推广了[J.Schliemann et al,Phys.Rev.A64,022303(2001)]中定义的Slater关联测度,暗示“粒子纠缠”可以看作是最小的“模纠缠”.我们还证明了前面定义的纠缠熵与这种并发性的关系,其表达式类似于两量子比特的情况。对于该系统的混合态,解析地计算了以前的纠缠度和纠缠熵的凸顶扩展,将以前的结果推广到一般态。

英文摘要:

We analyze the problem of quantifying entanglement in pure and mixed states of fermionic systems with fixed number parity yet not necessarily fixed particle number. The "mode entanglement" between one single-particle level and its orthogonal complement is first considered, and an entanglement entropy for such a partition of a particular basis of the single-particle Hilbert space $\mathcal H$ is defined. The sum over all single-particle modes of this entropy is introduced as a measure of the total entanglement of the system with respect to the chosen basis and it is shown that its minimum over all bases of $\mathcal H$ is a function of the one-body density matrix. Furthermore, we show that if minimization is extended to all bases related through a Bogoliubov transformation, then the entanglement entropy is a function of the generalized one-body density matrix. These results are then used to quantify entanglement in fermion systems with four single-particle levels. For general pure states of such a system a closed expression for the fermionic concurrence is derived, which generalizes the Slater correlation measure defined in [J. Schliemann et al, Phys. Rev. A 64, 022303 (2001)], implying that "particle entanglement" may be seen as minimum "mode entanglement". It is also shown that the entanglement entropy defined before is related to this concurrence by an expression analogous to that of the two-qubit case. For mixed states of this system the convex roof extension of the previous concurrence and entanglement entropy are evaluated analytically, extending the results of previous ref. to general states.

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