通过耗散和不平衡分束增加标准纠缠势量化的相对非经典性
Increasing relative nonclassicality quantified by standard entanglement potentials by dissipation and unbalanced beam splitting
摘要：
如果单模非经典光在分束器上与真空结合，则输出态是纠缠的。如[Phys. Rev. Lett. 94，173602（2005）]，通过测量平衡无损分束器的输出态纠缠，可以量化输入态的非经典性。这些非经典性的度量（称为纠缠势）原则上可以基于各种纠缠度量，导致负性（NP）和并发（CP）势，以及纠缠的相对熵（REEP）势。我们寻找最大的非经典性，这可以通过比较任意两个量子比特态的纠缠度量和那些可以通过平衡无损分束器从光子数量子比特产生的纠缠度量来实现，其中量子比特基态是真空和单光子态。令人惊讶的是，我们发现，最大的非经典性，测量的REEP为一个给定的NP值，可以增加（如果NP<0.527）通过使用可调分束器或通过振幅阻尼的平衡分束器的输出状态。我们还表明，最大的非经典性，测量的NP为一个给定的值的REEP，可以通过移相增加。纠缠本身并没有增加这些本地损失，但不同的措施可能的比例受到影响。此外，我们表明，部分混合态可以比纯态和完全混合态更非经典，通过比较NP对于一个给定的值的REEP。这意味着并非所有的纠缠度量都可以作为纠缠势。另外，一个平衡的无损分束器并不总是将其输入态的全部非经典性转换为其输出模的纠缠。至少在本文分析的情况下，应用有耗分束器可以解决这个问题。
英文摘要：
If a single-mode nonclassical light is combined with the vacuum on a beam splitter, then the output state is entangled. As proposed in [Phys. Rev. Lett. 94, 173602 (2005)], by measuring the output-state entanglement for a balanced lossless beam splitter, one can quantify the input-state nonclassicality. These measures of nonclassicality (referred to as entanglement potentials) can be based, in principle, on various entanglement measures, leading to the negativity (NP) and concurrence (CP) potentials, and the potential for the relative entropy of entanglement (REEP). We search for the maximal nonclassicality, which can be achieved by comparing two entanglement measures for arbitrary two-qubit states and those which can be generated from a photon-number qubit via a balanced lossless beam-splitter, where the qubit basis states are the vacuum and single-photon states. Surprisingly, we find that the maximal nonclassicality, measured by the REEP for a given value of the NP, can be increased (if NP<0.527) by using either a tunable beam splitter or by amplitude damping of the output state of the balanced beam splitter. We also show that the maximal nonclassicality, measured by the NP for a given value of the REEP, can be increased by dephasing. The entanglement itself is not increased by these local losses, but the possible ratios of different measures are affected. Moreover, we show that partially-mixed states can be more nonclassical than both pure states and completely-mixed states, by comparing the NP for a given value of the REEP. This implies that not all entanglement measures can be used as entanglement potentials. Alternatively, a single balanced lossless beam splitter is not always transferring the whole nonclassicality of its input state into the entanglement of its output modes. Applying a lossy beam splitter can solve this problem at least for the cases analyzed in this paper.
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