We present a significantly improved scheme of entanglement detection inspired by local uncertainty relations for a system consisting of two qubits. Developing the underlying idea of local uncertainty relations, namely correlations, we demonstrate that it's possible to define a measure which is invariant under local unitary transformations and which is based only on local measurements. It is quite simple to implement experimentally and it allows entanglement quantification in a certain range for mixed states and exactly for pure states, without first obtaining full knowledge (e.g. through tomography) of the state.
In this paper, we investigate how to reduce the number of measurement configurations needed for sufficiently precise entanglement quantification. Instead of analytical formulae, we employ artificial neural networks to predict the amount of entanglement in a quantum state based on results of collective measurements (simultaneous measurements on multiple instances of the investigated state). This approach allows us to explore the precision of entanglement quantification as a function of measurement configurations. For the purpose of our research, we consider general two-qubit states and their ne…
Gradual loss of the entanglement of a twin beam containing around 25 photon pairs with the increasing external noise is experimentally investigated. The entanglement is quantified by the non-classicality depths and the non-classicality counting parameters related to several non-classicality criteria. The reduction of intensity moments of the analyzed multi-mode twin beams to single-mode ones allows to determine the negativity as another quantifier of the entanglement. Both the raw photocount histograms and the reconstructed photon-number distributions are analyzed in parallel.
The certification of entanglement dimensionality is of great importance in characterizing quantum systems. Recently, it is pointed out that quantum correlation of high-dimensional states can be simulated with a sequence of lower-dimensional states. Such problem may render existing characterization protocols unreliable---the observed entanglement may not be a truly high-dimensional one. Here, we introduce the notion of irreducible entanglement to capture its dimensionality that is indecomposable in terms of a sequence of lower-dimensional entangled systems. We prove this new feature can be dete…
A quantitative witness for Greenberger-Horne-Zeilinger entanglement
随着实验量子技术的巨大进步，对三个或更多量子系统之间纠缠的量化需求越来越大。理论仍然没有为此目的提供足够的工具。我们的目标是，除了寻求准确的结果，开发操作方法，允许有效的纠缠量化。在这里，我们提出了一个分析方法，服务于这两个目标。我们提供了一个简单的方法来量化任意三比特态的Greenberger-Horne-Zeilinger型多体纠缠。对于两个量子位，该方法相当于Wootters的开创性结果[Phys. Rev. Lett. 80，2245（1998）]。它建立了纠缠量化和纠缠检测证人之间的密切联系，并可以推广到更高的维度和超过三方。
In this letter, we propose a method of dark count correction in quantum state tomography of entangled photon pairs. The framework is based on a linear model of dark counts, which is imposed on the measurement formalism. The method is tested on empirical data derived from an experiment on polarization-entangled photons. We demonstrate that the numerical approach to dark count correction guarantees more reliable state reconstruction compared with standard estimation. Most importantly, however, the conceptually simple theoretical approach proves to be more efficient at entanglement quantification…
The wedge product of vectors has been shown to yield the generalised entanglement measure I-concurrence, wherein the separability of the multiparty qubit system arises from the parallelism of vectors in the underlying Hilbert space of the subsystems. Here, we demonstrate the geometrical conditions of the post-measurement vectors which maximize the entanglement corresponding to the bi-partitions and can yield non-identical set of maximally entangled states. The Bell states for the two qubit case, GHZ and GHZ like states with superposition of four constituents for three qubits, naturally arise a…
Entanglement measures quantify nonclassical correlations present in a quantum system, but can be extremely difficult to calculate, even more so, when information on its state is limited. Here, we consider broad families of entanglement criteria that are based on variances of arbitrary operators and analytically derive the lower bounds these criteria provide for two relevant entanglement measures: the best separable approximation (BSA) and the generalized robustness (GR). This yields a practical method for quantifying entanglement in realistic experimental situations, in particular, when only f…
Operational Classification and Quantification of Multipartite Entangled States
本文形式化和推广了T. R.奥利维拉湾Rigolin和M. C. de Oliveira，Phys.Rev.A73，010305（R）（2006），通过全球纠缠（GE）的推广[D. A. Meyer和N. R. Wallach，J.Math.Phys.43，4273（2002）]。与通用电气的主要特点，这一措施在于，我们研究的平均线性熵的所有可能的分区的多部系统。这使得一个可操作的多体纠缠措施，这是能够区分不同的多体纠缠态，GE未能区分的建设。此外，它也是最大的伊辛链在横向磁场的临界点，从而能够检测到量子相变。
