Nonclassicality and entanglement are notions fundamental to quantum information processes involving continuous variable systems. That these two notions are intimately related has been intuitively appreciated for quite some time. An aspect of considerable interest is the behaviour of these attributes of a state under the action of a noisy channel. Inspired by the notion of entanglement-breaking channels, we define the concept of nonclassicality-breaking channels in a natural manner. We show that the notion of nonclassicality-breaking is essentially equivalent---in a clearly defined sense of the phrase `essentially'---to the notion of entanglement-breaking, as far as bosonic Gaussian channels are concerned. This is notwithstanding the fact that the very notion of entanglement-breaking requires reference to a bipartite system, whereas the definition of nonclassicality-breaking makes no such reference. Our analysis rests on our classification of channels into nonclassicality-based, as against entanglement-based, types of canonical forms. Our result takes ones intuitive understanding of the close relationship between nonclassicality and entanglement a step closer.
The quantum nature of the state of a bosonic quantum field manifests itself in its entanglement, coherence, or optical nonclassicality which are each known to be resources for quantum computing or metrology. We provide quantitative and computable bounds relating entanglement measures with optical nonclassicality measures. These bounds imply that strongly entangled states must necessarily be strongly optically nonclassical. As an application, we infer strong bounds on the entanglement that can be produced with an optically nonclassical state impinging on a beam splitter. For Gaussian states, we…
Characterizing genuine quantum resources and determining operational rules for their manipulation are crucial steps to appraise possibilities and limitations of quantum technologies. Two such key resources are nonclassicality, manifested as quantum superposition between reference states of a single system, and entanglement, capturing quantum correlations among two or more subsystems. Here we present a general formalism for the conversion of nonclassicality into multipartite entanglement, showing that a faithful reversible transformation between the two resources is always possible within a pre…
Increasing relative nonclassicality quantified by standard entanglement potentials by dissipation and unbalanced beam splitting
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。
Simple joint measurements of pairs of observables reveal that states considered universally as classical-like, such as SU(2) spin coherent states, Glauber coherent states, and thermal states are actually nonclassical. We show that this holds because we can find a joint measurement the statistics of which is not separable. Eventually this may be extended to all states different from the maximally mixed state.
Sudden vanishing and reappearance of nonclassical effects: General occurrence of finite-time decays and periodic vanishings of nonclassicality and entanglement witnesses
We investigate the performance of a certain nonclassicality identifier, expressed via integrated second-order intensity moments of optical fields, in revealing bipartite entanglement of quantum-optical frequency combs (QOFCs), which are generated in both spontaneous and stimulated parametric down-conversion processes. We show that, by utilizing that nonclassicality identifier, one can well identify the entanglement of the QOFC directly from the experimentally measured intensity moments without invoking any state reconstruction techniques or homodyne detection. Moreover, we demonstrate that the…
The concept of photon added two-mode Schrödinger cat states in which both modes are independent is introduced, their non-classical properties and entanglement are studied. The introduced states emerge as the eigenstates of $f_1f_2a_1a_2$, where $f_1, f_2$ are nonlinear functions of the number operator and $a_1, a_2$ are annihilation operators. We study the evolution of these states under the canonical transformation using the parity operator for the case of standard coherent states of the harmonic oscillator. The non-classical properties of these states are evaluated especially by considering …
Mode-entanglement based criteria and measures become insufficient for broadband emission, e.g. from spasers (plasmonic nano-lasers). We introduce criteria and measures for the (i) total entanglement of two wavepackets, (ii) entanglement of a wavepacket with an ensemble and (iii) total nonclassicality of a wavepacket~(WP). We discuss these criteria in the context of (i) entanglement of two WPs emitted from two initially entangled cavities (or two initially entangled atoms) and (ii) entanglement of an emitted WP with the ensemble/atom for the spontaneous emission and the single-photon superradia…
Lower- and higher-order nonclassicality in a Bose-condensed optomechanical-like system and a Fabry-Perot cavity with one movable mirror: squeezing, antibunching and entanglement
Thermal machines exploit interactions with multiple heat baths to perform useful tasks, such as work production and refrigeration. In the quantum regime, tasks with no classical counterpart become possible. Here, we consider the minimal setting for quantum thermal machines, namely two-qubit autonomous thermal machines that use only incoherent interactions with their environment, and investigate the fundamental resources needed to generate entanglement. Our investigation is systematic, covering different types of interactions, bosonic and fermionic environments, and different resources that can…
Dynamics of atom-field correlations and single-mode nonclassicalities present in the resonant Jaynes-Cummings model are investigated using negativity and entanglement potential for a set of initial states. The study has revealed the interplay between three different types of nonclassicality present in the model and established that the nonclassicality is continuously exchanged between the field and atom through the atom-field correlations. Further, it is observed that the entanglement potential does not capture all the single-mode nonclassicality and there exists some residual nonclassicality …
We introduce two independent quantifications for 3-mode and 4-mode entanglement. We investigate the conversion of one type of nonclassicality, i.e. single-mode nonclassicality, into another type of nonclassicality, i.e. multi-mode entanglement, in beam-splitters. We observe parallel behavior of the two quantifications. The methods can be generalized to the quantification of any multi-mode entanglement.
Twisted Gaussian Schell States in Quantum Optics: Twist-Assisted Nonclassicality and Entanglement
Universidade Federal do Rio de Janeiro（里约热内卢联邦大学）；Universidad del Bío-Bío（比奥比奥大学）；Universidade Federal Fluminense（弗鲁米嫩塞联邦大学）；Universidade Federal de Santa Catarina（圣卡塔琳娜联邦大学）；Universidad de Concepción（孔塞普西翁大学）；Millennium Institute for Research in Optics, Universidad de Concepción（孔塞普西翁大学光学研究千年研究所）
Singularity or negativity of Glauber P-function is a widespread notion of nonclassicality, with important implications in quantum optics and with the character of an irreducible resource. Here we explore how P-nonclassicality may be generated by conditional Gaussian measurements on bipartite Gaussian states. This nonclassical steering may occur in a weak form, which does not imply entanglement, and in a strong form that implies EPR-steerability and thus entanglement. We show that field quadratures are the best measurements to remotely generate nonclassicality, and exploit this result to derive…
A beam splitter can generate entanglement if the input field is nonclassical, e.g. squeezed. Thus, beam splitter transformation can be utilized for obtaining single-mode nonclassicality conditions from two-mode entanglement criteria. Here, we demonstrate that (i) the generalized quadrature-squeezing condition $\langle\hat{a}^\dagger\hat{a}\rangle < |\langle \hat{a}^2\rangle|$ can be obtained from the two-mode entanglement criterion of Duan-Giedke-Cirac-Zoller~[PRL 84, 2722]. (ii) Single-mode nonclassicality condition, obtained from the two-mode entanglement criterion of Simon~[PRL, 84 2726], c…
Nonclassicality according to the singularity or negativity of the Glauber P-function is a powerful resource in quantum information, with relevant implications in quantum optics. In a Gaussian setting, and for a system of two modes, we explore how P-nonclassicality may be conditionally generated or influenced on one mode by Gaussian measurements on the other mode. Starting from the class of two-mode squeezed thermal states (TMST), we introduce the notion of nonclassical steering (NS) and the graphical tool of Gaussian triangoloids. In particular, we derive a necessary and sufficient condition f…
Gravity-induced entanglement is not a theory-independent witness of nonclassicality of the gravitational field
Institute for Quantum Optics and Quantum Information (IQOQI) Vienna, Austrian Academy of Sciences（维也纳量子光学与量子信息研究所（IQOQI），奥地利科学院）；Basic Research Community for Physics e.V.（基础物理研究社区协会）
Higher-order nonclassical effects in fluctuating-loss channels
研究了大气起伏损耗信道中高阶非经典性和纠缠准则的演化。通过制定的矩量矩阵的输入输出关系，我们调查的影响，这样的通道上相应的量子化标准。本文推广了我们以前关于高斯纠缠的工作[M. Bohmann等人，Phys. Rev. A 94，010302（R）（2016）]不仅利用了基于二阶的场景，而且还提供了对非高斯和多模辐射场中经历波动衰减的非经典性和纠缠的详细研究。也就是说，详细研究了标准和状态的各种示例，意外效果，例如，发现了压缩转移对相干位移的依赖性，并证明了非高斯纠缠比高斯纠缠更能抵抗大气损耗.此外，我们提出了一种检测方案，用于测量通过大气传播后的考虑时刻。因此，我们的研究结果可能有助于开发，改进和优化非高斯量子光源在自由空间量子通信中的应用。
