Laura Ares, Alfredo Luis
We demonstrate the simple and deep equivalence between quantum coherence and nonclassicality and the definite way in which they determine metrological resolution. Moreover, we define a coherence observable consistent with a classical intuition relating coherence with phase statistics.
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We establish the nonclassicality of continuous-variable states as a resource for quantum metrology. Based on the quantum Fisher information of multimode quadratures, we introduce the metrological power as a measure of nonclassicality with a concrete operational meaning of displacement sensitivity beyond the classical limit. This measure belongs to the resource theory of nonclassicality, which is nonincreasing under linear optical elements. Our Letter reveals that a single copy, highly nonclassical quantum state is intrinsically advantageous when compared to multiple copies of a quantum state w…
Nonclassicality of Mixed States with Photon Number Coherence
本文首次计算了具有光子数相干性的混合态的ORT度量，揭示了非经典性和计量学能力在玻色子去相干过程中的行为特性。
In this review, we introduce the notion of quantum nonclassicality of light, and the role of nonclassicality in optical quantum metrology. The first part of the paper focuses on defining and characterizing the notion of nonclassicality and how it may be quantified in radiation fields. Several prominent examples of nonclassical light is also discussed. The second part of the paper looks at quantum metrology through the lens of nonclassicality. We introduce key concepts such as the Quantum Fisher information, the Cram{é}r-Rao bound, the standard quantum limit and the Heisenberg limit, and discus…
We examine whether metrological resolution beyond coherent states is a nonclassical effect. We show that this is true for linear detection schemes but false for nonlinear schemes, and propose a very simple experimental setup to test it. We find a nonclassicality criterion derived from quantum Fisher information.
We provide a coherence-based approach to nonclassical behavior by means of distance measures. We develop a quantitative relation between coherence and nonclassicality quantifiers, which establish the nonclassicality as the maximum quantum-coherence achievable. We compute the coherence of several representative examples and discuss whether the theory may be extended to reference observables with continuous spectra.
Coherence-based operational nonclassicality criteria
本文提出仅依赖实验可测数据的可操作判据，用于检测单个量子相干性的非经典性，并分析其在损耗与噪声信道下的鲁棒性，可广泛应用于多类量子实验。
Kirkwood discovered in 1933, and Dirac discovered in 1945, a representation of quantum states that has undergone a renaissance recently. The Kirkwood-Dirac (KD) distribution has been employed to study nonclassicality across quantum physics, from metrology to chaos to the foundations of quantum theory. The KD distribution is a quasiprobability distribution, a quantum generalization of a probability distribution, which can behave nonclassically by having negative or nonreal elements. Negative KD elements signify quantum information scrambling and potential metrological quantum advantages. Nonrea…
Nonclassicality of induced coherence witnessed by contextuality
本文针对诱导相干的量子本质争议，设计语境性检验，确定其产生超越非语境隐变量模型的量子预言的条件，从而见证其非经典性。
A coherence monotone from Kirkwood-Dirac nonclassicality with respect to mutually unbiased bases
本文研究互无偏基下的 Kirkwood-Dirac 经典性，刻画其与相对于基 $A$ 非相干的等价关系，并提出可由弱值表达的相干性单调量，从而用于检测异常弱值。
Quantum coherence from Kirkwood-Dirac nonclassicality, some bounds, and operational interpretation
本研究基于KD准概率的非实性与负性构建量子相干性量化指标，推导其上下界、纯态闭式表达式及非对易基下的权衡关系，提出基于弱值测量的估计方案并探讨与量子互文性的关联。
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…
Interference of two beams produced at separate biphoton sources was first observed more than two decades ago. The phenomenon, often called "induced coherence without induced emission", has recently gained attention after its applications to imaging, spectroscopy, and measuring biphoton correlations have been discovered. The sources used in the corresponding experiments are nonlinear crystals pumped by laser light. The use of a laser pump makes the occurrence of induced (stimulated) emission unavoidable and the effect of stimulated emission can be observed in the joint detection rate of the two…
In this article, we provide theoretical support for the use of geometric measures of nonclassicality as a general tool to identify quantum phase transitions. We argue that divergences in the susceptibility of any geometric measure of nonclassicality are sufficient conditions to identify phase transitions at arbitrary temperature. This establishes that geometric measures of nonclassicality, in any quantum resource theory, are generic tools to investigate phase transitions in quantum systems. At zero temperature, we show that geometric measures of quantum coherence are especially useful for iden…
In this paper, we show that coherence witness for a single qubit itself yields conditions for nonlocality and entanglement inequalities for multiqubit systems. It also yields a condition for quantum discord in two--qubit systems. It is shown by employing homomorphism among the stabiliser group of a single qubit and those of multi--qubit states. Interestingly, globally commuting homomorphic images of single qubit stabilisers do not allow for consistent assignments of outcomes of local observables. As an application, we show that CHSH inequality can be straightforwardly generalised to nonlocalit…
Hybrid Qubit-Qutrit Quantum Battery: Nonclassicality and Energy Performance
本文研究了混合量子比特-量子三重态量子电池的非经典性和能量性能，通过分析量子相干性和纠缠性，揭示了非经典性对能量存储效率的提升作用，并展示了实验可实现的镍自由基分子复合物在室温下的量子相干性和高效能量存储。
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 …
Quadrature Coherence Scale of Linear Combinations of Gaussian Functions in Phase Space
本文提出计算 Wigner 函数为高斯函数线性组合的量子态正交相干尺度的方法，并用于分析损耗下非经典性的可扩展性及培育协议输出态质量。
In this paper, we detail an orthogonalization procedure that allows for the quantification of the amount of coherence present an arbitrary superposition of coherent states. The present construction is based on the quantum coherence resource theory introduced by Baumgratz et al., and the coherence resource monotone that we identify is found to characterize the nonclassicality traditionally analyzed via the Glauber-Sudarshan $P$ distribution. This suggests that identical quantum resources underlie both quantum coherence in the discrete finite dimensional case and the nonclassicality of quantum l…
Coherence revival under the Unruh effect and its metrological advantage
本文研究加速Unruh-DeWitt探测器间的量子相干性提取，发现相干性随Unruh温度升高呈现先消失后复苏的现象，并通过量子Fisher信息增强验证其可作为量子计量学的物理资源。
Nonclassicality Analysis and Entanglement Witnessing in Spin-$1/2$ NMR Systems
该研究通过核磁共振系统分析量子临界点和纠缠特性，利用可实验验证的纠缠见证方法，将量子信息量度与核磁共振谱联系起来。
Nonclassicality is a key ingredient for quantum enhanced technologies and experiments involving macro- scopic quantum coherence. Considering various exactly-solvable quantum-oscillator systems, we address the role played by the anharmonicity of their potential in the establishment of nonclassical features. Specifically, we show that a monotonic relation exists between the the entropic nonlinearity of the considered potentials and their ground state nonclassicality, as quantified by the negativity of the Wigner function. In addition, in order to clarify the role of squeezing--which is not captu…
We use the exact calculation of the quantum mechanical, temporal characteristic function $χ(η)$ and the degree of second-order coherence $g^{(2)}(τ)$ for a single-mode, degenerate parametric amplifier for a system in the Gaussian state, viz., a displaced-squeezed thermal state, to study the different criteria for nonclassicality. In particular, we contrast criteria that involve only one-time functions of the dynamical system, for instance, the quasiprobability distribution $P(β)$ of the Glauber-Sudarshan coherent or P-representation of the density of state and the Mandel $Q_{M}(τ)$ parameter, …
Photonic Entanglement and Polarization Nonclassicality: Two Manifestations, One Nature
该研究从理论和实验层面证明非经典偏振与不可区分光子的纠缠严格等价，统一了两种看似不同的量子现象，验证了该量子性的偏振基无关性，指出非经典偏振可作为量子协议的等效资源，具备实际应用价值。
The Copenhagen interpretation has been long-lasted, whose core concepts are in the Heisenberg's uncertainty principle and nonlocal correlation of EPR. The second-order anticorrelation on a beam splitter represents these phenomena where it cannot be achieved classically. Here, the anticorrelation of nonclassicality on a beam splitter is interpreted in a purely coherence manner. Unlike a common belief in a particle nature of photons, the anticorrelation roots in pure wave nature of coherence optics, where quantum superposition between two input fields plays a key role. This interpretation may in…
We demonstrate that quantum Fisher information and superradiance can be formulated as coherence measures in accordance with the resource theory of coherence, thus establishing a direct link between metrological information, superradiance and coherence. We also show that the trivial coherence measure has a metrological interpretation. The arguments are then generalized to show that coherence may be considered as the underlying fundamental resource for any functional of state that is first of all faithful, and second of all concave or linear. Arguments are then presented that show quantum Fisher…
From Local Nonclassicality to Entanglement: A Convexity Law for Single-Excitation Dynamics
本文针对保激发相互作用证明了局域维格纳负度之和受初始态预算限制的凸性定律，解析推导两体结果并数值验证态传输链，揭示纠缠增长为资源重分配及相干性驱动的暗传输，提供硬件误差诊断度量。
Measuring small separations between two optical sources, either in space or in time, constitute an important metrological challenge as standard intensity-only measurements fail for vanishing separations. Contrarily, it has been established that appropriate coherent mode projections can appraise arbitrarily small separations with quantum-limited precision. However, the question of whether the optical coherence brings any metrological advantage to mode projections is still a point of debate. Here, we elucidate this problem by experimentally investigating the effect of varying coherence on estima…
We address the dynamics of nonclassicality for a quantum system interacting with a noisy fluctuating environment described by a classical stochastic field. As a paradigmatic example, we consider a harmonic oscillator initially prepared in a maximally nonclassical state, e.g. a Fock number state or a Schroedinger cat-like state, and then coupled to either resonant or non-resonant external field. Stochastic modeling allows us to describe the decoherence dynamics without resorting to approximated quantum master equations, and to introduce non- Markovian effects in a controlled way. A detailed com…
Nonclassical states of light are necessary resources for quantum technologies such as cryptography, computation and the definition of metrological standards. Observing signatures of nonclassicality generally requires inferring either the photon number distribution or a quasi-probability distribution indirectly from a set of measurements. Here, we report an experiment in which the nonclassical character of families of quantum states is assessed by direct inspection of the outcomes from a multiplexed photon counter. This scheme does not register the actual photon number distribution; the statist…
Coherence as a resource for $N$-box and quantum pigeonhole paradoxes
研究涉及混合预选择和后选择状态的非逻辑PPS场景，以N盒和量子鸽巢悖论为例，通过证明悖论与单个特定算符的弱值异常及相干性的关联来分析，阐明非经典性概念作用及与经典解释的偏差。
Interferometric measurement of the quadrature coherence scale using two replicas of a quantum optical state
本文提出一种利用量子态两个副本的线性干涉仪直接测量正交相干尺度的方法，无需完整态层析，扩展了Hong-Ou-Mandel效应，为非经典性评估提供了新途径。
We address the relation between quantum metrological resolution and coherence. We examine this dependence in two manners: we develop a quantum Wiener-Kintchine theorem for a suitable model of quantum ruler, and we compute the Fisher information. The two methods have the virtue of including both the contributions of probe and measurement on an equal footing. We illustrate this approach with several examples of linear and nonlinear metrology. Finally, we optimize resolution regarding coherence as a finite resource.
Quantum speedup from nonclassical polarization
提出框架识别极化系统中的非经典加速，通过对比角动量相干态经典极限与无限制量子速度极限，定量分析非经典性带来的加速，并在交叉Kerr相互作用中展示随光子数N的√N加速及奇偶效应。
Quantum teleportation and dynamics of quantum coherence and metrological non-classical correlations for open two-qubit systems: A study of Markovian and non-Markovian regimes
本研究探讨开放两量子比特系统中量子相干性与非经典关联的动力学，提出基于不同物理场景的量子隐形传态策略，并分析初始纯度和环境性质的影响。
Relation between nonclassical features through logical qudits
该研究探讨逻辑量子比特中的量子相干性与物理多量子比特系统中非经典关联的互易关系，并将框架推广至更高维系统，发现单一非经典性条件可检测多种非经典性。
We introduce, for each state of a bosonic quantum field, its quadrature coherence scale (QCS), a measure of the range of its quadrature coherences. Under coupling to a thermal bath, the purity and QCS are shown to decrease on a time scale inversely proportional to the QCS squared. The states most fragile to decoherence are therefore those with quadrature coherences far from the diagonal. We further show a large QCS is difficult to measure since it induces small scale variations in the state's Wigner function. These two observations imply a large QCS constitutes a mark of "macroscopic coherence…
Quantum coherence leveraged agnostic phase estimation
该研究针对旋转轴未知时量子比特旋转角度估计精度下降的问题，提出基于辅助比特初始量子相干性的协议，无需纠缠资源即可实现与轴参数无关的最优费希尔信息，提供了资源高效的不可知相位传感方案。
A simple proof that anomalous weak values require coherence
本文提出了一个简单证明，表明反常弱值要求前后选择状态在可观测量本征基中具有相干性，并给出了反常弱值作为广义语境性见证的条件，无需弱测量设置。
Coherence-enhanced single-qubit thermometry out of equilibrium
本文研究非平衡热化过程中量子温度计的计量极限，证明初始量子相干性可增强量子费希尔信息，并在量子比特情形下解析证明有限时间灵敏度可优于渐近值。
Estimating phase parameters of a three-level system interacting with two classical monochromatic fields in simultaneous and individual metrological strategies
本文研究三能级原子与两个经典单色场相互作用下的双激光相位估计，利用希尔伯特-施密特速度比较同时与单独计量策略，揭示量子相干性和失谐参数对提高多参数估计精度的关键作用。
Chirped-pulse interferometry (CPI) captures the metrological advantages of quantum Hong-Ou-Mandel (HOM) interferometry in a completely classical system. Modified HOM interferometers are the basis for a number of seminal quantum-interference effects. Here, the corresponding modifications to CPI allow for the first observation of classical analogues to the HOM peak and quantum beating. They also allow a new classical technique for generating phase super-resolution exhibiting a coherence length dramatically longer than that of the laser light, analogous to increased two-photon coherence lengths i…
Deviations from classical physics when distant quantum systems become correlated are interesting both fundamentally and operationally. There exist situations where the correlations enable collaborative tasks that are impossible within the classical formalism. Here, we consider the efficiency of quantum computation protocols compared to classical ones as a benchmark for separating quantum and classical resources and argue that the computational advantage of collaborative quantum protocols in the discrete variable domain implies the nonclassicality of correlations. By analysing a toy model, it t…
Trade-off between predictability and quantum coherence for multi-path interferometry and its operational interpretation
研究多路径设置下可预测性与量子相干性的互补关系，引入基于基的可预测性，推导二者权衡关系，该关系赋予相干性度量操作解释，为源独立量子随机数生成器的猜测概率提供最坏情况界限。
The transition from quantum to classical, in the case of a quantum harmonic oscillator, is typically identified with the transition from a quantum superposition of macroscopically distinguishable states, such as the Schrödinger cat state, into the corresponding statistical mixture. This transition is commonly characterized by the asymptotic loss of the interference term in the Wigner representation of the cat state. In this paper we show that the quantum to classical transition has different dynamical features depending on the measure for nonclassicality used. Measures based on an operatorial …
Making use of coherence and entanglement as metrological quantum resources allows to improve the measurement precision from the shot-noise- or quantum limit to the Heisenberg limit. Quantum metrology then relies on the availability of quantum engineered systems that involve controllable quantum degrees of freedom which are sensitive to the measured quantity. Sensors operating in the qubit mode and exploiting their coherence in a phase-sensitive measurement have been shown to approach the Heisenberg scaling in precision. Here, we show that this result can be further improved by operating the qu…
Spin coherence scale: operator-ordering sensitivity beyond the Heisenberg-Weyl group
研究引入自旋相干尺度度量自旋系统量子相干性，推广正交相干尺度，证明其诸多性质可沿用，在量子计量学有应用，还将框架推广到SU(n)系统，确定不变信道并给出李代数方法定义和表征相干尺度性质。
Two-Photon Induced Coherence without Induced Emission
本研究观测到无诱导发射的双光子诱导相干性，利用双光子Fock态实现干涉相位调制翻倍，验证了相位变化的倍频调制效应，为高分辨率量子计量应用开辟新路径。
Quantum systems can be exploited for disruptive technologies but in practice quantum features are fragile due to noisy environments. Quantum coherence, a fundamental such feature, is a basis-dependent property that is known to exhibit a resilience to certain types of Markovian noise. Yet, it is still unclear whether this resilience can be relevant in practical tasks. Here, we experimentally investigate the resilient effect of quantum coherence in a photonic Greenberger-Horne-Zeilinger state under Markovian bit-flip noise, and explore its applications in a noisy metrology scenario. In particula…
Quantum coherence is a fundamental aspect of quantum physics and plays a central role in quantum information science. This essential property of the quantum states could be fragile under the influence of the quantum operations. The extent to which quantum coherence is diminished depends both on the channel and the incoherent basis. Motivated by this, we propose a measure of nonclassicality of a quantum channel as the average quantum coherence of the state space after the channel acts on, minimized over all orthonormal basis sets of the state space. Utilizing the squared $l_1$-norm of coherence…
Symmetry: a fundamental resource for quantum coherence and metrology
本研究提出量子计量深度纠缠态制备新范式，证明对称本征态下的非对角可观测量纯量子涨落可作为计量资源，为多体系统非高斯关联的最优计量应用奠定基础。
To share and not share a singlet: control qubit and nonclassicality in teleportation
本文研究了控制量子比特叠加态在量子隐形传态中的作用，提出两种控制协议，发现叠加能在宽参数范围内提供超越经典混合的量子优势，且优势与量子相干性相关。
Theory-independent monitoring of the decoherence of a superconducting qubit with generalized contextuality
本文提出一种不依赖理论的过程重构方法，用于监测超导量子比特的退相干现象，展示了系统在不假设量子理论的情况下如何通过广义情境性分析其退相干过程，并揭示了非马尔可夫演化特性。
Interplay Between Quantum Coherence and Multiparameter Quantum Estimation in Graphene
研究石墨烯系统中量子相干与多参数量子估计的关系，聚焦温度\(T\)和波矢\(k_x\)，通过量子克莱姆 - 拉奥界评估最小方差，发现二者变化对相干和精度影响不同，还引入比率\(\Gamma\)比较估计方案差异。
Errors in quantum state identification with ultrashort pulses
本文从探测基本原理出发，证明超短脉冲等非平稳场的相干函数依赖场形状，导致超快源观测的聚束与超聚束不能指示场的量子态。
The quantumness of Dirac paticles for quantized fields in a dilatonic black hole is estimated by means of quantum channel. We develop a general Bloch vector representation of quantum channel in black hole spacetimes beyond single mode approximation. The nonclassicality of Dirac particles can be measured by the minimization of quantum coherence over all orthonormal basis sets. The quantumness of the channel decreases as the dilaton parameter increases. The interplay between the external reservoir noise and dilaton black hole on the dynamical behavior of quantum coherence and steerability is inv…
We show a general relationship between a superposition of macroscopically distinct states and sensitivity in quantum metrology. Generalized cat states are defined by using an index which extracts the coherence between macroscopically distinct states, and a wide variety of states, including a classical mixture of an exponentially large number of states, has been identified as the generalized cat state with this criterion. We find that, if we use the generalized cat states for magnetic field sensing without noise, we achieve the Heisenberg limited sensitivity. Moreover, we even show that sensiti…
Quantum-limited estimation of atmospheric turbulence via spatial mode decomposition
在量子计量学框架下，建立了光学空间相干半径（Fried参数）估计的终极精度极限，并证明在弱场条件下，当接收孔径小于相干半径时，空间模式分解方法比传统直接成像能实现更精确的估计。
Basing on the analogy between the coherent states of light and separable states of $N$ bosons, we demonstrate that the violation Cauchy-Schwarz inequality for any-order correlation function signals the entanglement among the constituent particles. Rather than restricting to the correlations between the positions of particles, we consider the broadest set of measurements allowed by quantum mechanics. Our result is general -- it applies to any quantum system of bosons, even when the number of particles is not fixed, provided that there is no coherence between different number states. We also dem…
Equivalence of quantum resources under ergodic dynamics
该研究证明在遍历动力学下，多体量子系统中不同量子资源的演化并非独立，而是受少数共同自由度支配，强 scrambling 可实现不同量子资源间的等价性，为理解量子资源的动力学行为提供了关键简化机制。
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