Nonlocal Correlations Research Articles

Quantum nonlocal modulation cancelation with distributed clocks

We demonstrate nonlocal modulation of entangled photons with truly distributed radio frequency (RF) clocks. Leveraging a custom radio-over-fiber (RFoF) system characterized via classical spectral interference, we validate its effectiveness for quantum networking by multiplexing the RFoF clock with one photon from a frequency-bin-entangled pair and distributing the coexisting quantum-classical signals over fiber. Phase modulation of the two photons reveals nonlocal correlations in excellent agreement with theory: in-phase modulation produces additional sidebands in the joint spectral intensity, while out-of-phase modulation is nonlocally canceled. Our simple, feedback-free design attains subpicosecond synchronization—namely, drift less than ∼0.5 ps in a 5.5 km fiber over 30 min (fractionally only ∼2×10−8 of the total fiber delay)—and should facilitate frequency-encoded quantum networking protocols such as high-dimensional quantum key distribution and entanglement swapping, unlocking frequency-bin qubits for practical quantum communications in deployed metropolitan-scale networks.

Hierarchical relationship of nonlocal correlations in Schwarzschild-de Sitter spacetime

We investigate the hierarchy of nonlocal correlations for Dirac fields in the Schwarzschild de Sitter (SdS) spacetime. We find that the hierarchical relationship between the nonlocal advantage of quantum coherence (NAQC) and Bell nonlocality remains in the multi-event horizon curved spacetime. It is demonstrated that the NAQC and Bell nonlocality provide a more rigorous characterization of physically inaccessible quantum properties than coherence, as classical information cannot be transmitted across the thermally opaque membrane. Furthermore, when considering an effective equilibrium temperature, an enhancement in quantum nonlocality with increasing temperature is observed, which challenges the conventional understanding that the Hawking effect solely diminishes quantum nonlocality. This study contributes to integrating quantum resource theory within the context of multi-event horizon black holes.

Experimental Test of Nonlocality Limits from Relativistic Independence

Quantum correlations, like entanglement, represent the characteristic trait of quantum mechanics and pose essential issues and challenges to the interpretation of this pillar of modern physics. Although quantum correlations are largely acknowledged as a major resource to achieve quantum advantage in many tasks of quantum technologies, their full quantitative description and the axiomatic basis underlying them are still under investigation. Previous works have suggested that the origin of nonlocal correlations is grounded in principles capturing (from outside the quantum formalism) the essence of quantum uncertainty. In particular, the recently introduced principle of relativistic independence has given rise to a new bound intertwining local and nonlocal correlations. Here, we test such a bound by realizing together sequential and joint weak measurements on entangled photon pairs, allowing us to simultaneously quantify both local and nonlocal correlations by measuring incompatible observables on the same quantum system without collapsing its state, a task typically forbidden in the traditional (projective) quantum measurement framework. Our results demonstrate the existence of a fundamental limit on the extent of quantum correlations, shedding light on the profound role of uncertainty in both enabling and balancing them. Published by the American Physical Society 2024

Incorporating edge convolution and correlative self-attention into graph neural network for material properties prediction

Abstract Predicting material properties is a crucial problem for new material designs. Traditional methods either based on trail-and-error experiments or large-scale density function theory calculations have their limitations. The recent machine learning methods shed light on resolving this problem efficiently. However, most machine learning models only consider the local atomic environment, while ignoring the non-local correlations between atoms. Even the periodic patterns of crystal structure are not taken into serious consideration. These issues lead to insufficient grasping of the feature information of atoms and bonds.In this paper, we propose a crystal graph convolutional neural network based on edge convolution and correlative self-attention, namely EdgeConv-GANN. The network is able to better extract atomic and bonding feature information and also effectively learning the importance weights of all neighboring nodes. Numerical experiments on predicting electronic structural properties of metal-organic frameworks show that our model achieves state-of-the-art performance. In addition, we applied the proposed model to predict the heat capacity and thermal decomposition temperature of materials, both results show that our method generalize well in multi-scale prediction tasks.

Detecting nonlocal correlations in chain-shaped quantum networks via a deep-learning method

Detecting nonlocal correlations in chain-shaped quantum networks via a deep-learning method

Time transfer and clock synchronization with ghost frequency comb

We report an experimental demonstration of a time transfer and distant clock synchronization scheme based on what we have labeled as a ghost frequency comb, observed from the nonlocal correlation measurements of a laser beam. Unlike a conventional frequency comb, the laser beam used in this work does not consist of a pulse train but instead it is in a continuous-wave operation. The laser beam, consisting of half a million longitudinal cavity modes from a fiber ring laser, is split into two beams, each sent to a distant observer. In their local measurements, both observers observe constant intensity with no pulse structure present. Surprisingly, a pulse train of comb-like, ultra-narrow peaks is observed from their nonlocal correlation function measurement. This observation makes an important contribution to the field of precision spectroscopy, as we show in optical correlation-based nonlocal timing and positioning.

Energy-Filtered Quantum States and the Emergence of Nonlocal Correlations

Energy-Filtered Quantum States and the Emergence of Nonlocal Correlations

Scaling limit of domino tilings on a pentagonal domain.

We consider the six-vertex model at its free-fermion point with domain wall boundary conditions, which is equivalent to random domino tilings of the Aztec diamond. We compute the scaling limit of a particular nonlocal correlation function, essentially equivalent to the partition function for the domino tilings of a pentagon-shaped domain, obtained by cutting away a triangular region from a corner of the initial Aztec diamond. We observe a third-order phase transition when the geometric parameters of the obtained pentagonal domain are tuned to have the fifth side exactly tangent to the arctic ellipse of the corresponding initial model.

Lithiation-Induced Volume Change of NCM-Type Battery Cathode Materials from First Principles

Volume change during lithiation of battery cathode materials is a critical input parameter for battery cell performance predictions. Here, our focus was on seven LiNixCoyMn1-x-yO2 (NCM)-type battery cathode materials: NCM811, NCM721, NCM622, NCM523, NCM111, NCM361, and NCM271. Lithium (Li) concentration was varied from 0 to 1 in a range of structures for each material. The convex hull was computed with the VASP plane wave density functional code from which was determined the lowest energy structure for each Li concentration. Using two different functionals, viz. the optB86b-vdW (an optimized van der Waals functional based upon the Becke 86 functional) and the SCAN+rVV10 (combines the strongly constrained and appropriately normed meta-GGA for short- and intermediate-range interactions with the long-range van der Waals interaction from rVV10, the revised Vydrov-van Voorhis nonlocal correlation functional), the following were computed: DV (volume change) vs. state of charge (SOC) (0 ≤ x ≤ 1), ΔV vs. open circuit voltage (OCV), and OCV vs. SOC (0 ≤ x ≤ 1), where x is Li concentration. Qualitative comparisons with room temperature experimental data revealed closer accord from the SCAN+rVV10 functional. Noted disparities for lower values of x are explained in terms of structural phase transitions and loss of oxygen during lithiation/de-lithiation with incorporation of these effects as next steps.

Interacting-Bath Dynamical Embedding for Capturing Nonlocal Electron Correlation in Solids.

Quantitative simulation of electronic structure of solids requires treating local and nonlocal electron correlations on an equal footing. We present a new abinitio formulation of Green's function embedding which, unlike dynamical mean-field theory that uses noninteracting bath, derives bath representation with general two-particle interactions in a systematically improvable manner. The resulting interacting-bath dynamical embedding theory (ibDET) utilizes an efficient real-axis coupled-cluster solver to compute the self-energy, approaching the full system limit at much reduced cost. When combined with the GW theory, GW+ibDET achieves good agreement with experimental spectral properties across a range of semiconducting, insulating, and metallic materials. Our approach also enables quantifying the role of nonlocal electron correlation in determining material properties and addressing the long-standing debate on the bandwidth narrowing of metallic sodium.

Exploring the Percolation Phenomena in Quantum Networks

Quantum entanglement as a non-local correlation between particles is critical to the transmission of quantum information in quantum networks (QNs); the key challenge lies in establishing long-distance entanglement transmission between distant targets. This issue aligns with percolation theory, and as a result, an entanglement distribution scheme called “Classical Entanglement Percolation” (CEP) has been proposed. While this scheme provides an effective framework, “Quantum Entanglement Percolation” (QEP) indicates a lower percolation threshold through quantum preprocessing strategies, which will modify the network topology. Meanwhile, an emerging statistical theory known as “Concurrence Percolation” reveals the unique advantages of quantum networks, enabling entanglement transmission under lower conditions. It fundamentally belongs to a different universality class from classical percolation. Although these studies have made significant theoretical advancements, most are based on an idealized pure state network model. In practical applications, quantum states are often affected by thermal noise, resulting in mixed states. When these mixed states meet specific conditions, they can be transformed into pure states through quantum operations and further converted into singlets with a certain probability, thereby facilitating entanglement percolation in mixed state networks. This finding greatly broadens the application prospects of quantum networks. This review offers a comprehensive overview of the fundamental theories of quantum percolation and the latest cutting-edge research developments.

Polarimetric image denoising via non-local based cube matching convolutional neural network

Due to rapid advances in deep learning, many polarimetric image denoising networks have been developed and achieved promising results. However, these methods are based on general network architectures that do not fully exploit problem-specific knowledge, leading to over-smoothing results and poor generalization. Inspired by the non-local, which is an effective prior for image restoration, we propose a cube matching convolutional neural network to incorporate non-local operations into denoising models for polarimetric images. Specifically, the cube matching technique allows the denoising network to simultaneously exploit the non-local correlation and polarization relationship between the corresponding voxels of similar cubes. Rather than applying self-similarity directly in an isolated manner, the proposed cube matching module can be flexibly integrated into existing deep networks by combining with 3D convolution, achieving an effect equivalent to non-local means. This design enhances the generalization ability against various types and levels of noise. Experimental results show that using cube matching operations significantly improves denoising performance.

Nonlocal correlations and entanglement in two coupled double quantum dots under external magnetic field

Abstract Double quantum dot (DQD) systems are highly regarded as promising candidates for advancements in the fields of quantum computing and quantum information processing. In this work, we investigate the behaviour of quantum correlations (QCs) in a system of two DQDs, AlGaAs/GaAs under various parameters such as temperature, energy offsets, coupling strength, and magnetic fields. We employ uncertainty-induced nonlocality and Bell nonlocality to quantify QC and logarithmic negativity to measure entanglement in the considered physical system. The findings demonstrate that QCs are significantly influenced by noisy equilibrium temperature and system parameters. In particular, lower temperatures enhance QCs, whereas higher temperatures result in a decrease due to thermal effects. In addition, the magnetic fields have a profound influence on the conditions of resonance. We also show that maximum QCs occur at resonance conditions with no energy offset between each DQD. However, introducing an energy offset or too-strong coupling reduces these correlations. Finally, our findings provide an understanding of the behaviour of entanglement and quantum nonlocality in DQD systems, offering potential implications for quantum technologies.

Exploring nonlocal correlations in arbitrarily high-dimensional systems

Abstract The utilization of higher-dimensional systems holds promise for unveiling novel phenomena and enhancing the efficacy of practical tasks, such as entanglement-based quantum cryptography. However, detecting quantum correlations within qudit systems poses a formidable challenge. In this study, we delve into the intricacies of Bell’s nonlocality within higher-dimensional systems. We introduce a novel correlation function and leverage it to construct a family of Bell inequalities adapted for quantum systems of arbitrary dimensionality. By gauging the probability of local realism violation under random measurements as our primary metric, we explore the relation between pure state entanglement and nonlocality. Our findings align closely with predictions derived from the comprehensive polytope analysis via linear programming methods. While our focus predominantly centers on the two-qudit scenario, our methodology readily extends to the N -partite case, accommodating an arbitrary number of measurements per party.

Information Causality as a Tool for Bounding the Set of Quantum Correlations.

Information causality was initially proposed as a physical principle aimed at deriving the predictions of quantum mechanics on the type of correlations observed in the Bell experiment. In the same work, information causality was famously shown to imply the Uffink inequality that approximates the set of quantum correlations and rederives Tsirelson's bound of the Clauser-Horne-Shimony-Holt inequality. This result found limited generalizations due to the difficulty of deducing implications of the information causality principle on the set of nonlocal correlations. In this Letter, we present a simple technique for obtaining polynomial inequalities from information causality bounding the set of physical correlations in any bipartite Bell scenario. This result makes information causality an efficient tool for approximating the set of quantum correlations. To demonstrate our method, we derive a family of inequalities which nontrivially constrains the set of nonlocal correlations in Bell scenarios with binary outcomes and equal number of measurement settings. Finally, we propose an improved statement of the information causality principle and obtain a tighter constraint for the simplest Bell scenario that goes beyond the Uffink inequality and recovers a part of the boundary of the quantum set.

Superconductivity in LaO: a density functional theory study with local, semilocal, and nonlocal exchange-correlation functionals

We studied the dynamical, electronic, and superconducting properties of LaO using a local (PZ), a semilocal (PBE), and three nonlocal correlation functionals (rVV10, vdW-DF2, and vdW-DF3) of the density-functional theory. We concluded that the nonlocal correlations have a marginal effect on the electronic band energies near the Fermi surface. However, the corresponding shifts in the density of states are relatively significant. In contrast, the phonon energies get modified by larger amounts, particularly for optical phonons. These variations in energies of optical phonons do not get reflected to the same degree in electron-phonon coupling constant, λ, from different functionals as their contribution in λ is minor that ranges from 21% to 33%. Moreover, the imperative role of Kohn anomalies and Fermi surface nesting for the electron-phonon coupling is reconfirmed. We also highlighted the role of the matrix elements that surpasses, in certain parts of the Brillouin Zone, all other factors including the Fermi surface nesting. The nonlocal correlation functional, vdW-DF2, gives significantly different results than other nonlocal as well as local/semilocal functionals due to excessive low energy phonons and larger phonon linewidth.

ScTCA: a hybrid Transformer-CNN architecture for imputation and denoising of scDNA-seq data.

Single-cell DNA sequencing (scDNA-seq) has been widely used to unmask tumor copy number alterations (CNAs) at single-cell resolution. Despite that arm-level CNAs can be accurately detected from single-cell read counts, it is difficult to precisely identify focal CNAs as the read counts are featured with high dimensionality, high sparsity and low signal-to-noise ratio. This gives rise to a desperate demand for reconstructing high-quality scDNA-seq data. We develop a new method called scTCA for imputation and denoising of single-cell read counts, thus aiding in downstream analysis of both arm-level and focal CNAs. scTCA employs hybrid Transformer-CNN architectures to identify local and non-local correlations between genes for precise recovery of the read counts. Unlike conventional Transformers, the Transformer block in scTCA is a two-stage attention module containing a stepwise self-attention layer and a window Transformer, and can efficiently deal with the high-dimensional read counts data. We showcase the superior performance of scTCA through comparison with the state-of-the-arts on both synthetic and real datasets. The results indicate it is highly effective in imputation and denoising of scDNA-seq data.

Entropy Bounds for Device-Independent Quantum Key Distribution with Local Bell Test.

One of the main challenges in device-independent quantum key distribution (DIQKD) is achieving the required Bell violation over long distances, as the channel losses result in low overall detection efficiencies. Recent works have explored the concept of certifying nonlocal correlations over extended distances through the use of a local Bell test. Here, an additional quantum device is placed in close proximity to one party, using short-distance correlations to verify nonlocal behavior at long distances. However, existing works have either not resolved the question of DIQKD security against active attackers in this setup, or used methods that do not yield tight bounds on the key rates. In this work, we introduce a general formulation of the key rate computation task in this setup that can be combined with recently developed methods for analyzing standard DIQKD. Using this method, we show that if the short-distance devices exhibit sufficiently high detection efficiencies, positive key rates can be achieved in the long-distance branch with lower detection efficiencies as compared to standard DIQKD setups. This highlights the potential for improved performance of DIQKD over extended distances in scenarios where short-distance correlations are leveraged to validate quantum correlations.

Emergent pair symmetries in systems with poor man's Majorana modes

Few-site Kitaev chains are promising for realizing Majorana zero modes without topological protection but fully nonlocal, which are known as poor man's Majorana modes. While several signatures have already been reported both theoretically and experimentally, it still remains unknown what is the nature of superconducting correlations in the presence of poor man's Majorana modes. In this paper, we study few-site Kitaev chains and demonstrate that they host pair correlations with distinct symmetries, entirely determined by the underlying quantum numbers. In particular, we find that a two-site Kitaev chain hosts local (odd-frequency) and nonlocal (odd- and even-frequency) pair correlations, both spin polarized and highly tunable by the system parameters. Interestingly, the odd-frequency pair correlations exhibit a divergent behavior around zero frequency when the nonlocal p-wave pair potential and electron tunneling are of the same order, an effect that can be controlled by the on-site energies. Since a divergent odd-frequency pairing is directly connected to the intrinsic spatial nonlocality of Majorana zero modes in topological superconductors, the divergent odd-frequency pairing here reflects the intrinsic Majorana nonlocality of poor man's Majorana modes but without any relation to topology. Our findings could help understanding the emergent pair correlations in few-site Kitaev chains. Published by the American Physical Society 2024

Enhancing quantum time transfer security: detecting intercept-resend attacks with energy-time entanglement

Abstract Quantum time transfer has emerged as a powerful technique, offering sub-picosecond precision and inherent security through the nonlocal temporal correlation property of energy-time entangled biphoton sources. In this paper, we demonstrate the inherent security advantage of quantum time transfer, and the utilization in detecting potential intercept-resend attacks. By investigating the impact of these attacks on the nonlocality identifier associated with nonlocal dispersion cancellation of energy-time entanglement, we establish a security threshold model for detecting intercept-resend attacks. Experimental verification on a 102 km fiber-optic link confirms that even a malicious delay as small as 25 ps can be identified. This investigation serves as a compelling illustration of secure two-way time transfer, safeguarding against intercept-resend attacks, and showcasing its potential applications in fields reliant on authentic time distribution between remote parties.