Nonclassical Correlations Research Articles

Classical and quantum orbital correlations in molecular electronic states

The quantum superposition principle has been extensively utilized in the quantum mechanical description of bonding phenomenon. It explains the emergence of delocalized molecular orbitals and provides a recipe for the construction of near-exact electronic wavefunctions. On the other hand, its existence in composite systems may give rise to nonclassical correlations that are regarded as a resource in quantum technologies. Here, we approach the electronic ground states of three prototypical molecules in the light of the framework set by fermionic information theory. By introducing the notion of orbital discord, we additively decompose the pairwise orbital correlations into their classical and quantum parts in the presence of superselection rules. We observe that quantum orbital correlations can be stronger than classical orbital correlations though not often. Moreover, quantum orbital correlations can survive even in the absence of orbital entanglement depending on the symmetries of the constituent orbitals. Finally, we demonstrate that orbital entanglement would be underestimated if the orbital density matrices were treated as qubit states.

Thermal non-classical correlation via skew information, quantum Fisher information, and quantum teleportation of a spin-1/2 Heisenberg trimer system

Thermal non-classical correlation via skew information, quantum Fisher information, and quantum teleportation of a spin-1/2 Heisenberg trimer system

Exploiting non-linear effects in optomechanical sensors with continuous photon-counting

Optomechanical systems are rapidly becoming one of the most promising platforms for observing quantum behaviour, especially at the macroscopic level. Moreover, thanks to their state-of-the-art methods of fabrication, they may now enter regimes of non-linear interactions between their constituent mechanical and optical degrees of freedom. In this work, we show how this novel opportunity may serve to construct a new generation of optomechanical sensors. We consider the canonical optomechanical setup with the detection scheme being based on time-resolved counting of photons leaking from the cavity. By performing simulations and resorting to Bayesian inference, we demonstrate that the non-classical correlations of the detected photons may crucially enhance the sensor performance in real time. We believe that our work may stimulate a new direction in the design of such devices, while our methods apply also to other platforms exploiting non-linear light-matter interactions and photon detection.

Nonlocal quantum correlations in a bipartite quantum system coupled to a bosonic non-Markovian reservoir

In this paper, we explore the dynamics of nonlocal correlations in a two-qubit system, that is, first prepared in a Gisin state and then interacts with a bosonic non-Markovian environment. We employ uncertainty-induced nonlocality (UIN) and the Horodecki measure (Bell function) to characterize the degree of nonclassical correlations and quantum nonlocality in the system, taking into account the influence of the non-Markovian reservoir. The dynamics of the UIN and the Bell nonlocality are next examined using the various parameters that define the non-Markovian reservoir and the initial Gisin state. Our results show that the amount of nonlocal correlations and the degree of violation of Bell’s inequality can be modulated by varying the physical parameters characterizing the non-Markovian reservoir and the initial Gisin state. We also show that in some specific cases, the system exhibits nonclassical correlations while the evolved Gisin state does not violate Bell’s inequality. Our results also confirm that UIN is robust than Bell’s nonlocality in the presence of decoherence induced by the interaction with the non-Markovian reservoir.

Experimental Investigation of Quantum Correlations in a Two-Qutrit Spin System.

We report an experimental investigation of quantum correlations in a two-qutrit spin system in a single nitrogen-vacancy center in diamond at room temperatures. Quantum entanglement between two qutrits was observed at room temperature, and the existence of nonclassical correlations beyond entanglement in the qutrit case has been revealed. Our work demonstrates the potential of the NV centers as the multiqutrit system to execute quantum information tasks and provides a powerful experimental platform for studying the fundamental physics of high-dimensional quantum systems in the future.

Quantum memory and coherence dynamics of two dipole-coupled qubits interacting with two cavity fields under decoherence effect

This paper investigates the intrinsic decoherence effect on quantum memory and coherence dynamics of two dipole-coupled qubits in a non-degenerate bimodel cavity. Entropic uncertainty, entropy, and concurrence are utilized to examine the dynamics of quantum-memory-assisted entropic uncertainty relations, mixedness, and entanglement. The results indicate that the nonlinear interactions between the two qubits and the cavity generate quantum-memory-assisted entropic uncertainty, mixedness, and entanglement, which are reliant not only on the qubit–qubit interaction but also on the intrinsic decoherence and coherence intensity of the initial two-mode cavity states. For specific values of the dipole–dipole interaction, non-classical correlations can be enhanced By reducing the initial intensity coherence, the stability of the quantum memory-assisted entropic uncertainty relations, as well as the two-qubit mixedness of the intrinsic decoherence, are accelerated.

Quantum coherence versus non-classical correlations in XXZ spin-chain under Dzyaloshinsky–Moriya (DM) and KSEA interactions

Quantum coherence versus non-classical correlations in XXZ spin-chain under Dzyaloshinsky–Moriya (DM) and KSEA interactions

Controllable Nonclassical Correlation by an Incoherent Pumping Field in a Λ‐Type Atom‐Cavity System

It is important to generate stable nonclassical correlation and wide-band quantum field in quantum optical communication. In this paper, a scheme is proposed to manipulate the nonclassical intensity correlation of two output fields by a coherent coupling laser and an incoherent pumping field. The core of the scheme is that the coherent field is applied to manipulate the intracavity polaritons, and the incoherent pumping field is used to control the linear absorption and the atomic coherence. With the incoherent field, atoms are repopulated on the excited state | 3 ⟩ $|3\rangle$ . It induces a wide-band zero absorption of a probe field and the negative dispersion that can be used to realize a white-light cavity. In addition, with the increasement of the incoherent pumping rate, the wide-band maximum stable intensity correlation and the nonclassical field with sub-Poissonian distribution are attained, which is verified by the evolution of the second-order correlation of two output channels.

Nonclassical correlations in subsystems of globally entangled quantum states

The relation between genuine multipartite entanglement in the pure state of a collection of N qubits and the nonclassical correlations in its two-qubit subsystems is studied. Quantum discord is used as the quantifier of nonclassical correlations in the subsystem while the generalised geometric measure (GGM) [Phys. Rev. A. 81, 012308 (2010)] is used to quantify global entanglement in the N-qubit state. While no definite discernible dependence between the two can be found for randomly generated global states, for those with additional structure like weighted graph states we find that local discord is indicative of global multipartite entanglement. Global states that admit efficient classical descriptions like stabilizer states furnish an exception in which despite multipartite entanglement, nonclassical correlation is absent in two qubit subsystems. We discuss these results in the context of mixed state quantum computation where nonclassical correlation is considered a candidate resource that enables exponential speedup over classical computers.

Behavior of local quantum Fisher information and local quantum uncertainty in dipolar spin system with DM and KSEA interactions

In this paper, a two-spin-1/2 particle coupled via dipolar interaction with the interplay of antisymmetric Dzyaloshinskii–Moriya and symmetric Kaplan–Shekhtman–Entin–Wohlman–Aharony interactions is considered at thermal equilibrium. The local quantum uncertainty and local quantum Fisher information are investigated as the pairwise nonclassical correlations quantifiers. The effects of the different system parameters on the quantified quantum correlation are analyzed in detail. Our results showed that the two studied thermal quantum correlation measures have similar qualitative behavior as well as quantitative sometimes. Also, it is shown that the quantumness of correlations is lost as the temperature rises sufficiently. Nevertheless, the antisymmetric and symmetric contributions of spin-orbit coupling considerably improve the amount of quantum correlations. Our analysis emphasized also that the local quantum uncertainty is bounded by local quantum Fisher information.

Dynamics of quantum correlations in a quantum dot system with intrinsic decoherence effects

In this paper, we explore the dynamics of quantum correlations in an isolated physical quantum dot under the influence of intrinsic decoherence (ID). We characterize quantum correlations in the quantum dot system using Concurrence to capture the amount of entanglement, and we use Trace Distance Discord (TDD) and Local Quantum Uncertainty (LQU) to measure nonclassical correlations. Likewise, we investigate the effect of ID on the temporal evolution of these quantifiers. In particular, the behavior of the two discord-like quantifiers in terms of the system parameters and the ID rate is investigated and analyzed in detail. We found that TDD and LQU behave similarly for different parameters characterizing the considered system and that TDD is more resistant than Concurrence and LQU against the ID effects. Our results also show that the robustness of quantum correlations can be modulated by adjusting the ID rate, quantum dot physical properties and initial conditions.

Non-classical correlations in a Heisenberg spin model with Heitler–London approach

Non-classical correlations in a Heisenberg spin model with Heitler–London approach

Nonclassical correlations of radiation in relation to Bell nonlocality

We analyze nonclassical correlations between outcomes of measurements conducted on two spatial radiation modes. These correlations cannot be simulated with statistical mixtures of coherent states or, more generally, with non-negative phase-space functions of quantum states and measurements. We argue that nonclassical correlations are naturally related to Bell nonlocality, the former being a more general class of quantum correlations. Indeed, it is known that local realistic as well as noncontextual models correspond to non-negative solutions to a system of linear equations for the joint probability distributions of all observables. We demonstrate that nonclassical correlations correspond to a particular solution to this system, which may have negative values even if local realism is not violated. A modification of Bell inequalities enables us to test such correlations. At the same time, our approach leads to a formulation of Bell inequalities applicable also to continuous variables. The results are illustrated with two-mode squeezed vacuum states and with hybrid entangled states (Schr\"odinger-Cat states), one mode being analyzed by balanced and the other one by unbalanced homodyne detection.

Robustness of nonlocality in many-body open quantum systems

Non-locality consists in the existence of non-classical correlations between local measurements. So far, it has been investigated mostly in isolated quantum systems. Here we show that non-local correlations are present, can be detected and might be robust against noise also in many-body open quantum systems, both in the steady-state and in the transient regime. We further discuss the robustness of non-local correlations when the open quantum system undergoes repeated measurements, a relevant scenario for quantum cryptography.

Intrinsic decoherence effects on nonclassical correlations in a symmetric spin–orbit model

Nonclassical correlations dynamics of local quantum Fisher information, trace-norm measurement-induced nonlocality, and log-negativity are investigated in a symmetric spin–orbit model. This model is described by a Heisenberg XYZ two-spin system exposed to the magnetic field and Kaplan–Shekhtman–Entin-Wohlman–Aharony (KSEA) interaction under the intrinsic decoherence effects. To evaluate the resourcefulness of this model for the generation and preservation of quantum correlations, we consider two qubits that are initially prepared in two different states, namely uncorrelated and maximally correlated states. The magnetic field is discovered to be effective at generating and preserving nonclassical correlations. Besides, we find that the KSEA interaction can alter the generation, preservation, and revival features of nonclassical correlations in the two-qubit system. However, the bipartite nonclassical correlations become extremely fragile when intrinsic decoherence effects appear.

Non-classical Correlations in Multipartite Generalized Coherent States

Non-classical Correlations in Multipartite Generalized Coherent States

Robust thermal correlations induced by spin–orbit interactions

In this work, we demonstrate that the thermal nonclassical correlations can enhance and become more stable under the magnetic field and second-order spin–orbit interaction effect. Also, we show that the local quantum uncertainty measure can reveal purely quantum correlations beyond other quantifiers of correlations. Our analytical model describes a Heisenberg XYZ two-spin system with an inhomogeneous magnetic field and spin–orbit interaction. A special emphasis is devoted to local quantum uncertainty, concurrence, and Bell-nonlocality measures to investigate the correlations of the bipartite-system at the equilibrium temperature. Our results indicate that temperature, magnetic field, and magnetic field inhomogeneity with DMI and Kaplan–Shekhtman–Entin-Wohlman–Aharony (KSEA) exchanges all play a role in determining the degree of correlation between the quantum spins to some extent. The KSEA interaction greatly enhances the quantum correlation in the system whereas quantum quantifiers are more stable under the influence of the external magnetic field, especially when inhomogeneous and homogeneous magnetic field are near to zero. Furthermore, these findings suggest that the thermal LQU captures a stronger quantum correlation than the Bell-nonlocality and entanglement measures.

Dynamics of quantum effects in a three-level system interacting with two-mode time-dependent fields including parametric down conversion and damping

In this article, the interaction between a time-dependent damping field and Λ-type of three-level atom is investigated. The effect of the atom and the fields' parameters on the behaviour of the atomic inversion, the generated entanglement between the atom and the cavity modes, the Pancharatnam geometric phase are discussed. It is shown that the phenomena of the collapses and revivals for the three physical quantities depend on the time-dependent coupling and the damping parameter. The long-lived behaviour of the non-classical correlation is observed either on the presence of only time-dependent coupling or at large values of the damping parameter. The damping effect has different features on the three phenomena, where the atomic inversion decays, the upper bounds of entanglement increases and increases periodic time of the Pancharatnam geometric phase.

Linear response theory of entanglement entropy

Linear response theory (LRT) is a powerful tool for investigating classical and quantum systems when perturbed by some external forces, connecting experimental observables with the correlation functions of the system in equilibrium states. On the other hand, the entanglement entropy (EE), or von Neumann entropy, is an important measure of non-classical correlations in quantum information science. As the EE is not a normal physical observable, developing a LRT for EE is valuable for understanding the changes of entanglement under an external perturbation. In this work, we present a framework of LRT of the von Neumann entropy. We found that for any composite quantum state the linear response of the von Neumann entanglement entropy can be quantified by a special correlation function. Consequently, we can derive the corresponding Kubo formula and the susceptibility of the EE, which have the same properties as its conventional counterpart. We further found that the linear response of the EE is zero for maximally entangled or separable states, a unique feature of entanglement dynamics. A numerical verification of our analytical results is also given based on the XX spin chain model. Overall, the LRT of EE provides a useful tool in investigating and understanding EE.

Nonclassical correlated deterministic single-photon pairs for a trapped atom in bimodal cavities

Single photons and single-photon pairs, inherently nonclassical in their nature, are fundamental elements of quantum sciences and technologies. Here, we propose to realize the nonclassical correlated deterministic photon pairs at the single-photon level for a single atom trapped in bimodal cavities. It is shown that the photon emissions for bimodal cavities exhibit a high single photon purity and relatively large intracavity photon numbers, which are ascribed to the combination of optical Stark shift enhanced energy-spectrum anharmonicity and quantum interference suppressed two-photon excitation. Our scheme generates photon-photon pairs with strong photon blockade for cavity modes via constructing a highly tunable cavity-enhanced deterministic parametric down-conversion process. Furthermore, we show that the cross-correlation function between the two cavity modes vastly violates the Cauchy-Schwarz inequality of the classical boundary, which unambiguously demonstrates the nonclassicality of the single-photon pairs. Our result reveals a prominent strategy to generate the high-quality two-mode single-photon sources with strong nonclassical correlation, which could provide versatile applications in distribution of quantum networks, long distance quantum communication, and fundamental tests of quantum physics.