Quantum Correlations Research Articles

Sharing asymmetric Einstein–Podolsky–Rosen steering with projective measurements

Recently, both global and local classical randomness-assisted projective measurement protocols have been employed to share Bell nonlocality of an entangled state among multiple sequential parties. Unlike Bell nonlocality, Einstein–Podolsky–Rosen (EPR) steering exhibits distinct asymmetric characteristics and serves as the necessary quantum resource for one-sided device-independent quantum information tasks. In this work, we propose a projective measurement protocol and investigate the shareability of EPR steering with steering radius criterion theoretically and experimentally. Our results reveal that arbitrarily many independent parties can share one-way steerability using projective measurements, even when no shared randomness is available. Furthermore, by leveraging only local randomness, asymmetric two-way steerability can also be shared. Our work not only deepens the understanding of the role of projective measurements in sharing quantum correlations but also opens up a new avenue for reutilizing asymmetric quantum correlations.

Variational data encoding and correlations in quantum-enhanced machine learning

Abstract Leveraging the extraordinary phenomena of quantum superposition and quantum correlation, quantum computing offers unprecedented potential for addressing challenges beyond the reach of classical computers. This paper tackles two pivotal challenges in the realm of quantum computing: firstly, the development of an effective encoding protocol for translating classical data into quantum states, a critical step for any quantum computation. Different encoding strategies can significantly influence quantum computer performance. Secondly, we address the need to counteract the inevitable noise that can hinder quantum acceleration. Our primary contribution is the introduction of a novel variational data encoding method, grounded in quantum regression algorithm models. By adapting the learning concept from machine learning, we render data encoding a learnable process. This allowed us to study the role of quantum correlation in data encoding. Through numerical simulations of various regression tasks, we demonstrate the efficacy of our variational data encoding, particularly post-learning from instructional data. Moreover, we delve into the role of quantum correlation in enhancing task performance, especially in noisy environments. Our findings underscore the critical role of quantum correlation in not only bolstering performance but also in mitigating noise interference, thus advancing the frontier of quantum computing.

Characterizing nonclassical correlation via local channels

Local operation is an important tool to characterize the nonlocal aspects of multipartite quantum system. Exploiting the notion of resource theory of coherence, in this article, we establish a quantum correlation measure as the difference between the bipartite coherence and marginal state coherence. We study the Tsallis α-entropy (TαE) coherence based quantum correlation of bipartite state relative to different channels such as unitary channel, the twirling (unitary-induced) channel, projective measurements and weak measurements. It is shown that the quantum channel helps us to discriminate the product and classical-quantum states. We provide the operational interpretation of the correlation measure relative to the measurement in terms of classical uncertainty of channel. A closer connection between the correlation relative to the projective measurements and weak measurements is obtained in terms of measurement strength. As an illustration, we have studied the quantum correlations of well-known two-qubit states.

Enhanced phase sensitivity in a feedback-assisted interferometer

The topology of feedback optical parametric amplifier (FOPA) renders a number of significant advantages over the topology of traditional optical parametric amplifier (TOPA) such as a higher degree of quantum correlation, all-phase entanglement enhancement, and the robustness of the losses. Here, we propose a feedback-assisted interferometer based on the topology of FOPA for quantum metrology. We theoretically study the phase sensitivity with the method of homodyne detection and product detection. By manipulating the feedback strength of the FOPA, the phase sensitivity can be further enhanced, and approach the quantum Cramér-Rao bound. Furthermore, we demonstrate that our proposal is superior to the SU(1,1) interferometer based on the topology of TOPA.

Nonclassicality and teleportation fidelity probes in amplitude-tailored superconducting charge qubits

Optimal procedures and appropriate designs of transmitting channels are required to preserve quantum correlations (QCs) in quantum teleportation schemes. In pursuit of this objective, a comprehensive investigation into the intricate dynamics of a quantum channel comprising two singular Cooper-pair charge qubits, mutually coupled and influenced by Josephson energy, has been undertaken. The use of the Gibbs density operator has provided valuable insights into the thermal quantum characteristics and teleportation fidelity of the system. Furthermore, this study underscores the critical interplay between the energy parameters and the utilization of quantum features to establish conducive conditions for the preservation of nonclassicality and the success of the teleportation scheme. Additionally, the study reveals that trace distance discord is the most resilient in the thermal output state, surpassing both concurrence and the degree of violation of Bell’s inequality. Notably, precise control of the input state amplitude and adept manipulation of the energy parameters have enhanced QCs and resulted in the teleportation fidelity exceeding the classical limit.

Error-tolerant quantum convolutional neural networks for symmetry-protected topological phases

The analysis of noisy quantum states prepared on current quantum computers is getting beyond the capabilities of classical computing. Quantum neural networks based on parametrized quantum circuits, measurements and feed-forward can process large amounts of quantum data to reduce measurement and computational costs of detecting nonlocal quantum correlations. The tolerance of errors due to decoherence and gate infidelities is a key requirement for the application of quantum neural networks on near-term quantum computers. Here we construct quantum convolutional neural networks (QCNNs) that can, in the presence of incoherent errors, recognize different symmetry-protected topological phases of generalized cluster-Ising Hamiltonians from one another as well as from topologically trivial phases. Using matrix product state simulations, we show that the QCNN output is robust against symmetry-breaking errors below a threshold error probability and against symmetry-preserving errors provided the error channel is invertible. This is in contrast to string order parameters and the output of previously designed QCNNs, which vanish in the presence of any symmetry-breaking errors. To facilitate the implementation of the QCNNs on near-term quantum computers, the QCNN circuits can be shortened from logarithmic to constant depth in system size by performing a large part of the computation in classical postprocessing. These constant-depth QCNNs reduce sample complexity exponentially with system size in comparison to the direct sampling using local Pauli measurements. Published by the American Physical Society 2024

Quantum chaos in the presence of nonconformality

The behavior of a chaotic system and its effect on existing quantum correlation has been holographically studied in the presence of nonconformality. Keeping in mind the gauge/gravity duality framework, the nonconformality in the dual field theory has been introduced by considering a Liouville type dilaton potential for the gravitational theory. The resulting black brane solution is associated with a parameter η which represents the deviation from conformality. The parameters of chaos, namely, the Lyapunov exponent and butterfly velocity are computed by following the well-known shock wave analysis. The obtained results reveal that the presence of nonconformality leads to suppression of the chaotic nature of a system. Further, for a particular value of the nonconformal parameter η, the system achieves Lyapunov stability resulting from the vanishing of both the Lyapunov exponent as well as butterfly velocity. Interestingly, this particular value of η matches with the previously given upper bound of η known as Gubser bound in the literature. The effects of chaos and nonconformality on the existing correlation of a thermofield doublet state have been quantified by holographically computing the thermomutual information in both the presence and absence of the shock wave. Furthermore, the entanglement velocity is also computed, and the effect of nonconformality on it has been observed. Finally, the obtained results for the Lyapunov exponent and the butterfly velocity have also been computed from the pole-skipping analysis. The results from the two approaches agree with each other. Published by the American Physical Society 2024

Compressing continuous variable quantum measurements

We generalize the notion of joint measurability to continuous variable systems by extending a recently introduced compression algorithm of quantum measurements to this realm. The extension results in a property that asks for the minimal dimensional quantum system required for representing a given set of quantum measurements. To illustrate the concept, we show that the canonical pair of position and momentum is completely incompressible. We translate the concept of measurement compression to the realm of quantum correlations, where it results in a generalization of continuous variable quantum steering. In contrast to the steering scenario, which detects entanglement, the generalization detects the dimensionality of entanglement. We illustrate the bridge between the concepts by showing that an analogue of the original EPR argument is genuinely infinite-dimensional with respect to our figure of merit, and that a fundamental discrete variable result on preparability of unsteerable state assemblages with separable states does not directly carry over to the continuous variable setting. We further prove a representation result for partially entanglement breaking channels that can be of independent interest.

Influences of 2-ethylanthraquinone on lignin degradation in alkaline cooking of Eucalyptus pellita wood

The impact of 2-ethylanthraquinone (2-EAQ) on alkaline cooking of Eucalyptus pellita wood was examined by analyzing pulp yield, kappa number, the structure and weight-averaged molecular weight (Mw) of dissolved lignin in black liquor and residual lignin in pulp (RL). During soda cooking under 23% of active alkaline (AA) condition, adding 0.06% 2-EAQ reduced the kappa number by 12 points compared to cooking without it. The addition of 0.06% 2-EAQ to kraft cooking increased pulp yield by about 4% at about 19 of kappa number compared to the no addition. The analysis of lignin structure by using semiquantitative heteronuclear single quantum correlation nuclear magnetic resonance (HSQC NMR) showed that the addition of 2-EAQ promoted β-ether cleavage during alkaline cooking and reduced peeling reaction of polysaccharides and prevention of the elimination of γ-OH of lignin to form enol ethers and stilbenes. Overall, the addition of 2-EAQ promoted the degradation of lignin to lower molecular weight compounds, with a more pronounced effect observed in soda cooking than in kraft cooking. Under optimal conditions, the delignification efficiency and pulp yield of soda-EAQ pulping were comparable to kraft cooking.

Quantum fluctuation dynamics of open quantum systems with collective operator-valued rates, and applications to Hopfield-like networks

We consider a class of open quantum many-body systems that evolves in a Markovian fashion, the dynamical generator being in GKS-Lindblad form. Here, the Hamiltonian contribution is characterized by an all-to-all coupling, and the dissipation features local transitions that depend on collective, operator-valued rates, encoding average properties of the system. These types of generators can be formally obtained by generalizing, to the quantum realm, classical (mean-field) stochastic Markov dynamics, with state-dependent transitions. Focusing on the dynamics emerging in the limit of infinitely large systems, we build on the exactness of the mean-field equations for the dynamics of average operators. In this framework, we derive the dynamics of quantum fluctuation operators, that can be used in turn to understand the fate of quantum correlations in the system. We then apply our results to quantum generalized Hopfield associative memories. Here we show that, asymptotically and at the description level of quantum fluctuations, only a very weak amount of quantum correlations, in the form of quantum discord, emerges beyond classical correlations.

Measurement device-independent quantum state discrimination

Quantum state discrimination depicts the general progress of extracting classical information from quantum systems. We show that quantum state discrimination can be realized in a measurement device-independent scenario using tools of self-testing results. That is, the states can be discriminated credibly with the untrusted experiment devices by the correspondence between quantum correlations and states. In detail, we show that two states that are not conjugate with each other can be discriminated without additional trusted input states, and the discrimination of others require a kind of trusted inputs. Further we show the guessing probability analysis of this protocol for minimum error discrimination, which is acceptable based on numerical results.

Dynamical Coulomb blockade: An all electrical probe of the ultrastrong light-matter coupling regime

The regime of ultrastrong light-matter coupling (USC) has attracted considerable interest recently, as it predicts fascinating quantum phenomena such as the presence of virtual photons in the ground state. Here we propose theoretically an approach to probe the quantum correlations in USC systems based on an electrical transport experiment. Namely, we consider the case in which the light-matter coupled system constitutes the electromagnetic environment of an ultrasmall tunnel junction operating in the regime of a dynamical Coulomb blockade (DCB). Unlike optical spectroscopy, such a probe can be seen as a nondemolition measurement of the USC system ground state. We have developed two approaches in order to evaluate the effect of the light-matter coupled states on the tunneling process. The first is a generalized, multimode Peierls substitution for the tunneling Hamiltonian. The second is based on the P(E) theory describing a DCB, where the USC system is treated as an equivalent circuit. The two approaches are consistent, whereas the second one enables us explicitly to take into account the presence of dissipation. This study has been performed for the zero-temperature case, which is pertinent for the terahertz spectral range where excitation energies are typically much higher compared to the microwave domain, where DCB was first studied. Published by the American Physical Society 2024

Persistence of Correlations in Neurotransmitter Transport through the Synaptic Cleft.

The "quantum brain" proposal can revolutionize our understanding of cognition if proven valid. The core of the most common "quantum brain" mechanism is the appearance of correlated neuron triggering induced by quantum correlations between ions. In this work, we examine the preservation of the correlations created in the pre-synaptic neurons through the transfer of neurotransmitters across the synaptic cleft, a critical ingredient for the validity of the "quantum brain" hypothesis. We simulated the transport of two neurotransmitters at two different clefts, with the only assumption that they start simultaneously, and determined the difference in their first passage times. We show that in physiological conditions, the correlations are persistent even if the parameters of the two neurons are different.

Time-bin entanglement in the deterministic generation of linear photonic cluster states

We theoretically investigate strategies for the deterministic creation of trains of time-bin entangled photons using an individual quantum emitter described by a Λ-type electronic system. We explicitly demonstrate the theoretical generation of linear cluster states with substantial numbers of entangled photonic qubits in full microscopic numerical simulations. The underlying scheme is based on the manipulation of ground state coherences through precise optical driving. One important finding is that the most easily accessible quality metrics, the achievable rotation fidelities, fall short in assessing the actual quantum correlations of the emitted photons in the face of losses. To address this, we explicitly calculate stabilizer generator expectation values as a superior gauge for the quantum properties of the generated many-photon state. With widespread applicability in other emitter and excitation–emission schemes also, our work lays the conceptual foundations for an in-depth practical analysis of time-bin entanglement based on full numerical simulations with predictive capabilities for realistic systems and setups, including losses and imperfections. The specific results shown in the present work illustrate that with controlled minimization of losses and realistic system parameters for quantum-dot type systems, useful linear cluster states of significant lengths can be generated in the calculations, discussing the possibility of scalability for quantum information processing endeavors.

Quantum correlation, entanglement in the kagome lattice non-Hermitian quantum systems

Quantum correlation in the antiferromagnetic XXZ model on kagome lattice and non-Hermitian tight binding model on kagome lattice are investigated. For the Hermitian XXZ model on kagome lattice, the calculations are performed for different extensions of the model with single-ion anisotropy and out-of-plane Dzyaloshinskii–Moriya interaction (DMI). We get the von Neumann entropy as a function of the DMI interaction, anisotropy and single-ion anisotropy with aim to verify the effect of these corrections that occurs in real magnetic systems on quantum correlation. Moreover, the effect of DMI interaction, anisotropy and single-ion anisotropy is investigated on concurrence and entanglement negativity as well. For the non-Hermitian tight binding model on kagome lattice, we analyzed quantum correlation using the entanglement negativity as entanglement measure which is more adequate in this case.

Enhanced Gaussian interferometric power, entanglement and Gaussian quantum steering in magnonics system with squeezed light

In this study, we propose a scheme to improve quantum correlations (QCs) between two magnons in a tripartite magnonical system. We use Gaussian interferometric power (GIP) to quantify QCs beyond entanglement. Additionally, Gaussian quantum steering is discussed. We investigate the enhancement of QCs via a squeezing parameter and an optical parametric amplifier (OPA). The Mancini criterion is considered to confirm the presence of shared entanglement between the two magnons. We observed a squeezing of about 7 dB for the first magnon. Additionally, the squeezed vacuum field and the OPA improve the generation of genuine tripartite entanglement. We hope that current experimental technology will allow the proposed scheme to be implemented.

Entropy production rate and correlations in a cavity magnomechanical system

We present the irreversibility generated by a stationary cavity magnomechanical system composed of a yttrium iron garnet (YIG) sphere with a diameter of a few hundred micrometers inside a microwave cavity. In this system, the magnons, i.e., collective spin excitations in the sphere, are coupled to the cavity photon mode via magnetic dipole interaction and to the phonon mode via magnetostrictive force (optomechanical-like). We employ the quantum phase-space formulation of the entropy change to evaluate the steady-state entropy production rate and associated quantum correlation in the system. We find that the behavior of the entropy flow between the cavity photon mode and the phonon mode is determined by the magnon-photon coupling and the cavity photon dissipation rate. Interestingly, the entropy production rate can increase/decrease depending on the strength of the magnon-photon coupling and the detuning parameters. We further show that the amount of correlations between the magnon and phonon modes is linked to the irreversibility generated in the system for small magnon-photon coupling. Our results demonstrate the possibility of exploring irreversibility in driven magnon-based hybrid quantum systems and open a promising route for quantum thermal applications. Published by the American Physical Society 2024

Enhancement and manipulation of nonreciprocity via dissipative coupling.

Classical and quantum nonreciprocity have important applications in information processing due to their special one-way controllability for physical systems. In this paper we investigate the nonreciprocal transmission and quantum correlation by introducing the dissipative coupling into a linear coupling system consisting of two microdisk resonators. Our research results demonstrate that even in the case of a stationary resonator, dissipative coupling can effectively induce nonreciprocity within the system. Moreover, the degree of nonreciprocity increases with the dissipative coupling strength. Importantly, the phase shift between the dissipative coupling and coherent coupling serves as a critical factor for controlling both nonreciprocal transmision and one-way quantum steering. Consequently, the introduction of dissipative coupling not only enhances the nonreciprocal transmission and nonreciprocal quantum correlation but also enables on-demand manipulation of nonreciprocity. This highlights dissipation as an effective means for manipulating classical and quantum nonreciprocity, thus playing a favorable role in chiral quantum networks.

Crossover behavior at an exceptional point for quantum entanglement and correlation in a non-Hermitian XY spin system

Crossover behavior at an exceptional point for quantum entanglement and correlation in a non-Hermitian XY spin system

Efficient and reliable detection of nonlocal quantum correlations in continuous-variable and hybrid states via random measurements

We investigate the violation of nonlocal realism using various entangled continues- and hybrid-variable states under dichotomic observables. In particular, we consider two cases of dichotomic observables (1) described by a pseudospin operator and (2) given in terms of the Wigner representation of the state in phase space, parity measurement and displacement operation. We address the recently proposed operational measure of nonlocality which describes the probability of local-realism violation under randomly sampled observables. We show the usefulness and limitations of the probability of local-realism violation for the detection of nonlocality. A simple procedure to detect such nonlocal correlations for randomly chosen settings with efficiencies of up to 100% is proposed. The practical advantage of applying random measurements that considerably lowers the experimental requirements is mentioned.