Nonclassical Correlations Research Articles

Two-time quantities as elements of physical reality

In recent years, time correlators have received renewed attention, especially under the guise of identifiers of nonclassical correlations. However, the physical interpretation of these objects, and more generally of multi-times variables, remains ambiguous, which may be one of the reasons why they are so difficult to measure. In this work, we introduce and advance the perspective that a two-time correlator should actually be regarded as an average involving a novel single physical observable, one that cannot be rephrased in terms of the primitive ones, according to quantum principles. In particular, we provide examples showing that the presumed constituents of a two-time correlator and the proposed two-time operator itself cannot be simultaneous elements of the physical reality.

Nonclassical Correlations between Photons and Phonons of Center-of-Mass Motion of a Mechanical Oscillator.

We demonstrate nonclassical correlations between phonons and photons created using opto-mechanical spontaneous parametric down-conversion in a system based on a soft-clamped ultracoherent membrane oscillator inside of a Fabry-Pérot optical resonator. Non-Gaussian quantum features are demonstrated for the center-of-mass motion of a submillimeter nanogram-scale mechanical oscillator. We show that phonons stored in the mechanical oscillator, when subsequently read out, display strong signs of quantum coherence, which we demonstrate by single-photon counting enabled by our state-of-the-art optical filtering system. We observe a violation of the classical two-time Cauchy-Schwarz inequality between a heralding write photon and a stored phonon with a confidence of >92%.

Nonequilibrium energy transport and nonclassical correlations of photons in a two-mode circuit quantum electrodynamic system

Nonequilibrium energy transport and nonclassical correlations of photons in a two-mode circuit quantum electrodynamic system

Vacuum entanglement probes for ultra-cold atom systems

Abstract This study explores the transfer of nonclassical correlations from an ultra-cold atom system to a pair of pulsed laser beams. Through nondestructive local probe measurements, we introduce an alternative to destructive techniques for mapping Bose–Einstein Condensate (BEC) entanglement. Operating at ultra-low temperatures, BEC density fluctuations emulate a relativistic vacuum field. We show that lasers can serve as Unruh–DeWitt detectors for vacuum BEC phonons. A quantum vacuum holds intrinsic entanglement, transferable to distant probes briefly interacting with it—a phenomenon termed ‘entanglement harvesting’. Our study accomplishes two primary objectives: first, establishing a mathematical connection between a pair of pulsed laser probes interacting with an effective relativistic field and the entanglement harvesting protocol; and second, to closely examine the potential and persisting obstacles for realising this protocol in an ultra-cold atom experiment.

Transfer of nonclassical correlations between bosonic field and atomic ensemble through electromagnetically induced transparency

Dark state polariton, as an important concept in the mechanism of electromagnetic induced transparency (EIT), can map the state of bosonic fields to atomic ensembles. To reflect the mapping ability of dark state polariton, we choose the odd and even bosonic coherent states as the probe field in EIT process, and employ spin squeezing, entanglement, and quantum correlation to characterize nonclassical correlations of atomic ensembles during the manipulation of the driving field. It is shown that the differences between the odd and even coherent states are comprehensively reflected in the three characterizations of nonclassical correlations generated through dark state polaritons. The even bosonic coherent states can perfectly transfer bosonic squeezing into atomic ensembles, resulting in spin squeezing. Although the odd bosonic coherent states cannot induce the spin squeezing, they have an advantage over the even bosonic coherent states in generating quantum entanglement and quantum correlations. Furthermore, we demonstrate that atomic ensembles can achieve significant spin squeezing with squeezing degree ∝ 1/N 2/3 through the one-axis twisting (OAT) model and two-axis twisting (TAT) model under the large N limit with the low excitation conditions, and the EIT mechanism was used to transfer the generated spin squeezing to the bosonic field, providing a feasible strategy for obtaining significant bosonic squeezing.

Non-classical correlations and coherence in a two-dimensional electron gas under the influence of Rashba spin–orbit coupling

This investigation delves into the dynamics of quantum correlations and coherence within a two-dimensional electron gas (2DEG) system, taking into account the influence of Rashba spin–orbit coupling (SOC). We utilize metrics such as logarithmic negativity ([Formula: see text]), local quantum uncertainty (LQU ([Formula: see text])), and relative entropy of coherence ([Formula: see text]) to specifically assess these quantum resources among electron spins within non-interacting electron gases. Our study investigates how the separation distance (R) between electrons and the intensity of Rashba SOC ([Formula: see text]), impact the behavior of quantum properties within the 2DEG system. We find that specific strengths of Rashba SOC can effectively regulate quantum correlations and coherence within this system. Particularly significant is our observation that optimal quantum indicators emerge when electron proximity is minimized, underscoring the substantial influence of electron distance on quantum characteristics. Conversely, as electron separation increases, these quantum metrics diminish. Therefore, by adjusting the Rashba parameter, we can enhance the resilience of these quantum measurements against increasing electron separations, providing valuable insights into the potential for controlling and manipulating quantum behavior in similar 2DEG systems.

Bayesian optimization of non-classical optomechanical correlations

Bayesian optimization of non-classical optomechanical correlations

Research on Intraparticle to Interparticle Entanglement Swapping Protocols

Entanglement is one of the most striking features of quantum systems, whereby its non-classical correlation is an essential resource in numerous quantum protocols. Entanglement can be divided into two categories: interparticle and intraparticle entanglement. There are both distinctions and similarities between these two kinds of entangled states. This work delves into these distinctions and similarities from the following aspects: correlation and non-locality, robustness, the mechanisms of generation and separation, and practical applications. Entanglement swapping is a technique based on quantum entanglement. As entanglement has different categories, entanglement swapping also has various types, including interparticle to interparticle and intraparticle to interparticle. Swapping protocols from intraparticle entanglement to interparticle entanglement can be applied to super quantum dense encoding, quantum information transmission, quantum teleportation, etc. Thus, this work proposes three swapping protocols, from spin–orbit intraparticle entanglement to spin–spin interparticle entanglement, based on Bell state joint measurement, the cross-Kerr medium, and linear optical elements. This work can help us better understand entanglement by analyzing the differences and similarities between the two types of entangled states. It can also enhance entanglement swapping protocols, from spin–orbit intraparticle to spin–spin interparticle entanglement, for use in quantum information transfer.

Quantum State Combinatorics.

This paper concerns the analysis of large quantum states. It is a notoriously difficult problem to quantify separability of quantum states, and for large quantum states, it is unfeasible. Here we posit that when quantum states are large, we can deduce reasonable expectations for the complex structure of non-classical multipartite correlations with surprisingly little information about the state. We show, with pegagogical examples, how known results from combinatorics can be used to reveal the expected structure of various correlations hidden in the ensemble described by a state.

Metrological non-classical correlations and quantum coherence in hybrid (1/2,1) system under decoherence channels

Abstract Our research focuses on the interplay between thermal noise and decoherence channels on quantum coherence and nonclassical correlations in a hybrid ( 1 / 2 , 1 ) Heisenberg model. This hybrid system integrates the Dzyaloshinsky–Moriya interaction (DMI) and operates under the influence of an external magnetic field. We use local quantum Fisher information (LQFI) for correlation estimation and relative entropy of coherence for coherence measurement in the considered system. Our investigation encompasses various parameters of the hybrid system, the strength of the DMI and the intensity of the external magnetic field. Our findings underscore that elevated temperature compromises both nonclassical correlations and coherence. On the other hand, the robustness of the DMI mitigates the impact of thermal noise on quantum Fisher information correlations and relative entropy of coherence in the hybrid system. Additionally, we inspect the impact of decohering channels-specifically, dephasing, phase flip, and bit- and trit-flip channels-on thermal coherence and quantum correlations. The introduction of various decoherence processes into the hybrid qubit-qutrit system leads to a competition with thermal fluctuations, thereby giving rise to out-of-equilibrium states. Our results indicate that as the decoherence strength parameter (p) increases, both LQFI and relative entropy of coherence exhibit similar behaviors in the dephasing and phase flip channels. These resources gradually diminish, eventually disappearing entirely at p = 1. In the context of the bit- and trit-flip channels, quantum coherence displays notable distinctions compared to what is observed under dephasing and phase flip channels, revealing that coherence can be preserved if the DMI is strong and the intensity of the external magnetic field is reduced. These findings are important since it is crucial to first understand the decoherence process, arising from the interaction with the environment, and then to find ways to hinder this decoherence in order to avoid the complete loss of the quantum resources necessary to quantum information processing.

Hierarchical certification of nonclassical network correlations

Hierarchical certification of nonclassical network correlations

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.

Unveiling non-classical correlations, quantum coherence, and steering measurement uncertainty in two-qubit $$XY-\Gamma$$ spin model

Unveiling non-classical correlations, quantum coherence, and steering measurement uncertainty in two-qubit $$XY-\Gamma$$ spin model

Non-classical correlations between a quantum probe and complex quantum systems in presence of noise

Non-classical correlations generated within a quantum probe system when it interacts with a large, macroscopic system can signal the presence of quantum features in the latter. Theoretical models have considered how entanglement generated in photosynthetic bacteria can be probed using light that interacts with them. More recently, a tardigrade was entangled to a transmon qubit. We consider a detailed model including noise for such systems wherein a small quantum probe interacts with a large system in order to delineate the regimes with respect to coupling strengths and noise levels in which such signatures of quantumness in macroscopic systems can realistically be detected.

Nonlocal advantage of quantum coherence under Garfinkle-Horowitz-Strominger dilation space-time

Nonlocal advantage of quantum coherence (NA-QC) is a relatively robust non-classical correlation and can be achieved by coherence complementary relations. In this work, we investigate NA-QC based on various coherence measures within the theoretical framework of a Garfinkle-Horowitz-Strominger dilation black hole. Interestingly, NA-QC exhibits monotonically decreasing evolutionary features with the growing dilation parameter in the physically accessible regions. Furthermore, the hierarchical relationship among NA-QC, quantum steerability and Bell-nonlocality is analyzed under the influence of Hawking radiation. It is shown that NA-QC is a subset of Bell-nonlocality and quantum steering is a superset of Bell-nonlocality. Additionally, to achieve more quantum resources, we propose a feasible physical scheme to control NA-QC by using optimal parity-time ( PT )-symmetric operations. The amount of NA-QC can be effectively enhanced in the certain time interval for the physical accessible state in the context of black hole. These results might play important roles for understanding quantum information theory under the curved space-time.

Non-classical correlations of light in the Jaynes-Cummings model

The problems concerning the influence of spectral filters on the quantum properties of light have recently attracted great attention in connection with quantum cryptography and quantum data transmission. In this paper, we consider the influence of a spectral filter on the second-order coherence function of a field of a resonator mode and a two-level atom in the framework of the Jaynes-Cummings model. Since the Heisenberg equations for the operators of the field of the resonator mode and the atom can be solved exactly, it is possible to obtain exact analytical Fourier transformation of the dynamics of operators of the resonator mode and two-level atom. We demonstrate that the second-order coherence function of the resonator mode and the two-level atom is equal to zero for all possible frequencies in the spectrum of operator oscillations. We find the interbeam second-order coherence function between different frequencies of the Fourier spectrum and show that in the limit of a large number of quanta, it can take the values in the range from zero to two. Thus, non-classical correlations are formed between certain frequencies in the Fourier spectrum of emitted light. We demonstrate that in the limit of a large number of quanta in the resonator mode, when the filter sums up the frequencies near the resonator eigenfrequency, the second-order coherence function of the field of the resonator mode is not affected by the interaction with the two-level atom.

Nonlocal coherence harvesting from quantum vacuum

It is well known that nonlocal coherence reflects nonclassical correlations better than quantum entanglement. Here, we analyze nonlocal coherence harvesting from the quantum vacuum to particle detectors adiabatically interacting with a quantum scalar field in Minkowski spacetime. We find that the harvesting-achievable separation range of nonlocal coherence is larger than that of quantum entanglement. As the energy gap grows sufficiently large, the detectors harvest less quantum coherence, while the detectors could extract more quantum entanglement from the vacuum state. Compared with the linear configuration and the scalene configuration, we should choose the model of equilateral triangle configuration to harvest tripartite coherence from the vacuum. Finally, we find a monogamous relationship, which means that tripartite l1-norm of coherence is essentially bipartite types.

Entangled two-photon quantum heat engine: Dissipative nonequilibrium dynamics and correlated statistics

Recently, the thermodynamics of open quantum systems driven by light fields has been investigated in the framework of quantum heat engines, whose output work can be measured as a spectroscopic signal. In this work, we investigate a four-level quantum heat engine that generates cascaded entangled photon pairs, treating the hot bath as an incoherent thermal pump and focusing on the correlation statistics of the output work and photon indistinguishability. We show that the dissipative nonequilibrium dynamics of this thermodynamic open quantum system can be reconstructed by correlation measurements under carefully mediated optical control. Comparing our findings with the traditional coherently pumped model, we find that the thermal pumping has the potential to generate nonclassical correlations resulting in photon indistinguishability, displaying an advantage in probing the nonequilibrium effects induced by the baths at different temperatures. Our work also demonstrates that incoherent pumping can optimize such correlations in output work beyond the classical limit. Lastly, we show that nonclassical correlations and resonant power at steady state cannot be simultaneously maximized in the weak coupling regime. Published by the American Physical Society 2024

Tracing quantum correlations back to collective interferences

In this paper, we investigate the possibility of explaining nonclassical correlations between two quantum systems in terms of quantum interferences between collective states of the two systems. We achieve this by mapping the relations between different measurement contexts in the product Hilbert space of a pair of two-level systems onto an analogous sequence of interferences between paths in a single-particle interferometer. The relations between different measurement outcomes are then traced to the distribution of probability currents in the interferometer, where paradoxical relations between the outcomes are identified with currents connecting two states that are orthogonal and should therefore exclude each other. We show that the relation between probability currents and correlations can be represented by continuous conditional (quasi)probability currents through the interferometer, given by weak values; the violation of the noncontextual assumption is expressed by negative conditional currents in some of the paths. Since negative conditional currents correspond to the assignment of negative conditional probabilities to measurements results in different measurement contexts, the necessity of such negative probability currents represents a failure of noncontextual local realism. Our results help to explain the meaning of nonlocal correlations in quantum mechanics, and support Feynman’s claim that interference is the origin of all quantum phenomena.

Chiral photon blockade in the spinning Kerr resonator

We propose how to achieve chiral photon blockade by spinning a nonlinear optical resonator. We show that by driving such a device at a fixed direction, completely different quantum effects can emerge for the counter-propagating optical modes, due to the spinning-induced breaking of time-reversal symmetry, which otherwise is unattainable for the same device in the static regime. Also, we find that in comparison with the static case, robust non-classical correlations against random backscattering losses can be achieved for such a quantum chiral system. Our work, extending previous works on the spontaneous breaking of optical chiral symmetry from the classical to purely quantum regimes, can stimulate more efforts towards making and utilizing various chiral quantum effects, including applications for chiral quantum networks or noise-tolerant quantum sensors.