Search

Within quantum information frameworks, managing decoherence stands as a pivotal task. The present work delves into decoherence dynamics of a dressed qubit, represented by a spinless fermion hopping between two lattice sites that are strongly coupled to a collective bosonic bath. To simplify calculations under strong coupling, we adopt the Lang-Firsov transformation, effectively minimizing system-bath interactions. Within the polaron perspective using Ohmic bath spectral density with a Gaussian cutoff, we identify a fundamental timescale s (equivalently a length scale l), dictating coherence decay. Utilizing a quantum master equation in the energy eigen basis while maintaining fixed particle number, we demonstrate that coherence persists for small s values but diminishes for larger ones. Additionally, we explore the utilization of π -pulses to manipulate decoherence within the system. [ABSTRACT FROM AUTHOR]

Self-interactions and interaction with the environment inevitably push quantum systems towards highly mixed states, which is the definition of decoherence. However, if decoherence is completely suppressed, then any quantum system evolves unitarily through pure states towards maximally entangled configurations. We argue that these maximally entangled states of the systems fall into the well-defined classes that can be uniquely described by the values of certain entanglement invariants. After discussing these ideas, we present examples of maximally entangled states for a number of generic systems, construct compact states in the most entangled classes for tripartite systems, and suggest how they may be constructed for other n -partite systems. We study random walks through the space of entanglement classes to see how unitary evolution along paths to the maximally entangled states, i.e. entanglement growth, works in practice. [ABSTRACT FROM AUTHOR]

Abstract The inherent association between real quantum systems and their surrounding environment invariably results in decoherence, leading to the loss of entanglement. This diminution in entanglement coincides with a decline in the fidelity of transmitted information using the entangled quantum resource. This study scrutinizes the impact of the squeezed generalized amplitude damping (SGAD) channel on quantum Fisher information (QFI) parameters. The SGAD channel model, a versatile framework, is also employed to simulate other dissipative channels, including amplitude damping (AD) and generalized amplitude damping (GAD). Kraus operators facilitate the modeling of noisy channels. The results reveal that, within the SGAD channel, the QFI remains impervious to the squeezing variables (r and $$\Phi$$ Φ ). In the GAD channel, $$F\theta _{GAD}$$ F θ GAD undergoes enhancement to a constant value with an upswing in temperature (T), while the $$\phi$$ ϕ parameter in the GAD channel, $$F\phi _{GAD}$$ F ϕ GAD , akin to the SGAD channel, surges around T = 2 before complete loss ensues. Concerning the AD channel, the $$\theta$$ θ component of the QFI initially experiences decoherence with an augmentation in the AD noise parameter ( $$\lambda$$ λ ). Subsequently, it is restored to its initial value with a further escalation in $$\lambda$$ λ . Conversely, the $$\phi$$ ϕ component of the QFI in the AD channel experiences decoherence with an elevation in the AD noise parameter ( $$\lambda$$ λ ).

The inherent association between real quantum systems and their surrounding environment invariably results in decoherence, leading to the loss of entanglement. This diminution in entanglement coincides with a decline in the fidelity of transmitted information using the entangled quantum resource. This study scrutinizes the impact of the squeezed generalized amplitude damping (SGAD) channel on quantum Fisher information (QFI) parameters. The SGAD channel model, a versatile framework, is also employed to simulate other dissipative channels, including amplitude damping (AD) and generalized amplitude damping (GAD). Kraus operators facilitate the modeling of noisy channels. The results reveal that, within the SGAD channel, the QFI remains impervious to the squeezing variables (r and Φ ). In the GAD channel, F θ GAD undergoes enhancement to a constant value with an upswing in temperature (T), while the ϕ parameter in the GAD channel, F ϕ GAD , akin to the SGAD channel, surges around T = 2 before complete loss ensues. Concerning the AD channel, the θ component of the QFI initially experiences decoherence with an augmentation in the AD noise parameter (λ ). Subsequently, it is restored to its initial value with a further escalation in λ . Conversely, the ϕ component of the QFI in the AD channel experiences decoherence with an elevation in the AD noise parameter (λ ). [ABSTRACT FROM AUTHOR]

We consider a recently introduced framework for the description of memory effects based on quantum state distinguishability quantifiers, in which entropic quantifiers can be included. After briefly presenting the approach, we validate it considering the performance of different quantifiers in the characterization of the reduced dynamics of a two-level system undergoing decoherence. We investigate the different behaviors of these quantifiers in the dependence on physical features of the model, such as environmental temperature and coupling strength. It appears that the performance of the different quantifiers conveys the same physical information, though with different sensitivities, thus supporting robustness of the approach. [ABSTRACT FROM AUTHOR]

Tensor network quantum machine learning (QML) models are promising applications on near-term quantum hardware. While decoherence of qubits is expected to decrease the performance of QML models, it is unclear to what extent the diminished performance can be compensated for by adding ancillas to the models and accordingly increasing the virtual bond dimension of the models. We investigate here the competition between decoherence and adding ancillas on the classification performance of two models, with an analysis of the decoherence effect from the perspective of regression. We present numerical evidence that the fully decohered unitary tree tensor network (TTN) with two ancillas performs at least as well as the non-decohered unitary TTN, suggesting that it is beneficial to add at least two ancillas to the unitary TTN regardless of the amount of decoherence may be consequently introduced.

A study is made of the progressive 'decoherence' of cosmic ray extensive air-shower particle-detector signals in small air showers through measurements of coincidence rates for pairs of detectors versus the detector separation. Measurements are made both when only the two separated detectors themselves trigger in coincidence, and when that coincidence trigger also requires the detection of a local air shower by a small external air-shower array. The addition of the explicit air-shower trigger ensures that the latter data correspond to showers of a larger particle size, and triggering by very localised shower cores is then unlikely. When including a shower trigger, the decoherence results appear substantially different in form. The coincidence rate between two detectors only can be approximated by a power-law variation with separation distance. When triggering involves an air-shower array, the variation becomes close to an exponential form with characteristic exponent distances varying systematically with increasing detector and air-shower size thresholds. A result is that one can see that small air showers will exhibit clear non-Poissonian density fluctuations near their cores, out to distances of ~5 m, or at shower energies below ~0.05 PeV. These ideas can be helpful in understanding the statistical properties of signals when using large detectors in air-shower arrays. [ABSTRACT FROM AUTHOR]

At present, decoherence continuous-time quantum walks are only based on measurement decoherence, but a new broken-line noise model for continuous-time quantum walks on the cycle is proposed. In this scheme, we first construct the Hamiltonian H according to four cases including no broken links, the link to the left of the node is broken, the link to the right of the node is broken, and both links are broken, and then, the probability of a walker is reconstructed. We analyze the probability distribution, mixing time, diffusion and localization, quantum coherence, and non-Markovian properties under different transition rates and broken-line probabilities. We observe that walkers exhibit different dynamics for two transition rates (slow transition rate and fast transition rate), the slow transition rate localizes the walker near the initial position, while the fast transition rate exhibits a shift to classical diffusion behavior. In particular, we use negentropy to evaluate the degree of classicization. Research has shown that a greater broken-line probability induces a transition to classicization. Our results contribute to a deeper understanding of the effects of broken-line noise and decoherence on continuous-time quantum walks and provide some reference value for the application research of broken-line decoherence continuous-time quantum walking in other fields. [ABSTRACT FROM AUTHOR]

This article explores the use of art to make quantum mechanics more intuitive and accessible. The authors organized a hackathon event where participants created artworks that illustrate quantum principles such as superposition and entanglement. Examples of these artworks include a quantum mosaic, quantum vortex art, and musical compositions. The authors argue that these artistic representations help to make quantum concepts more understandable. The article also discusses other projects that explore the intersection of quantum science and art, including generating art based on quantum architectures and using quantum principles to enhance machine learning visualization. The text raises questions about the definition of art and includes the perspective of a Japanese contemporary artist. Overall, these projects aim to make quantum concepts more accessible through artistic expression. [Extracted from the article]

It is generally assumed that compatibility with special relativity is guaranteed by the invariance of the fundamental equations of quantum physics under Lorentz transformations and the impossibility of transferring energy or information faster than the speed of light. Despite this, various contradictions persist, which make us suspect the solidity of that compatibility. This paper focuses on collapse theories—although they are not the only way of interpreting quantum theory—in order to examine what seems to be insurmountable difficulties we encounter when trying to construct a space–time picture of such typically quantum processes as state vector reduction or the non-separability of entangled systems. The inescapable nature of such difficulties suggests the need to go further in the search for new formulations that surpass our current conceptions of matter and space–time. [ABSTRACT FROM AUTHOR]

Quasi-equilibrium states that can be prepared in solids through Nuclear Magnetic Resonance (NMR) techniques are out-of-equilibrium states that slowly relax towards thermodynamic equilibrium with the lattice. In this work, we use the quantum discord dynamics as a witness of the quantum correlation in this kind of state. The studied system is a set of dipole interacting spin pairs whose initial state is prepared with the NMR Jeener–Broekaert pulse sequence, starting from equilibrium at high temperature and high external magnetic field. It then evolves as an open quantum system within two consecutive dynamic scenarios: adiabatic decoherence driven by the coupling of the pairs to a common phonon field, described within a non-Markovian approach, and spin–lattice relaxation represented by the high-temperature limit of the Born–Markov master equation, and driven by thermal fluctuations. In this way, the studied model is endowed with the dynamics of a realistic solid sample. The quantum discord rapidly increases during the preparation of the initial state, escalating several orders of magnitude compared with thermal equilibrium at room temperature. During decoherence—despite the decay of coherences—the quantum discord oscillates upon this high value, holding the same value as the initial state. Finally, the quantum discord dissipates within a time scale shorter than but comparable to spin–lattice relaxation. [ABSTRACT FROM AUTHOR]

We analyze in detail a procedure of entangling of two singlet–triplet (S– T 0 ) qubits operated in a regime when energy associated with the magnetic field gradient, Δ B z , is an order of magnitude smaller than the exchange energy, J, between singlet and triplet states (Shulman et al. in Science 336:202, 2012). We have studied theoretically a single S– T 0 qubit in free induction decay and spin echo experiments. We have obtained analytical expressions for the time dependence of components of its Bloch vector for quasistatic fluctuations of Δ B z and quasistatic or dynamical 1 / f β -type fluctuations of J. We have then considered the impact of fluctuations of these parameters on the efficiency of the entangling procedure which uses an Ising-type coupling between two S– T 0 qubits. In particular, we have obtained an analytical expression for evolution of two qubits affected by 1 / f β -type fluctuations of J. This expression indicates the maximal level of entanglement that can be generated by performing the entangling procedure. Our results deliver also an evidence that in the above-mentioned experiment S– T 0 qubits were affected by uncorrelated 1 / f β charge noises. [ABSTRACT FROM AUTHOR]

It was recently argued that a neuron would decohere very rapidly from any quantum superposition of its "on" and "off" states. We argue that a hot, wet, biological device such as a neuron is not a two-state device but is better modeled as a "quasi-analog" device with a very large number of basis states, using the presence of electric fields. Furthermore, the neuron is shown to have a decoherence time on the order of 1 ms, the timescale of an action potential. This would imply that quantum information may be stored and processed by a neuron. [ABSTRACT FROM AUTHOR]

Quantum states containing records of incompatible outcomes of quantum measurements are valid states in the tensor-product Hilbert space. Since they contain false records, they conflict with the Born rule and with our observations. I show that excluding them requires a fine-tuning to an extremely restricted subspace of the Hilbert space that seems "conspiratorial", in the sense that (1) it seems to depend on future events that involve records (including measurement settings) and on the dynamical law (normally thought to be independent of the initial conditions), and (2) it violates Statistical Independence, even when it is valid in the context of Bell's theorem. To solve the puzzle, I build a model in which, by changing the dynamical law, the same initial conditions can lead to different histories in which the validity of records is relative to the new dynamical law. This relative validity of the records may restore causality, but the initial conditions still must depend, at least partially, on the dynamical law. While violations of Statistical Independence are often seen as non-scientific, they turn out to be needed to ensure the validity of records and our own memories and, by this, of science itself. A Past Hypothesis is needed to ensure the existence of records and turns out to require violations of Statistical Independence. It is not excluded that its explanation, still unknown, ensures such violations in the way needed by local interpretations of quantum mechanics. I suggest that an as-yet unknown law or superselection rule may restrict the full tensor-product Hilbert space to the very special subspace required by the validity of records and the Past Hypothesis. [ABSTRACT FROM AUTHOR]

Grover search algorithm provides a quadratic speedup over all classical algorithms for unstructured data search. The origin of this quantum advantage has been explored and studied from various angles. In this work, we investigate this issue from the perspective of decoherence induced by the quantum channel associated to Grover search algorithm. We establish a complementary relation between coherence and success probability for the generalized Grover search algorithm with the register initialized in arbitrary pure state or pseudo-pure state. We provide an operational illustration of decoherence as success probability for the Grover search algorithm with unital noise and the register initialized in the maximal superposition state, which includes standard Grover search algorithm as a special case. To understand the dynamic of decoherence and its relation with success probability intuitively, we evaluate it for the Grover search algorithm with bit flip noise, phase flip noise, amplitude damping noise, and phase damping noise, and observe that both decoherence and success probability exhibit similar behaviors with the time of iterations. [ABSTRACT FROM AUTHOR]

Device-independent quantum secure direct communication (DI-QSDC) is a promising primitive in quantum cryptography aimed towards addressing the problems of device imperfections and key management. However, significant effort is required to tackle practical challenges such as the distance limitation due to the decohering effects of quantum channels. Here, we explore the constructive effect of non-Markovian noise to improve the performance of DI-QSDC. Considering two different environmental dynamics modelled by the amplitude damping and the dephasing channels, we show that for both cases non-Markovianty leads to a considerable improvement over Markovian dynamics in terms of three benchmark performance criteria of the DI-QSDC task. Specifically, we find that non-Markovian noise (i) enhances the protocol security measured by Bell violation, (ii) leads to a lower quantum bit error rate, and (iii) enables larger communication distances by increasing the capacity of secret communication. [ABSTRACT FROM AUTHOR]

We consider a metric describing the difference between real (noisy) and ideal processes that is based on the operator norm of the maximum deviation between the final real and ideal states of a quantum system. We discuss the properties as well as geometric and experimental interpretations of the metric. [ABSTRACT FROM AUTHOR]

Dicke state is one of the most useful multipartite quantum states. In this paper, by utilizing the method of designing the evolution operators to construct shortcuts to adiabatic passages (STAP), we have implemented the fast preparation of Dicke states |D3(2)〉 and |D4(2)〉 with one step. Specifically, with the assistance of quantum Zeno dynamics, the original Hamiltonian of the system is simplified into an effective Hamiltonian. Under the dynamic evolution of this effective Hamiltonian, the pulse control is realized by selecting two appropriate auxiliary parameters. This not only greatly shortens the evolution time but also optimizes the operation of the quantum system. In addition, we have discussed the effects of decoherence, parameter fluctuations and noise on fidelity. The results indicate that the proposed protocol maintains high fidelity. Finally, the fast preparation of Dicke state |Dn(2)〉 can be achieved based on the same method.

We investigate the amplification of the genuine tripartite nonlocality (GTN) and the genuine tripartite entanglement (GTE) of Dirac particles in the background of a Schwarzschild black hole by a local filtering operation under decoherence. It is shown that the physically accessible GTN will be completely destroyed by decoherence, and the local filtering operation can cause the physically accessible GTN to be generated in the system coupled with the environment, which has not been discovered before and is beneficial for quantum information processing. Furthermore, we also find that the physically accessible GTE approaches a stable value in the limit of infinite Hawking temperature for most cases, and the nonzero stable value of GTE can be increased by performing the local filtering operation, even in the presence of decoherence. In particular, if the decoherence parameter p is less than 1, the “sudden death” of GTE will take place when the decoherence strength is large enough. Finally, we have shown that physically inaccessible GTE can be enhanced with the operation of the local filter, whereas inaccessible GTN are not allowed to occur in the presence of decoherence.

Religion teaches about the soul as the essence of human being, but modern neuroscience and physics are getting closer to this concept in their research. The soul is immortal by grace. From the Orthodox Christian point of view, the highest level of development of the human soul is the discovery and direction of the soul towards God. At the base of all physics lies the principle of action from which all the basic laws of physics are derived, and the utmost form of this principle would correspond to biological behavior. Living organisms would behave as a consequence of causally determined quantum processes. The PenroseHameroff model considers the "soul" as a quantum computer that exists in the material world while the human individual is alive, but after biological death continues life in the fine quantum structure of the created world. The process of decoherence, which occurs when a quantum object comes into contact with the environment, takes part in the suppression of quantum effects on the macroplane. The Penrose-Hameroff model assumes that tubulins are the quantum units in microtubule quantum computers. Derandomization (in the sense of order), as opposed to entropy (in the sense of disorder) is an essential prerequisite of life. Derandomization (orderliness) is enabled by enformy. The necessity of introducing a "third" basic principle into the theory of "everything" reflects the unsustainability of an exclusively materialistic approach. They could say that both science and religion talk about the same phenomena, known in different ways, with different terminology but the same essence.

In positron annihilation, exploration of the polarization correlations of the emerging gamma quanta has gained interest, since they offer a possibility to improve signal-to-background in medical imaging using positron emission tomography. The annihilation quanta, which are predicted to be in an entangled state, have orthogonal polarizations and this property may be exploited to discriminate them from two uncorrelated gamma photons contributing to the background. Recent experimental studies of polarization correlations of the annihilation quanta after a prior Compton scattering of one of them, had rather different conclusions regarding the strength of the correlation after the scattering, showing its puzzling nature. The scattering was described as a decoherence process. In the present work, we perform for the first time, a study of the polarization correlations of annihilation quanta after decoherence via Compton scattering in the angular range 0∘−50∘ using single-layer gamma ray polarimeters. In addition, we compare the measured polarization correlations after Compton scattering at 30∘ with an active and a passive scatterer element. The measured azimuthal correlation of back-to-back annihilation quanta is consistent with the Pryce-Ward formulation, as confirmed by Monte Carlo simulations. Further, the results indicate that the correlation, expressed in terms of the polarimetric modulation factor, shows no significant difference at small scattering angles (0∘−30∘) compared to the correlation measured for direct photons, while a moderate indication of a lower modulation is observed for 50∘ scattering angle. The measured modulation is larger at all scattering angles than the one expected from the simulation of orthogonally polarized, independent annihilation quanta.

We consider the spectral and initial value problem for the Lindblad–Gorini–Kossakowski–Sudarshan master equation describing an open quantum system of bosons and spins, where the bosonic parts of the Hamiltonian and Lindblad jump operators are quadratic and linear respectively, while the spins couple to bosons via mutually commuting spin operators. Needless to say, the spin degrees of freedom can be replaced by any set of finite-level quantum systems. A simple, yet non-trivial example of a single open spin-boson model is worked out in some detail. [ABSTRACT FROM AUTHOR]

In this paper, we study the decaying dynamics in the mirror-field interaction by means of the intrinsic decoherence scheme. Factorization of the mirror-field Hamiltonian with the use of displacement operators allows us to calculate the explicit solution to Milburn's equation for arbitrary initial conditions. We show expectation values, correlations, and Husimi functions for the solutions obtained. [ABSTRACT FROM AUTHOR]

We develop a mathematical formalism that allows to study decoherence with a great level generality, so as to make it appear as a geometrical phenomenon between reservoirs of dimensions. It enables us to give quantitative estimates of the level of decoherence induced by a purely random environment on a system according to their respectives sizes, and to exhibit some links with entanglement entropy. [ABSTRACT FROM AUTHOR]

The One, by theoretical physicist Heinrich Pas, reviews the history of monism from the Greeks to the modern period. Pas proposes a monism he calls "the One." His One follows from quantum mechanics. It is a universal wave function. He is then faced with how our everyday world arises. To answer he relies on a version of Evert's many-worlds interpretation of quantum mechanics and a process of decoherence that results in a dualism of consciousness and the quasi-classical world of space, time, and matter. C. G. Jung proposed a dual-aspect monism influenced by synchronicity. A comparison between Pas's monism and dualism and Jung's finds some apparent differences, but in the end they are found to be remarkably similar. [ABSTRACT FROM AUTHOR]

Quantum computers have been proposed to solve a number of important problems such as discovering new drugs, new catalysts for fertilizer production, breaking encryption protocols, optimizing financial portfolios, or implementing new artificial intelligence applications. Yet, to date, a simple task such as multiplying 3 by 5 is beyond existing quantum hardware. This article examines the difficulties that would need to be solved for quantum computers to live up to their promises. I discuss the whole stack of technologies that has been envisioned to build a quantum computer from the top layers (the actual algorithms and associated applications) down to the very bottom ones (the quantum hardware, its control electronics, cryogeny, etc.) while not forgetting the crucial intermediate layer of quantum error correction. [ABSTRACT FROM AUTHOR]

We consider a well-known, exactly solvable model of an open quantum system with pure decoherence. The aim of this paper is twofold. Firstly, decoherence is a property of open quantum systems important for both quantum technologies and the fundamental question of the quantum–classical transition. It is worth studying how the long-term rate of decoherence depends on the spectral density characterising the system–bath interaction in this exactly solvable model. Secondly, we address a more general problem of the Markovian embedding of non-Markovian open system dynamics. It is often assumed that a non-Markovian open quantum system can be embedded into a larger Markovian system. However, we show that such embedding is possible only for Ohmic spectral densities (for the case of a positive bath temperature) and is impossible for both sub- and super-Ohmic spectral densities. On the other hand, for Ohmic spectral densities, an asymptotic large-time Markovianity (in terms of the quantum regression formula) takes place. [ABSTRACT FROM AUTHOR]

One of the outstanding questions in modern physics is how to test whether gravity is classical or quantum in a laboratory. Recently there has been a proposal to test the quantum nature of gravity by creating quantum superpositions of two nearby neutral masses, close enough that the quantum nature of gravity can entangle the two quantum systems, but still sufficiently far away that all other known Standard Model interactions remain negligible. However, preparing superposition states of a neutral mass (the light system) requires the vicinity of laboratory apparatus (the heavy system). We will suppose that such a heavy system can be modelled as another quantum system; since gravity is universal, the lighter system can get entangled with the heavier system, providing an inherent source of gravitational decoherence. In this paper, we will consider a toy model composed of two light and two heavy quantum oscillators prepared in the motional ground state, forming pairs of probe-detector systems, and study under what conditions the entanglement between two light systems evades the decoherence induced by the heavy systems. We conclude by estimating the decoherence in the proposed experiment for testing the quantum nature of gravity. [ABSTRACT FROM AUTHOR]

A Multilayer network is a potent platform that paves the way for the study of the interactions among entities in various networks with multiple types of relationships. This study explores the dynamics of discrete-time quantum walks on a multilayer network. We derive a recurrence formula for the coefficients of the wave function of a quantum walker on an undirected graph with a finite number of nodes. By extending this formula to include extra layers, we develop a simulation model to describe the time evolution of the quantum walker on a multilayer network. The time-averaged probability and the return probability of the quantum walker are studied with Fourier, and Grover walks on multilayer networks. Furthermore, we analyze the impact of decoherence on quantum transport, shedding light on how environmental interactions may impact the behavior of quantum walkers on multilayer network structures. [ABSTRACT FROM AUTHOR]

Quantum Darwinism explains the emergence of classical objectivity within a quantum universe. However, to date, most research on quantum Darwinism has focused on specific models and their stationary properties. To further our understanding of the quantum-to-classical transition, it appears desirable to identify the general criteria a Hamiltonian has to fulfill to support classical reality. To this end, we categorize all N-qubit models with two-body interactions, and show that only those with separable interaction of the system and environment can support a pointer basis. We further demonstrate that "perfect" quantum Darwinism can only emerge if there are no intra-environmental interactions. Our analysis is complemented by solving the ensuing dynamics. We find that in systems exhibiting information scrambling, the dynamical emergence of classical objectivity directly competes with the non-local spread of quantum correlations. Our rigorous findings are illustrated through the numerical analysis of four representative models. [ABSTRACT FROM AUTHOR]

We briefly review a number of major features of the approach to quantum measurement theory based on environment-induced decoherence of the measuring apparatus, and summarise our observations in the form of a couple of general principles that, unlike the wave function collapse hypothesis, emerge as the ones consistent with the unitary Schrödinger evolution of wave functions. We conclude with a few observations of a philosophical nature, to the effect that that quantum theory does not purport to describe reality but constitutes an interpretation of our phenomenal reality within a context – one where the Planck scale is not crossed. Beyond the Planck scale, a radically new interpretation of reality is likely to emerge. [ABSTRACT FROM AUTHOR]

Abstract The dynamics of the Gaudin magnet (“central‐spin model”) is studied using machine‐learning methods. This model is of practical importance, for example, for studying non‐Markovian decoherence dynamics of a central spin interacting with a large bath of environmental spins and for studies of nonequilibrium superconductivity. The Gaudin magnet is also integrable, admitting many conserved quantities. Machine‐learning methods may be well suited to exploiting the high degree of symmetry in integrable problems, even when an explicit analytic solution is not obvious. Motivated in part by this intuition, a neural‐network representation (restricted Boltzmann machine) is used for each variational eigenstate of the model Hamiltonian. Accurate representations of the ground state and of the low‐lying excited states of the Gaudin‐magnet Hamiltonian are then obtained through a variational Monte Carlo calculation. From the low‐lying eigenstates, the non‐perturbative dynamic transverse spin susceptibility is found, describing the linear response of a central spin to a time‐varying transverse magnetic field in the presence of a spin bath. Having an efficient description of this susceptibility opens the door to improved characterization and quantum control procedures for qubits interacting with an environment of quantum two‐level systems.

By varying the magnitude of the effective interaction between spins in relation to the perturbations, we study the decoherence behavior in a connected proton system. Making use of the Magnus expansion, we introduce a NMR pulse sequence that generates an average Hamiltonian with Double Quantum terms multiplied by a scaling factor, δ, with the possibility to take positive and negative values. The performance of the pulse sequence for different values of the scaling factors was validated in polycrystalline adamantane, by observing the evolution of the polarization. A time reversal procedure, accessible through the change of sign in the controlled Hamiltonian, was necessary to observe multiple quantum coherences. The spin counting develops a characteristic growth in two species of clusters for the scaled time. The influence of the scaling factor on the reversibility was observed through the behavior of the Loschmidt echoes, which decayed faster as the scaling factor increases. From the analysis of dynamics and its reversibility, we extracted characteristic times for the spin diffusion, T2δ and the intrinsic decoherence decay, T3δ for each scaling factor δ, and perturbation time scale, TΣ. Observing the dependence of reversibility vs. perturbation rates, both normalized with the spin diffusion rate, we find that in the limit of low perturbations, T2δ/T3δ deviates from the linear dependence on T2δ/TΣ that corresponds to strong perturbation. The asymptotic value T2/T3≈0.15 as T2δ/TΣ vanishes, gives evidence that the main source of irreversibility is the intrinsic decoherence associated to the chaotic many-body dynamics of the system.

Here we correct an expression in a recent paper Jeschke (2023).

In this paper, we investigate a system composed of two spatially separated cavities, each with a magnon mode of a yttrium iron garnet (YIG) sphere coupled to a microwave (MW) cavity and phonon modes, respectively, via linear beam splitter and magnetostrictive interactions. In addition, two-mode squeezed vacuum fields drive the two cavities. We investigate and compare the behavior of three nonclassicality indicators in two subsystems (i.e., magnon–magnon and phonon–phonon) under the influences of the temperature, the cavity–magnon damping rate, and the magnomechanical coupling rate. We use the entanglement of formation (EoF) to measure the degree of entanglement, the Gaussian quantum discord (GQD) to characterize the quantum correlations beyond entanglement and Gaussian quantum coherence (GQC) to quantify coherence. Considering that the quantifiers share the same entropic definition, we compare the three quantifiers and test the validity of the hypothesis that quantum states with nonzero discord are inherently entangled. We find, on the one hand, that both GQC and GQD exhibit freezing behavior and that they are more robust to the decoherence effect than the EoF. On the other hand, the EoF and the GQD are always upper bounded by GQC, and there is no simple dominance relationship between EoF and GQD; hence these two quantifiers should not be compared. The effect of other parameters is also discussed in detail. [ABSTRACT FROM AUTHOR]

This paper further explores the recent scheme of switched quantum channels with indefinite causal order applied to the reference metrological task of quantum phase estimation in the presence of noise. We especially extend the explorations, previously reported with depolarizing noise and thermal noise, to the class of Pauli noises, important to the qubit and not previously addressed. Nonstandard capabilities, not accessible with standard quantum phase estimation, are exhibited and analyzed, with significant properties that are specific to the Pauli noises, while other properties are found in common with the depolarizing noise or the thermal noise. The results show that the presence and the type of quantum noise are both crucial to the determination of the nonstandard capabilities from the switched channel with indefinite causal order, with a constructive action of noise reminiscent of stochastic resonance phenomena. The study contributes to a more comprehensive and systematic characterization of the roles and specificities of quantum noise in the operation of the novel devices of switched quantum channels with indefinite causal order. [ABSTRACT FROM AUTHOR]

We focus our study on the quantum correlations of coupled photon pairs produced in an open atomic laser system, where quantum coherence is brought about by the superposition of a coherent atomic state and a coherent classical field. Quantum properties produced by photon–photon correlations are a long sought-after goal in quantum information science and technology, because photons combine at room temperature with high speed and long coherence times. The openness of the system under consideration allows quantum decoherence due to temperature and phase fluctuations to influence the quantum correlations generated. The competition between these quantum coherence and quantum decoherence leads to temporal quantum correlations, which we analyze using the time evolution of the density operator. Strong quantum correlations can be achieved by choosing an appropriate amplitude of the classical fields, treating temperature and phase fluctuations, and increasing the atomic injection rate over time. We also show that quantum entanglement is short-lived, quantum steering slowly decreases, but quantum discord increases with increasing heat bath temperature and atomic phase fluctuations. In this study, we explore the behavior of quantum correlations in an open atomic laser system and investigate the dynamics of entanglement, discord, and steering in this system and examine how they evolve over time. [ABSTRACT FROM AUTHOR]