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Squeezed reservoir engineering is a powerful technique in quantum information that combines the features of squeezing and reservoir engineering to create and stabilize non-classical quantum states. In this paper, we focus on the previously neglected aspect of the impact of the squeezing phase on the precision of quantum phase and amplitude estimation based on a simple model of a two-level system (TLS) interacting with a squeezed reservoir. We derive the optimal squeezed phase-matching conditions for phase ϕ and amplitude θ parameters, which are crucial for enhancing the precision of quantum parameter estimation. The robustness of the squeezing-enhanced quantum Fisher information against departures from these conditions is examined, demonstrating that minor deviations from phase-matching can still result in remarkable precision of estimation. Additionally, we provide a geometric interpretation of the squeezed phase-matching conditions from the classical motion of a TLS on the Bloch sphere. Our research contributes to a deeper understanding of the operational requirements for employing squeezed reservoir engineering to advance quantum parameter estimation. [ABSTRACT FROM AUTHOR]

Quantum coherence and quantum steering are extremely useful resources for quantum information technology. Based on the correlation between quantum coherence and quantum steering, the steered quantum coherence (SQC) is formed, which is a new quantum correlation measurement method. In this paper, the SQC is applied to explore the dynamic evolution of quantum correlation in non-Markovian systems and it is also compared with typical quantum entanglement (QE). The results show that same as the QE, the SQC can also measure the quantum correlation of quantum system. Further research shows that there are obvious differences between the two measurement methods: on the one hand, in the process of non-Markovian dynamics evolution, death and rebirth occur for QE, but not for SQC, which indicates that SQC is more robust than QE; on the other hand, the superposition effect of the channel is not conducive to maintaining the QE of the system, but it is beneficial to maintaining the SQC. [ABSTRACT FROM AUTHOR]

We investigated the quantum speed limit (QSL) time of a spin qubit coupled to an XXZ Heisenberg spin chain environment. The spins of the environment are coupled to the system qubit through Heisenberg like interaction. We carried out our investigations under approximate solution and we evaluate the minimal evolution time from initial state to one of its final target state. QSL time becomes uniform in the low temperature limit and at higher temperature different oscillatory behaviors are observed. Maximal speed of evolution of quantum systems is determined by QSL time. We use Margolus-Levitin (ML) types bound to investigate the minimal evolution time of quantum states. We also discuss the dependence of QSL time upon temperature, driving time and anisotropy of spin chain environment. [ABSTRACT FROM AUTHOR]

Quantum nonlocality is a very important quantum resource. Different types of quantum correlation can present the quantum nonlocality in varying degrees. In this paper, we investigate the dynamic evolution of steered quantum coherence (SQC) for a system of interacting qubits in contact with various models of the environment, and emphatically compare the robustness of SQC and quantum entanglement (QE). The results show that from the given initially state, due to the decoherence effect, both SQC and QE evolve with time in the form of oscillation. In the process of dynamic evolution, the QE will appear death and rebirth for all different decay modes. Differently, the SQC will not emerge the sudden death or the dark and bright, which shows a stronger robustness than QE. Moreover, we find that, under the long-term limit condition, the SQC will approach the steady value in different decay modes, which provides a reliable resource foundation for the completion of quantum information processing. [ABSTRACT FROM AUTHOR]

We study the exceptional-point (EP) structure and the associated quantum dynamics in a system consisting of a non-Hermitian qubit and a Hermitian qubit. We find that the system possesses two sets of EPs, which divide the system-parameter space into P T -symmetry unbroken, partially broken and fully broken regimes, each with distinct quantum-dynamics characteristics. Particularly, in the partially broken regime, while the P T -symmetry is generally broken in the whole four-dimensional Hilbert space, it is preserved in a two-dimensional subspace such that the quantum dynamics in the subspace are similar to those in the P T -symmetry unbroken regime. In addition, we reveal that the competition between the inter-qubit coupling and the intra-qubit driving gives rise to a complex pattern in the EP variation with system parameters. [ABSTRACT FROM AUTHOR]

Dynamical decoupling (DD) is normally ineffective when applied to DC measurement. In its straightforward implementation, DD nulls out DC signal as well while suppressing noise. This work proposes a phase relay method that is capable of continuously interrogating the DC signal over many DD cycles. We illustrate its efficacy when applied to the measurement of a weak DC magnetic field with an atomic spinor Bose–Einstein condensate. Sensitivities approaching standard quantum limit or Heisenberg limit are potentially realizable for a coherent spin state or a squeezed spin state of 10000 atoms, respectively, while ambient laboratory level noise is suppressed by DD. Our work offers a practical approach to mitigate the limitations of DD to DC measurement and would find other applications for resorting coherence in quantum sensing and quantum information processing research. [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]

The expectation that quantum computation might bring performance advantages in machine learning algorithms motivates the work on the quantum versions of artificial neural networks. In this study, we analyse the learning dynamics of a quantum classifier model that works as an open quantum system which is an alternative to the standard quantum circuit model. According to the obtained results, the model can be successfully trained with a gradient descent (GD)-based algorithm. The fact that these optimisation processes have been obtained with continuous dynamics, shows promise for the development of a differentiable activation function for the classifier model. [ABSTRACT FROM AUTHOR]

Quantum coherence and entanglement are two basic aspects of nonclassicality. With the specific bipartite quantum systems which entanglement and coherence can be calculated, we provide a quantitative connection between the coherence and the dynamical entanglement that be created with the help of incoherent operations. We show that the coherence upper bounds the dynamical entanglement, and in particular conditions of the initial quantum states, the coherence of a single-party is equal to the dynamical entanglement. We extend these results to multipartite quantum systems. [ABSTRACT FROM AUTHOR]

The non-Hermitian systems with the non-Hermitian skin effect (NHSE) are very sensitive to the imposed boundary conditions and lattice sizes, which lead to size-dependent non-Hermitian skin effects. Here, we report the experimental observation of NHSE with different boundary conditions and different lattice sizes in the unidirectional hopping model based on a circuit platform. The circuit admittance spectra and corresponding eigenstates are very sensitive to the presence of the boundary. Meanwhile, our experimental results show how the lattice sizes and boundary terms together affect the strength of NHSE. Therefore, our electric circuit provides a good platform to observe size-dependent boundary effects in non-Hermitian systems. [ABSTRACT FROM AUTHOR]

We develop an approach using coherent states and path integral to investigate the dynamics of entanglement in a simple two-dimensional non-commutative harmonic oscillator. We start by employing a Bopp shift to convert the Hamiltonian describing the system into a commutative equivalent one. This allows us to construct coherent states and calculate the propagator in standard way. By deriving the explicit expression of the time-dependent coherent states and considering its connection with the number states, we provide exact results for evaluating the degree of entanglement between the ground state and any excited state through the purity function. The interesting emerging result is that, as long as the non-commutativity parameter is non-zero, our system exhibits the phenomenon of collapse and revival of entanglement. [ABSTRACT FROM AUTHOR]

The XY spin-chain in transverse fields under the modulation of δ -function periodic kickings has been studied in this paper. We have obtained the exact expression of the Floquet effective Hamiltonian, and analyzed the corresponding properties of the criticality. We also considered the central spin system’s Loschmidt echo coupling to this spin-chain, and gave the relation between the decoherence factor and the periodic kicking parameters. Our results show that, periodic kickings on the spin-chain can modulate the critical points and the phase diagram of the spin chain, which can be detected by the decoherence of the coupling central spin system. [ABSTRACT FROM AUTHOR]

By using the complete solution of the Milburn equation (beyond the Lindblad form that it is generally used) that describes intrinsic decoherence, we studied the decaying dynamics of a displaced harmonic oscillator. We calculated the expectation values of position quadrature and the number operator in the initial coherent and squeezed states. [ABSTRACT FROM AUTHOR]

We introduce four new entangled quantum states which are generated by applying symmetric photon operations to two-mode entangled coherent states. As a comparison, we investigate quantum statistical properties, squeezing, entanglement affected by photon operations. Through our numerical and theoretical analysis, it is interesting to find that the symmetric photon-addition operation enhances the squeezing and weakens the entanglement of two-mode entangled coherent states. [ABSTRACT FROM AUTHOR]

The quantumness of Dirac particles for quantized fields in a dilatonic black hole is estimated by means of quantum channel. A general Bloch vector representation of quantum channel in black hole spacetimes beyond single mode approximation is developed. The nonclassicality of Dirac particles can be measured by the minimization of quantum coherence over all orthonormal basis sets. The quantumness of the channel decreases as the dilaton field increases. The interplay between the external reservoir noise and dilaton black hole on the dynamical behavior of quantum coherence and steerability is investigated in the Pauli basis. The external environment is modeled by a random telegraph noise channel. The monotonous decay of quantum nonlocality occurs in the weak coupling case. The degradation and revival of quantum nonlocality are observed in the strong coupling condition. It is found that quantum fluctuation effects of the external reservoir can protect quantum coherence and steerability from the information loss of the black hole. [ABSTRACT FROM AUTHOR]

Based on the resource theory for quantifying the coherence of quantum channels, we introduce a new coherence quantifier for quantum channels via maximum relative entropy. We prove that the maximum relative entropy for coherence of quantum channels is directly related to the maximally coherent channels under a particular class of superoperations, which results in an operational interpretation of the maximum relative entropy for coherence of quantum channels. We also introduce the conception of sub-superchannels and sub-superchannel discrimination. For any quantum channels, we show that the advantage of quantum channels in sub-superchannel discrimination can be exactly characterized by the maximum relative entropy of coherence for quantum channels. Similar to the maximum relative entropy of coherence for channels, the robustness of coherence for quantum channels has also been investigated. We show that the maximum relative entropy of coherence for channels provides new operational interpretations of robustness of coherence for quantum channels and illustrates the equivalence of the dephasing-covariant superchannels, incoherent superchannels, and strictly incoherent superchannels in these two operational tasks. [ABSTRACT FROM AUTHOR]

A quantum network is a promising quantum many-body system because of its tailored geometry and controllable interaction. Here, we propose an external control scheme for the qubit-photon interaction and multiqubit reset in a dissipative quantum network, which comprises superconducting circuit chains with microwave drives and filter-filter couplings. The traditional multiqubit reset of the quantum network requires physically disconnected qubits to prevent their entanglement. However, we use an original effect of dissipation, i.e., consuming the entanglement generated by qubits' interaction, to achieve an external control of the multiqubit reset in an always-connected superconducting circuit. The reset time is independent of the number of qubits in the quantum network. Our proposal can tolerate considerable fluctuations in the system parameters and can be applicable to higher-dimensional quantum networks. [ABSTRACT FROM AUTHOR]

We study the entanglement and the Bell nonlocality of a coupled two-qubit system, in which each qubit is coupled with one individual environment. We study how the nonequilibrium environments (with different temperatures or chemical potentials) influence the entanglement and the Bell nonlocality. The nonequilibrium environments can have constructive effects on the entanglement and the Bell nonlocality. Nonequilibrium thermodynamic cost can sustain the thermal energy or particle current and enhance the entanglement and the Bell nonlocality. However, the nonequilibrium conditions (characterized by the temperature differences or the thermodynamic cost quantified by the entropy production rates) which give the maximal violation of the Bell inequalities are different from the nonequilibrium conditions which give the maximal entanglement. When the Bell inequality has asymmetric observables (between Alice and Bob), for example the I 3322 inequality, such asymmetry can also be reflected from the effects under the nonequilibrium environments. The spatial asymmetric two-qubit system coupled with nonequilibrium bosonic environments shows the thermal rectification effect, which can be witnessed by the Bell nonlocality. Different spatial asymmetric factors can be linearly cancelled with each other in the thermal rectification effect, which is also reflected on the changes of the entanglement and the Bell nonlocality. Our study demonstrates that the nonequilibrium environments are both valuable for the entanglement and Bell nonlocality resources, based on different optimal nonequilibrium conditions though. [ABSTRACT FROM AUTHOR]

In this paper, we study detection of the state non-classicality for a quantum harmonic oscillator by a qubit in the presence of dissipation effects. We show that dissipation can enhance the effectiveness of the method in case of using the corrected form of the related nonclassicality witness. Such an improvement is attributed to the fact that dissipation leads to probing a surface, instead of a curve, of the complex plane for non-classicality condition on normally-ordered characteristic function. [ABSTRACT FROM AUTHOR]

In this work, we studied the relaxation dynamics of coherences of different orders present in a system of two coupled nuclear spins. We used a previously designed model for intrinsic noise present in such systems which considers the Lindblad master equation for Markovian relaxation. We experimentally created zero-, single- and double-quantum coherences in several two-spin systems and performed a complete state tomography and computed state fidelity. We experimentally measured the decay of zero- and double-quantum coherences in these systems. The experimental data fitted well to a model that considers the main noise channels to be a correlated phase damping (CPD) channel acting simultaneously on both spins in conjunction with a generalised amplitude damping channel acting independently on both spins. The differential relaxation of multiple-quantum coherences can be ascribed to the action of a CPD channel acting simultaneously on both the spins. [ABSTRACT FROM AUTHOR]

In this paper, we study a model of two two-level atoms interacting with a quantum field. An analytical solution is obtained which is used to study the information entropy of the system. It is shown that the nonlinear term plays a significant role in the behaviour of the minimum uncertainty (MU) compared with the concurrence (C). Our extensive study of information entropy of atoms–field interaction demonstrates that using the coupling strength between the atoms and the field as a controller parameter, one can control the dynamics of the system by increasing the lower bound of the entropic uncertainty relation or decreasing the entanglement. [ABSTRACT FROM AUTHOR]

Fast state transfer of quantum system is one of the effective robust control problems that often appear in practice, which is good for efficiency improving and decoherence suppression caused by the environment. In this paper, we mainly discuss the fast transfer of quantum state in a two-qubit system. By analyzing the advantages and disadvantages of the existing Lyapunov control, we propose that the quantum state transfer can be accelerated by minimizing the Lyapunov function, and give three optimized control field design methods. For the problem of a two-qubit open quantum system, numerical simulation results indicate the effectiveness of the method. Not only the state transfer time is shortened, but also the high-frequency oscillation that occurs in bang–bang control is effectively avoided. [ABSTRACT FROM AUTHOR]

Fast control of the state of quantum system is one of the robust control problems that often appear in practice, which is effective for decoherence suppression caused by the environment. In this paper, on the basis of an analysis of the advantages and disadvantages of the existing Lyapunov control, we propose three improved bang-bang control designs that are composed of the theory of bang-bang control and parameters optimization method and can solve fast control of the state transfer problem for quantum systems. Numerical simulation shows the effectiveness of the method for the problem of a closed two-level quantum system. Not only the decrease of Lyapunov function is fastened, the optimization of control time is realized and the coherence is protected, but also the high-frequency oscillation that occurs in bang-bang control is effectively avoided. [ABSTRACT FROM AUTHOR]

Controlling transport in quantum systems holds the key to many promising quantum technologies. Here we review the power of symmetry as a resource to manipulate quantum transport and apply these ideas to engineer novel quantum devices. Using tools from open quantum systems and large deviation theory, we show that symmetry-mediated control of transport is enabled by a pair of twin dynamic phase transitions in current statistics, accompanied by a coexistence of different transport channels. By playing with the symmetry decomposition of the initial state, one can modulate the importance of the different transport channels and hence control the flowing current. Motivated by the problem of energy harvesting, we illustrate these ideas in open quantum networks, an analysis that leads to the design of a symmetry-controlled quantum thermal switch. We review an experimental setup recently proposed for symmetry-mediated quantum control in the lab based on a linear array of atom-doped optical cavities, and the possibility of using transport as a probe to uncover hidden symmetries, as recently demonstrated in molecular junctions, is also discussed. Other symmetry-mediated control mechanisms are also described. Overall, these results demonstrate the importance of symmetry not only as an organizing principle in physics but also as a tool to control quantum systems. [ABSTRACT FROM AUTHOR]

In this paper we investigate the entanglement dynamics between two two-level atoms interacting with two coherent fields in two spatially separated cavities which are filled with a Kerr-like medium. We examine the effect of nonlinear medium on the dynamical properties of entanglement and atomic occupation probabilities in the case of even and odd deformed coherent states. The results show that the deformed fields play important roles in the evolution of entanglement. Also, the results demonstrate that entanglement sudden death, sudden birth and long-distance can be controlled by the deformation and nonlinear parameters. [ABSTRACT FROM AUTHOR]

We have studied the transition probability and decoherence time of levitating polaron in helium film thickness. By using a variational method of Pekar type, the ground and the first excited states of polaron are calculated above the liquid-helium film placed on the polar substrate. It is shown that the polaron transits from the ground to the excited state in the presence of an external electromagnetic field in the plane. We have seen that, in the helium film, the effects of the magnetic and electric fields on the polaron are opposite. It is also shown that the energy, transition probability and decoherence time of the polaron depend sensitively on the helium film thickness. We found that decoherence time decreases as a function of increasing electron-phonon coupling strength and the helium film thickness. It is seen that the film thickness can be considered as a new confinement in our system and can be adjusted in order to reduce decoherence. [ABSTRACT FROM AUTHOR]

In quantum state transfer and tracking, control of quantum states are effective against decoherence caused by external environment and information processing. In this paper, based on model reference adaptive control theory, the orbit tracking based on Lyapunov approach in Liouville equation for open quantum systems is investigated under various dissipative modes. Meanwhile, through counteracting the shift term by simplification, the control law avoids oscillating state trajectory in a two-level Markovian quantum system. Finally, numerical simulations for a two-level open quantum system validate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

Laser control of open quantum systems (OQS) is a challenging issue as compared to its counterpart in isolated small size molecules, basically due to very large numbers of degrees of freedom to be accounted for. Such a control aims at appropriately optimising decoherence processes of a central two-level system (a given vibrational mode, for instance) towards its environmental bath (including, for instance, all other normal modes). A variety of applications could potentially be envisioned, either to preserve the central system from decaying (long duration molecular alignment or orientation, qubit decoherence protection) or, to speed up the information flow towards the bath (efficient charge or proton transfers in long chain organic compounds). Achieving such controls requires some quantitative measures of decoherence in relation with memory effects in the bath response, actually given by the degree of non-Markovianity. Characteristic decoherence rates of a Spin-Boson model are calculated using a Nakajima–Zwanzig type master equation with converged hierarchical equations of motion expansion for the memory kernel. It is shown that, by adequately tuning the two-level transition frequency through a controlled Stark shift produced by an external laser field, non-Markovianity can be enhanced in a continuous way leading to a first attempt towards the control of OQS. [ABSTRACT FROM AUTHOR]

In quantum state transfer and tracking, fast control of quantum states are effective against the decoherence caused by external environment and information processing. The variational method and maximum principle are often used to solve the time-optimal control problems. However, analytic results cannot be obtained for many cases and it is necessary to find methods that can solve optimal control problems very efficiently. By comparing the advantages and disadvantages of the existing Lyapunov control and bang–bang control, we apply in this paper a new method that combines bang–bang control and Lyapunov control and can solve the time-optimal control problems in the open quantum system. Numerical simulations for a two-level open quantum system validate the effectiveness of the method. Furthermore, the optimized scheme avoids high frequency oscillation that occurs in bang–bang control. [ABSTRACT FROM AUTHOR]

The interaction between a two-level atom trapped in symmetric double-well potential and a single mode cavity field is investigated. By solving Schrödinger equation, the analytical expressions of the total wave function and inversion of the atomic energy level for the whole system are given. Time evolution of the atomic population inversion is analyzed for different initial states of the cavity field, such as fock and coherent states. Influence of atomic mass centre motion on population inversion is considered. Results show that by choosing appropriate cavity field initial state, potential well location and related factors, the spontaneous emission rate of an atom can be effectively controlled and the atomic moving centre of mass can suppress Rabi oscillation. [ABSTRACT FROM AUTHOR]

The current study investigated the time evolution of entanglement in an open quantum system. This system includes two independent harmonic oscillators interacting with a general environment. This study reports the solution of the time evolution of the covariance matrix by using the Markovian master equation. It was found that the entanglement for a preferred Gaussian state, is a continuous variable system. This study examined the time evolution of the entanglement by using Simon's separability criterion for continuous variable systems and computing covariance matrix with considering environmental factors such as temperature for two initial state of system (separable and entangled) with drawing Simon's criterion and logarithmic negativity. The results demonstrated that for a certain value of dispersion and dissipation coefficient, the initial state of the system is saved over the time. But for other amounts of the above factors, entanglement birth, entanglement death and repeated entanglement birth and entanglement death happen in the system. Furthermore, the present study investigated the behavior of system's purity under the effects of environmental factors, such as temperature and environment parameter with regard to the relation between purity and covariance matrix for two-mode Gaussian state. [ABSTRACT FROM AUTHOR]

The effect of the quantum feedback on the tightness of the variance-based uncertainty, the possibility of using quantum feedback to prepare the state with a better tightness, and the relationship between the tightness of the uncertainty and the mixedness of the system are studied. It is found that the tightness of Schrodinger-Robertson uncertainty (SUR) relation has a strict liner relationship with the mixedness of the system. As for the Robertson uncertainty relation (RUR), we find that the tightness can be enhanced by tuning the feedback at the beginning of the evolution. In addition, we deduce that the tightness of RUR has an inverse relationship with the mixedness and the relationship turns into a strict linear one when the system reach the steady state. [ABSTRACT FROM AUTHOR]

Diamond is an attractive material due to its extreme physical properties, and has recently found a key role in the developing field of quantum bits based upon long spin coherence times at room temperature. In both natural and synthetic diamond, nitrogen is probably the dominant impurity, with the simplest configuration being substitution of a host carbon atom, which has a single unpaired electron. This defect, seen as the P1 paramagnetic resonance center, has been the subject of detailed experimental investigation, revealing the extensive interaction of the electron spin with nearby 13 C nuclei. The interaction between the electron and nuclear spins is not only of importance in determining defect structure, but is of technological importance, such as in the main decoherence mechanism for NV centers, so understanding the coupling between 13 C and the electron spin is of critical importance. In this paper we assess the assignment of hyperfine interactions to the various carbon sites in the vicinity of the nitrogen atom. We show that although the experimentally derived model is correct in the main, the best fit to the calculated data requires reassignment of at least one of the carbon sites. [ABSTRACT FROM AUTHOR]

In this paper, we consider the 1D two-qubit isotropic antiferromagnetic Heisenberg XXX model in an external magnetic field B. Firstly, we pay our attention to study the properties of thermal entanglement in XXX model, and in comparison with XX model, we found that the thermal entanglement could be easily obtained and quantum phase transition could not easily occur in XXX model. Finally, we calculate the fidelity of teleportation of one-qubit pure state via thermally entangled state of a two-qubit Heisenberg XXX chain, in the same way, we compare XXX model with XX model, we found that teleportation via thermally entangled state of a two-qubit Heisenberg XXX chain may be better. [ABSTRACT FROM AUTHOR]

In this paper, we introduce useful scheme of the atom–atom interaction in the case of absence and presence of the atomic motion effect. The quantum entanglement between the two two-level atoms based on concurrence quantifier will be investigated. The dependence atom–atom entanglement on the different system parameters will be examined. Moreover, the influence of the atomic motion and initial state setting on the dynamical behavior of the entanglement between two atoms will be discussed. Furthermore, we have explored some important phenomena related to the entanglement between two atoms such as entanglement sudden death (ESD) and entanglement sudden birth (ESB). Finally, we have obtained a high amount of the atom–atom entanglement by a proper choice of the system parameters in the case of stationary and moving atoms. [ABSTRACT FROM AUTHOR]

The optimal control function for an open system is designed by taking the expectation value of the state under control reaching the target state as the performance indicator. Utilizing Runge Kutta method, the open quantum system with a superconducting qubit is numerically investigated. The influences of parameters on the population transfer probability and control time are compared. The simulation experiments indicate that the state transfer and state maintenance is realized with a success rate of more than 98 percent, which validating the application of optimal control method in the state transfer and maintenance for open quantum system containing superconducting qubits. Moreover, proper increase of the proportional coefficients can accelerate the qubits flip and reduce the vibration frequency of control function. [ABSTRACT FROM AUTHOR]

We studied the behaviour under decoherence of four different measures of the distance between quantum states and classical states for the harmonic oscillator coupled to a linear Markovian bath. Three of these are relative measures, using different definitions of the distance between the given quantum states and the set of all classical states. The fourth measure is an absolute one, the negative volume of the Wigner function of the state. All four measures are found to agree, in general, with each other. When applied to the eigenstates | n〉, all four measures behave non-trivially as a function of time during dynamical decoherence. First, we find that the first set of classical states to which the set of eigenstate evolves is (by all measures used) closest to the initial set. That is, all the states decohere to classicality along the 'shortest path'. Finding this closest classical set of states helps improve the behaviour of all the relative distance measures. Second, at each point in time before becoming classical, all measures have a state n with maximal quantum-classical distance; the value n decreases as a function of time. Finally, we explore the dynamics of these non-classicality measures for more general states. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of entanglement given by the concurrence of a two-qubit system in the non-Markovian setting. A quantum master equation is derived, which is solved in the eigenbasis of the system Hamiltonian for X-type initial states. A closed formula for time evolution of concurrence is presented for a pure state. It is shown that under the influence of dissipation non-zero entanglement is created in unentangled two-qubit states which decay in the same way as pure entangled states. We also show that under real circumstances, the decay rate of concurrence is strongly modified by the non-Markovianity of the evolution. [ABSTRACT FROM AUTHOR]

In this paper, we study the decoherence of quantum excitation (photon-added) even /odd coherent states, $((\hat a{^{\dagger }})^{m} \left | {\alpha _ \pm } \right \rangle )$, in a thermal environment by investigating the variation of negative part of the Wigner quasidistribution function vs. the rescaled time. For this purpose, at first we obtain the time-dependent Wigner function corresponding to the mentioned states in the framework of standard master equation. Then, the time evolution of the Wigner function associated with photon-added even /odd coherent states, as well as the number of added photons m are analysed. It is shown that, in both states, the negative part of the Wigner function decreases with time. By deriving the threshold value of the rescaled time for single photon-added even /odd coherent states, it is also found that, if the rescaled time exceeds the threshold value, the associated Wigner function becomes positive, i.e., the decoherence occurs completely. [ABSTRACT FROM AUTHOR]

We investigate the dynamics of charge-state coherence in a degenerate double-dot Aharonov–Bohm interferometer with finite inter-dot Coulomb interactions. The quantum coherence of the charge states is found to be sensitive to the transport setup configurations, involving both the single-electron impurity channels and the Coulomb-assisted ones. We numerically demonstrate the emergence of a complete coherence between the two charge states, with the relative phase being continuously controllable through the magnetic flux. Interestingly, a fully coherent charge qubit arises at the double-dots electron pair tunneling resonance condition, where the chemical potential of one electrode is tuned at the center between a single-electron impurity channel and the related Coulomb-assisted channel. This pure quantum state of charge qubit could be experimentally realized at the current–voltage characteristic turnover position, where differential conductance sign changes. We further elaborate the underlying mechanism for both the real-time and the stationary charge-states coherence in the double-dot systems of study.

The paper investigates the effects of a two-level quantum system coupled to transversal and longitudinal dissipative environment. The time-dependent phase accumulation, LZ transition probability and entropy in the presence of fast-ohmic, sub-ohmic and super-ohmic quantum noise are derived. Analytical results are obtained in terms of temperature, dissipation strength, LZ parameter and bath cutoff frequency. The bath is observed to modify the standard occupation difference by a decaying random phase factor and also produces dephasing during the transfer of population. The dephasing characteristics or the initial non-zero decoherence rate are observed to increase in time with the bath temperature and depend on the system-bath coupling strength and cutoff frequency. These parameters are found to strongly affect the memory and thus tailor the coherence process of the system. [ABSTRACT FROM AUTHOR]

We examine the dynamical evolution of wave packets in a cubical billiard where three quantum numbers ( n, n, n) determine its energy spectrum and consequently its dynamical behaviour. We have constructed the wave packet in the cubical billiard and have observed its time evolution for various closed orbits. The closed orbits are possible for certain specific values of quantum numbers ( n, n, n) and initial momenta ( k, k, k). We observe that a cubical billiard exhibits degenerate energy levels and the path lengths of the closed orbits for these degenerate energy levels are identical. In spite of the identical path lengths, the shapes of the closed orbits for degenerate levels are different and depend upon angles 휃 and ϕ which we term as the sweep and the elevation angles, respectively. These degenerate levels owe their origin to the symmetries prevailing in the cubical billiard and these levels disappear completely or partially for a parallelepiped billiard as the symmetry breaks due to commensurate or incommensurate ratio of sides. [ABSTRACT FROM AUTHOR]

Quantum information technology largely relies on a sophisticated and fragile resource, called quantum entanglement, which exhibits a highly nontrivial manifestation of the coherent superposition of the states of composite quantum systems. In this paper, we study the interaction between the general and even coherent fields with moving and stationary two two-level atoms. In this regard, this paper investigates the von Neumann entropy and the atoms-field tangle as a measure of entanglement between the general and even coherent fields with the two atoms. Also, the entanglement between the two atoms using concurrence and negativity during time evolution is discussed. This paper examines the effects of multiphoton transitions and initial state setting on the entanglement for the system under consideration. Finally, the results demonstrate an important phenomenon such as the sudden death and birth of entanglement when the two atoms are initially in entangled states. [ABSTRACT FROM AUTHOR]

We set up the fermionic thermal entangled state approach to address Fermi operator master equations for the amplitude- and phase-decay processes and find the explicit Kraus operator-sum representations (KOSR) describing time-evolution of Fermi systems. As an application of KOSR, the evolution law of the two-level atomic system interacting with a photon field in these two decay processes is analytically derived, which shows that the interaction system has a quantum jump as a result of dissipation, but it always remains unchanged in the phase-decay process. [ABSTRACT FROM AUTHOR]

We study the dynamic of the Gaussian quantum discord in a continuous-variable system subject to a common non-Markovian environment with zero-temperature. By considering an initial two-mode Gaussian symmetric squeezed thermal state, we show that Gaussian discord has a very different dynamic characteristic in a non-Markovian evolution versus a Markov process, and can be created by the memory effect, which features non-Markovianity. We also study the relationship between Gaussian discord and the non-Markovian degree of the environment. The results may offer us an effective experimental method to get more quantum correlations. [ABSTRACT FROM AUTHOR]

We propose a new photon field in quantum field theory, named Laguerre-polynomial-weighted chaotic field. This field will emerge when an initial photon-added chaotic field, which is represented by its density operator , dissipates in an amplitude damping channel described by time evolution equation , where κ is a damping coefficient, that is, the initial field will evolve into with , where L s is the s-order Laguerre-polynomial, and : : denotes normal ordering. We employ the method of summation within an ordered product of operators to obtain our result. [ABSTRACT FROM AUTHOR]