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Quantum privacy query (QPQ) is a cryptographic application that protects the privacy of both users and databases while querying the database secretly. In most existing QPQ protocols, the protection of user privacy can only be cheat-sensitive. Cheat-sensitive means that Bob will be found later with a certain probability if he tries to get the address queried by Alice. On the premise of cheat-sensitive, although Alice can discover Bob's malicious behavior after a query (transaction), the secret information of Alice was leaked in the completed query, which is likely to be a fatal blow to Alice. Or, to prevent Bob's malicious behavior, Alice executes one or more additional queries to test Bob's honesty. However, to bypass Alice's honesty test, Bob can also provide several honest queries before performing dishonest queries. Therefore, cheat-sensitive should not be the ultimate goal of user privacy protection in QPQ. In this paper, we propose a two-way QKD-based QPQ protocol with better user privacy protection than cheat-sensitive based on order rearrangement of qubits. The proposed QPQ protocol can resist the Trojan horse attack even without wavelength filter and photon number splitter equipped with auxiliary monitoring detectors. [ABSTRACT FROM AUTHOR]

We propose a scheme of controlled cyclic quantum teleportation of an arbitrary two-qubit entangled state by using a ten-qubit entangled state. In our scheme, Alice can teleport an arbitrary two-qubit entangled state to Bob, Bob can teleport an arbitrary two-qubit entangled state to Charlie and at the same time Charlie can teleport an arbitrary two-qubit entangled state to Alice with the help of the controller David. [ABSTRACT FROM AUTHOR]

We consider the case that the two-level atom off-resonantly interacts with cavity field via a one-photon hopping in double Jaynes-Cummings system. Geometrical quantum discord (GQD) is adopted to quantify the degree of correlation between two subsystems. GQD dynamics between two atoms, between cavity fields, as well as between atom and cavity fields are studied. The influences of detuning on GQD are discussed. The results obtained using the numerical method show that GQD between two atoms, between cavity fields, and between atom and cavity fields all display periodic oscillations. Furthermore, their periodic oscillations frequencies increase with the increase of detuning. On the other hand, GQD between two cavity fields and that between atom and cavity are weakened with increase of detuning. [ABSTRACT FROM AUTHOR]

We investigate how the efficiency is affected on the noise environments in the controlled remote state preparation protocol, where the several realistic scenarios, i.e., a part or all of the qubits are subjected to the same or different types of noise, are considered. We find that more noise or less entanglement of qubits environment lead to more efficiency in terms of average fidelity. We show that it is better way to subject the qubits in different noise channels in order to increase the fidelity of the controlled remote state preparation protocol. By using a non-maximally three-qubit pure entangled state as quantum channel, furthermore, we could realize a perfect controlled remote state preparation by choosing the right noisy environments and adjusting their relations in terms of noisy rates. [ABSTRACT FROM AUTHOR]

In this paper, we consider the situation that three identical two-level atoms are separately trapped in the three single-mode cavities. Each atom resonantly interacts with cavity via a one-photon hopping. The dynamics of nonlocality in the system is investigated via Mermin-Ardehali-Belinksii-Klyshko inequality. The results show that when three atoms are initially in W state and three-cavity fields are in vacuum states both the quantum state of three atoms and that of three cavities all display nonlocality On the other hand, when three atoms are initially in Greenberger-Horne-Zeilinger state and three-cavity fields are in vacuum states, the quantum state of three atoms and that of three cavities all do not display nonlocality. [ABSTRACT FROM AUTHOR]

Recently, Xu et al. [Phys. Rev. A 2012, 86, 012113] have explored the behaviour of the entropic uncertainty relation under the influence of local unital and nonunital noisy channels for a class of Bell-diagonal states. We here improve their results and investigate the entropic uncertainty relation under the influence of unital and nonunital noisy channels with memory. Different types of noisy channels with memory, such as amplitude damping channel (nonunital channel), phase-damping and depolarizing channels (unital channels) have been taken into account. Analytical or numerical results is presented. The effect of channels with memory on dynamics of the entropic uncertainty (or its lower bound) has been discussed in detail. Compare with previous results, our results show that, the entropic uncertainty (or its lower bound) subjecting to amplitude damping channel with memory will be reduced at first and then be lifted with the memory coefficient of channel μ increasing, however they will be only reduced under phase-damping and depolarizing channels in the presence of memory. Especially, in the limit of , the entropic uncertainty (or its lower bound) could be well protected and immune to decoherence of channels. [ABSTRACT FROM AUTHOR]

In this paper we consider the situation that four identical two-level atoms are separately trapped in four single-mode optical cavities, which are placed at the vertices of a tetrahedron and coupled by six fibers. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the schrödinger equation when the total excitation number of the system equals one. The geometrical quantum discords between atoms and between cavities are investigated. The effects of cavity-fiber coupling coefficient on the geometrical quantum discords between atoms and between cavities are discussed. The results obtained using the numerical method indicate that the geometrical quantum discords between atoms and between cavities are all weakened with increase of cavity-fiber coupling coefficient. [ABSTRACT FROM AUTHOR]

In a recent article in this journal Guo-Hui (Int. J. Theor. Phys. 56, 600, 2017) has investigated the quantum correlation properties in the two qubits one-axis spin squeezing model with an external magnetic field by means of concurrence (C) and quantum discord (QD) criteria. We are to reveal numerically and analytically that in the limiting case T→ 0 (ground state) the sudden disappearance phenomenon (SDP) does not occur in the behavior of C and the values obtained for C are incorrect. [ABSTRACT FROM AUTHOR]

In this paper, we consider the situation that four identical two-level atoms are separately trapped in separated tetrahedral structure single-mode optical cavities, which are placed at the vertices of a tetrahedron and are coupled by four fibres. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the Schrödinger equation when the total excitation number of the system equals one. Negativity is adopted to quantify the degree of entanglement between two subsystems. The entanglement dynamics between atoms and between cavities is studied. The influences of atom-cavity coupling coefficient on the entanglement between atoms and that between cavities are discussed. The results obtained using the numerical method show that the atom-atom entanglement and the cavity-cavity entanglement are all strengthened with increase of atom-cavity coupling coefficient. [ABSTRACT FROM AUTHOR]

The well-known anisotropic XY spin chains in a transverse magnetic field are studied in the context of bipartite entanglement and quantum discord. By using Jordan-Wigner transformation, the model mapped onto a noninteracting spinless fermion model then, by accessing the energy spectrum we have found analytical formulas for concurrence and quantum discord based on reduced density matrix. We used the approach to go beyond the trivial nearest neighbours information, namely the second and third nearest neighbours. It is shown that far distance derivatives of quantum discord are able to capture XY-critical line and furthermore a signature of factorization points is observed. [ABSTRACT FROM AUTHOR]

In this paper, the linear entropy and collapse-revival phenomenon through the relation (⟨a^+a^⟩-n¯) in a system of N-configuration four-level atom interacting with a single-mode field with additional forms of nonlinearities of both the field and the intensity-dependent atom-field coupling functional are investigated. A factorization of the initial density operator is assumed, considering the field to be initially in a squeezed coherent states and the atom initially in its most upper excited state. The dynamical behavior of the linear entropy and the time evolution of (⟨a^+a^⟩-n¯) are analyzed. In particular, the effects of the mean photon number, detuning, Kerr-like medium and the intensity-dependent coupling functional on the entropy and the evolution of (⟨a^+a^⟩-n¯) are examined. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the preservation of quantum Fisher information (QFI) of a single-qubit system coupled to a common zero temperature reservoir through the addition of noninteracting qubits. The results show that, the QFI is completely protected in both Markovian and non-Markovian regimes by increasing the number of additional qubits. Besides, the phenomena of QFI display monotonic decay or non-monotonic with revival oscillations depending on the number of additional qubits N − 1 in a common dissipative reservoir. If N < Nc (a critical number depending on the reservoirs parameters), the behavior of QFI with monotonic decay occurs. However, if N ≥ Nc, QFI exhibits non-monotonic behavior with revival oscillations. Moreover, we extend this model to investigate the effect of additional qubits and the initial conditions of the system on the geometric phase (GP). It is found that, the robustness of GP against the dissipative reservoir has been demonstrated by increasing gradually the number of additional qubits N − 1. Besides, the GP is sensitive to the initial parameter 휃, and possesses symmetric in a range regime [0,2π]. [ABSTRACT FROM AUTHOR]

Normalized quantum Stokes operators introduced in Phys. Rev. A 95, 042113 (2017) enable one to better observe non-classical correlations of entangled states of optical fields with undefined photon numbers. For a given run of an experiment the new quantum Stokes operators are defined by the differences of the measured intensities (or photon numbers) at the exits of a polarizer divided by their sum. It is this ratio that is to be averaged, and not the numerator and the denominator separately, as it is in the conventional approach. The new approach allows to construct more robust entanglement indicators against photon-loss noise, which can detect entangled optical states in situations in which witnesses using standard Stokes operators fail. Here we show an extension of this approach beyond phenomena linked with polarization. We discuss EPR-like experiments involving correlations produced by optical beams in a multi-mode bright squeezed vacuum state. EPR-inspired entanglement conditions for all prime numbers of modes are presented. The conditions are much more resistant to noise due to photon loss than similar ones which employ standard Glauber-like intensity, correlations. [ABSTRACT FROM AUTHOR]

We consider two independent models which consist of an ion interacting with laser field. The influence of the initial degree of entanglement of the two ions on the entanglement of internal states of ions, ion-ion vibration entanglement and ion vibration-ion vibration entanglement is discussed. We examine entanglement via concurrence and negativity, and find that the sum of entanglement between two ions and entanglement between two ion vibration states is conserved. It is shown that entanglement sudden death (ESD) can happen and the less the value of initial degree of entanglement of the two ions is, the longer the ESD time cost. The initial degree of entanglement of the two ions has no influence on the trends of the evolution of entanglement. The less value of the initial degree of entanglement of the two ions is, the smaller the peak value of the entanglement achieved. [ABSTRACT FROM AUTHOR]

In quantum information processing, rapid control is a basic requirement for performance improvement because a realistic quantum system cannot be perfectly separated from its environment, which will cause relaxation or decoherence effect. Rapid control may make the control law more robust to uncertainties in the model or in the control process. From the time optimization standpoint, bang-bang control is an excellent method for engineering practice. However, quantum bang-bang control relies on quantum Zeno effect, quantum anti-Zeno effect will occur under low measurement frequency, which intensifies the decoherence and leads to high-frequency oscillation phenomenon. In order to realize rapid state transfer and avoid high-frequency oscillation with an infinitesimal period in bang-bang Lyapunov control, we design control fields to realize arbitrary state transfer for non-Markovian system with phase relaxation and energy dissipative relaxation by Lyapunov stability theory. The numerical simulations illustrate that arbitrary state (eigenstate, superposition state or mixed state) transfer and maintenance for non-Markovian system can be realized under the approximate bang-bang Lyapunov control. [ABSTRACT FROM AUTHOR]

In most models and theoretical calculations describing multiphoton or tunnelling ionisation by infrared light, and in most part of the spectroscopy, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. We review and complete our recent theoretical investigations which showed the importance of photon momentum in photoionisation of atoms in various regimes. We report signatures of the photon momentum in one photon, multiphoton and tunnelling regimes and compare them to the predictions of a new variant a simple standard tunnelling three-step model in which the Lorentz force is included in the final (classical) ionisation step. Importance of the magnetic quiver motion is underlined. In each regime, the photon momentum effect shows up in a specific way, in particular, the dependence of the photon effect on the laser polarisation is reviewed in detail. We also outline how this effect shows up in molecular photoionisation. [ABSTRACT FROM AUTHOR]

We introduce local filters as a means to detect the entanglement of bound entangled states which do not yield to detection by witnesses based on positive maps which are not completely positive. We demonstrate how such non-detectable bound entangled states can be locally filtered into detectable bound entangled states. Specifically, we show that a bound entangled state in the orthogonal complement of the unextendible product bases (UPB), can be locally filtered into another bound entangled state that is detectable by the Choi map. We reinterpret these filters as local measurements on locally extended Hilbert spaces. We give explicit constructions of a measurement-based implementation of these filters for $$2 \otimes 2$$ and $$3 \otimes 3$$ systems. This provides us with a physical mechanism to implement such local filters. [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 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]

The entanglement evolution of a mixed three-qubit W-type state in three independent non-Markovian reservoirs with Lorentzian spectral density is investigated. The effects of the non-Markovianity of environment, initial state and the detuning on the entanglement dynamics in three-qubit system are presented. The results show that the entanglement between qubits in many-body system is more complicated than that in two-body system. First, the disentanglement criterion for two qubits is not suitable for multi qubits. Second, in many-body system, the effects of weight number characterizing the initial state distribution on the entanglement between two arbitrary qubits may be opposite. Further numerical calculations show that increasing the amount of non-Markovianity λ / γ , the purity of the initial quantum state or the detuning are all conducive to maintain the entanglement between qubits. [ABSTRACT FROM AUTHOR]

Super-quantum discord (SQD) with weak measurement is regarded as a kind of quantum correlation in quantum information processing. We investigate the dynamical evolution of SQD under weak measurement in circuit quantum electrodynamics (circuit QED). The results indicate that in the correlated dephasing decay channel, the SQD monotonously tends to a stable value depending on the purity of initial quantum state, and the freezing phenomenon occurs. In the correlated instantaneous decay channel, however, not only the detuning sudden change but also the freezing phenomenon appear, and the freezing stable value is unrelated to the purity of the initial quantum state. The results reveal that the dynamical behavior of SQD entirely differs from the quantum discord, and it could become one important resource of quantum information. [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]

The purpose of this paper is a proposal on entanglement concentration protocol for cluster states. The protocol uses CNOT gate operation and is assisted with a single qubit. Moreover, the local and non-local operations are performed by a single party. We also make a comparative numerical study of the residual entanglement left out after the execution of each step of the protocol. [ABSTRACT FROM AUTHOR]

We present two schemes for transforming bipartite non-maximally entangled states into a W state in cavity QED system, by using highly detuned interactions and the resonant interactions between two-level atoms and a single-mode cavity field. A tri-atom W state can be generated by adjusting the interaction times between atoms and the cavity mode. These schemes demonstrate that two bipartite non-maximally entangled states can be merged into a maximally entangled W state. So the scheme can, in some sense, be regarded as an entanglement concentration process. The experimental feasibility of the schemes is also discussed. [ABSTRACT FROM AUTHOR]

We investigate the relation between dissipation and coherent effects in the dynamics of quantum entanglement (QE) between two qubits by means of weak measurement. Under weak measurement, the increase or decrease of QE closely relates with the dissipation. The amplitude of QE will only be elevated through enhancing the measurement strength in weak correlated or no correlated instantaneous modes, and the entanglement sudden death (ESD) is retarded effectively. In the correlated dephasing mode, however, the amplitude of QE will decrease due to the increases of measurement strength. The periods for entanglement death, as well as the dark and bright in the concurrence are not changed. Therefore, the environmental relevance helps to enhance the amplitude of QE and delay the entanglement death. [ABSTRACT FROM AUTHOR]

The idea of secure quantum information exchange (SQIE) [ J. Phys. B: At. Mol. Opt. Phys. 44, 115504 (2011)] is introduced for the secure exchange of single qubit information states between two legitimate users, Alice and Bob. In the present paper, we extend this original SQIE protocol by presenting a scheme, which enables the secure exchange of n-single qubit information states among the n nodes of a quantum network, with the aid of a special kind of 4 n-qubit entangled state and the classical assistance of an extra participant Charlie. For experimental realization of our extended SQIE protocol, we suggest an efficient scheme for the generation of a special kind of 4 n-qubit entangled state using the interaction between highly detuned Λ-type three-level atoms and optical coherent field. Further, by discussing the various experimental parameters, we show that the special kind 4 n-qubit entangled state can be generated with the presently available technology. [ABSTRACT FROM AUTHOR]

A scheme is proposed to prepare W -type entangled squeezed vacuum states (ESVSs) via free-travelling optical fields with simple linear optical devices, photo detectors and cross-Kerr medium. We investigate the influence of an inexact nonlinear phase factor between two modes on the fidelity of the prepared three-mode W -type ESVSs. By adjusting the appropriate reflectivity, the scheme can be extended to create 2 n + 1 -mode W -type ESVSs. [ABSTRACT FROM AUTHOR]

Some properties of three-qubit systems interacting with a noisy environment are discussed. The amount of the survival entanglement is quantified for the Greenberger-Horne-Zeilinger (GHZ) and W states. It is shown that the entanglement decay depends on the noise type (correlated or non-correlated), the number of interacting qubits with the environment, and the initial state that passes through this noisy environment. In general, the GHZ state is more fragile than the W state. The phenomenon of entanglement sudden death appears in the GHZ state only for non-correlated noise. [ABSTRACT FROM AUTHOR]

We propose two schemes to generate GHZ-type and W-type entangled squeezed vacuum states with free-travelling optical fields via a single-photon source, the Kerr medium, balanced beam-splitters and single-photon detectors. The effects of a fluctuation of nonlinear phase shift between two optical fields on the fidelity and on the success probability are discussed. We also discuss the effects of the imperfection of photon detectors on the success probability and the experimental feasibility of the present schemes. [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 mainly study on the effect of decoherence of the multiple photon annihilation-then-creation coherent state (MPACCS) in two different channels, i.e., amplitude decay and phase damping, by virtue of the time-evolution of the Wigner functions (WFs) of such state. Based on deriving the analytical expression of MPACCS's normalization factor related to the Bell-polynomial, the time-evolution of its WFs in each channel is derived analytically and discussed numerically. After undergoing the amplitude decay channel, the partial negativity of WFs diminishes gradually and the WFs have no negative region when the decay time exceeds a threshold value, while the negativity of WFs in phase damping channel varies more slowly than that of the amplitude decay. Especially, at long times WF evolves into the case of vacuum state in amplitude decay channel and a sum of weight WFs of all the Fock states in phase damping channel. [ABSTRACT FROM AUTHOR]

Quantum entanglement between two qubits was obtained by solving the model of two qubits coupled to two independent reservoirs. This paper studied the influences of dissipative environment, detuning between transition frequency of the qubit and center frequency of a cavity. Results showed that: (1) In the non-Markovian channel, a certain amount of detuning was conducive to maintain and improve the entanglement of a system. (2) On the other hand, in a two-sided amplitude damping channel, the composite effect of two independent dissipative channels would appear through the entanglement between two qubits. Given the composite effect of two independent channels, the entanglement dynamics not only affected the Markovian process but also hindered the backflow effect of the non-Markovian channel, which was not conducive to maintain the entanglement between qubits. [ABSTRACT FROM AUTHOR]

In this paper, we study how to fuse two W states into a larger W state in a cavity quantum electrodynamic system. Two atoms are collided within a detuned vacuum cavity mode. One atom is from the first N-atom W state system, and the other is from the second M-atom W state system. The two atoms will be detected after flying out of the cavity mode. If only one excitation is detected between the two atoms, an ( N + M − 2)-atom W state can be generated with some probability. If no excitation is detected between the two atoms, the remaining ( N − 1)-atom W state and ( M − 1)-atom W state can be re-fused using the same procedure, continuing until two excitations are detected between the two atoms. Using this procedure, the chance of entanglement of the two W states is as large as possible. A feasibility analysis indicates that this scheme can be implemented using existing experimental technologies. [ABSTRACT FROM AUTHOR]