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Quantum information and quantum foundations are becoming popular topics for advanced undergraduate courses. Many of the fundamental concepts and applications in these two fields, such as delayed choice experiments and quantum encryption, are comprehensible to undergraduates with basic knowledge of quantum mechanics. In this paper, we show that the quantum eraser, usually used to study the duality between wave and particle properties, can also serve as a generic platform for quantum key distribution. We present a pedagogical example of an algorithm to securely share random keys using the quantum eraser platform and propose its implementation with quantum circuits. [ABSTRACT FROM AUTHOR]

Encoding and decoding are the two key steps in information processing. In this work, we study the encoding and decoding capabilities of operational theories in the context of information-storability game, where the task is to freely choose a set of states from which one state is chosen at random and by measuring the state it must be identified; a correct guess results in as many utiles as the number of states in the chosen set and an incorrect guess means a penalty of a fixed number of utiles. We connect the optimal winning strategy of the game to the amount of information that can be stored in a given theory, called the information storability of the theory, and show that one must use so-called nondegradable sets of states and nondegradable measurements whose encoding and decoding properties cannot be reduced. We demonstrate that there are theories where the perfect discrimination strategy is not the optimal one so that the introduced game can be used as an operational test for super information storability. We further develop the concept of information storability by giving new useful conditions for calculating it in specific theories. [ABSTRACT FROM AUTHOR]

Quantum simulation has become a promising avenue of research that allows one to simulate and gain insight into the models of high energy physics whose experimental realizations are either complicated or inaccessible with current technology. We demonstrate the quantum simulation of such a model, a quantum mechanical system with spatial noncommutativity, which is inspired by the works in noncommutative geometry and noncommutative field theory for a universal quantum computer. We use the novel group theoretical formalism to map the Hamiltonian of such a noncommutative quantum system into the ordinary quantum mechanical Hamiltonian and then carry out the quantum simulation using the Trotter–Suzuki product formula. Furthermore, we distinguish the impact of the noncommutativity parameter on the quantum simulation, especially on the Trotter error, and point out how its sizable value affects the simulation. [ABSTRACT FROM AUTHOR]

Entanglement plays a major role in quantum information process and is a principal resource for various quantum applications like quantum teleportation, quantum key distribution, quantum communication, etc. Strong entangled pairs are required for efficient information process but system impurities during the transportation diminishes the entanglement by reducing the fidelity of the entangled pair. In order to reduce this, purification techniques can be used. In this paper, we propose an efficient entanglement purification method to distill the entanglement using entanglement swapping. The proposed method increases the fidelity of the entanglement and can be a proficient for various applications of quantum computing. [ABSTRACT FROM AUTHOR]

In this paper, we study the influence of nonabelian superselection rules on the transfer of quantum information with the help of qubits on the base of an algebraic model and formulate quantum protocols. We pay the main attention to the superselection structure of the algebra of observables O G defined by the Cuntz algebra O d (a field algebra) that contains O G as a pointwise fixed subalgebra with respect to the action of the gauge group G. We prove that it is possible to code information only with the help of states such that projectors on them belong to the algebra of observables. These projectors commute with the elements of the representation of the group G , and therefore allow the recipient to restore the obtained information. [ABSTRACT FROM AUTHOR]

We conduct a pair of quasirandom estimations of the separability probabilities with respect to 10 measures on the 15-dimensional convex set of two-qubit states, using its Euler-angle parametrization. The measures include the (nonmonotone) Hilbert–Schmidt one, plus nine others based on operator monotone functions. Our results are supportive of previous assertions that the Hilbert–Schmidt and Bures (minimal monotone) separability probabilities are 8 3 3 ≈ 0. 2 4 2 4 2 4 and 2 5 3 4 1 ≈ 0. 0 7 3 3 1 3 8 , respectively, as well as suggestive of the Wigner–Yanase counterpart being 1 2 0 . However, one result appears inconsistent (much too small) with an earlier claim of ours that the separability probability associated with the operator monotone (geometric-mean) function x is 1 − 2 5 6 2 7 π 2 ≈ 0. 0 3 9 3 2 5 1. But a seeming explanation for this disparity is that the volume of states for the x -based measure is infinite. So, the validity of the earlier conjecture — as well as an alternative one, 1 9 (5 9 3 − 6 0 π 2) ≈ 0. 0 9 1 5 2 6 2 , we now introduce — cannot be examined through the numerical approach adopted, at least perhaps not without some truncation procedure for extreme values. [ABSTRACT FROM AUTHOR]

In this study, we investigated the academic literature on quantum technologies (QT) using bibliometric tools. We used a set of 49,823 articles obtained from the Web of Science (WoS) database using a search query constructed through expert opinion. Analysis of this revealed that QT is deeply rooted in physics, and the majority of the articles are published in physics journals. Keyword analysis revealed that the literature could be clustered into three distinct sets, which are (i) quantum communication/cryptography, (ii) quantum computation, and (iii) physical realizations of quantum systems. We performed a burst analysis that showed the emergence and fading away of certain key concepts in the literature. This is followed by co-citation analysis on the "highly cited" articles provided by the WoS, using these we devised a set of core corpus of 34 publications. Comparing the most highly cited articles in this set with respect to the initial set we found that there is a clear difference in most cited subjects. Finally, we performed co-citation analyses on country and organization levels to find the central nodes in the literature. Overall, the analyses of the datasets allowed us to cluster the literature into three distinct sets, construct the core corpus of the academic literature in QT, and to identify the key players on country and organization levels, thus offering insight into the current state of the field. Search queries and access to figures are provided in the appendix. [ABSTRACT FROM AUTHOR]

Dynamical high-dimensional quantum states can be tracked and manipulated in many cases. Using a new theoretical framework approach of manipulating quantum systems, we will show how one can manipulate and introduce parameters that allow tracking and descriptive insight in the dynamics of states. Using quantum topology and other novel mathematical representations, we will show how quantum states behave in critical points when the shift of probability distribution introduces changes. [ABSTRACT FROM AUTHOR]

Understanding temporal processes and their correlations in time is of paramount importance for the development of near-term technologies that operate under realistic conditions. Capturing the complete multi-time statistics that define a stochastic process lies at the heart of any proper treatment of memory effects. This is well understood in classical theory, where a hierarchy of joint probability distributions completely characterizes the process at hand. However, attempting to generalize this notion to quantum mechanics is problematic: observing realizations of a quantum process necessarily disturbs the state of the system, breaking an implicit, and crucial, assumption in the classical setting. This issue can be overcome by separating the experimental interventions from the underlying process, enabling an unambiguous description of the process itself and accounting for all possible multi-time correlations for any choice of interrogating instruments. In this paper, using a novel framework for the characterization of quantum stochastic processes, we first solve the long standing question of unambiguously describing the memory length of a quantum processes. This is achieved by constructing a quantum Markov order condition, which naturally generalizes its classical counterpart for the quantification of finite-length memory effects. As measurements are inherently invasive in quantum mechanics, one has no choice but to define Markov order with respect to the interrogating instruments that are used to probe the process at hand: different memory effects are exhibited depending on how one addresses the system, in contrast to the standard classical setting. We then fully characterize the structural constraints imposed on quantum processes with finite Markov order, shedding light on a variety of memory effects that can arise through various examples. Finally, we introduce an instrument-specific notion of memory strength that allows for a meaningful quantification of the temporal correlations between the history and the future of a process for a given choice of experimental intervention. These findings are directly relevant to both characterizing and exploiting memory effects that persist for a finite duration. In particular, immediate applications range from developing efficient compression and recovery schemes for the description of quantum processes with memory to designing coherent control protocols that efficiently perform information-theoretic tasks, amongst a plethora of others. [ABSTRACT FROM AUTHOR]

Analogous to the notion of mutually unbiased bases for Hilbert spaces, we consider mutually unbiased unitary bases (MUUBs) for the space of operators, M (d , ℂ) , acting on such Hilbert spaces. The notion of MUUB reflects the equiprobable guesses of unitary operators in one basis of M (d , ℂ) when estimating a unitary operator in another. Though, for prime dimension d , the maximal number of MUUBs is known to be d 2 − 1 , there is no known recipe for constructing them, assuming they exist. However, one can always construct a minimum of three MUUBs, and the maximal number is approached for very large values of d. MUUBs can also exist for some d -dimensional subspace of M (d , ℂ) with the maximal number being d. [ABSTRACT FROM AUTHOR]

We study the dynamic of quantum speed limit (QSL) time of a qubit coupled to a bath of noninteracting spins. The investigations are carried under both exact and approximate solutions of the model and the results of the two approaches are compared with each other. Under the exact solution and in the low temperature limit, QSL time becomes independent of the number of spins in the environment with potential capacity for quantum speed up. Beyond a critical value of temperature, there exists a frequency of the spins that reduces the capacity for speeding up quantum evolution. The effects of driving time, nonuniform frequencies of the bath's spins and coupling strength on QSL time are also investigated. [ABSTRACT FROM AUTHOR]

In this paper, we proposed novel plans of quaternary quantum reversible half and full subtractor circuits. The subtractor element is the essential part of the ALU in the digital computational devices. Thus, the improvement of subtractor block has a significant impact on the overall system performance. According to the comparison results, the proposed quaternary quantum half and full subtractor circuits show tremendous improvement in quantum cost, hardware complexity, number of constant input and garbage output as compared to their counterparts. Moreover, for the first time, the quaternary quantum borrow ripple subtractor structure is realized using the proposed quaternary quantum half and full subtractor circuits. [ABSTRACT FROM AUTHOR]

It was recently emphasized by Byrnes, Forster and Tessler [Phys. Rev. Lett. 120 (2018) 060501] that the continuous-time formulation of Grover's quantum search algorithm can be intuitively understood in terms of Rabi oscillations between the source and the target subspaces. In this work, motivated by this insightful remark and starting from the consideration of a time-independent generalized quantum search Hamiltonian as originally introduced by Bae and Kwon [Phys. Rev. A 66 (2002) 012314], we present a detailed investigation concerning the physical connection between quantum search Hamiltonians and exactly solvable time-dependent two-level quantum systems. Specifically, we compute in an exact analytical manner the transition probabilities from a source state to a target state in a number of physical scenarios specified by a spin- 1 ∕ 2 particle immersed in an external time-dependent magnetic field. In particular, we analyze both the periodic oscillatory as well as the monotonic temporal behaviors of such transition probabilities and, moreover, explore their analogy with characteristic features of Grover-like and fixed-point quantum search algorithms, respectively. Finally, we discuss from a physics standpoint the connection between the schedule of a search algorithm, in both adiabatic and nonadiabatic quantum mechanical evolutions, and the control fields in a time-dependent driving Hamiltonian. [ABSTRACT FROM AUTHOR]

For parameter estimation from an N -component composite quantum system, it is known that a separable preparation leads to a mean-squared estimation error scaling as 1 / N while an entangled preparation can in some conditions afford a smaller error with 1 / N 2 scaling. This quantum superefficiency is however very fragile to noise or decoherence, and typically disappears with any small amount of random noise asymptotically at large N. To complement this asymptotic characterization, here we characterize how the estimation efficiency evolves as a function of the size N of the entangled system and its degree of entanglement. We address a generic situation of qubit phase estimation, also meaningful for frequency estimation. Decoherence is represented by the broad class of noises commuting with the phase rotation, which includes depolarizing, phase-flip and thermal quantum noises. In these general conditions, explicit expressions are derived for the quantum Fisher information quantifying the ultimate achievable efficiency for estimation. We confront at any size N the efficiency of the optimal separable preparation to that of an entangled preparation with arbitrary degree of entanglement. We exhibit the 1 / N 2 superefficiency with no noise, and prove its asymptotic disappearance at large N for any nonvanishing noise configuration. For maximizing the estimation efficiency, we characterize the existence of an optimum N opt of the size of the entangled system along with an optimal degree of entanglement. For nonunital noises, maximum efficiency is usually obtained at partial entanglement. Grouping the N qubits into independent blocks formed of N opt entangled qubits restores at large N a nonvanishing efficiency that can improve over that of N independent qubits optimally prepared. Also, one inactive qubit included in the entangled probe sometimes stands as the most efficient setting for estimation. The results further attest with new characterizations the subtlety of entanglement for quantum information in the presence of noise, showing that when entanglement is beneficial, maximum efficiency is not necessarily obtained by maximum entanglement but instead by a controlled degree and finite optimal amount of it. [ABSTRACT FROM AUTHOR]

Entanglement is one of the pillars of quantum mechanics and quantum information processing, and as a result, the quantumness of nonentangled states has typically been overlooked and unrecognized until the last decade. We give a robust definition for the classicality versus quantumness of a single multipartite quantum state, a set of states, and a protocol using quantum states. We show a variety of nonentangled (separable) states that exhibit interesting quantum properties, and we explore the 'zoo' of separable states; several interesting subclasses are defined based on the diagonalizing bases of the states, and their nonclassical behavior is investigated. [ABSTRACT FROM AUTHOR]

We propose multi-party quantum summation protocols based on single particles, in which participants are allowed to compute the summation of their inputs without the help of a trusted third party and preserve the privacy of their inputs. Only one participant who generates the source particles needs to perform unitary operations and only single particles are needed in the beginning of the protocols. [ABSTRACT FROM AUTHOR]

An overview is given of the nature of the quantum mechanical wave function. [ABSTRACT FROM AUTHOR]

We firstly propose a simultaneous dense coding protocol with two-photon four-qubit cluster states in which two receivers can simultaneously get their respective classical information sent by a sender. Because each photon has two degrees of freedom, the protocol will achieve a high transmittance. The security of the simultaneous dense coding protocol has also been analyzed. Secondly, we investigate how to simultaneously teleport two different quantum states with polarization and path degree of freedom using cluster states to two receivers, respectively, and discuss its security. The preparation and transmission of two-photon four-qubit cluster states is less difficult than that of four-photon entangled states, and it has been experimentally generated with nearly perfect fidelity and high generation rate. Thus, our protocols are feasible with current quantum techniques. [ABSTRACT FROM AUTHOR]

We propose an adynamical interpretation of quantum theory called Relational Blockworld (RBW) where the fundamental ontological element is a 4D graphical amalgam of space, time and sources called a ' spacetimesource element.' These are fundamental elements of space, time and sources, not source elements in space and time. The transition amplitude for a spacetimesource element is computed using a path integral with discrete graphical action. The action for a spacetimesource element is constructed from a difference matrix K and source vector J on the graph, as in lattice gauge theory. K is constructed from graphical field gradients so that it contains a non-trivial null space and J is then restricted to the row space of K, so that it is divergence-free and represents a conserved exchange of energy-momentum. This construct of K and J represents an adynamical global constraint between sources, the spacetime metric and the energy-momentum content of the spacetimesource element, rather than a dynamical law for time-evolved entities. To illustrate this interpretation, we explain the simple EPR-Bell and twin-slit experiments. This interpretation of quantum mechanics constitutes a realist, psi-epistemic model that might underwrite certain information-theoretic accounts of the quantum. [ABSTRACT FROM AUTHOR]

We present an explicit reexamination of Gisin's 1991 original proof concerning the violation of Bell's inequality for any pure entangled state of two-particle systems. Given the relevance of Gisin's work, our analysis is motivated by pedagogical reasons and allows the straightening of a few mathematical points in the original proof that in no way change the physical conclusions reached by Gisin. [ABSTRACT FROM AUTHOR]

Reversible logic is one of the indispensable aspects of emerging technologies for reducing physical entropy gain, since reversible circuits do not lose information in the form of internal heat during computation. This paper aimed to initiate constructing parity preserving reversible circuits. A novel parity preserving reversible block, HB is presented. Then a new design of a cost-effective parity preserving reversible full adder/subtractor (PPFA/S) is proposed. Next, we suggested a new parity preserving binary to BCD converter. Finally, we proposed new realization of parity preserving reversible BCD adder. The proposed designs are cost-effective in terms of quantum cost and delay. All the scales are in the NANO-metric area. [ABSTRACT FROM AUTHOR]

We studied in this paper the bipartite quantum systems in finite Hilbert space. We applied the measurement on the first subsystem only and studied how this measurement is affecting the entropy of the second subsystem. The analytic work carried out in this paper shows that the post measurement state is factorizable when measurement is applied on the first subsystem. This result shows that the disorder of the system is reduced and hence the entropy of the whole system is reduced as expected. However, it is also shown in this work that the entropy of the second subsystem of the post-measurement state could be greater than that of the original state. Increasing entropy after measurement means that the disorder in the second subsystem is increased. The information we gain from measuring the first subsystem reduces our information about the second subsystem rather than increasing it or at least keeping it the same as expected. This means that more factors may be required in order to calculate the probability of finding the second subsystem in a certain state. [ABSTRACT FROM AUTHOR]

We study the effect of the interference between two independent non-Markovian reservoirs on the non-Markovianity of a two-level system. We find that the interference can enhance the non-Markovianity of the system. The additive master equation (ME) is invalid even if only one of the two reservoirs is non-Markovian, while it is a good approximation when all reservoirs are Markovian. We also derive the two states uniqueness for a two-level system coupled to structured reservoirs which maximize the measure of non-Markovianity defined by Breuer et al. [ Phys. Rev. Lett. 103 (2009) 210401]. [ABSTRACT FROM AUTHOR]

We argue that the dimensionality of the space of quantum systems' states should be considered as a legitimate resource for quantum information tasks. The assertion is supported by the fact that quantum states with discord-like capacities can be obtained from classically-correlated states in spaces of dimension large enough. We illustrate things with some simple examples that justify our claim. [ABSTRACT FROM AUTHOR]

Reversible logic has been recently considered as an interesting and important issue in designing combinational and sequential circuits. The combination of reversible logic and multi-valued logic can improve power dissipation, time and space utilization rate of designed circuits. Only few works have been reported about sequential reversible circuits and almost there are no paper exhibited about quantum ternary reversible counter. In this paper, first we designed 2-qutrit and 3-qutrit quantum reversible ternary up-counters using quantum ternary reversible T-flip-flop and quantum reversible ternary gates. Then we proposed generalized quantum reversible ternary n-qutrit up-counter. We also introduced a new approach for designing any type of n-qutrit ternary and reversible counter. According to the results, we can conclude that applying second approach quantum reversible ternary up-counter is better than the others. [ABSTRACT FROM AUTHOR]

We present a scheme for transferring quantum state between atom and cavity field in Jaynes-Cummings model (JC) in the aid of spin-echo-like technique. It is based on the facts that the atom in a cavity can induce the generation of modified coherent states (MCSs), which can be shown to be macroscopically distinguishable, and the anti-commutation relation between the Hamiltonian and the z-component Pauli matrix. We show that this scheme is applicable for a class of cavity field states. The application on two-cavity system provides an alternative scheme for preparation of non-local superpositions of quasi-classical light states. Numerical simulation shows that the proposed schemes are efficient. [ABSTRACT FROM AUTHOR]

We provide a statistical communication model for the phenomenon of quantum information evaporation from black holes (BHs). A BH behaves as a reflecting quantum channel in a very special regime, which allows for a receiver to perfectly recover the absorbed quantum information. The quantum channel of a perfectly reflecting (PR) BH is the probabilistically weighted sum of infinitely many qubit cloning channels. In this work, we reveal the statistical communication background of the information evaporation process of PR BHs. We show that the density of the cloned quantum particles in function of the PR BH's mass approximates a Chi-square distribution, while the stimulated emission process is characterized by zero-mean, circular symmetric complex Gaussian random variables. The results lead to the existence of Rayleigh random distributed coefficients in the probability density evolution, which confirms the presence of Rayleigh fading (a special type of random fluctuation) in the statistical communication model of BH information evaporation. [ABSTRACT FROM AUTHOR]

We report a systematic study of asymptotic behavior of entanglement between the position and coin degrees of freedom for a one-dimensional discrete quantum walk. Effects of coin bias, initial coin asymmetry and phase difference and non-locality of initial position state on the fluctuations and asymptotic value of entanglement are investigated by using Fourier approach. Fluctuations in entropy are found to die out as power law in time as t-α with α = 1/2,3/2 and 5/2 depending on the initial state parameters of the system. [ABSTRACT FROM AUTHOR]

We introduce new mathematical aspects of the Bell states using matrix factorizations, non-noetherian singularities, and noncommutative blowups. A matrix factorization of a polynomial p consists of two matrices ϕ1, ϕ2 such that ϕ1ϕ2 = ϕ2ϕ1 = p id. Using this notion, we show how the Bell states emerge from the separable product of two mixtures, by defining pure states over complex matrices rather than just the complex numbers. We then show in an idealized algebraic setting that pure states are supported on non-noetherian singularities. Moreover, we find that the collapse of a Bell state is intimately related to the representation theory of the noncommutative blowup along its singular support. This presents an exchange in geometry: the nonlocal commutative spacetime of the entangled state emerges from an underlying local noncommutative spacetime. [ABSTRACT FROM AUTHOR]

This paper presents the INRIM effort related to the implementation of an optical time domain reflectometer (OTDR) that exploits backscattered light to test an optical fiber operating at single photon level, using two different avalanche photodetectors: a free-running Id220 and a gated Id201. Our tests show that the use of the Id220-based OTDR could be very helpful for QKD application, since it allows testing much longer portions of an optical link. This represents a great advantage for QKD systems, where single components and connections are often embedded in a non-separable optical system. [ABSTRACT FROM AUTHOR]

We investigate spin squeezing dynamics in a two-component Bose-Einstein condensate (BEC) in the presence of the nonlinear interatomic interaction, interspecies interaction and Josephson-like tunneling interaction. In particular, we are interesting in the dependence of spin squeezing parameter on the interspecies interaction and the numbers of atom. By adopting the two-mode approximation and the rotating wave approximation, we succeed in obtaining analytical solutions for the optimally squeezed angle and spin squeezing parameter. It is shown that the stronger interspecies interaction induces faster spin squeezing and the more atoms or the larger population imbalance induces stronger squeezing; while the Josephson-like tunneling gives vanishing contribution to spin squeezing. [ABSTRACT FROM AUTHOR]

We analyze phase interferometry realized with a bosonic Josephson junction (BJJ) made of trapped dilute and ultracold atoms. By using a suitable phase sensitivity indicator we study the zero temperature junction-states useful to achieve sub shot-noise precisions. Sub shot-noise phase shift sensitivities can be reached even at finite temperature under a suitable choice of the junction state. We infer a scaling law in terms of the size system (that is, the number of particles) for the temperature at which the shot-noise limit is not overcome anymore. [ABSTRACT FROM AUTHOR]

We demonstrate two entanglement concentration protocols (ECPs) for photon pairs in less-hyperentanglement, the simultaneous entanglement in multiple degrees of freedom. Using these ECPs, some maximally entangled states in polarization can be reconstructed deterministically from less-hyperentangled ones shared between remote participants. Both of the success probabilities are 100% in principle, and they do not require additional single photons. The former is implemented with the passive linear optics, which is achievable with current technology. The later adopts the cross-Kerr nonlinearity media to complete this task, which can increase the efficiency of the entanglement concentration process since it does not require the sophisticated single-photon detectors for measurements. The two ECPs are useful for practical long-distance quantum communication due to the fact that the entangled state in either the spatial degree of freedom or the frequency degree of freedom suffers little from channel noise in optical fiber. [ABSTRACT FROM AUTHOR]

We revise the Dirac equation for a free particle and investigate Lorentz transformations on spinors. We study how the spin quantization axis changes under Lorentz transformations, and evince the interplay between spin and momentum in this context. [ABSTRACT FROM AUTHOR]

In semiclassical gravity, the final state of black-hole evaporation cannot be described by a pure state. Nevertheless, we point out that the system can be described by a generalized pure state, which is not defined on a three-dimensional hypersurface but on the four-dimensional spacetime describing the whole Universe at all times. Unlike the conventional quantum state, such a generalized state treats time on an equal footing with space, which makes it well suited for systems that are both quantum and relativistic. In particular, such a generalized state contains a novel type of information encoded in the correlations between future and past, which avoids the black-hole information paradox. [ABSTRACT FROM AUTHOR]

Based on maximally entangled states in the full- and sub-spaces of two-qutrits, we present an alternative decomposition of two-qutrit pure states in a form $|\Psi\rangle = \frac{p_{1}}{\sqrt{3}}(|00\rangle + |11\rangle + |22\rangle) + \frac{p_{2}}{\sqrt{2}}(|01\rangle + |12\rangle) + p_{3}e^{i\theta}|02\rangle$. Similar to the Schmidt decomposition, all two-qutrit pure states can be transformed into the alternative decomposition under local unitary transformations, and the parameter p1 is shown to be an entanglement invariant. [ABSTRACT FROM AUTHOR]

We have studied the symmetric and non-symmetric pairwise ground state and thermal entanglement in three-qubits system. We have considered the anisotropic Heisenberg (XXZ) model in the presence of Dzyaloshinskii-Moriya (DM) interaction in addition to the Ising model in a magnetic field with DM interaction. We have found that the increment of DM interaction and magnetic field can enhance and reduce the entanglement of the system. We have shown that the non-symmetric pairwise state has higher value concurrence and critical temperature (above which the entanglement vanishes) than the symmetric pairwise one. For the negative anisotropy, the non-symmetric entanglement is a monotonic function of DM interaction while for positive anisotropy, it has a maximum versus DM parameter and vanishes for larger values of DM interaction. The conditions for the existence of thermal entanglement are discussed in detail. The most remarkable result appears at zero temperature where the three-qubits ground state entanglement of the system (in spite of two-qubits counterpart) shows the fingerprint of the quantum phase transition for a system of infinite number of qubits. [ABSTRACT FROM AUTHOR]

We demonstrate that a genuine six-qubit entangled state introduced by Tapiador et al. [J. Phys. A42 (2009) 415301] can be used to realize the deterministic controlled teleportation of an arbitrary three-qubit state by performing only the Bell-state measurements. [ABSTRACT FROM AUTHOR]

We discuss "partial" quantum cloning of pure two-partite states, where only the "part" of the initial state related to one qubit is copied. The same approach gives the possibility of designing a quantum copying machine for mixed qubit states. [ABSTRACT FROM AUTHOR]

We studied the entanglement of a photon and an exciton in semiconductors. Our model shows a superhigh degree of entanglement between the exciton and the emitted photon, higher than that between the atom and the photon.11 We obtained the simple analytical formulas for both R and K which are the specific parameters of the entanglement. We also investigated the dependence of entanglement on the main parameters of the exciton in semiconductors. These results are in a good agreement with previous works. [ABSTRACT FROM AUTHOR]

In the past decades, various schemes of teleportation of quantum states through different types of quantum channels (a prior shared entangled state between the sender and the receiver), e.g. EPR pairs, generalized Bell states, qubit GHZ states, standard W states and its variations, genuine multiqubit entanglement states, etc., have been developed. Recently, three-qutrit quantum states and two-qudit quantum states have also been considered as quantum channels for teleportation. In this paper, we investigate the teleportation of an unknown qudit using a d level GHZ state, i.e. a three-qudit maximally entangled state, as quantum channel. We design a general scheme of faithful teleportation of an unknown qudit using a d-level GHZ state shared between the sender and the receiver, or among the sender, the receiver and the controller; an unknown two-qudit of Schmidt form using a d level GHZ state shared between the sender and the receiver; as well as an unknown arbitrary two-qudit using two shared d level GHZ states between the sender, the receiver and the controller, or using one shared d level GHZ state and one shared generalized Bell state. We obtain the general formulas of Alice's measurement basis, Charlie's measurement basis and Bob's unitary operations to recover the input state of Alice. It is intuitionistic to generalize the protocols of teleporting an arbitrary two-qudit state to teleporting an arbitrary n-qudit state. [ABSTRACT FROM AUTHOR]

We study the possibility of preventing finite-time disentanglement caused by dissipation by making use of non-local quantum error correction. This is made in comparison to previous results, where it was shown that local quantum error correction can delay disentanglement, but can also cause entanglement sudden death when it is not originally present. [ABSTRACT FROM AUTHOR]

In this work, we have studied the post-measurement dynamics of several electrons in a double quantum dot system using a new method of calculation. We have solved the effective-mass non-linear Schrödinger equation for two electron wave packets in a double quantum well taking into account the different number of projected electrons in each quantum well after an observation. It is found the existence of a critical density that reduces strongly the tunneling dynamics between both quantum wells. In addition, we have shown the possibility of having a new kind of electromagnetic radiation emerging from a semiconductor device after a quantum observation. [ABSTRACT FROM AUTHOR]

We study decoherence of a three-qubit array coupled to substrate phonons. Assuming an initial three-qubit entangled state that would be decoherence-free for identical qubit positions, allows us to focus on non-Markovian effects of the inevitable spatial qubit separation. It turns out that the coherence is most affected when the qubits are regularly spaced. Moreover, we find that up to a constant scaling factor, two-qubit entanglement is not influenced by the presence of the third qubit, even though all qubits interact via the phonon field. [ABSTRACT FROM AUTHOR]

Quantum entanglement, like other resources, is now considered to be a resource. It can be produced, concentrated if required, swapped, transported and consumed. During recent years, various schemes of quantum state teleportation have been proposed using different types of quantum channels. Not restricting to qubit based systems, qutrit states and channels have also been of considerable interest. In the present paper, we investigate the teleportation of an unknown single qutrit state, as well as a two qutrit state through a three qutrit quantum channel, along with the required operations to recover the state. This is further generalized to the case of teleportation of an n-qutrit system. [ABSTRACT FROM AUTHOR]

The time that a given initial quantum state requires to evolve to an orthogonal state is used to explore the role played by entanglement in the dynamical evolution of composite fermionic systems. We study two different applications of free spin fermions in a four site cluster. It is shown that through selection of the hopping matrix elements, the relationship between entanglement and speed of quantum evolution can be controlled. Consequently, the result recently pointed out [J. Batle et al., Phys. Rev. A72 (2005) 032337] represents only a particular choice of the energy spectrum of the system. [ABSTRACT FROM AUTHOR]

A scheme for remote preparation of field (atomic) states is proposed. Protocol execution requires cavity QED based atom-field interactions successively supplemented with Ramsey interferometry. The state to be remotely prepared at the receiver's end is acquired by deterministically manipulating the sender's component of the pre-shared entangled state. In the case of field entanglement, it is carried out with the help of an atom that passes through the sender's cavity and then traverses a classical external field for specified times prior to detection. However, for atomic entangled states, only interactions with the classical field suffice to complete the task. The scheme guarantees good success probability with high fidelity and requires one bit of classical communication. [ABSTRACT FROM AUTHOR]

A qubit which is prepared in one of two non-orthogonal states and subjected to bit-flipping noise is considered. The objective is to use measurement and feedback control to correct the state of the qubit. Three classical schemes using projective measurements, i.e. discrimination and re-preparation, do nothing and random preparation, have been discussed, and are not optimal with respect to a performance which is quantified by the average fidelity of the corrected state compared to the initial state. In addition, one quantum scheme using a non-projective measurement with an optimum measurement strength achieves the best trade-off between gaining information about the system and disturbing it through measurement back-action. The performance of a quantum control scheme outperforms the classical schemes. Furthermore, no universal corrected scheme is discussed. [ABSTRACT FROM AUTHOR]

A true quantum signature algorithm based on continuous-variable entanglement state is proposed. In the suggested algorithm, a key-pair, i.e. private signature key and public verification key, is generated based on a one-way function. By employing the signature key, a message state is encoded into a 2k-particle entangled state and a two-particle entangled state is prepared. The resulting states are exploited as a signature of the message state. The signature can be decoded under the verification key when it needs to be verified. Subsequently, a decoded message state and a two-particle entangled state are obtained. To compare the decoded states and the original states, a quantum circuit for comparing these states is exploited. Making use of measurement results of the quantum circuit one can judge the authenticity of the received signature. According to the security requirement of the signature scheme, the suggested algorithm has been proven to be theoretically secure by using the Shannon information theory. [ABSTRACT FROM AUTHOR]