International Journal of Quantum Information

We discuss the relation between the no-cloning theorem from quantum information and the doubling procedure used in the formalism of thermofield dynamics (TFD). We also discuss how to apply the no-cloning theorem in the context of thermofield states defined in TFD. Consequences associated to mixed states, von Neumann entropy and thermofield vacuum are also addressed.

We propose the simulation for an effective scheme to realize a spin network with tunable long-range couplings in the coupled cavity array with external multi-driving lasers. Via this scheme, the linear photon-like dispersion relation is achievable, which could be employed to perform a perfect quantum state transfer. Numerical results show that when applying two lasers in each cavity, the fidelity is higher than the highest fidelity of a classical transfer even for the transfer distance l increases up to 100 sites. In the simulation, as the number of lasers increases, the fidelity will be evidently enhanced for a wide range of l.

We propose a scheme for the deterministic sharing arbitrary qudit states among three distant parties and characterize the set of ideal quantum channels. We also show that the use of non-ideal quantum channels for quantum state sharing can be related to the problem of quantum state discrimination. This allows us to formulate a protocol which leads to perfect quantum state sharing with a finite success probability.

We propose a scheme for implementing a three-qubit controlled-Not-Not (CNNOT) gate and a two-qubit SWAP gate between atoms and single-photon pulse through cavity QED. In the scheme, we can one-step realize multiple-qubit GHZ state and two-qubit Bell state by applying multiple-qubits CNNOT gate. We have also shown that our scheme would be robust against practical imperfections in current cavity QED experiment setup through simple numerical estimates. Finally, we provide the current parameters to show that our scheme is feasible.

A phase controlled wave packet, recently used in experiment of wave packet interferometry of a diatomic molecule, is investigated to obtain mesoscopic superposition structures, useful in quantum metrology. This analysis provides a new way of obtaining sub-Planck scale structures at smaller time scale of revival dynamics. We study a number of situations for delineating the smallest interference structures and their control by tailoring the relative phase between two subsidiary wave packets. We also find the most appropriate state, so far, for high precision parameter estimation in a system of diatomic molecule.

In this work, we present an implementation of quantum logic gates and algorithms in a three effective qubits system, represented by a (I = 7/2) NMR quadrupolar nuclei. To implement these protocols we have used the strong modulating pulses (SMP) and the various stages of each implementation were verified by quantum state tomography (QST). The results for the computational base states, Toffolli logic gates, and Deutsch–Jozsa and Grover algorithms are presented here. Also, we discuss the difficulties and advantages of implementing such protocols using the SMP technique in quadrupolar systems.

We develop the concept of quantum carrier and show that messages can be uploaded and downloaded from this carrier and while in transit, these messages are hidden from external agents. We explain in detail the working of the quantum carrier for different communication tasks, including quantum key distribution, classical secret and quantum state sharing among a set of n players according to general threshold schemes. The security of the protocol is discussed and it is shown that only the legitimate subsets can retrieve the secret messages, the collaboration of all the parties is needed for the continuous running of the protocol and maintaining the carrier.

We give asymptotic behaviors of the von Neumann entropy and the Shannon entropy of discrete-time time quantum walks on Z².

In this paper, a set of topological basis realization is presented. By acting on the different subspaces, we obtain the new nontrivial six-dimensional (6D) and four-dimensional Temperley–Lieb matrix representations. Then it is shown that XXZ model can be constructed from the Temperley–Lieb algebra (TLA) generators. The eigenstates of XXZ model can be denoted by combinating the set of topological bases. It is interesting that the ground state is closely related to parameter Δ.

Two schemes for controlled dense coding with a one-dimensional four-particle cluster state are investigated. In this protocol, the supervisor (Cliff) can control the channel and the average amount of information transmitted from the sender (Alice) to the receiver (Bob) by adjusting the local measurement angle θ. It is shown that the results for the average amounts of information are unique from the different two schemes.

We investigate the possibility of appearance of new phenomena derived from the influence of noise on quantum systems. As a starting point we study the entanglement evolution of two coupled qubits in interaction with an external environment and in the presence of an external magnetic field with a stochastic component. The results show the expected degradation of entanglement due to the noise. The new effect is that for particular initial states the time of disentanglement depends in a non-monotonous way on the strength of the noise. We find that it is shortest for an intermediate strength value of the noise. This we call "stochastic anti-resonance." Our new result indicates that there are noise values which are particularly harmful and should be avoided. This could lead to a better understanding of noisy perturbations and their role for optimal designing of quantum devices.

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.

We analyze continuous-time quantum walks on necklace graphs — cyclical graphs consisting of many copies of a smaller graph (pearl). Using a Bloch-type ansatz for the eigenfunctions, we block-diagonalize the Hamiltonian, reducing the effective size of the problem to the size of a single pearl. We then present a general approach for showing that the mixing time scales (with growing size of the necklace) similarly to that of a simple walk on a cycle. Finally, we present results for mixing on several necklace graphs.

Both classical and quantum computations operate with the registers of bits. At nanometer scale the quantum fluctuations at the position of a given bit, say, a quantum dot, not only lead to the decoherence of quantum state of this bit, but also affect the quantum states of the neighboring bits, and therefore affect the state of the whole register. That is why the requirement of reliable separate access to each bit poses the limit on miniaturization, i.e. constrains the memory capacity and the speed of computation. In the present paper we suggest an algorithmic way to tackle the problem of constructing reliable and compact registers of quantum bits. We suggest accessing the states of a quantum register hierarchically, descending from the state of the whole register to the states of its parts. Our method is similar to quantum wavelet transform, and can be applied to information compression, quantum memory, quantum computations.

We show that, under the influence of finite temperature reservoirs, for a board class of states, Bell nonlocality always endures sudden death. Furthermore, for two states evolved from two different maximally entangled states, the larger the concurrence and quantum discord, the better the resource for dense coding. However, the more usefulness of a state for teleportation could not be owed to the larger negativity and geometric measure of discord.