International Journal of Quantum Information

Traditionally, quantum information is formulated and implemented in terms of elementary quantum systems e.g. qubits. Now, it has been shown that certain appealing properties of entangled systems, like for instance nonlocality, are more robust against noise when they are realized at the level of higher dimensional systems, like qutrits (or more generally qudits with d strictly larger than 2). It has also been shown, in the case of the generalized Ekert protocol, (a so-called entanglement-based protocol) that confidentiality is enhanced when Quantum Key Distribution is realized with carriers of information of dimensionality strictly higher than 2 (this is because optimal cloning is less effective when the dimension d increases). This brought us to consider the possible generalization of the so-called Singapore protocol, a full tomographic qubit protocol, to dimensions strictly higher than 2. As we shall show, dimension 3 is, like dimension 2, privileged regarding the connections of the protocol with Wigner tomography. We also show that, although the qutrit version of the Singapore protocol is, just like Ekert's protocol, more robust against noise than its qubit counterpart, it is less appealing for what concerns the effective information rate (e.g. the number of qudits to be sent in average in order to exchange one classical bit of the key).

We introduce a new control strategy on a two-way deterministic cryptographic scheme, which relies on a suitable unitary transformation rather than a quantum measurement. The study is developed for d-ary alphabets and the particular choice of the transformation works when d is an odd prime power. It leads to an improvement of the protocol security, which we prove to increase with the alphabet order d.

Brown et al. and Borras et al. have explored numerical optimization procedures to search for highly entangled five- and six-qubit states. Here, we give a relation between the genuine multiqubit entangled state and Bell state for 4-, 5-, and 6-qubit systems. And the general method to construct maximally multipartite entangled state is also presented.

We consider an application of the Burshtein equation to Jaynes–Cummings interactions possessing a field-noise modeled as the two-state random phase telegraph noise in the presence of two extra physical cases. These cases are the incorporation of respectively, (i) the atomic-noise regarded as phase damping and (ii) the Stark shift. We obtain the exact solutions to these systems to investigate the dynamical behavior of their entanglements. We observed those for the case (i), phase damping does not affect the field-noise-induced nonmonotonic behavior of the entanglement other than the amplitude of the entanglement and the optimal value of the mean dwell time at which the minimum entanglement occurs. Also, phase damping naturally accelerates the decoherence of the entanglement caused by the field-noise. For the case (ii), the Stark shift reduces the maximum value of the entanglement and induce more oscillations of the entanglement in time under the influence of the noise. This shift delays the time for the complete destruction of the entanglement and restrains the influence of the noise. We also consider the time-average of the entanglement as depending on the shift parameters. The average entanglement is a nonmonotonic function of these parameters.

We propose a scheme for preparing a highly entangled six qubit genuine state [J. Phys. A40 (2007) 13407] in an ion-trap system. We evaluate the influence of lasers' small intensity fluctuation and find it is negligible in our scheme. Our scheme is feasible with the present techniques.

Lorentz transformation of the spin density matrix as well as the momentum density matrix for a quantum system of three massive spin-1/2 particles that have an appropriate discrete momentum distribution is investigated. In a rest frame, both the spin and the momentum parts are supposed to be maximally entangled as the Greenberger–Horne–Zeilinger state. Provided the particles move with sufficiently high speeds, we can find boosts that completely remove both the spin and momentum entanglements. We can even find boosts that completely preserve both entanglements. For the considered boosts, there is no mixing between the spin and the momentum parts.

Classical communication plays an important role in quantum information processing such as remote state preparation and quantum teleportation. First, in this paper, we present some simple faithful remote state preparation of an arbitrary n-qubit state by constructing entanglement resources and special measurement basis for the sender. Then to weigh the classical resource required, we present an information-theoretical model to evaluate the classical communication cost. By optimizing the classical communication in quantum protocols, we obtain the optimal classical communication cost. This model can also be applied to the quantum teleportation. Moreover, based on the present computation model, we reinvestigate some remote state preparation and teleportation protocols in which the classical communication cost was imperfectly computed. Finally, some problems will be presented.

In terms of a suitable variant of the EPR–Bohm example, we argue that the quantum mechanically predicted and experimentally verified violation of a Bell-type path–spin noncontextual realist inequality for an "intraparticle" path–spin entanglement involving single neutrons can be used to infer a form of nonlocality, distinct from Bell-type nonlocality, that is required for any relevant hidden variable model to be compatible with the quantum mechanical treatment of an EPR–Bohm-type "interparticle" entanglement.

We propose two schemes for realizing dense coding in linear optical system. These schemes are based on a genuine four-qubit entangled state |Ψ〉 as a shared resource of entanglement. There are many interesting properties and possible applications in quantum information processing and fundamental tests of quantum physics by this state. By distinguishing Alice's operation we can easily achieve the dense-coding.

Recently, a genuine five-qubit entangled state has been proposed by Man et al. In this paper, we present a simple scheme for generating such a state in ion trap and cavity QED, respectively. This study offers a new approach to faithful controlled teleportation of an arbitrary two-qubit state in these two systems.

By assuming a hierarchical interaction with the environments within the Houtappel approximation, decoherence effects on a Quantum Information System (QIS) are studied. In order to avoid a harmful "always on" effect it is assumed that such interaction happens in a single one step. As a result the decoherence times are quantized and inversely proportional to the strength of the couplings of the QIS with the environment. The decoherence is manifested as a liberated heat by the QIS. By Landauer's principle this effect erases the information. Our theoretical results are applied to three different Nuclear Magnetic Resonance systems. Bounds to the probability of erasing the information are imposed for these systems.

A scheme is proposed for generating a polarization four-photon cluster, which is believed to be suitable to achieve the one-way quantum computing via single-qubit projective measurements, with the help of only cross-Kerr nonlinearity and current linear optical systems. Compared with the existing schemes, the distinct advantage of the present scheme is that cluster states can be achieved with high success probability close to unity. Our scheme is experimentally demanding but efficient. Based on the present scheme, the cluster state of 3N + 1 photons can be obtained with the help of linear optical elements.

The security of the second (STF-B) protocol of quantum cryptography proposed by Shimizu, Tamaki and Fukasaka (STF) [Phys. Rev. A80 (2009) 022323] is analyzed in a general way, which shows that the STF-B protocol is vulnerable under a kind of generalized attack. Sufficient and necessary condition for which Eve can successfully attack the message M without introducing errors is deduced. It shows that the correlation-elicitation (CE) attack strategy found by Qin et al. [Phys. Rev. A82 (2010) 036301] is a special case of the sufficient and necessary condition. Next, we advance an effective and efficient strategy to prevent the general attack. Hence, the STF-B protocol is revised to a secure one by improving the control mode.

The relative error of cloning of quantum states with arbitrary prior probabilities is considered. It is assumed that the ancilla may contain some a priori information about the input state to be cloned. The lower bound on the relative error for general cloning scenario is derived. Both the case of two-state set and case of multistate set are analyzed in detail. The treated figure of merit is compared with other optimality criteria. It is posed that the relative error is more expressive in the case of nearly indistinguishable states. The quantum circuit for optimal cloning of a pair of pure states is built.