International Journal of Quantum Information

A quantum key distribution (QKD) network is currently being implemented in Vienna by integrating seven QKD-link devices that connect five subsidiaries of Siemens Austria. We give an architectural overview of the network and present the enabling QKD technologies, as well as the novel QKD network protocols.

Logics for reasoning about quantum states and their evolution have been given in the literature. In this paper, we consider quantum computation tree logic (QCTL), which adds temporal modalities to exogenous quantum propositional logic. We give a sound and complete axiomatization of QCTL and combine the standard CTL model-checking algorithm with the dEQPL model-checking algorithm to obtain a model-checking algorithm for QCTL. Finally, we illustrate the use of the logic by reasoning about the BB84 key distribution protocol.

This work reviews and extends recent results concerning the distribution of entanglement, as well as nonlocality (in terms of inequality violations) in tripartite qubit systems. With recourse to a Monte Carlo generation of pure and mixed states of three-qubits, we explore several features related to the distribution of entanglement (expressed in the form of different measures of multiqubit entanglement based upon bipartitions). Also, special interest is paid to maximally entangled states (such as the GHZ for three-qubits) and W states. This study also sheds some light on the interesting relation existing between some entanglement measures and perfect state discrimination in LOCC measurements relevant to cryptographic protocols. We round off the results by studying the distribution of entanglement between Alice and Bob in a modified teleportation protocol toy model over three-qubit states.

The semantics of a language for reasoning about finite-dimensional quantum systems is presented. This language can express most important classes of assertions about quantum systems, including formulas for outputs of all combinational quantum circuits/algorithms. The main result of this paper is an algorithm for efficient translation of a formula of language into an equivalent formula in another decidable language ℝℂ, which is the language of reals and its complex extension. An important consequence is a descriptive characterization of quantum circuits that can be efficiently simulated classically. We illustrate this with examples of two classes of quantum circuits which are known to have efficient classical simulation. The algorithm for deciding the satisfiability of a general formula can be adapted for the simulation.

We show how quantum tamper-proof devices (QTPD's) can be used to attack and to develop security protocols. On one hand, we prove that it is possible to transfer proofs of zero-knowledge protocols using QTPD's. This attack can be extended to other security schemes where privacy is important. On the other hand, we present a fair contract signing protocol using QTPD's where there is no communication with Judge during the exchange phase (which is impossible classically). In the latter case, we make use of decoherence in the quantum state of the QTPD to implement a global clock over the asynchronous network. QTPD's seem to be possible to implement with existing quantum hardware, due to the fact that it is hard to isolate quantum memory from interference. These theoretical results contribute to justify the implementation of QTPD's.

We studied the dynamics of the entanglement for two electron atoms with initial states created from a superposition of the eigenstates of the two-electron Hamiltonian. We present numerical evidence that the pairwise entanglement for the two electrons evolves in a way that is strongly related with the time evolution of the Coulombic interaction between the two electrons.

We propose an experimentally feasible scheme to implement the optimal asymmetric economical 1 → 2 phase-covariant telecloning, which works without ancilla, based on Cavity-QED. Our scheme is insensitive to the cavity field states and cavity decay. During the telecloning process, the cavity is only virtually excited, thus it greatly prolongs the efficient decoherent time. Therefore, the scheme can be experimentally realized in the range of current cavity QED techniques.

Classical communications are used in the post-processing procedure of quantum key distribution. Since the security of quantum key distribution is based on the principles of quantum mechanics, intuitively, the secret key can only be derived from the quantum states. We find that classical communications are incorrectly used in the so-called quantum dialogue type protocols. In these protocols, public communications are used to transmit secret messages. Our calculations show that half of Alice's and Bob's secret message is leaked through the classical channel. By applying the Holevo bound, we can see that the quantum efficiency claimed in the quantum dialogue type of protocols is not achievable.

The time evolution of the atomic Wehrl entropy and long-lived entanglement generation using a single trapped ion interacting with a laser field are analyzed. Starting from the Heisenberg equation of motion, an exact solution of the system is obtained by indicating that there are some interesting features when a time-dependent modulating function is considered. We demonstrate that the long-living quantum entanglement can be obtained using the time-independent interaction when the field is initially in a pair cat states.

We study the dynamics of entanglement for a four-qubit system in cavity QED. Two initially entangled atoms A and B are coupled respectively with spatially separate cavities a and b with coupling strengths g_A and g_B. We show that when g_A ≠ g_B, the entanglement will oscillate in the period of entanglement sudden death (ESD) for g_A = g_B, and the oscillation times are related to the ratios between g_A and g_B. Also, we show that the coupling strengths have the same effects on the entanglement evolution and energy transfer.

Quantum cryptography is used to find practical and useful applications. Recently, some first quantum cryptographic solutions became available on the market. For clients, it is important to be able to compare the quality and properties of the proposed products. To this end, one needs to elaborate on specifications and standards of solutions in quantum cryptography. We propose and discuss a list of characteristics for the specification, which includes numerical evaluations of the security of solution and can be considered as a standard for quantum key distribution solutions. The list is based on the average time of key generation, depending on some parameters. In the simplest case for the user, the list includes three characteristics: the security degree, the length of keys and the key refresh rate.

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.

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.

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.

We propose a measure of entanglement through the quantum trajectory approach. The advantage of this measure is that it can avoid the problem of finding all possible decompositions for mixed states, which is usually complicated. This method can be applied to any physical system whose time evolution is governed by the master equation. As an example, we calculate the entanglement of the following cases: (A) entangled system composed of a pair of two-level atoms coupling to two separate baths and, (B) two entangled modes subjected to two separate baths.

With the importance of quantum information science being widely recognized, introductory textbooks on quantum control will be required by many students from diversified backgrounds. Intended for undergraduate and beginning graduate students in mathematics, engineering, and physics, this book by Dr. D'Alessandro is the first mathematician-written textbook on the subject of quantum control. Representative quantum control problems from various contexts are introduced with clarity and strictness. Emphasis is placed on the applications of Lie algebra and Lie group theory in quantum control.