International Journal of Quantum Information

This paper is a short survey on intermediate states (IS) of the light field studied in the last fifteen years in the literature; they coincide with two basic states in two different limits of the involved parameters. We consider their main characteristics, distinguishing them in two classes: the class of interpolating states, and a special class of superposition states. A new IS is proposed to compare properties of distinct states mapping between the same extremes.

The adiabatic quantum evolution method has generally been studied for the case where the linear interpolation from an initial to a final Hamiltonian is taken. However, this approach fails even for the case of a single qubit, when the initial and final states are orthogonal or almost orthogonal. In this case, the evolution time becomes infinite. We show that by using a simple nonlinear interpolation method the adiabatic evolution can be done in finite time, regardless of the orthogonality of the initial and final states.

We present three different types of the evolution of a time-averaged probability distribution of the quantum walk on even cycles. We concentrate on initial states which lead to non-uniform limiting distributions.

An efficient technique to generate ensembles of spins that are highly polarized by external magnetic fields is the Holy Grail in Nuclear Magnetic Resonance (NMR) spectroscopy. Since spin-half nuclei have steady-state polarization biases that increase inversely with temperature, spins exhibiting high polarization biases are considered cool, even when their environment is warm. Existing spin-cooling techniques are highly limited in their efficiency and usefulness. Algorithmic cooling is a promising new spin-cooling approach that employs data compression methods in open systems. It reduces the entropy of spins on long molecules to a point far beyond Shannon's bound on reversible entropy manipulations, thus increasing their polarization. Here we present an efficient and experimentally feasible algorithmic cooling technique that cools spins to very low temperatures even on short molecules. This practicable algorithmic cooling could lead to breakthroughs in high-sensitivity NMR spectroscopy in the near future, and to the development of scalable NMR quantum computers in the far future. Moreover, while the cooling algorithm itself is classical, it uses quantum gates in its implementation, thus representing the first short-term application of quantum computing devices.

A measure to quantify the degree of entanglement for two qubits (2×2) in a pure state is presented in Phys. Rev.A65, 044303 (2002). We generalize their results to 2×n quantum systems. An analytical expression of the entanglement of formation for quantum mixed states of 2×n quantum systems is obtained. Moreover for multipartite quantum systems a generalized "measure" is presented to judge if a pure state is separable or maximally entangled.

The Holevo–Schumacher–Westermoreland (HSW) channel capacity for the diagonal unital qudit channels is considered. In Phys. Rev.A69, 022302, the HSW channel capacity for the diagonal unital qudit channels Φ is given as χ(Φ)=1-min_ρH(Φ(ρ)), where minimization is over the input states of the channel. In this paper, using the concavity of von Neumann entropy, we show that using only pure states we can work out its HSW channel capacity. Hence, our result simplifies the computation.

Recent developments in quantum computation and quantum information theory allow one to extend the scope of game theory for the quantum world. The authors have recently proposed a quantum description of financial market in terms of quantum game theory. This paper contains an analysis of such markets that shows that there are advantages to be gained from using quantum computers and quantum strategies.

Prediction of the correlation between the data does not cause any problems in an ideal Einstein–Podolsky–Rosen (EPR) experiment, but if both measurements are not really performed at the same time one should take into account that a particle is irreversibly absorbed by a detector during the measurement. We calculate with full details the destruction of quantum states during the spin measurement of a particle involving localization of a particle in a detector in a system of two distinguishable particles of spin-½ and consider an example of the destruction in the configuration typical for EPR experiments.

In this paper, by introducing the additional classical information and taking advantage of the property of quantum entanglement swapping of two photon pairs instead of the property of quantum entanglement of a photon pair, the Bostrom–Felbinger protocol [Phys. Rev. Lett.89, 187902 (2002)] is improved. In the improved protocol, when the photon travels from the message receiver's site to the message sender's site, any attack can be detected in the control mode. Moreover, it is deterministically successful to transmit the secret messages in the message mode. The security of the present improved protocol against the eavesdropping is ensured by the Bostrom–Felbinger protocol, and besides, the attacks proposed by Cai [Phys. Rev. Lett.90, 109801 (2003)] to attack the Bostrom–Felbinger protocol without eavesdropping can be prevented.

Time-dependent dynamical systems with a particular emphasis on models attaining the minimum value of uncertainty formula are considered. The role of the Bogolubov coefficients, in general and in the context of the loss of minimum uncertainty, is analyzed. Different fluctuation values on squeezed states are performed. The decoherence energy is parametrized by an angle ϕ and turns out to vanish whenever ϕ=π. An application to the Paul trap theory is discussed.

The present methods for obtaining the optimal Lewenestein–Sanpera decomposition of a mixed state are difficult to handle analytically. We provide a simple analytical expression for the optimal Lewenstein–Sanpera decomposition by using semi-definite programming. In particular, we obtain the optimal Lewenstein–Sanpera decomposition for some examples such as: the Bell decomposable state, the iso-concurrence state, the generic two-qubit state in the Wootters basis, the 2⊗3 Bell decomposable state, the d⊗d Werner and isotropic states, a one parameter 3⊗3 state, and finally a multi-partite isotropic state.