We analyse dissipation in quantum computation and its destructive impact on the efficiency of quantum algorithms. Using a general model of decoherence, we study the time evolution of a quantum register of arbitrary length coupled with an environment of arbitrary coherence length. We discuss relations between decoherence and computational complexity and show that the quantum factorization algorithm must be modified in order to be regarded as efficient and realistic.

We study the dynamics of quantum correlations in a class of exactly solvable Ising-type models. We analyze in particular the time evolution of initial Bell states created in a fully polarized background and on the ground state. We find that the pairwise entanglement propagates with a velocity proportional to the reduced interaction for all the four Bell states. Singletlike states are favored during the propagation, in the sense that tripletlike states change their character during the propagation under certain circumstances. Characteristic for the anisotropic models is the instantaneous creation of pairwise entanglement from a fully polarized state; furthermore, the propagation of pairwise entanglement is suppressed in favor of a creation of different types of entanglement. The “entanglement wave” evolving from a Bell state on the ground state turns out to be very localized in space time. Our findings agree with a recently formulated conjecture on entanglement sharing; some results are interpreted in terms of this conjecture.

We propose an experimental realization of cryptographic-key-sharing scheme exploiting quantum correlations between pair photons. Our experimental setup consists of an external source of correlated photon pairs which propagate to two widely separated unbalanced Mach-Zehnder interferometers. The probability of detection of photon pairs in any two outputs of the interferometers can be fully modulated by phase plates in either interferometer.

We show that entanglement is a useful resource to enhance the mutual information of the depolarizing channel when the noise on consecutive uses of the channel has some partial correlations. We obtain a threshold in the degree of memory above which a higher amount of classical information is transmitted with entangled signals.