We propose a feasible method of coherent-state information concentration and purification utilizing quantum memory. The method allows us to optimally concentrate and purify information carried by many noisy copies of an unknown coherent state (randomly distributed in time) to a single copy. Thus non-classical resources and operations can be saved, if we compare information processing with many noisy copies and a single copy with concentrated and purified information.

In this paper the quantum-statistical properties of light beams propagating in a directional nonlinear coupler are investigated. This device is composed of a pair of linearly coupled nonlinear waveguides operating by means of degenerate parametric downconversion. The possibility of generation of nonclassical states of light in single and compound modes, using the short-length approximation, is discussed. Influence of linear and nonlinear mismatches for generation of nonclassical light is taken into account.

We investigate a compact source of entanglement. This device is composed of a pair of linearly coupled nonlinear waveguides operating by means of degenerate parametric downconversion. For the vacuum state at the input the generalized squeeze variance and logarithmic negativity are used to quantify the amount of nonclassicality and entanglement of output beams. Squeezing and entanglement generation for various dynamical regimes of the device are discussed. The influence of losses is investigated in detail.

In the framework of the Heisenberg-Langevin theory the statistical effects arising from a mutual interaction of two non-degenerate down-conversion processes are investigated. Primarily, we focus on the possibility to control the intensity and statistical properties of the light generate din one nonlinear medium by the light entering the other one. We also show that the model discussed is an interesting generalization of the famous experiments by Zou, Wang, and Mandel on the induced coherence without induced emission.

The quantum-statistical properties of light beams in a directional symmetric nonlinear coupler composed of two nonlinear waveguides operating by the down-conversion processes are examined. By means of short-length approximation non-classical behaviour of single and compound modes in such a device is analyzed. Linear and nonlinear mismatches are taken into account.