Geometric phase of open two-level systems

Geometric phase of open quantum systems is reviewed. An emphasis is given on specific features of the geometric phase which can serve as an indicator of type and strength of interaction between two-level system (qubit) and its bosonic environment. We study three examples: (i) a single qubit dephasingly coupled to the environment, (ii) a qubit being a part of quantum register, and (iii) a neutrino interacting with matter and environment

Binary communication with Gazeau-Klauder coherent states

We investigate advantages and disadvantages of using Gazeau–Klauder coherent states for optical communication. In this short paper we show that using an alphabet consisting of coherent Gazeau–Klauder states related to a Kerr-type nonlinear oscillator instead of standard Perelomov coherent states results in lowering of the Helstrom bound for error probability in binary communication. We also discuss trace distance between Gazeau–Klauder coherent states and a standard coherent state as a quantifier of distinguishability of alphabets

Disentanglement of qubits in classical limit of interaction

Two qubits coupled by integral spin object are studied in the semi-classical limit of interaction intermediary. It is shown that initial entanglement of qubits becomes more robust when mediated by semi-classical interaction and does not decay below certain value at a given time. The statements are supported by numerical averaging with respect to a set of randomly chosen initial preparations. There are evidences that such a robustness holds true also for different types of quantum correlations

Quantum cloning disturbed by thermal Davies environment

A network of quantum gates designed to implement universal quantum cloning machine is studied.We analyze how thermal environment coupled to auxiliary qubits, ‘blank paper’ and ‘toner’ required at the preparation stage of copying, modifies an output fidelity of the cloner. Thermal environment is described in terms of the Markovian Davies theory. We show that such a cloning machine is not universal any more but its output is independent of at least a part of parameters of the environment. As a case study, we consider cloning of states in a six-state cryptography’s protocol. We also briefly discuss cloning of arbitrary input states