On the degree of non-Markovianity of quantum evolution

We propose a new characterization of non-Markovian quantum evolution based on the concept of non-Markovianity degree. It provides an analog of a Schmidt number in the entanglement theory and reveals the formal analogy between quantum evolution and the entanglement theory: Markovian evolution corresponds to a separable state and non-Markovian one is further characterized by its degree. It enables one to introduce a non-Markovinity witness -- an analog of an entanglement witness -- and a family of measures -- an analog of Schmidt coefficients -- and finally to characterize maximally non-Markovian evolution being an analog of maximally entangled state. Our approach allows to classify the non-Markovianity measures introduced so far in a unified rigorous mathematical framework.Comment: 5 page

Non-Markovian dynamics of a damped driven two-state system

We study a driven two-state system interacting with a structured environment. We introduce the non-Markovian master equation ruling the system dynamics, and we derive its analytic solution for general reservoir spectra. We compare the non-Markovian dynamics of the Bloch vector for two classes of reservoir spectra: the Ohmic and the Lorentzian reservoir. Finally, we derive the analytic conditions for complete positivity with and without the secular approximation. Interestingly, the complete positivity conditions have a transparent physical interpretation in terms of the characteristic timescales of phase diffusion and relaxation processes.Comment: 12 pages, 5 figures, final versio

Decoherence control in different environments

We investigate two techniques for controlling decoherence, focusing on the crucial role played by the environmental spectrum. We show how environments with different spectra lead to very different dynamical behaviours. Our study clearly proves that such differences must be taken into account when designing decoherence control schemes. The two techniques we consider are reservoir engineering and quantum-Zeno control. We focus on a quantum harmonic oscillator initially prepared in a nonclassical state and derive analytically its non-Markovian dynamics in presence of different bosonic thermal environments. On the one hand we show how, by modifying the spectrum of the environment, it is possible to prolong or reduce the life of a Schr\"odinger cat state. On the other hand we study the effect of nonselective energy measurements on the degradation of quantumness of initial Fock states. In this latter case we see that the crossover between Zeno (QZE) and anti-Zeno (AZE) effects, discussed by Maniscalco et al. [Phys. Rev. Lett. 97, 130402 (2006)], is highly sensitive to the details of the spectrum. In particular, for certain types of spectra, even very small variations of the system frequency may cause a measurement-induced acceleration of decoherence rather than its inhibition.Comment: 8 pages, 4 figures, new figures added, text modified for clarit

Effect of temperature on non-Markovian dynamics in Coulomb crystals

In this paper we generalize the results reported in Phys. Rev. A 88, 010101 (2013) and investigate the flow of information induced in a Coulomb crystal in presence of thermal noise. For several temperatures we calculate the non-Markovian character of Ramsey interferometry of a single 1/2 spin with the motional degrees of freedom of the whole chain. These results give a more realistic picture of the interplay between temperature, non-Markovianity and criticality.Comment: 5 pages, 3 figures. Accepted for publication in Special Issue of the International Journal of Quantum Information devoted to IQIS2013 conferenc

Non-Markovian Quantum Probes

We review the most recent developments in the theory of open quantum systems focusing on situations in which the reservoir memory effects, due to long-lasting and non-negligible correlations between system and environment, play a crucial role. These systems are often referred to as non-Markovian systems. After a brief summary of different measures of non-Markovianity that have been introduced over the last few years we restrict our analysis to the investigation of information flow between system and environment. Within this framework we introduce an important application of non-Markovianity, namely its use as a quantum probe of complex quantum systems. To illustrate this point we consider quantum probes of ultracold gases, spin chains, and trapped ion crystals and show how properties of these systems can be extracted by means of non-Markovianity measures.Comment: 31 pages, 7 figures. To appear in a special volume of OSID on open system