Probing the spectral density of a dissipative qubit via quantum synchronization

The interaction of a quantum system, which is not accessible by direct measurement, with an external probe can be exploited to infer specific features of the system itself. We introduce a probing scheme based on the emergence of spontaneous quantum synchronization between an out-of-equilibrium qubit, in contact with an external environment, and a probe qubit. Tuning the frequency of the probe leads to a transition between synchronization in phase and antiphase. The sharp transition between these two regimes is locally accessible by monitoring the probe dynamics alone and allows one to reconstruct the shape of the spectral density of the environment

Quantum Darwinism and non-Markovian dissipative dynamics from quantum phases of the spin-1/2 XX model

Quantum Darwinism explains the emergence of a classical description of objects in terms of the creation of many redundant registers in an environment containing their classical information. This amplification phenomenon, where only classical information reaches the macroscopic observer and through which different observers can agree on the objective existence of such object, has been revived lately for several types of situations, successfully explaining classicality. We explore quantum Darwinism in the setting of an environment made of two level systems which are initially prepared in the ground state of the XX model, which exhibits different phases; we find that the different phases have different ability to redundantly acquire classical information about the system, being the "ferromagnetic phase" the only one able to complete quantum Darwinism. At the same time we relate this ability to how non-Markovian the system dynamics is, based on the interpretation that non-Markovian dynamics is associated to back flow of information from environment to system, thus spoiling the information transfer needed for Darwinism. Finally, we explore mixing of bath registers by allowing a small interaction among them, finding that this spoils the stored information as previously found in the literature

Genuine quantum and classical correlations in multipartite systems

Generalizing the quantifiers used to classify correlations in bipartite systems, we define genuine total, quantum, and classical correlations in multipartite systems. The measure we give is based on the use of relative entropy to quantify the "distance" between two density matrices. Moreover, we show that, for pure states of three qubits, both quantum and classical bipartite correlations obey a ladder ordering law fixed by two-body mutual informations, or, equivalently, by one-qubit entropies.Comment: Accepted for publication in Phys. Rev. Let

Maximally discordant mixed states of two qubits

We study the relative strength of classical and quantum correlations, as measured by discord, for two-qubit states. Quantum correlations appear only in the presence of classical correlations, while the reverse is not always true. We identify the family of states that maximize the discord for a given value of the classical correlations and show that the largest attainable discord for mixed states is greater than for pure states. The difference between discord and entanglement is emphasized by the remarkable fact that these states do not maximize entanglement and are, in some cases, even separable. Finally, by random generation of density matrices uniformly distributed over the whole Hilbert space, we quantify the frequency of the appearance of quantum and classical correlations for different ranks

Synchronization, quantum correlations and entanglement in oscillator networks

Synchronization is one of the paradigmatic phenomena in the study of complex systems. It has been explored theoretically and experimentally mostly to understand natural phenomena, but also in view of technological applications. Although several mechanisms and conditions for synchronous behavior in spatially extended systems and networks have been identified, the emergence of this phenomenon has been largely unexplored in quantum systems until very recently. Here we discuss synchronization in quantum networks of different harmonic oscillators relaxing towards a stationary state, being essential the form of dissipation. By local tuning of one of the oscillators, we establish the conditions for synchronous dynamics, in the whole network or in a motif. Beyond the classical regime we show that synchronization between (even unlinked) nodes witnesses the presence of quantum correlations and entanglement. Furthermore, synchronization and entanglement can be induced between two different oscillators if properly linked to a random network.Comment: 10 pages, 5 figures, submitted to Scientific Report

Landau-Zener topological quantum state transfer

Fast and robust quantum state transfer (QST) is a major requirement in quantum control and in scalable quantum information processing. Topological protection has emerged as a promising route for the realization of QST robust against sizable imperfections in the network. Here we present a scheme for robust QST of topologically protected edge states in a dimeric Su-Schrieffer-Heeger spin chain assisted by Landau-Zener tunneling. As compared to topological QST protocols based on Rabi flopping proposed in recent works, our method is more advantageous in terms of robustness against both diagonal and off-diagonal disorder in the chain, without a substantial increase of the interaction time.Comment: 10 pages, 8 figures, to appear in Advanced Quantum Technologie

Sof’ja Andreevna Tolstaja e la ‘verità’ di Moja žizn’

This paper focuses on the most relevant work by Sof’ya Andreevna Tolstaya (1844-1919), Moya žizn’, an extensive family chronicle, which was written between 1904 and 1916, and published only in 2011 in its first Russian complete version. The Author aims at analyzing the structure and genre of her work shedding light on the reasons that led Sofia Andreevna to write her own memoirs. Her book can be seen as a combined genre, in that it includes her autobiography as well as Tolstoy’s biography. Indeed, the whole story revolves around Tolstoy. The ‘moya’ in the title highlights her perspective, from which the author conceived her work. Andreevna Tostaya’s book represents an opportunity for her to tell her 'truth' about Tolstoy and their life together. Thus, her memories provide evidence for a true tragedy

Robustness of different indicators of quantumness in the presence of dissipation

The dynamics of a pair of coupled harmonic oscillators in separate or common thermal environments is studied, focusing on different indicators of quantumness, such as entanglement, twin oscillators correlations and quantum discord. We compare their decay under the effect of dissipation and show, through a phase diagram, that entanglement is more likely to survive asymptotically than twin oscillators correlations