E. Ferraro

Non-Markovian dynamics of a single electron spin coupled to a nuclear spin bath

We apply the time-convolutionless (TCL) projection operator technique to the model of a central spin, which is coupled to a spin bath via nonuniform Heisenberg interaction. The second-order results of the TCL method for the coherences and populations of the central spin are determined analytically and compared to numerical simulations of the full von Neumann equation of the total system. The TCL approach is found to yield an excellent approximation in the strong field regime for the description of both the short-time dynamics and the long time behavior.

Non-Markovian dissipative dynamics of two coupled qubits in independent reservoirs: Comparison between exact solutions and master-equation approaches

The reduced dynamics of two interacting qubits coupled to two independent bosonic baths is investigated. The one-excitation dynamics is derived and compared with that based on the resolution of appropriate non-Markovian master equations. The Nakajima-Zwanzig and the time-convolutionless projection operator techniques are exploited to provide a description of the non-Markovian features of the dynamics of the two-qubit system. The validity of such approximate methods and their range of validity in correspondence to different choices of the parameters describing the system are brought to light.

Entanglement sudden death and sudden birth in two uncoupled spins

We investigate the entanglement evolution of two qubits interacting with a common environment through a Heisenberg XX mechanism. We reveal the possibility of realizing the phenomenon of entanglement sudden death as well as the entanglement sudden birth acting on the environment. Such analysis is of maximal interest in the light of the large applications that spin systems have in quantum information theory.

Exact dynamics of XX central spin models

The dynamical behavior of a star network of spins, wherein each N decoupled spin interacts with a central spin through non-uniform Heisenberg XX interaction, is exactly studied. The time-dependent Schrodinger equation of the spin system model is solved starting from an arbitrary initial state. The resulting solution is analyzed and briefly discussed.

A criterion for entanglement in two two-level systems

The exact entanglement evolution of two two-level systems, coupled to N surrounding spin , is analysed with the help of a new criterion relying on the ability of measuring a few simple observables only.

Effective Hamiltonian approach to the non-Markovian dynamics in a spin bath

We investigate the dynamics of a central spin that is coupled to a bath of spins through a non-uniform distribution of coupling constants. Simple analytical arguments based on master equation techniques as well as numerical simulations of the full von Neumann equation of the total system show that the short-time damping and decoherence behaviour of the central spin can be modelled accurately through an effective Hamiltonian involving a single effective coupling constant. The reduced short-time dynamics of the central spin is thus reproduced by an analytically solvable effective Hamiltonian model.

On the validity of non-Markovian master equation approaches for the entanglement dynamics of two-qubit systems

In the framework of the dissipative dynamics of coupled qubits interacting with independent reservoirs, a comparison between non-Markovian master equation techniques and an exact solution is presented here. We study various regimes in order to find the limits of validity of the Nakajima?Zwanzig and the time-convolutionless master equations in the description of the entanglement dynamics. A comparison between the performances of the concurrence and the negativity as entanglement measures for the system under study is also presented.

Entanglement dynamics in a spin star system

The implementation of more and more efficient nanodevices exploitable in applicative contexts like for example quantum computers often requires a highly challenging miniaturizing process aimed at packing a huge number of point-like basic elements whose dynamics mimics indeed that of a qubit. Stimulated by such a requirement, over the last few years theoretical schemes using the language of the spin ½ system models have been investigated. The main reason is that besides the simple dynamical behaviour of each elementary constituent these Hamiltonian models do indeed capture basic ingredients of several physical situations differing one another mainly for the numerical values of some relevant parameters appearing in the same model.

Dynamical behaviour of an XX central spin model through Bethe ansatz techniques

Following the Bethe ansazt procedure the exact dynamics of an XX central spin model is revealed. Particular initial conditions are analyzed and the consequent time evolution is compared with the exact solution obtained by solving the time-dependent Schrudinger equation. The interest towards spin systems and in particular central spin systems, is motivated by the recent developments in more applicative contexts.

W-like states of N uncoupled spins 1/2

The exact dynamics of a disordered spin star system, describing a central spin coupled to N distinguishable and non interacting spins 1 2, is reported. Exploiting their interaction with the central single spin system, we present possible conditional schemes for the generation of W-like states, as well as of well-defined angular momentum states, of the N uncoupled spins. We provide in addition a way to estimate the coupling intensity between each of the N spins and the central one. Finally the feasibility of our procedure is briefly discussed.