arXiv subjects

Alessandro Cuccoli

Publications and source records attributed to Alessandro Cuccoli.

At least 37 records · Page 2Linked to original sources

arXiv2015

Getting through to a qubit by magnetic solitons

We propose a method for acting on the spin state of a spin-1/2 localized particle, or `qubit', by means of a magnetic signal effectively generated by the nearby transit of a magnetic soliton, there conveyed through a transmission line. We first introduce the specific magnetic soliton of which we will make use, and briefly review the properties that make it apt to represent a signal. We then show that a Heisenberg spin chain can serve as transmission line, and propose a method for injecting a soliton into the chain by acting just on one of its ends. We finally demonstrate that the resulting magnetic pulse can indeed cause, just passing by the spin-1/2 localized particle embodying the qubit, a permanent change in its spin state, thus realizing the possibility of getting through to a single, localized qubit, and manipulate its state. A thorough analysis of how the overall dynamical system operates depending on the setting of its parameters demonstrates that fine tuning is not necessary as it exists an extended region in the parameters space that corresponds to an effective functioning. Moreover, we show that possible noise on the transmission line does not invalidate the scheme.

cond-mat.mtrl-sci↗

arXiv2015

Parametric description of the quantum measurement process

We present a description of the measurement process based on the parametric representation with environmental coherent states. This representation is specifically tailored for studying quantum systems whose environment needs being considered through the quantum-to-classical cross-over. Focusing upon projective measures, and exploiting the connection between large-$N$ quantum theories and the classical limit of related ones, we manage to push our description beyond the pre-measurement step. This allows us to show that the outcome production follows from a global-symmetry breaking, entailing the observed system's state reduction, and that the statistical nature of the process is brought about, together with the Born's rule, by the macroscopic character of the measuring apparatus.

quant-ph↗

arXiv2015

Getting information via a quantum measurement: the role of decoherence

In this work we investigate the relation between quantum measurements and decoherence, in order to formally express the necessity of the latter for obtaining an informative output from the former. To this aim, referring to the Von Neumann scheme for ideal quantum measurements, we first look for the minimal structure that the interaction between principal system and measurement apparatus must have for properly describing the process, beyond the quantum measurement limit, and then analyze the dynamical evolution induced by one such interaction. The analysis is developed by means of a recently introduced method for studying open quantum systems, namely the parametric representation with environmental coherent states, that allows us to determine a necessary condition that the quantum state of the apparatus must fullfil in order to give information on the observable being measured. We find that this condition strictly implies decoherence in the principal system, with respect to the eigenstates of the hermitian operator that represents the measured observable, thus establishing that there cannot be information flux from a quantum system towards a readable analyser unless decoherence occurs. The relevance of dynamical entanglement generation is highlighted, and consequences of the possible macroscopic structure of the measurement apparatus are also commented upon.

quant-ph↗

arXiv2013

Parametric representation of open quantum systems and crossover from quantum to classical environment

We propose an approach to the study of open quantum systems based on a parametric representation of the principal system. The representation is obtained introducing generalized coherent states for the environment, and is such that the evolution from a fully quantum environment to a classical one can be inherently followed. The method is applied to the specific case where the principal system is a central qubit and the environment is a surrounding ring of interacting spins, with the overall Hamiltonian being that of the $s=1/2$ Heisenberg star with frustration. In this case the quantum character of the environment, embodied by the total spin $S$ of the ring, can be physically varied by acting on the ratio between the couplings entering the Hamiltonian, and formally followed by increasing $S$. We find that when the star is in any of its eigenstates, the qubit behaves as if it were under the effect of an external magnetic field, whose direction in real space is set by the variables defining the environmental coherent states, and is hence broadened according to their quantum probability distribution. When the quantum character of the environment is reduced, such distribution becomes narrower, finally collapsing into a Dirac-$δ$ as $S\to\infty$. The entanglement between the central qubit and its environment remains well defined even in this limit, where it comes across as the binary entropy of the $2π$-normalized Berry phase characterizing the dynamics of a qubit in an adiabatically precessing magnetic field.

quant-ph↗

arXiv2012

Reentrant enhancement of quantum fluctuations for symmetric environmental coupling

The `system-plus-reservoir' (SPR) model is the most common and effective approach to study quantum dissipative effects. Indeed, it makes quantization possible by considering the whole energy-conserving system, while the reservoir's degrees of freedom, assumed to be harmonic, can be `traced out' by the path-integral technique, leading to a formulation that only includes the system of interest. In the standard SPR model the environment is only coupled with the system's coordinate and turns out to quench its quantum fluctuations. However, there are physical systems coupled with an environment whose `coordinates' and `momenta' can be completely interchangeable (e.g., magnets), so an SPR coupling must symmetrically affect both canonical variables. In this paper such a general environmental coupling is studied in the case of a harmonic oscillator. It is found that quantum fluctuations are generally enhanced by environmental coupling, with an unexpected nonmonotonic behavior. This leads one to speculate about the possibility that spin-lattice coupling could drive the 2D Heisenberg antiferromagnet to reach its quantum-critical point.

cond-mat.stat-mech↗

arXiv2011

Long quantum channels for high-quality entanglement transfer

High-quality quantum-state and entanglement transfer can be achieved in an unmodulated spin bus operating in the ballistic regime, which occurs when the endpoint qubits A and B are coupled to the chain by an exchange interaction $j_0$ comparable with the intrachain exchange. Indeed, the transition amplitude characterizing the transfer quality exhibits a maximum for a finite optimal value $j_0^{opt}(N)$, where $N$ is the channel length. We show that $j_0^{opt}(N)$ scales as $N^{-1/6}$ for large $N$ and that it ensures a high-quality entanglement transfer even in the limit of arbitrarily long channels, almost independently of the channel initialization. For instance, the average quantum-state transmission fidelity exceeds 90% for any chain length. We emphasize that, taking the reverse point of view, should $j_0$ be experimentally constrained, high-quality transfer can still be obtained by adjusting the channel length to its optimal value.

quant-ph↗

arXiv2010

Optimal dynamics for quantum-state and entanglement transfer through homogeneous quantum wires

It is shown that effective quantum-state and entanglement transfer can be obtained by inducing a coherent dynamics in quantum wires with homogeneous intrawire interactions. This goal is accomplished by tuning the coupling between the wire endpoints and the two qubits there attached, to an optimal value. A general procedure to determine such value is devised, and scaling laws between the optimal coupling and the length of the wire are found. The procedure is implemented in the case of a wire consisting of a spin-1/2 XY chain: results for the time dependence of the quantities which characterize quantum-state and entanglement transfer are found of extremely good quality and almost independent of the wire length. The present approach does not require `ad hoc' engineering of the intrawire interactions nor a specific initial pulse shaping, and can be applied to a vast class of quantum channels.

quant-ph↗

arXiv2010

Interplay among helical order, surface effects and range of interacting layers in ultrathin films

The properties of helical thin films have been thoroughly investigated by classical Monte Carlo simulations. The employed model assumes classical planar spins in a body-centered tetragonal lattice, where the helical arrangement along the film growth direction has been modeled by nearest neighbor and next-nearest neighbor competing interactions, the minimal requirement to get helical order. We obtain that, while the in-plane transition temperatures remain essentially unchanged with respect to the bulk ones, the helical/fan arrangement is stabilized at more and more low temperature when the film thickness, n, decreases; in the ordered phase, increasing the temperature, a softening of the helix pitch wave-vector is also observed. Moreover, we show also that the simulation data around both transition temperatures lead us to exclude the presence of a first order transition for all analyzed sizes. Finally, by comparing the results of the present work with those obtained for other models previously adopted in literature, we can get a deeper insight about the entwined role played by the number (range) of interlayer interactions and surface effects in non-collinear thin films.

cond-mat.stat-mech↗

arXiv2009

Monte Carlo simulations on rare-earth Holmium ultra-thin films

Motivated by recent experimental results in ultra-thin helimagnetic Holmium films, we have performed an extensive classical Monte Carlo simulation of films of different thickness, assuming a Hamiltonian with six inter-layer exchange constants. Both magnetic structure and critical properties have been analyzed. For n>16 (n being the number of spin layers in the film) a correct bulk limit is reached, while for lower n the film properties are clearly affected by the strong competition among the helical pitch and the surface effects, which involve the majority of the spin layers: In the thickness range n=9-16 three different magnetic phases emerge, with the high-temperature, disordered, paramagnetic phase and the low-temperature, long-range ordered one separated by an intriguing intermediate-temperature block phase, where outer ordered layers coexist with some inner, disordered ones. The phase transition of these inner layers displays the signatures of a Kosterlitz-Thouless one. Finally, for n<~7 the film collapse once and for all to a quasi-collinear order. A comparison of our Monte Carlo simulation outcomes with available experimental data is also proposed, and further experimental investigations are suggested.

cond-mat.stat-mech↗

arXiv2008

Vortex-enhanced alternating order around impurities in antiferromagnets

It has been recently pointed out that the presence of a nonmagnetic impurity in a Heisenberg antiferromagnet generates an alternating order of the surrounding spins which is independent of temperature in a wide range. Quantum Monte Carlo simulations in the two-dimensional $S=1/2$ case confirmed this picture, but showed a counterintuitive enhancement of the alternating order around the Kosterlitz-Thouless transition. We propose here an explanation in terms of the effect of vortex excitations.

cond-mat.str-el↗

arXiv2008

Staggered magnetization and entanglement enhancement by magnetic impurities in $S=1/2$ spin chain

We study the effects of a magnetic impurity on the behavior of a $S=1/2$ spin chain. At T=0, both with and without an applied uniform magnetic field, an oscillating magnetization appears, whose decay with the distance from the impurity is ruled by a power law. As a consequence, pairwise entanglement is either enhanced or quenched, depending on the distance of the spin pair with respect to the impurity and on the values of the magnetic field and the intensity of the impurity itself. This leads us to suggest that acting on such control parameters, an adiabatic manipulation of the entanglement distribution can be performed. The robustness of our results against temperature is checked, and suggestions about possible experimental applications are put forward.

cond-mat.stat-mech↗

arXiv2007

Extracting signatures of quantum criticality in the finite-temperature behavior of many-body systems

We face the problem of detecting and featuring footprints of quantum criticality in the finite-temperature behavior of quantum many-body systems. Our strategy is that of comparing the phase diagram of a system displaying a T=0 quantum phase transition with that of its classical limit, in order to single out the genuinely quantum effects. To this aim, we consider the one-dimensional Ising model in a transverse field: while the quantum S=1/2 Ising chain is exactly solvable and extensively studied, results for the classical limit (infinite S) of such model are lacking, and we supply them here. They are obtained numerically, via the Transfer-matrix method, and their asymptotic low-temperature behavior is also derived analytically by self-consistent spin-wave theory. We draw the classical phase-diagram according to the same procedure followed in the quantum analysis, and the two phase diagrams are found unexpectedly similar: Three regimes are detected also in the classical case, each characterized by a functional dependence of the correlation length on temperature and field analogous to that of the quantum model. What discriminates the classical from the quantum case are the different values of the exponents entering such dependencies, a consequence of the different nature of zero-temperature quantum fluctuations with respect to thermal ones.

cond-mat.stat-mech↗

arXiv2005

Dissipation-driven phase transition in 2D Josephson arrays

We analyze the interplay of dissipative and quantum effects in the proximity of a quantum phase transition. The prototypical system is a resistively shunted two-dimensional Josephson junction array, studied by means of an advanced Fourier path-integral Monte Carlo algorithm. The reentrant superconducting-to-normal phase transition driven by quantum fluctuations, recently discovered in the limit of infinite shunt resistance, persists for moderate dissipation strength but disappears in the limit of small resistance. For large quantum coupling our numerical results show that, beyond a critical dissipation strength, the superconducting phase is always stabilized at sufficiently low temperature. Our phase diagram explains recent experimental findings.

cond-mat.stat-mech↗

arXiv2005

Quantum Heisenberg antiferromagnets: a survey of the activity in Firenze

Over the years the research group in Firenze has produced a number of theoretical results concerning the statistical mechanics of quantum antiferromagnetic models, which range from the theory of two-magnon Raman scattering to the characterization of the phase transitions in quantum low-dimensional antiferromagnetic models. Our research activity was steadily aimed to the understanding of experimental observations.

cond-mat.str-el↗

arXiv2004

Berezinskii-Kosterlitz-Thouless transition in Josephson junction arrays

The quantum XY model shows a Berezinskii-Kosterlitz-Thouless (BKT) transition between a phase with quasi long-range order and a disordered one, like the corresponding classical model. The effect of the quantum fluctuations is to weaken the transition and eventually to destroy it. However, in this respect the mechanism of disappearance of the transition is not yet clear. In this work we address the problem of the quenching of the BKT in the quantum XY model in the region of small temperature and high quantum coupling. In particular, we study the phase diagram of a 2D Josephson junction array, that is one of the best experimental realizations of a quantum XY model. A genuine BKT transition is found up to a threshold value $g^\star$ of the quantum coupling, beyond which no phase coherence is established. Slightly below $g^\star$ the phase stiffness shows a reentrant behavior at lowest temperatures, driven by strong nonlinear quantum fluctuations. Such a reentrance is removed if the dissipation effect of shunt resistors is included.

cond-mat.mes-hall↗

arXiv2003

Reentrant behavior of the phase stiffness in Josephson junction arrays

The phase diagram of a 2D Josephson junction array with large substrate resistance, described by a quantum XY model, is studied by means of Fourier path-integral Monte Carlo. A genuine Berezinskii-Kosterlitz-Thouless transition is found up to a threshold value g* of the quantum coupling, beyond which no phase coherence is established. Slightly below g* the phase stiffness shows a reentrant behavior with temperature, in connection with a low-temperature disappearance of the superconducting phase, driven by strong nonlinear quantum fluctuations.

cond-mat.mes-hall↗

arXiv2003

Detection of XY behaviour in weakly anisotropic quantum antiferromagnets on the square lattice

We consider the Heisenberg antiferromagnet on the square lattice with S=1/2 and very weak easy-plane exchange anisotropy; by means of the quantum Monte Carlo method, based on the continuous-time loop algorithm, we find that the thermodynamics of the model is highly sensitive to the presence of tiny anisotropies and is characterized by a crossover between isotropic and planar behaviour. We discuss the mechanism underlying the crossover phenomenon and show that it occurs at a temperature which is characteristic of the model. The expected Berezinskii-Kosterlitz-Thouless transition is observed below the crossover: a finite range of temperatures consequently opens for experimental detection of non-critical 2D XY behaviour. Direct comparison is made with uniform susceptibility data relative to the S=1/2 layered antiferromagnet Sr2CuO2Cl2.

cond-mat.stat-mech↗

arXiv2003

Quantum 2D Heisenberg antiferromagnet: bridging the gap between field-theoretical and semiclassical approaches

The field-theoretical result for the low-$T$ behaviour of the correlation length of the quantum Heisenberg antiferromagnet on the square lattice was recently improved by Hasenfratz [Eur. Phys. J. B {\bf 13}, 11 (2000)], who corrected for cutoff effects. We show that starting from his expression, and exploiting our knowledge of the classical thermodynamics of the model, it is possible to take into account non-linear effects which are responsible for the main features of the correlation length at intermediate temperature. Moreover, we find that cutoff effects lead to the appearance of an effective exchange integral depending on the very same renormalization coefficients derived in the framework of the semiclassical {\em pure-quantum self-consistent harmonic approximation}: The gap between quantum field-theoretical and semiclassical results is here eventually bridged.

cond-mat.stat-mech↗