arXiv subjects

Giuseppe Compagno

Publications and source records attributed to Giuseppe Compagno.

At least 37 records · Page 2Linked to original sources

arXiv2014

Entanglement dynamics in superconducting qubits affected by local bistable impurities

We study the entanglement dynamics for two independent superconducting qubits each affected by a bistable impurity generating random telegraph noise (RTN) at pure dephasing. The relevant parameter is the ratio $g$ between qubit-RTN coupling strength and RTN switching rate, that captures the physics of the crossover between Markovian and non-Markovian features of the dynamics. For identical qubit-RTN subsystems, a threshold value $g_\mathrm{th}$ of the crossover parameter separates exponential decay and onset of revivals; different qualitative behaviors also show up by changing the initial conditions of the RTN. We moreover show that, for different qubit-RTN subsystems, when both qubits are very strongly coupled to the RTN an increase in entanglement revival amplitude may occur during the dynamics.

quant-ph↗

arXiv2014

Spin-echo entanglement protection from random telegraph noise

We analyze local spin-echo procedures to protect entanglement between two non-interacting qubits, each subject to pure-dephasing random telegraph noise. For superconducting qubits this simple model captures characteristic features of the effect of bistable impurities coupled to the device. An analytic expression for the entanglement dynamics is reported. Peculiar features related to the non-Gaussian nature of the noise already observed in the single qubit dynamics also occur in the entanglement dynamics for proper values of the ratio $g=v/γ$, between the qubit-impurity coupling strength and the switching rate of the random telegraph process, and of the separation between the pulses $Δt$. We find that the echo procedure may delay the disappearance of entanglement, cancel the dynamical structure of entanglement revivals and dark periods, and induce peculiar plateau-like behaviors of the concurrence.

quant-ph↗

arXiv2014

Preserving entanglement and nonlocality in solid-state qubits by dynamical decoupling

In this paper we study how to preserve entanglement and nonlocality under dephasing produced by classical noise with large low-frequency components, as $1/f$ noise, by Dynamical Decoupling techniques. We first show that quantifiers of entanglement and nonlocality satisfy a closed relation valid for two independent qubits locally coupled to a generic environment under pure dephasing and starting from a general class of initial states. This result allows to assess the efficiency of pulse-based dynamical decoupling for protecting nonlocal quantum correlations between two qubits subject to pure-dephasing local random telegraph and $1/f$-noise. We investigate the efficiency of an "entanglement memory" element under two-pulse echo and under sequences of periodic, Carr-Purcell and Uhrig dynamical decoupling. The Carr-Purcell sequence is shown to outperform the other sequences in preserving entanglement against both random telegraph and $1/f$ noise. For typical $1/f$ flux-noise figures in superconducting nanocircuits, we show that entanglement and its nonlocal features can be efficiently stored up to times one order of magnitude longer than natural entanglement disappearance times employing pulse timings of current experimental reach.

quant-ph↗

arXiv2014

Characterization of the degree of Musical non-Markovianity

Recently, as an aid for musical analysis, in computational musicology mathematical and informatics tools have been developed to quantitatively characterize some aspects of musical compositions. To a musical composition can be attributed by ear a certain amount of memory. This results associated to repetitions and similarities of the patterns in musical scores. To higher variations, a lower amount of memory is perceived. However musical memory of a score has never been quantitatively defined. Here we aim to give such a measure following an approach similar to that used in Physics to quantify the memory (non-Markovianity) of open quantum systems. We have applied this measure to some existing musical compositions, showing that the results obtained via this quantifier agree with what one expects by ear. The Musical non-Markovianity quantifier can thus be used as a new tool that can aid quantitative musical analysis.

physics.data-an↗

arXiv2013

Hierarchy and dynamics of trace distance correlations

We define and analyze measures of correlations for bipartite states based on trace distance. For Bell diagonal states of two qubits, in addition to the known expression for quantum correlations using this metric, we provide analytic expressions for the classical and total correlations. The ensuing hierarchy of correlations based on trace distance is compared to the ones based on relative entropy and Hilbert-Schmidt norm. Although some common features can be found, the trace distance measure is shown to differentiate from the others in that the closest uncorrelated state to a given bipartite quantum state is not given by the product of the marginals, and further, the total correlations are strictly smaller than the sum of the quantum and classical correlations. We compare the various correlation measures in two dynamical non-Markovian models, locally applied phase-flip channels and random external fields. It is shown that the freezing behavior, observed across all known valid measures of quantum correlations for Bell diagonal states under local phase-flip channels, occurs for a larger set of starting states for the trace distance than for the other metrics.

quant-ph↗

arXiv2012

Comparison of non-Markovianity criteria in a qubit system under random external fields

We give the map representing the evolution of a qubit under the action of non-dissipative random external fields. From this map we construct the corresponding master equation that in turn allows us to phenomenologically introduce population damping of the qubit system. We then compare, in this system, the time-regions when non-Markovianity is present on the basis of different criteria both for the non-dissipative and dissipative case. We show that the adopted criteria agree both in the non-dissipative case and in the presence of population damping.

quant-ph↗

arXiv2012

Dynamics of quantum correlations in two-qubit systems within non-Markovian environments

Knowledge of the dynamical behavior of correlations with no classical counterpart, like entanglement, nonlocal correlations and quantum discord, in open quantum systems is of primary interest because of the possibility to exploit these correlations for quantum information tasks. Here we review some of the most recent results on the dynamics of correlations in bipartite systems embedded in non-Markovian environments that, with their memory effects, influence in a relevant way the system dynamics and appear to be more fundamental than the Markovian ones for practical purposes. Firstly, we review the phenomenon of entanglement revivals in a two-qubit system for both independent environments and a common environment. We then consider the dynamics of quantum discord in non-Markovian dephasing channel and briefly discuss the occurrence of revivals of quantum correlations in classical environments.

quant-ph↗

arXiv2012

Dynamics of geometric and entropic quantifiers of correlations in open quantum systems

We extend the Hilbert-Schmidt (square norm) distance, previously used to define the geometric quantum discord, to define also geometric quantifiers of total and classical correlations. We then compare the dynamics of geometric and entropic quantifiers of the different kinds of correlations in a non-Markovian open two-qubit system under local dephasing. We find that qualitative differences occur only for quantum discords. This is taken to imply that geometric and entropic discords are not, in general, equivalent in describing the dynamics of quantum correlations. We then show that also geometric and entropic quantifiers of total correlations present qualitative disagreements in the state space. This aspect indicates that the differences found for quantum discord are not attributable to a different separation, introduced by each measure, between the quantum and classical parts of correlations. Finally, we find that the Hilbert-Schmidt distance formally coincides with a symmetrized form of linear relative entropy.

quant-ph↗

arXiv2012

Revival of quantum correlations without system-environment back-action

Revivals of quantum correlations have often been explained in terms of back-action on quantum systems by their quantum environment(s). Here we consider a system of two independently evolving qubits, each locally interacting with a classical random external field. The environments of the qubits are also independent, and there is no back-action on the qubits. Nevertheless, entanglement, quantum discord and classical correlations between the two qubits may revive in this model. We explain the revivals in terms of correlations in a classical-quantum state of the environments and the qubits. Although classical states cannot store entanglement on their own, they can play a role in storing and reviving entanglement. It is important to know how the absence of back-action, or modelling an environment as classical, affects the kind of system time evolutions one is able to describe. We find a class of global time evolutions where back-action is absent and for which there is no loss of generality in modelling the environment as classical. Finally, we show that the revivals can be connected with the increase of a parameter used to quantify non-Markovianity of the single-qubit dynamics.

quant-ph↗

arXiv2012

Unified view of correlations using the square norm distance

The distance between a quantum state and its closest state not having a certain property has been used to quantify the amount of correlations corresponding to that property. This approach allows a unified view of the various kinds of correlations present in a quantum system. In particular, using relative entropy as a distance measure, total correlations can be meaningfully separated in a quantum and a classical part thanks to an additive relation involving only distances between states. Here, we investigate a unified view of correlations using as distance measure the square norm, already used to define the so-called geometric quantum discord. We thus consider geometric quantifiers also for total and classical correlations finding, for a quite general class of bipartite states, their explicit expressions. We analyze the relationship among geometric total, quantum and classical correlations and we find that they do not satisfy anymore a closed additivity relation.

quant-ph↗

arXiv2011

Dynamics of correlations due to a phase noisy laser

We analyze the dynamics of various kinds of correlations present between two initially entangled independent qubits, each one subject to a local phase noisy laser. We give explicit expressions of the relevant quantifiers of correlations for the general case of single-qubit unital evolution, which includes the case of a phase noisy laser. Although the light field is treated as classical, we find that this model can describe revivals of quantum correlations. Two different dynamical regimes of decay of correlations occur, a Markovian one (exponential decay) and a non-Markovian one (oscillatory decay with revivals) depending on the values of system parameters. In particular, in the non-Markovian regime, quantum correlations quantified by quantum discord show an oscillatory decay faster than that of classical correlations. Moreover, there are time regions where nonzero discord is present while entanglement is zero.

quant-ph↗

arXiv2011

Entanglement between quantum fields

We analyze entanglement between quantum interacting fields. In particular, we use Rényi entropy to quantify the entanglement between the fields in the ground state of the linear $σ$ model. We adopt Rényi entropy because the failure of the replica trick method does not permit to obtain the Von Neumann entropy. We find that the degree of entanglement is larger in the spontaneously broken symmetry case and a generalization of geometric entropy area law is obtained. Finally, we suggest that entanglement between fields can be used as a signature of the presence of unobserved fields, such as is the Higgs model case with its unobserved sector.

quant-ph↗

arXiv2010

Entanglement Dynamics of Two Independent Cavity-Embedded Quantum Dots

We investigate the dynamical behavior of entanglement in a system made by two solid-state emitters, as two quantum dots, embedded in two separated micro-cavities. In these solid-state systems, in addition to the coupling with the cavity mode, the emitter is coupled to a continuum of leaky modes providing additional losses and it is also subject to a phonon-induced pure dephasing mechanism. We model this physical configuration as a multipartite system composed by two independent parts each containing a qubit embedded in a single-mode cavity, exposed to cavity losses, spontaneous emission and pure dephasing. We study the time evolution of entanglement of this multipartite open system finally applying this theoretical framework to the case of currently available solid-state quantum dots in micro-cavities.

quant-ph↗

arXiv2010

Distillation by repeated measurements: continuous spectrum case

Repeated measurements on a part of a bipartite system strongly affect the other part not measured, whose dynamics is regulated by an effective contracted evolution operator. When the spectrum of this operator is discrete, the latter system is driven into a pure state irrespective of the initial state, provided the spectrum satisfies certain conditions. We here show that even in the case of continuous spectrum an effective distillation can occur under rather general conditions. We confirm it by applying our formalism to a simple model.

quant-ph↗

arXiv2010

Connection among entanglement, mixedness and nonlocality in a dynamical context

We investigate the dynamical relations among entanglement, mixedness and nonlocality, quantifed by concurrence C, purity P and maximum of Bell function B, respectively, in a system of two qubits in a common structured reservoir. To this aim we introduce the C-P-B parameter space and analyze the time evolution of the point representative of the system state in such a space. The dynamical interplay among entanglement, mixedness and nonlocality strongly depends on the initial state of the system. For a two-excitation Bell state the representative point draws a multi-branch curve in the C-P-B space and we show that a closed relation among these quantifers does not hold. By extending the known relation between C and B for pure states, we give an expression among the three quantifers for mixed states. In this equation we introduce a quantity, vanishing for pure states which has not in general a closed form in terms of C, P and B. Finally we demonstrate that for an initial one-excitation Bell state a closed C-P-B relation instead exists and the system evolves remaining always a maximally entangled mixed state.

quant-ph↗

arXiv2009

Optimized experimental settings for the best detection of quantum nonlocality

Nonlocality lies at the core of quantum mechanics from both a fundamental and applicative point of view. It is typically revealed by a Bell test, that is by violation of a Bell inequality, whose success depends both on the state of the system and on parameters linked to experimental settings. This leads to find, given the state, optimized parameters for a successful test. Here we provide, for a quite general class of quantum states, the explicit expressions of these optimized parameters and point out that, for a continuous change of the state, the corresponding suitable experimental settings may unexpectedly vary discontinuously. We finally show in a paradigmatic open quantum system that this abrupt "jump" of the experimental settings may even occur during the time evolution of the system. These jumps must be taken into account in order not to compromise the correct detection of nonlocality in the system.

quant-ph↗

arXiv2009

Two-qubit entanglement dynamics for two different non-Markovian environments

We study the time behavior of entanglement between two noninteracting qubits each immersed in its own environment for two different non-Markovian conditions: a high-$Q$ cavity slightly off-resonant with the qubit transition frequency and a nonperfect photonic band-gap, respectively. We find that revivals and retardation of entanglement loss may occur by adjusting the cavity-qubit detuning, in the first case, while partial entanglement trapping occurs in non-ideal photonic-band gap.

quant-ph↗

arXiv2009

Thwarted dynamics by partial projective measurements

The dynamics of a system, made of a particle interacting with a field mode, thwarted by the action of repeated projective measurements on the particle, is examined. The effect of the partial measurements is discussed by comparing it with the dynamics in the absence of the measurements.

quant-ph↗