5,242 research outputs found
Simple Non-Markovian Microscopic Models for the Depolarizing Channel of a Single Qubit
The archetypal one-qubit noisy channels ---depolarizing, phase-damping and
amplitude-damping channels--- describe both Markovian and non-Markovian
evolution. Simple microscopic models for the depolarizing channel, both
classical and quantum, are considered. Microscopic models which describe phase
damping and amplitude damping channels are briefly reviewed.Comment: 13 pages, 2 figures. Title corrected. Paper rewritten. Added
references. Some typos and errors corrected. Author adde
Forza gravitazionale e forza elettrostatica: storia e confronto
In questo articolo viene proposta un\u2019attivit\ue0 didattica riguardante il confronto tra la forza gravitazionale, ovvero la legge di gravitazione universale di Newton, e la forza elettrostatica, ovvero l\u2019interazione tra due cariche puntiformi scoperta da Charles Coulomb. Inizialmente si introduce uno schema di progettazione didattica, rivolta a studenti del quinto anno di un liceo scientifico, basato su metodologie didattiche standard. Successivamente gli argomenti della proposta vengono trattati in dettaglio, riassumendo i passaggi storici essenziali. Infine, un confronto su scala microscopica e macroscopica dei due tipi di forze \ue8 presentato, mettendo in luce analogie e differenze utili ai fini didattici
Quantumness and memory of one qubit in a dissipative cavity under classical control
Hybrid quantum–classical systems constitute a promising architecture for useful control strategies of quantum systems by means of a classical device. Here we provide a comprehensive study of the dynamics of various manifestations of quantumness with memory effects, identified by non-Markovianity, for a qubit controlled by a classical field and embedded in a leaky cavity. We consider both Leggett–Garg inequality and quantum witness as experimentally-friendly indicators of quantumness, also studying the geometric phase of the evolved (noisy) quantum state. We show that, under resonant qubit-classical field interaction, a stronger coupling to the classical control leads to enhancement of quantumness despite a disappearance of non-Markovianity. Differently, increasing the qubit-field detuning (out-of-resonance) reduces the nonclassical behavior of the qubit while recovering non-Markovian features. We then find that the qubit geometric phase can be remarkably preserved irrespective of the cavity spectral width via strong coupling to the classical field. The controllable interaction with the classical field inhibits the effective time-dependent decay rate of the open qubit. These results supply practical insights towards a classical harnessing of quantum properties in a quantum information scenari
Unifying approach to the quantification of bipartite correlations by Bures distance
The notion of distance defined on the set of states of a composite quantum
system can be used to quantify total, quantum and classical correlations in a
unifying way. We provide new closed formulae for classical and total
correlations of two-qubit Bell-diagonal states by considering the Bures
distance. Complementing the known corresponding expressions for entanglement
and more general quantum correlations, we thus complete the quantitative
hierarchy of Bures correlations for Bell-diagonal states. We then explicitly
calculate Bures correlations for two relevant families of states: Werner states
and rank-2 Bell-diagonal states, highlighting the subadditivity which holds for
total correlations with respect to the sum of classical and quantum ones when
using Bures distance. Finally, we analyse a dynamical model of two independent
qubits locally exposed to non-dissipative decoherence channels, where both
quantum and classical correlations measured by Bures distance exhibit freezing
phenomena, in analogy with other known quantifiers of correlations.Comment: 18 pages, 4 figures; published versio
Relations between entanglement and purity in non-Markovian dynamics
Knowledge of the relationships among different features of quantumness, like
entanglement and state purity, is important from both fundamental and practical
viewpoints. Yet, this issue remains little explored in dynamical contexts for
open quantum systems. We address this problem by studying the dynamics of
entanglement and purity for two-qubit systems using paradigmatic models of
radiation-matter interaction, with a qubit being isolated from the environment
(spectator configuration). We show the effects of the corresponding local
quantum channels on an initial two-qubit pure entangled state in the
concurrence-purity diagram and find the conditions which enable dynamical
closed formulas of concurrence, used to quantify entanglement, as a function of
purity. We finally discuss the usefulness of these relations in assessing
entanglement and purity thresholds which allow noisy quantum teleportation. Our
results provide new insights about how different properties of composite open
quantum systems behave and relate each other during quantum evolutions.Comment: 16 Pages, 10 Figures. One author added. Improved version with more
references and comment
Hidden entanglement in the presence of random telegraph dephasing noise
Entanglement dynamics of two noninteracting qubits, locally affected by
random telegraph noise at pure dephasing, exhibits revivals. These revivals are
not due to the action of any nonlocal operation, thus their occurrence may
appear paradoxical since entanglement is by definition a nonlocal resource. We
show that a simple explanation of this phenomenon may be provided by using the
(recently introduced) concept of "hidden" entanglement, which signals the
presence of entanglement that may be recovered with the only help of local
operations.Comment: 8 pages, 1 figure, submitted to Physica Scripta on September 17th
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Hidden entanglement, system-environment information flow and non-Markovianity
It is known that entanglement dynamics of two noninteracting qubits, locally
subjected to classical environments, may exhibit revivals. A simple explanation
of this phenomenon may be provided by using the concept of hidden entanglement,
which signals the presence of entanglement that may be recovered without the
help of nonlocal operations. Here we discuss the link between hidden
entanglement and the (non-Markovian) flow of classical information between the
system and the environment.Comment: 9 pages, 2 figures; proceedings of the conference IQIS 2013,
September 24-26 2013, Como, Ital
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.Comment: 7 pages, 3 figures, accepted for publication in Phys. Scripta,
special issue for CEWQO 2011 proceeding
Hilbert–Schmidt speed as an efficient figure of merit for quantum estimation of phase encoded into the initial state of open n-qubit systems
Hilbert–Schmidt speed (HSS) is a special type of quantum statistical speed which is easily computable, since it does not require diagonalization of the system state. We find that, when both HSS and quantum Fisher information (QFI) are calculated with respect to the phase parameter encoded into the initial state of an n-qubit register, the zeros of the HSS dynamics are actually equal to those of the QFI dynamics. Moreover, the signs of the time-derivatives of both HSS and QFI exactly coincide. These findings, obtained via a thorough investigation of several paradigmatic open quantum systems, show that HSS and QFI exhibit the same qualitative time evolution. Therefore, HSS reveals itself as a powerful figure of merit for enhancing quantum phase estimation in an open quantum system made of n qubits. Our results also provide strong evidence for both contractivity of the HSS under memoryless dynamics and its sensitivity to system-environment information backflows to detect the non-Markovianity in high-dimensional systems, as suggested in previous studies
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