552 research outputs found
Characterizing Nonclassical Correlations via Local Quantum Uncertainty
Quantum mechanics predicts that measurements of incompatible observables
carry a minimum uncertainty which is independent of technical deficiencies of
the measurement apparatus or incomplete knowledge of the state of the system.
Nothing yet seems to prevent a single physical quantity, such as one spin
component, from being measured with arbitrary precision. Here we show that an
intrinsic quantum uncertainty on a single observable is ineludible in a number
of physical situations. When revealed on local observables of a bipartite
system, such uncertainty defines an entire class of bona fide measures of
nonclassical correlations. For the case of 2 x d systems, we find that a unique
measure is defined, which we evaluate in closed form. We then discuss the role
that these correlations, which are of the 'discord' type, can play in the
context of quantum metrology. We show in particular that the amount of discord
present in a bipartite mixed probe state guarantees a minimum precision, as
quantified by the quantum Fisher information, in the optimal phase estimation
protocol.Comment: Published in PRL, Editors' Suggestio
The classical-quantum boundary for correlations: discord and related measures
One of the best signatures of nonclassicality in a quantum system is the
existence of correlations that have no classical counterpart. Different methods
for quantifying the quantum and classical parts of correlations are amongst the
more actively-studied topics of quantum information theory over the past
decade. Entanglement is the most prominent of these correlations, but in many
cases unentangled states exhibit nonclassical behavior too. Thus distinguishing
quantum correlations other than entanglement provides a better division between
the quantum and classical worlds, especially when considering mixed states.
Here we review different notions of classical and quantum correlations
quantified by quantum discord and other related measures. In the first half, we
review the mathematical properties of the measures of quantum correlations,
relate them to each other, and discuss the classical-quantum division that is
common among them. In the second half, we show that the measures identify and
quantify the deviation from classicality in various
quantum-information-processing tasks, quantum thermodynamics, open-system
dynamics, and many-body physics. We show that in many cases quantum
correlations indicate an advantage of quantum methods over classical ones.Comment: Close to the published versio
Signatures of non-classicality in mixed-state quantum computation
We investigate signatures of non-classicality in quantum states, in
particular, those involved in the DQC1 model of mixed-state quantum computation
[Phys. Rev. Lett. 81, 5672 (1998)]. To do so, we consider two known
non-classicality criteria. The first quantifies disturbance of a quantum state
under locally noneffective unitary operations (LNU), which are local unitaries
acting invariantly on a subsystem. The second quantifies measurement induced
disturbance (MID) in the eigenbasis of the reduced density matrices. We study
the role of both figures of non-classicality in the exponential speedup of the
DQC1 model and compare them vis-a-vis the interpretation provided in terms of
quantum discord. In particular, we prove that a non-zero quantum discord
implies a non-zero shift under LNUs. We also use the MID measure to study the
locking of classical correlations [Phys. Rev. Lett. 92, 067902 (2004)] using
two mutually unbiased bases (MUB). We find the MID measure to exactly
correspond to the number of locked bits of correlation. For three or more MUBs,
it predicts the possibility of superior locking effects.Comment: Published version, containing additional discussion on the role of
non-classicality in the locking of classical correlation
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