59 research outputs found
Interpreting Quantum Discord in Quantum Metrology
Multipartite quantum systems show properties which do not admit a classical
explanation. In particular, even nonentangled states can enjoy a kind of
quantum correlations called quantum discord. I discuss some recent results on
the role of quantum discord in metrology. Given an interferometric phase
estimation protocol where the Hamiltonian is initially unknown to the
experimentalist, the quantum discord of the probe state quantifies the minimum
precision of the estimation. This provides a physical interpretation to a
widely investigated information-theoretic quantity.Comment: Contribution to the conference "DICE 2014: Spacetime - Matter -
Quantum Mechanics
Witnessing multipartite entanglement by detecting asymmetry
The characterization of quantum coherence in the context of quantum
information theory and its interplay with quantum correlations is currently
subject of intense study. Coherence in an Hamiltonian eigenbasis yields
asymmetry, the ability of a quantum system to break a dynamical symmetry
generated by the Hamiltonian. We here propose an experimental strategy to
witness multipartite entanglement in many-body systems by evaluating the
asymmetry with respect to an additive Hamiltonian. We test our scheme by
simulating asymmetry and entanglement detection in a three-qubit GHZ-diagonal
state.Comment: more examples and discussion in the open access published versio
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
How difficult is it to prepare a quantum state?
Consider a quantum system prepared in an input state. One wants to drive it
into a target state. Assuming classical states and operations as free
resources, I identify a geometric cost function which quantifies the difficulty
of the protocol in terms of how different it is from a classical process. The
quantity determines a lower bound to the number of commuting unitary
transformations required to complete the task. I then discuss the link between
the quantum character of a state preparation and the amount of coherence and
quantum correlations that are created in the target state.Comment: Published versio
An axiomatic measure of one-way quantum information
I introduce an algorithm to detect one-way quantum information between two
interacting quantum systems, i.e. the direction and orientation of the
information transfer in arbitrary quantum dynamics. I then build an
information-theoretic quantifier of one-way information which satisfies a set
of desirable axioms. In particular, it correctly evaluates whether correlation
implies one-way quantum information, and when the latter is transferred between
uncorrelated systems. In the classical scenario, the quantity measures
information transfer between random variables. I also generalize the method to
identify and rank concurrent sources of quantum information flow in many-body
dynamics, enabling to reconstruct causal patterns in complex networks.Comment: LA-UR-19-2631
Quantum discord for general two--qubit states: Analytical progress
We present a reliable algorithm to evaluate quantum discord for general
two--qubit states, amending and extending an approach recently put forward for
the subclass of X--states. A closed expression for the discord of arbitrary
states of two qubits cannot be obtained, as the optimization problem for the
conditional entropy requires the solution to a pair of transcendental equations
in the state parameters. We apply our algorithm to run a numerical comparison
between quantum discord and an alternative, computable measure of non-classical
correlations, namely the geometric discord. We identify the extremally
non-classically correlated two--qubit states according to the (normalized)
geometric discord, at fixed value of the conventional quantum discord. The
latter cannot exceed the square root of the former for systems of two qubits.Comment: 8 pages, 2 figure
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