22 research outputs found
Extracting Information from Qubit-Environment Correlations
Most works on open quantum systems generally focus on the reduced physical
system by tracing out the environment degrees of freedom. Here we show that the
qubit distributions with the environment are essential for a thorough analysis,
and demonstrate that the way that quantum correlations are distributed in a
quantum register is constrained by the way in which each subsystem gets
correlated with the environment. For a two-qubit system coupled to a common
dissipative environment , we show how to optimise interqubit
correlations and entanglement via a quantification of the qubit-environment
information flow, in a process that, perhaps surprisingly, does not rely on the
knowledge of the state of the environment. To illustrate our findings, we
consider an optically-driven bipartite interacting qubit system under the
action of . By tailoring the light-matter interaction, a
relationship between the qubits early stage disentanglement and the
qubit-environment entanglement distribution is found. We also show that, under
suitable initial conditions, the qubits energy asymmetry allows the
identification of physical scenarios whereby qubit-qubit entanglement minima
coincide with the extrema of the and entanglement
oscillations.Comment: 4 figures, 9 page
Model-Independent Quantum Phases Classifier
Machine learning has revolutionized many fields of science and technology.
Through the -Nearest Neighbors algorithm, we develop a model-independent
classifier, where the algorithm can classify phases of a model to which it has
never had access. For this, we study three distinct spin- models with some
common phases: the XXZ chains with uniaxial single-ion-type anisotropy, the
bound alternating XXZ chains, and the bilinear biquadratic chain. We show that,
with high probability, algorithms trained with two of these models can
determine common phases with the third. It is the first step toward a universal
classifier, where an algorithm is able to detect any phase with no knowledge
about the Hamiltonian, only knowing partial information about the quantum
state
Entanglement Irreversibility From Quantum Discord And Quantum Deficit.
We relate the problem of irreversibility of entanglement with the recently defined measures of quantum correlation--quantum discord and one-way quantum deficit. We show that the entanglement of formation is always strictly larger than the coherent information and the entanglement cost is also larger in most cases. We prove irreversibility of entanglement under local operations and classical communication for a family of entangled states. This family is a generalization of the maximally correlated states for which we also give an analytic expression for the distillable entanglement, the relative entropy of entanglement, the distillable secret key, and the quantum discord.10702050
Non-unitary versus unitary optimization in the control of open quantum systems
In this work, we compare the performance of the Krotov method for open
quantum systems (non-unitary optimization) with the Krotov method for closed
quantum systems (unitary optimization) in finding optimal controls aimed at
manipulating qubits and qutrits in the presence of the environment. In the case
of unitary optimization, the Krotov method is applied to quantum system
neglecting its interaction with the environment, afterwards the resulting
controls are used to manipulate the system along with the eviromental noise. We
consider two distinct control problems: target-state preparation from a given
initial state and quantum gate implementation. For the state preparation, we
{have found that the performance of the controls obtained from the non-unitary
optimization outperform that of the controls obtained from the unitary
optimization}. {However, in the case of the implementation of quantum gates, we
have found that the optimal controls obtained from the unitary evolution
exhibit a mean fidelity similar to that obtained from the non-unitary
evolution. Since unitary optimization does not depend on decay rates nor on
specific kinds of noise, besides being less computationally demanding, our
results suggest that the best current practice to implement quantum gates in
open quantum systems is to employ unitary optimization.Comment: 7 pages, 9 figure