119 research outputs found
Mediated Homogenization
Homogenization protocols model the quantum mechanical evolution of a system
to a fixed state independently from its initial configuration by repeatedly
coupling it with a collection of identical ancillas. Here we analyze these
protocols within the formalism of "relaxing" channels providing an easy to
check sufficient condition for homogenization. In this context we describe
mediated homogenization schemes where a network of connected qudits relaxes to
a fixed state by only partially interacting with a bath. We also study
configurations which allow us to introduce entanglement among the elements of
the network. Finally we analyze the effect of having competitive configurations
with two different baths and we prove the convergence to dynamical equilibrium
for Heisenberg chains.Comment: 6 pages, 6 figure
Use of dynamical coupling for improved quantum state transfer
We propose a method to improve quantum state transfer in transmission lines.
The idea is to localize the information on the last qubit of a transmission
line, by dynamically varying the coupling constants between the first and the
last pair of qubits. The fidelity of state transfer is higher then in a chain
with fixed coupling constants. The effect is stable against small fluctuations
in the system parameters.Comment: 5 pages, 7 figure
Hamiltonian Determination with Restricted Access in Transverse Field Ising Chain
We propose a method to evaluate parameters in the Hamiltonian of the Ising
chain under site-dependent transverse fields, with a proviso that we can
control and measure one of the edge spins only. We evaluate the eigenvalues of
the Hamiltonian and the time-evoultion operator exactly for a 3-spin chain,
from which we obtain the expectation values of of the first spin.
The parameters are found from the peak positions of the Fourier transform of
the expectation value. There are four assumptions in our method, which are mild
enough to be satisfied in many physical systems.Comment: 15pages, 4 figure
Signal Propagation, Termination, Crosstalk and Losses in Resistive Plate Chambers
We discuss the signal propagation, strip termination and crosstalk in Resistive Plate Chambers (RPCs) by analyzing the explicit time domain solution of a two dimensional multi-conductor transmission line. It is shown that all the effects can be calculated by elementary matrix manipulations
Interfacing with Hamiltonian Dynamics
Simple constructions and protocols are demonstrated to allow the
implementation of universal quantum computation on an arbitrarily large quantum
system by controlling a fixed number of spins, vastly reducing the engineering
requirements in comparison to a direct implementation of the circuit model.
Analytic pulse sequences are derived for deterministic gate operation and
system cooling which only require pulse strengths that are comparable to the
intrinsic Hamiltonian of the system.Comment: 9 pages, 4 figures v2: small correction to cooling sectio
Superballistic Diffusion of Entanglement in Disordered Spin Chains
We study the dynamics of a single excitation in an infinite XXZ spin chain,
which is launched from the origin. We study the time evolution of the spread of
entanglement in the spin chain and obtain an expression for the second order
spatial moment of concurrence, about the origin, for both ordered and
disordered chains. In this way, we show that a finite central disordered region
can lead to sustained superballistic growth in the second order spatial moment
of entanglement within the chain.Comment: 5 pages, 1 figur
Estimation of Spin-Spin Interaction by Weak Measurement Scheme
Precisely knowing an interaction Hamiltonian is crucial to realize quantum
information tasks, especially to experimentally demonstrate a quantum computer
and a quantum memory. We propose a scheme to experimentally evaluate the
spin-spin interaction for a two-qubit system by the weak measurement technique
initiated by Yakir Aharonov and his colleagues. Furthermore, we numerically
confirm our proposed scheme in a specific system of a nitrogen vacancy center
in diamond. This means that the weak measurement can also be taken as a
concrete example of the quantum process tomography.Comment: 4 pages, 1 table, 2 figures, to appear in Europhysics Letter
Quantum control theory and applications: A survey
This paper presents a survey on quantum control theory and applications from
a control systems perspective. Some of the basic concepts and main developments
(including open-loop control and closed-loop control) in quantum control theory
are reviewed. In the area of open-loop quantum control, the paper surveys the
notion of controllability for quantum systems and presents several control
design strategies including optimal control, Lyapunov-based methodologies,
variable structure control and quantum incoherent control. In the area of
closed-loop quantum control, the paper reviews closed-loop learning control and
several important issues related to quantum feedback control including quantum
filtering, feedback stabilization, LQG control and robust quantum control.Comment: 38 pages, invited survey paper from a control systems perspective,
some references are added, published versio
The Generalized Lyapunov Theorem and its Application to Quantum Channels
We give a simple and physically intuitive necessary and sufficient condition
for a map acting on a compact metric space to be mixing (i.e. infinitely many
applications of the map transfer any input into a fixed convergency point).
This is a generalization of the "Lyapunov direct method". First we prove this
theorem in topological spaces and for arbitrary continuous maps. Finally we
apply our theorem to maps which are relevant in Open Quantum Systems and
Quantum Information, namely Quantum Channels. In this context we also discuss
the relations between mixing and ergodicity (i.e. the property that there exist
only a single input state which is left invariant by a single application of
the map) showing that the two are equivalent when the invariant point of the
ergodic map is pure.Comment: 13 pages, 3 figure
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