2,477 research outputs found
Decompositions of unitary evolutions and entanglement dynamics of bipartite quantum systems
We describe a decomposition of the Lie group of unitary evolutions for a
bipartite quantum system of arbitrary dimensions. The decomposition is based on
a recursive procedure which systematically uses the Cartan classification of
the symmetric spaces of the Lie group SO(n). The resulting factorization of
unitary evolutions clearly displays the local and entangling character of each
factor.Comment: 11 pages, revtex
Optimal generation of entanglement under local control
We study the optimal generation of entanglement between two qubits subject to
local unitary control. With the only assumptions of linear control and unitary
dynamics, by means of a numerical protocol based on the variational approach
(Pontryagin's Minimum Principle), we evaluate the optimal control strategy
leading to the maximal achievable entanglement in an arbitrary interaction
time, taking into account the energy cost associated to the controls. In our
model we can arbitrarily choose the relative weight between a large
entanglement and a small energy cost.Comment: 4 page
Tailoring many-body entanglement through local control
We construct optimal time-local control pulses based on a multipartite
entanglement measure as target functional. The underlying control Hamiltonians
are derived in a purely algebraic fashion, and the resulting pulses drive a
composite quantum system rapidly into that highly entangled state which can be
created most efficiently for a given interaction mechanism, and which bears
entanglement that is robust against decoherence. Moreover, it is shown that the
control scheme is insensitive to experimental imperfections in first order.Comment: 12 pages, 11 figure
Controllability of Symmetric Spin Networks
We consider a network of n spin 1/2 systems which are pairwise interacting
via Ising interaction and are controlled by the same electro-magnetic control
field. Such a system presents symmetries since the Hamiltonian is unchanged if
we permute two spins. This prevents full (operator) controllability in that not
every unitary evolution can be obtained. We prove however that controllability
is verified if we restrict ourselves to unitary evolutions which preserve the
above permutation invariance. For low dimensional cases, n=2 and n=3, we
provide an analysis of the Lie group of available evolutions and give explicit
control laws to transfer between any two permutation invariant states. This
class of states includes highly entangled states such as GHZ states and W
states, which are of interest in quantum information
Quantum Control Theory for State Transformations: Dark States and their Enlightenment
For many quantum information protocols such as state transfer, entanglement
transfer and entanglement generation, standard notions of controllability for
quantum systems are too strong. We introduce the weaker notion of accessible
pairs, and prove an upper bound on the achievable fidelity of a transformation
between a pair of states based on the symmetries of the system. A large class
of spin networks is presented for which this bound can be saturated. In this
context, we show how the inaccessible dark states for a given
excitation-preserving evolution can be calculated, and illustrate how some of
these can be accessed using extra catalytic excitations. This emphasises that
it is not sufficient for analyses of state transfer in spin networks to
restrict to the single excitation subspace. One class of symmetries in these
spin networks is exactly characterised in terms of the underlying graph
properties.Comment: 14 pages, 3 figures v3: rewritten for increased clarit
Ultra-Efficient Cooling of Resonators: Beating Sideband Cooling with Quantum Control
The present state-of-the-art in cooling mechanical resonators is a version of
"sideband" cooling. Here we present a method that uses the same configuration
as sideband cooling --- coupling the resonator to be cooled to a second
microwave (or optical) auxiliary resonator --- but will cool significantly
colder. This is achieved by varying the strength of the coupling between the
two resonators over a time on the order of the period of the mechanical
resonator. As part of our analysis, we also obtain a method for fast,
high-fidelity quantum information-transfer between resonators.Comment: 4 pages, revtex4-1, 2 png figure
Optimal control technique for Many Body Quantum Systems dynamics
We present an efficient strategy for controlling a vast range of
non-integrable quantum many body one-dimensional systems that can be merged
with state-of-the-art tensor network simulation methods like the density Matrix
Renormalization Group. To demonstrate its potential, we employ it to solve a
major issue in current optical-lattice physics with ultra-cold atoms: we show
how to reduce by about two orders of magnitudes the time needed to bring a
superfluid gas into a Mott insulator state, while suppressing defects by more
than one order of magnitude as compared to current experiments [1]. Finally, we
show that the optimal pulse is robust against atom number fluctuations.Comment: 5 pages, 4 figures, published versio
Quantum measurement of a mesoscopic spin ensemble
We describe a method for precise estimation of the polarization of a
mesoscopic spin ensemble by using its coupling to a single two-level system.
Our approach requires a minimal number of measurements on the two-level system
for a given measurement precision. We consider the application of this method
to the case of nuclear spin ensemble defined by a single electron-charged
quantum dot: we show that decreasing the electron spin dephasing due to nuclei
and increasing the fidelity of nuclear-spin-based quantum memory could be
within the reach of present day experiments.Comment: 8 pages, 2 figures; minor changes, published versio
The Renewable Energy In A Led Standalone Streetlight
This work deals with the design of a standalone streetlight provided with a solar panel and a multiple vertical axis wind turbine (VAWT) along the structure. A prototype was built and is currently being tested in the Monte Dago campus of the UniversitĂ Politecnica delle Marche. The ongoing focus of the project is to improve the overall efficiency and the manufacturing details for the industrialization. A battery bank allows delaying the energy delivering from the energy production, while a central process unit on board collects the data from every component in the equipment. This unit allows to monitor the day-by-day efficiency of the energy-lighting system, and to send the information wirelessly with the purpose of integrating into a smart grid-like management platform. The test site includes a meteorological mast, which can measure the weather conditions, such as wind speed and solar radiation. The wind turbines included in the streetlight have been studied from an aerodynamic point of view through an extensive experimental analysis in the wind tunnel. Moreover, the structural design of the wind rotors was carried out together with the security system including a mechanical brake, which prevents the damage of the components during high wind speed conditions. The control of the hybrid energy unit, designed to track the optimal performance, has been analyzed throughout the local wind conditions. Also, it is discussed the effectiveness of this streetlight concept in various climate situations
Continuous quantum feedback of coherent oscillations in a solid-state qubit
We have analyzed theoretically the operation of the Bayesian quantum feedback
of a solid-state qubit, designed to maintain perfect coherent oscillations in
the qubit for arbitrarily long time. In particular, we have studied the
feedback efficiency in presence of dephasing environment and detector
nonideality. Also, we have analyzed the effect of qubit parameter deviations
and studied the quantum feedback control of an energy-asymmetric qubit.Comment: 11 page
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