146 research outputs found
Coherent and radiative couplings through 2D structured environments
We study coherent and radiative interactions induced among two or more
quantum units, by coupling them to two-dimensional lattices acting as
structured environments. This model can be representative of atoms trapped near
photonic crystal slabs, trapped ions in Coulomb crystals or to surface acoustic
waves on piezoelectric materials, cold atoms on state-dependent optical
lattices, or even circuit QED architectures, to name a few. We compare coherent
and radiative contributions for the isotropic and directional regimes of
emission into the lattice, for infinite and finite lattices, highlighting their
differences and existing pitfalls, e.g. related to long-time or large-lattice
limits. We relate the phenomenon of directionality of emission with
linear-shaped isofrequency manifolds in the dispersion relation, showing a
simple way to disrupt it. For finite lattices, we study further details as the
scaling of resonant number of lattice modes for the isotropic and directional
regimes, and relate this behavior with known van Hove singularities in the
infinite lattice limit. Further we export the understanding of emission
dynamics with the decay of entanglement for two quantum, atomic or bosonic,
units coupled to the 2D lattice. We analyze in some detail completely
subradiant configurations of more than two atoms, which can occur in the finite
lattice scenario, in contrast with the infinite lattice case. Finally we
demonstrate that induced coherent interactions for dark states are zero for the
finite lattice.Comment: 10 page
Completely subradiant multi-atom architectures through 2D photonic crystals
Inspired by recent advances in the manipulation of atoms trapped near 1D
waveguides and pro- posals to use surface acoustic waves on piezoelectric
substrates for the same purpose, we show the potential of two-dimensional
platforms. We exploit the directional emission of atoms near photonic crystal
slabs with square symmetry to build perfect subradiant states of 2 distant
atoms, possible in 2D only for finite lattices with reflecting boundaries. We
also show how to design massively parallel 1D arrays of atoms above a single
crystal, useful for multi-port output of nonclassical light, by ex- ploiting
destructive interference of guided resonance modes due to finite size effects.
Directionality of the emission is shown to be present whenever a linear
iso-frequency manifold is present in the dispersion relation of the crystal.
Multi-atom radiance properties can be obtained from a simple cross-talk
coefficient of a master equation, which we compare with exact atom-crystal
dynamics, showing its predictive power
Multi-ion sensing of dipolar noise sources in ion traps
Trapped-ion quantum platforms are subject to `anomalous' heating due to
interactions with electric-field noise sources of nature not yet completely
known. There is ample experimental evidence that this noise originates at the
surfaces of the trap electrodes, and models assuming fluctuating point-like
dipoles are consistent with observations, but the exact microscopic mechanisms
behind anomalous heating remain undetermined. Here we show how a two-ion probe
displays a transition in its dissipation properties, enabling experimental
access to the mean orientation of the dipoles and the spatial extent of
dipole-dipole correlations. This information can be used to test the validity
of candidate microscopic models, which predict correlation lengths spanning
several orders of mag- nitude. Furthermore, we propose an experiment to measure
these effects with currently-available traps and techniques
Discording power of quantum evolutions
We introduce the discording power of a unitary transformation, which assesses
its capability to produce quantum discord, and analyze in detail the generation
of discord by relevant classes of two-qubit gates. Our measure is based on the
Cartan decomposition of two-qubit unitaries and on evaluating the maximum
discord achievable by a unitary upon acting on classical-classical states at
fixed purity. We found that there exist gates which are perfect discorders for
any value of purity, and that they belong to a class of operators that includes
the $\sqrt{{SWAP}}. Other gates, even those universal for quantum computation,
do not posses the same property: the CNOT, for example, is a perfect discorder
only for states with low or unit purity, but not for intermediate values. The
discording power of a two-qubit unitary also provides a generalization of the
corresponding measure defined for entanglement to any value of the purity.Comment: accepted for publication in Physical Review Letter
Quantum Otto cycle with inner friction: finite-time and disorder effects
The concept of inner friction, by which a quantum heat engine is unable to
follow adiabatically its strokes and thus dissipates useful energy, is
illustrated in an exact physical model where the working substance consists of
an ensemble of misaligned spins interacting with a magnetic field and
performing the Otto cycle. The effect of this static disorder under a
finite-time cycle gives a new perspective of the concept of inner friction
under realistic settings. We investigate the efficiency and power of this
engine and relate its performance to the amount of friction from misalignment
and to the temperature difference between heat baths. Finally we propose an
alternative experimental implementation of the cycle where the spin is encoded
in the degree of polarization of photons.Comment: Published version in the Focus Issue on "Quantum Thermodynamics
Natural risk management for industrial plants and infrastructures: the DaBo system
Natural risk management on complex critical infrastructures often requires integration of data coming out from
a huge number of sensors. Solutions are sometimes derived by classical supervisory control and data acquisition
systems (SCADAs), usually employed in manufacturing and industrial plants environment. This “control room”
approach often proves to be ineffective when the system to be monitored goes beyond the limits of the single plant
and it is extended to the surrounding environment including buildings and public infrastructures in a strong interaction
with local communities. The paper presents the case study of a hydroelectric plant extended over a territory
of a few tens of square kilometers and subject to hydrogeological problems of various kinds, with interactions
with buildings and infrastructures. The huge number of sensors installed for production control proved to be far
to monitor the safety of the plant in its environmental context. We present here the risk assessment procedure and
the proposed actions, also in terms of sensor installation. DaBo platform work as a data integrator. The structural
and hydraulic “ordinary state” is continuously generated by means of numerical modeling basing upon real time
observed boundary conditions. This state, via a suitable set of state variables, is compared with sensor data allowing
a clear synthesis of the safety of the infrastructure and its natural and anthropic context. DaBo poses itself as
a systems integrator both from a conceptual and an operational point of view, able to activate direct measures to
reduce the risk in case of emergency, involving also local civil protection authorities.
The platform integrates information from a wide range of sensors (viz. temperature, water level, strain, water content),
weather alerts, weather forecast from high resolution limited area models. The main innovation of DaBo
consists in the dashboard designed to provide communication of risk to the end user and to link the warnings to
action procedures. It is technically a responsive single page web application that is based on an information storage
and management layer by a high capacity relational database, a powerful scalable business logic tier for decision
support and early warning system, and a multi profiled responsive user interface. The goal is to ensure the operation
of the entire supply chain that connects the various sources of information to the entire user range
Orthogonal measurements are {\it almost} sufficient for quantum discord of two qubits
The common use in literature of orthogonal measurements in obtaining quantum
discord for two-qubit states is discussed and compared with more general
measurements. We prove the optimality of orthogonal measurements for rank 2
states. While for rank 3 and 4 mixed states they are not optimal, we present
strong numerical evidence showing that they give the correct quantum discord up
to minimal corrections. Based on the connection, through purification with an
ancilla, between discord and entanglement of formation (EoF), we give a tight
upper bound for the EoF of a mixed state of rank 2, given by an
optimal decomposition of 2 elements. We also provide an alternative way to
compute the quantum discord for two qubits based on the Bloch vectors of the
state.Comment: EPL 96, 40005 (2011
Microscopic description for the emergence of collective dissipation in extended quantum systems
Practical implementations of quantum technology are limited by unavoidable effects of decoherence and dissipation. With achieved experimental control for individual atoms and photons, more complex platforms composed by several units can be assembled enabling distinctive forms of dissipation and decoherence, in independent heat baths or collectively into a common bath, with dramatic consequences for the preservation of quantum coherence. The cross-over between these two regimes has been widely attributed in the literature to the system units being farther apart than the bath's correlation length. Starting from a microscopic model of a structured environment (a crystal) sensed by two bosonic probes, here we show the failure of such conceptual relation, and identify the exact physical mechanism underlying this cross-over, displaying a sharp contrast between dephasing and dissipative baths. Depending on the frequency of the system and, crucially, on its orientation with respect to the crystal axes, collective dissipation becomes possible for very large distances between probes, opening new avenues to deal with decoherence in phononic baths
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