136 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
Signal amplification and control in optical cavities with off-axis feedback
We consider a large class of optical cavities and gain media with an off-axis
external feedback which introduces a two-point nonlocality. This nonlocality
moves the lasing threshold and opens large windows of control parameters where
weak light spots can be strongly amplified while the background radiation
remains very low. Furthermore, transverse phase and group velocities of a
signal can be independently tuned and this enables to steer it non
mechanically, to control its spatial chirping and to split it into two
counter-propagating ones.Comment: 4 pages, 4 picture
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
Quantum synchronization as a local signature of super- and subradiance
We study the relationship between the collective phenomena of super and
subradiance and spontaneous synchronization of quantum systems. To this aim we
revisit the case of two detuned qubits interacting through a pure dissipative
bosonic environment, which contains the minimal ingredients for our analysis.
By using the Liouville formalism, we are able to find analytically the ultimate
connection between these phenomena. We find that dynamical synchronization is
due to the presence of long standing coherence between the ground state of the
system and the subradiant state. We finally show that, under pure dissipation,
the emergence of spontaneous synchronization and of subradiant emission occur
on the same time scale. This reciprocity is broken in the presence of dephasing
noise.Comment: 12 pages, 6 figure
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
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