107 research outputs found
Scaling behavior in the adiabatic Dicke Model
We analyze the quantum phase transition for a set of -two level systems
interacting with a bosonic mode in the adiabatic regime. Through the
Born-Oppenheimer approximation, we obtain the finite-size scaling expansion for
many physical observables and, in particular, for the entanglement content of
the system.Comment: 4 pages, 3 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
Dicke model and environment-induced entanglement in ion-cavity QED
We investigate realistic experimental conditions under which the collective
Dicke model can be implemented in ion-cavity QED context. We show how ideal
subradiance and superradiance can be observed and we propose an experiment to
generate entanglement exploiting the existence of the subradiant state. We
explore the conditions to achieve optimal entanglement generation and we show
that they are reachable with current experimental technology.Comment: 17 pages, 11 figures. V2: published version, one reference added,
typos correcte
Exact spectral function of a Tonks-Girardeau gas in a lattice
The single-particle spectral function of a strongly correlated system is an
essential ingredient to describe its dynamics and transport properties. We
develop a general method to calculate the exact spectral function of a strongly
interacting one-dimensional Bose gas in the Tonks-Girardeau regime, valid for
any type of confining potential, and apply it to bosons on a lattice to obtain
the full spectral function, at all energy and momentum scales. We find that it
displays three main singularity lines. The first two can be identified as the
analogs of Lieb-I and Lieb-II modes of a uniform fluid; the third one, instead,
is specifically due to the presence of the lattice. We show that the spectral
function displays a power-law behaviour close to the Lieb-I and Lieb-II
singularities, as predicted by the non-linear Luttinger liquid description, and
obtain the exact exponents. In particular, the Lieb-II mode shows a divergence
in the spectral function, differently from what happens in the dynamical
structure factor, thus providing a route to probe it in experiments with
ultracold atoms.Comment: 10 pages, 3 figure
Quantum-state transfer via resonant tunnelling through local field induced barriers
Efficient quantum-state transfer is achieved in a uniformly coupled spin-1/2
chain, with open boundaries, by application of local magnetic fields on the
second and last-but-one spins, respectively. These effective \textit{barriers}
induce appearance of two eigenstates, bi-localized at the edges of the chain,
which allow a high quality transfer also at relatively long distances. The same
mechanism may be used to send an entire e-bit (e.g., an entangled qubit pair)
from one to the other end of the chain
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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