5,704 research outputs found
Continuous variable entanglement dynamics in structured reservoirs
We address the evolution of entanglement in bimodal continuous variable
quantum systems interacting with two independent structured reservoirs. We
derive an analytic expression for the entanglement of formation without
performing the Markov and the secular approximations and study in details the
entanglement dynamics for various types of structured reservoirs and for
different reservoir temperatures, assuming the two modes initially excited in a
twin-beam state. Our analytic solution allows us to identify three dynamical
regimes characterized by different behaviors of the entanglement: the
entanglement sudden death, the non-Markovian revival and the non-secular
revival regimes. Remarkably, we find that, contrarily to the Markovian case,
the short-time system-reservoir correlations in some cases destroy quickly the
initial entanglement even at zero temperature.Comment: 12 pages, 8 figure
Information/disturbance trade-off in continuous variable Gaussian systems
We address the information/disturbance trade-off for state-measurements on
continuous variable Gaussian systems and suggest minimal schemes for
implementations. In our schemes, the symbols from a given alphabet are encoded
in a set of Gaussian signals which are coupled to a probe excited in a known
state. After the interaction the probe is measured, in order to infer the
transmitted state, while the conditional state of the signal is left for the
subsequent user. The schemes are minimal, {\em i.e.} involve a single
additional probe, and allow for the nondemolitive transmission of a continuous
real alphabet over a quantum channel. The trade-off between information gain
and state disturbance is quantified by fidelities and, after optimization with
respect to the measurement, analyzed in terms of the energy carried by the
signal and the probe. We found that transmission fidelity only depends on the
energy of the signal and the probe, whereas estimation fidelity also depends on
the alphabet size and the measurement gain. Increasing the probe energy does
not necessarily lead to a better trade-off, the most relevant parameter being
the ratio between the alphabet size and the signal width, which in turn
determine the allocation of the signal energy.Comment: 9 pages, 6 figures, revised version, title changed, accepted PR
Characterization of qubit chains by Feynman probes
We address the characterization of qubit chains and assess the performances
of local measurements compared to those provided by Feynman probes, i.e.
nonlocal measurements realized by coupling a single qubit regis- ter to the
chain. We show that local measurements are suitable to estimate small values of
the coupling and that a Bayesian strategy may be successfully exploited to
achieve optimal precision. For larger values of the coupling Bayesian local
strategies do not lead to a consistent estimate. In this regime, Feynman probes
may be exploited to build a consistent Bayesian estimator that saturates the
Cram\'er-Rao bound, thus providing an effective characterization of the chain.
Finally, we show that ultimate bounds to precision, i.e. saturation of the
quantum Cram\'er-Rao bound, may be achieved by a two-step scheme employing
Feynman probes followed by local measurements.Comment: 8 pages, 5 figure
Non-Gaussian quantum discord for Gaussian states
In recent years the paradigm based on entanglement as the unique measure of
quantum correlations has been challenged by the rise of new correlation
concepts, such as quantum discord, able to reveal quantum correlations that are
present in separable states. It is in general difficult to compute quantum
discord, because it involves a minimization over all possible local
measurements in a bipartition. In the realm of continuous variable (CV)
systems, a Gaussian version of quantum discord has been put forward upon
restricting to Gaussian measurements. It is natural to ask whether non-Gaussian
measurements can lead to a stronger minimization than Gaussian ones. Here we
focus on two relevant classes of two-mode Gaussian states: squeezed thermal
states (STS) and mixed thermal states (MTS), and allow for a range of
experimentally feasible non-Gaussian measurements, comparing the results with
the case of Gaussian measurements. We provide evidence that Gaussian
measurements are optimal for Gaussian states.Comment: 12 pages, 9 figures (3 appendices
Nonlocal compensation of pure phase objects with entangled photons
We suggest and demonstrate a scheme for coherent nonlocal compensation of
pure phase objects based on two-photon polarization and momentum entangled
states. The insertion of a single phase object on one of the beams reduces the
purity of the state and the amount of shared entanglement, whereas the original
entanglement can be retrieved by adding a suitable phase object on the other
beam. In our setup polarization and momentum entangled states are generated by
spontaneous parametric downconversion and then purified using a programmable
spatial light modulator, which may be also used to impose arbitrary space
dependent phase functions to the beams. As a possible application, we suggest
and demonstrate a quantum key distribution protocol based on nonlocal phase
compensation.Comment: 7 pages, 5 figure
Experimental pre-assessing entanglement in Gaussian states mixing
We suggest and demonstrate a method to assess entanglement generation schemes
based on mixing of Gaussian states at a beam splitter (BS). Our method is based
on the fidelity criterion and represents a tool to analyze the effect of losses
and noise before the BS in both symmetric and asymmetric channels with and
without thermal effects. More generally, our scheme allows one to pre-assess
entanglement resources and to optimize the design of BS-based schemes for the
generation of continuous variable entanglement.Comment: 10 pages, 15 figure
Quantum non-Gaussianity witnesses in the phase space
We address detection of quantum non-Gaussian states, i.e. nonclassical states
that cannot be expressed as a convex mixture of Gaussian states, and present a
method to derive a new family of criteria based on generic linear functionals.
We then specialise this method to derive witnesses based on -parametrized
quasiprobability functions, generalising previous criteria based on the Wigner
function. In particular we discuss in detail and analyse the properties of
Husimi Q-function based witnesses and prove that they are often more effective
than previous criteria in detecting quantum non-Gaussianity of various kinds of
non-Gaussian states evolving in a lossy channel.Comment: 9 pages, 6 figure
Quantifying non-Gaussianity for quantum information
We address the quantification of non-Gaussianity of states and operations in
continuous-variable systems and its use in quantum information. We start by
illustrating in details the properties and the relationships of two recently
proposed measures of non-Gaussianity based on the Hilbert-Schmidt (HS) distance
and the quantum relative entropy (QRE) between the state under examination and
a reference Gaussian state. We then evaluate the non-Gaussianities of several
families of non-Gaussian quantum states and show that the two measures have the
same basic properties and also share the same qualitative behaviour on most of
the examples taken into account. However, we also show that they introduce a
different relation of order, i.e. they are not strictly monotone each other. We
exploit the non-Gaussianity measures for states in order to introduce a measure
of non-Gaussianity for quantum operations, to assess Gaussification and
de-Gaussification protocols, and to investigate in details the role played by
non-Gaussianity in entanglement distillation protocols. Besides, we exploit the
QRE-based non-Gaussianity measure to provide new insight on the extremality of
Gaussian states for some entropic quantities such as conditional entropy,
mutual information and the Holevo bound. We also deal with parameter estimation
and present a theorem connecting the QRE nonG to the quantum Fisher
information. Finally, since evaluation of the QRE nonG measure requires the
knowledge of the full density matrix, we derive some {\em experimentally
friendly} lower bounds to nonG for some class of states and by considering the
possibility to perform on the states only certain efficient or inefficient
measurements.Comment: 22 pages, 13 figures, comments welcome. v2: typos corrected and
references added. v3: minor corrections (more similar to published version
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