450 research outputs found
Entanglement generation in the ultra-strongly coupled Rabi model
We analyze the dynamics of the quantum Rabi model for two qubits interacting
through a common bosonic field mode (resonator), focusing on the generation and
detection of maximally entangled Bell states. We obtain analytical results for
the unitary dynamics of this system in the slow-qubit (or degenerate) regime,
considering ultra-strong coupling between qubits and resonator mode, for which
the rotating wave approximation is no longer applicable. We also numerically
investigate the dynamics beyond the slow-qubit condition in order to study the
validity of the model in the presence of less strict conditions.Comment: 11 pages, 6 figures; submitted to the special issue of International
Journal of Quantum Information as contribution to the Quantum 2014 conferenc
Selective cloning of Gaussian states by linear optics
We investigate the performances of a selective cloning machine based on
linear optical elements and Gaussian measurements, which allows to clone at
will one of the two incoming input states. This machine is a complete
generalization of a 1 to 2 cloning scheme demonstrated by U. L. Andersen et al.
[Phys. Rev. Lett. vol. 94, 240503 (2005)]. The input-output fidelity is studied
for generic Gaussian input state and the effect of non-unit quantum efficiency
is also taken into account. We show that if the states to be cloned are
squeezed states with known squeezing parameter, then the fidelity can be
enhanced using a third suitable squeezed state during the final stage of the
cloning process. A binary communication protocol based on the selective cloning
machne is also discussed.Comment: 6 pages, 6 figure
Degaussification of twin-beam and nonlocality in the phase space
We show that inconclusive photon subtraction (IPS) on twin-beam produces
non-Gaussian states that violate Bell's inequality in the phase-space. The
violation is larger than for the twin-beam itself irrespective of the IPS
quantum efficiency. The explicit expression of IPS map is given both for the
density matrix and the Wigner function representations.Comment: 7 pages, 6 figure
De-Gaussification by inconclusive photon subtraction
We address conditional de-Gaussification of continuous variable states by
inconclusive photon subtraction (IPS) and review in details its application to
bipartite twin-beam state of radiation. The IPS map in the Fock basis has been
derived, as well as its counterpart in the phase-space. Teleportation assisted
by IPS states is analyzed and the corresponding fidelity evaluated as a
function of the involved parameters. Nonlocality of IPS states is investigated
by means of different tests including displaced parity, homodyne detection,
pseudospin, and displaced on/off photodetection. Dissipation and thermal noise
are taken into account, as well as non unit quantum efficiency in the detection
stage. We show that the IPS process, for a suitable choice of the involved
parameters, improves teleportation fidelity and enhances nonlocal properties.Comment: 17 pages, 30 figure
Gaussian and Non-Gaussian operations on non-Gaussian state: engineering non-Gaussianity
Multiple photon subtraction applied to a displaced phase-averaged coherent
state, which is a non-Gaussian classical state, produces conditional states
with a non trivial (positive) Glauber-Sudarshan -representation. We
theoretically and experimentally demonstrate that, despite its simplicity, this
class of conditional states cannot be fully characterized by direct detection
of photon numbers. In particular, the non-Gaussianity of the state is a
characteristics that must be assessed by phase-sensitive measurements. We also
show that the non-Gaussianity of conditional states can be manipulated by
choosing suitable conditioning values and composition of phase-averaged states
Experimental quantification of non-Gaussianity of phase-randomized coherent states
We present the experimental investigation of the non-Gaussian nature of some
mixtures of Fock states by reconstructing their Wigner function and exploiting
two recently introduced measures of non-Gaussianity. In particular, we
demonstrate the consistency between the different approaches and the
monotonicity of the two measures for states belonging to the class of phase
randomized coherent states. Moreover, we prove that the exact behavior of one
measure with respect to the other depends on the states under investigation and
devise possible criteria to discriminate which measure is more useful for the
characterization of the states in realistic applications.Comment: 9 pages, 4 figure
Matter-wave interferometry: towards antimatter interferometers
Starting from an elementary model and refining it to take into account more
realistic effects, we discuss the limitations and advantages of matter-wave
interferometry in different configurations. We focus on the possibility to
apply this approach to scenarios involving antimatter, such as positrons and
positronium atoms. In particular, we investigate the Talbot-Lau interferometer
with material gratings and discuss in details the results in view of the
possible experimental verification.Comment: 18 pages; 8 figure
High-precision innovative sensing with continuous-variable optical states
When applied to practical problems, the very laws of quantum mechanics can
provide a unique resource to beat the limits imposed by classical physics: this
is the case of quantum metrology and high-precision sensing. Here we review the
main results obtained in the recent years at the Quantum Technology Lab of the
Department of Physics "Aldo Pontremoli" of the University of Milan, also in
collaboration with national and international institutions. In particular we
focus on the application of continuous-variable optical quantum states and
operations to improve different field of research, ranging from interferometry
to more fundamental problems, such as the testing of quantum gravity.Comment: 42 pages, 26 figures, review paper, the original publication is
available on https://www.sif.it/riviste/sif/nc
Photon subtracted states and enhancement of nonlocality in the presence of noise
We address nonlocality of continuous variable systems in the presence of
dissipation and noise. Three nonlocality tests have been considered, based on
the measurement of displaced-parity, field-quadrature and pseudospin-operator,
respectively. Nonlocality of twin beam has been investigated, as well as that
of its non-Gaussian counterparts obtained by inconclusive subtraction of
photons. Our results indicate that: i) nonlocality of twin beam is degraded but
not destroyed by noise; ii) photon subtraction enhances nonlocality in the
presence of noise, especially in the low-energy regime.Comment: 12 pages, 7 figure
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