3,762 research outputs found
Self-Consistent Projection Operator Theory in Nonlinear Quantum Optical Systems: A case study on Degenerate Optical Parametric Oscillators
Nonlinear quantum optical systems are of paramount relevance for modern
quantum technologies, as well as for the study of dissipative phase
transitions. Their nonlinear nature makes their theoretical study very
challenging and hence they have always served as great motivation to develop
new techniques for the analysis of open quantum systems. In this article we
apply the recently developed self-consistent projection operator theory to the
degenerate optical parametric oscillator to exemplify its general applicability
to quantum optical systems. We show that this theory provides an efficient
method to calculate the full quantum state of each mode with high degree of
accuracy, even at the critical point. It is equally successful in describing
both the stationary limit and the dynamics, including regions of the parameter
space where the numerical integration of the full problem is significantly less
efficient. We further develop a Gaussian approach consistent with our theory,
which yields sensibly better results than the previous Gaussian methods
developed for this system, most notably standard linearization techniques.Comment: Comments are welcom
Enhancement of the immunoregulatory potency of mesenchymal stromal cells by treatment with immunosuppressive drugs
Background aims Multipotent mesenchymal stromal cells (MSCs) are distinguished by their ability to differentiate into a number of stromal derivatives of interest for regenerative medicine, but they also have immunoregulatory properties that are being tested in a number of clinical settings. Methods We show that brief incubations with rapamycin, everolimus, FK506 or cyclosporine A increase the immunosuppressive potency of MSCs and other cell types. Results The treated MSCs are up to 5-fold more potent at inhibiting the induced proliferation of T lymphocytes in vitro. We show that this effect probably is due to adsorption of the drug by the MSCs during pre-treatment, with subsequent diffusion into co-cultures at concentrations sufficient to inhibit T-cell proliferation. MSCs contain measurable amounts of rapamycin after a 15-min exposure, and the potentiating effect is blocked by a neutralizing antibody to the drug. With the use of a pre-clinical model of acute graft-versus-host disease, we demonstrate that a low dose of rapamycin-treated but not untreated umbilical cord–derived MSCs significantly inhibit the onset of disease. Conclusions The use of treated MSCs may achieve clinical end points not reached with untreated MSCs and allow for infusion of fewer cells to reduce costs and minimize potential side effects
General linearized theory of quantum fluctuations around arbitrary limit cycles
The theory of Gaussian quantum fluctuations around classical steady states in
nonlinear quantum-optical systems (also known as standard linearization) is a
cornerstone for the analysis of such systems. Its simplicity, together with its
accuracy far from critical points or situations where the nonlinearity reaches
the strong coupling regime, has turned it into a widespread technique, which is
the first method of choice in most works on the subject. However, such a
technique finds strong practical and conceptual complications when one tries to
apply it to situations in which the classical long-time solution is time
dependent, a most prominent example being spontaneous limit-cycle formation.
Here we introduce a linearization scheme adapted to such situations, using the
driven Van der Pol oscillator as a testbed for the method, which allows us to
compare it with full numerical simulations. On a conceptual level, the scheme
relies on the connection between the emergence of limit cycles and the
spontaneous breaking of the symmetry under temporal translations. On the
practical side, the method keeps the simplicity and linear scaling with the
size of the problem (number of modes) characteristic of standard linearization,
making it applicable to large (many-body) systems.Comment: Constructive suggestions and criticism are welcom
The Applegate mechanism in Post-Common-Envelope Binaries: Investigating the role of rotation
Eclipsing time variations (ETVs) are observed in many close binary systems.
In particular, for several post-common-envelope binaries (PCEBs) that consist
of a white dwarf and a main sequence star, the O-C diagram suggests that real
or apparent orbital period variations are driven by Jupiter-mass planets or as
a result of magnetic activity, the so-called Applegate mechanism. The latter
explains orbital period variations as a result of changes in the stellar
quadrupole moment due to magnetic activity. We explore the feasibility of
driving ETVs via the Applegate mechanism for a sample of PCEB systems,
including a range of different rotations. Using the MESA code we evolve 12
stars with different masses and rotation rates. We apply a simple dynamo model
to their radial profiles to investigate on which scale the predicted activity
cycle matches the observed modulation period, and quantify the uncertainty, and
further calculate the required energies to drive que Applegate mechanism. We
show that the Applegate mechanism is energetically feasible in 5 PCEB systems,
and note that these are the systems with the highest rotation rate compared to
the critical rotation rate of the main-sequence star. The results suggest that
the ratio of physical to critical rotation in the main sequence star is an
important indicator for the feasibility of Applegate's mechanism, but exploring
larger samples will be necessary to probe this hypothesis.Comment: 9 pages, 5 figures. Accepted for publication in A&
Floquet theory for temporal correlations and spectra in time-periodic open quantum systems: Application to squeezed parametric oscillation beyond the rotating-wave approximation
Open quantum systems can display periodic dynamics at the classical level
either due to external periodic modulations or to self-pulsing phenomena
typically following a Hopf bifurcation. In both cases, the quantum fluctuations
around classical solutions do not reach a quantum-statistical stationary state,
which prevents adopting the simple and reliable methods used for stationary
quantum systems. Here we put forward a general and efficient method to compute
two-time correlations and corresponding spectral densities of time-periodic
open quantum systems within the usual linearized (Gaussian) approximation for
their dynamics. Using Floquet theory we show how the quantum Langevin equations
for the fluctuations can be efficiently integrated by partitioning the time
domain into one-period duration intervals, and relating the properties of each
period to the first one. Spectral densities, like squeezing spectra, are
computed similarly, now in a two-dimensional temporal domain that is treated as
a chessboard with one-period x one-period cells. This technique avoids
cumulative numerical errors as well as efficiently saves computational time. As
an illustration of the method, we analyze the quantum fluctuations of a damped
parametrically-driven oscillator (degenerate parametric oscillator) below
threshold and far away from rotating-wave approximation conditions, which is a
relevant scenario for modern low-frequency quantum oscillators. Our method
reveals that the squeezing properties of such devices are quite robust against
the amplitude of the modulation or the low quality of the oscillator, although
optimal squeezing can appear for parameters that are far from the ones
predicted within the rotating-wave approximation.Comment: Comments and constructive criticism are welcom
Enhancing quantum entanglement by photon addition and subtraction
The non-Gaussian operations effected by adding or subtracting a photon on the
entangled optical beams emerging from a parametric down-conversion process have
been suggested to enhance entanglement. Heralded photon addition or subtraction
is, as a matter of fact, at the heart of continuous-variable entanglement
distillation. The use of such processes has recently been experimentally
demonstrated in the context of the generation of optical coherent-state
superpositions or the verification of the canonical commutation relations.
Here, we carry out a systematic study of the effect of local photon additions
or subtractions on a two-mode squeezed vacuum state, showing that the
entanglement generally increases with the number of such operations. This is
analytically proven when additions or subtractions are restricted to one mode
only, while we observe that the highest entanglement is achieved when these
operations are equally shared between the two modes. We also note that adding
photons typically provides a stronger entanglement enhancement than subtracting
photons, while photon subtraction performs better in terms of energy
efficiency. Furthermore, we analyze the interplay between entanglement and
non-Gaussianity, showing that it is more subtle than previously expected.Comment: 10 pages, 6 figure
Inventari espeleològic dels termes d'Andratx i Estellencs
[cat] En aquest treball es presenta la primera part d'una actualització de les cavitats en els termes municipals del sud-oest de l’illa de Mallorca. Hem catalogat 76 cavitats en el terme d'Andratx, 8 a Sa Dragonera i 25 en el terme d'Estellencs.[eng] This note is the first part of the cavities up-dating task in the Southwestern area of the Mallorca island. Seventy-six cavities in Andratx municipality, eight in Sa Dragonera island and twenty-five in Estellencs municipality have been catalogued
Noncritical quadrature squeezing through spontaneous polarization symmetry breaking
We discuss the possibility of generating noncritical quadrature squeezing by
spontaneous polarization symmetry breaking. We consider first type-II
frequency-degenerate optical parametric oscillators, but discard them for a
number of reasons. Then we propose a four-wave mixing cavity in which the
polarization of the output mode is always linear but has an arbitrary
orientation. We show that in such a cavity complete noise suppression in a
quadrature of the output field occurs, irrespective of the parameter values
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