[Show description][Hide description] DESCRIPTION: A sideband cooling strategy that incorporates (i) the dynamics induced by structured (non- Markovian) environments in the target and auxiliary systems and (ii) the optimally-time-modulated interaction between them is developed. For the context of cavity optomechanics, when non-Markovian dynamics are considered in the target system, ground state cooling is reached at much faster rates and at much lower phonon occupation number than previously reported. In contrast to similar current strategies, ground state cooling is reached here for coupling-strength rates that are experimentally accesible for the state-of-the-art implementations. After the ultrafast optimal-ground-state-cooling protocol is accomplished, an additional optimal control strategy is considered to maintain the phonon number as closer as possible to the one obtained in the cooling procedure. Contrary to the conventional expectation, when non-Markovian dynamics are considered in the auxiliary system, the efficiency of the cooling protocol is undermined.
[Show abstract][Hide abstract] ABSTRACT: A sideband cooling strategy that incorporates (i) the dynamics induced by
structured (non-Markovian) environments in the target and auxiliary systems and
(ii) the optimally-time-modulated interaction between them is developed. For
the context of cavity optomechanics, when non-Markovian dynamics are considered
in the target system, ground state cooling is reached at much faster rates and
at much lower phonon occupation number than previously reported. In constrast
to similar current strategies, ground state cooling is reached here for
coupling-strength rates that are experimentally accesible for the
state-of-the-art implementations. After the ultrafast
optimal-ground-state-cooling protocol is accomplished, an additional optimal
control strategy is considered to maintain the phonon number as closer as
possible to the one obtained in the cooling procedure. Contrary to the
conventional expectation, when non-Markovian dynamics are considered in the
auxiliary system, the efficiency of the cooling protocol is undermined.
[Show abstract][Hide abstract] ABSTRACT: Frame dragging (Lense-Thirring effect) is generally associated with rotating
astrophysical objects. However, it can also be generated by electromagnetic
fields if electric and magnetic fields are simultaneously present. In most
models of astrophysical objects, macroscopic charge neutrality is assumed and
the entire electromagnetic field is characterized in terms of a magnetic dipole
component. Hence, the purely electromagnetic contribution to the frame dragging
vanishes. However, strange stars may posses independent electric dipole and
neutron stars independent electric quadrupole moments that may lead to the
presence of purely electromagnetic contributions to the frame dragging.
Moreover, recent observations have shown that in stars with strong
electromagnetic fields, the magnetic quadrupole may have a significant
contribution to the dynamics of stellar processes. As an attempt to
characterized and quantify the effect of electromagnetic frame-dragging in this
kind of astrophysical objects, an analytic solution to the Einstein-Maxwell
equations is constructed here on the basis that the electromagnetic field is
generated by the combination of arbitrary magnetic and electric dipoles plus
arbitrary magnetic and electric quadrupole moments. The effect of each
multipole contribution on the vorticity scalar and the Poynting vector is
described in detail. Corrections on important quantities such the innermost
stable circular orbit (ISCO) and the epyciclic frequencies are also considered.
Physical Review D 04/2015; 91(12). DOI:10.1103/PhysRevD.91.124047 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Reconstruction of the dynamics (quantum process tomography) of the
single-exciton manifold in energy transfer systems is proposed here on the
basis of two-dimensional fluorescence spectroscopy (2D-FS) with
phase-modulation. The quantum-process-tomography protocol introduced here
benefits from, e.g., the sensitivity enhancement and signal-to-noise ratio
ascribed to 2D-FS. Although the isotropically averaged spectroscopic signals
depend on the quantum yield parameter $\Gamma$ of the doubly-excited-exciton
manifold, it is shown that the reconstruction of the dynamics is insensitive to
this parameter. Applications to foundational and applied problems, as well as
further extensions, are discussed.
The Journal of Chemical Physics 02/2015; 142(21). DOI:10.1063/1.4919954 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A classical formulation of the quantum multichromophoric theory of resonance
energy transfer is developed on the basis of classical electrodynamics. The
theory allows for the identification of a variety of processes of different
order-in-the-interactions that contribute to the energy transfer in molecular
aggregates with intra-coupling in donors and acceptor chromophores. Enhanced
rates in multichromophoric resonance energy transfer are shown to be well
described by this theory. Specifically, in a coupling configuration between
N_{\mathrm{A}}$ acceptors and $N_{\mathrm{D}}$ donors, the theory correctly
predicts an enhancement of the energy transfer rate dependent on the total
number of donor-acceptor pairs. As an example, the theory, applied to the
transfer rate in LH~II, gives results in excellent agreement with experiment.
Finally, it is explicitly shown that as long as linear response theory holds,
the classical multichromophoric theory formally coincides with the quantum
formulation.
[Show abstract][Hide abstract] ABSTRACT: The interplay between non-Markovian dynamics and driving fields in the
survival of entanglement between two non-degenerate oscillators is considered
here. Based on exact analytical results for the non-Markovian dynamics of two
parametrically coupled non-degenerate oscillators in contact to non-identical
independent thermal baths, the out-of-equilibrium quantum limit derived in
[Phys. Rev. Lett. 105, 180501 (2010)] is generalized to the non-Markovian
regime. Specifically, it is shown that non-Markovian dynamics, when compared to
the Markovian case, allow for the survival of stationary entanglement at higher
temperatures, with larger coupling strength to the baths and at smaller driving
rates. The effect of the asymmetry of the (i) coupled oscillators, (ii)
coupling strength to the baths at equal temperature and (iii) temperature at
equal coupling strength is discussed.
New Journal of Physics 11/2014; 17(3). DOI:10.1088/1367-2630/17/3/033038 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synchronization is a ubiquitous phenomenon occurring in social, biological, and technological systems when the internal rythms of their constituents are adapted to be in unison as a result of their coupling. This natural tendency towards dynamical consensus has spurred a large body of theoretical and experimental research in recent decades. The Kuramoto model constitutes the most studied and paradigmatic framework in which to study synchronization. In particular, it shows how synchronization appears as a phase transition from a dynamically disordered state at some critical value for the coupling strength between the interacting units. The critical properties of the synchronization transition of this model have been widely studied and many variants of its formulations have been considered to address different physical realizations. However, the Kuramoto model has been studied only within the domain of classical dynamics, thus neglecting its applications for the study of quantum synchronization phenomena. Based on a system-bath approach and within the Feynman path-integral formalism, we derive equations for the Kuramoto model by taking into account the first quantum fluctuations. We also analyze its critical properties, the main result being the derivation of the value for the synchronization onset. This critical coupling increases its value as quantumness increases, as a consequence of the possibility of tunneling that quantum fluctuations provide.
Physical Review E 11/2014; 90(5-1):052904. DOI:10.1103/PhysRevE.90.052904 · 2.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Determining the spectral density of a molecular system immersed in a
proteomic scaffold and in contact to a solvent is a fundamental challenge in
the coarse-grained description of, e.g., electron and energy transfer dynamics.
Once the spectral density is characterized, all the time scales are captured
and no artificial separation between fast and slow processes need be invoked.
Based on the fluorescence Stokes shift function, we utilize a simple and robust
strategy to extract the spectral density of a number of molecular complexes
from available experimental data. Specifically, we show that experimental data
for dye molecules in several solvents, amino acid proteins in water, and some
photochemical systems (e.g., rhodopsin and green fluorescence proteins), are
well described by a three-parameter family of sub-Ohmic spectral densities that
are characterized by a fast initial Gaussian-like decay followed by a slow
algebraic-like decay rate at long times.
The Journal of Chemical Physics 10/2014; 141(17). DOI:10.1063/1.4900512 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We show that an ensemble of organic dye molecules with permanent electric
dipole moments embedded in a microcavity can lead to strong optical
nonlinearities at the single photon level. The strong long-range electrostatic
interaction between chromophores due to their permanent dipoles introduces the
desired nonlinearity of the light-matter coupling in the microcavity. We obtain
the absorption spectra of a weak probe field under the influence of strong
exciton-photon coupling with the cavity field. Using realistic parameters, we
demonstrate that a single cavity photon can significantly modify the absorptive
and dispersive response of the medium to a probe photon at a different
frequency. Finally, we show that the system is in the regime of cavity-induced
transparency with a broad transparency window for dye dimers. We illustrate our
findings using pseudoisocyanine chloride (PIC) J-aggregates in
currently-available optical microcavities.
[Show abstract][Hide abstract] ABSTRACT: Contrary to the conventional wisdom that deviations from standard
thermodynamics originate from the strong coupling to the bath, it is shown that
these deviations are intimately linked to the power spectrum of the thermal
bath. Specifically, it is shown that the lower bound of the dispersion of the
total energy of the system, imposed by the uncertainty principle, is dominated
by the bath power spectrum and therefore, quantum mechanics inhibits the system
thermal-equilibrium-state from being described by the canonical Boltzmann's
distribution. This is in sharp contrast to the classical case, for which the
thermal equilibrium distribution of a system interacting via central forces
with pairwise-self-interacting environment, irrespective of the interaction
strength, is shown to be \emph{exactly} characterized by the canonical
Boltzmann distribution. As a consequence of this analysis, we define an
\emph{effective coupling} to the environment that depends on all energy scales
in the system and reservoir interaction. Sample computations in regimes
predicted by this effective coupling are demonstrated. For example, for the
case of strong effective coupling, deviations from standard thermodynamics are
present and, for the case of weak effective coupling, quantum features such as
stationary entanglement are possible at high temperatures.
[Show abstract][Hide abstract] ABSTRACT: The ability of an environment to assist in one-photon phase control relies upon entanglement between the system and bath and on the breaking of the time reversal symmetry. Here, one-photon phase control is examined analytically and numerically in a model system, allowing an analysis of the relative strength of these contributions. Further, the significant role of non-Markovian dynamics and of moderate system-bath coupling in enhancing one-photon phase control is demonstrated, and an explicit role for quantum mechanics is noted in the existence of initial non-zero stationary coherences. Finally, desirable conditions are shown to be required to observe such environmentally assisted control, since the system will naturally equilibrate with its environment at longer times, ultimately resulting in the loss of phase control.
The Journal of Chemical Physics 10/2013; 139(16):164123. DOI:10.1063/1.4825358 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A full-relativistic approach is used to compute the radius of the innermost
stable circular orbit (ISCO), the Keplerian, frame-dragging, precession and
oscillation frequencies of the radial and vertical motions of neutral test
particles orbiting the equatorial plane of a magnetized neutron star. The
space-time around the star is modelled by the six parametric solution derived
by Pachon et al. It is shown that the inclusion of an intense magnetic field,
such as the one of a neutron star, have non-negligible effects on the above
physical quantities, and therefore, its inclusion is necessary in order to
obtain a more accurate and realistic description of the physical processes
occurring in the neighbourhood of this kind of objects such as the dynamics of
accretion disk. The results discussed here also suggest that the consideration
of strong magnetic fields may introduce non-negligible corrections in, e.g.,
the relativistic precession model and therefore on the predictions made on the
mass of neutron stars.
[Show abstract][Hide abstract] ABSTRACT: The underlying mechanisms for one photon phase control are revealed through a master equation approach. Specifically, two mechanisms are identified, one operating on the laser time scale and the other on the time scale of the system-bath interaction. The effects of the secular and non-secular Markovian approximations are carefully examined.
[Show abstract][Hide abstract] ABSTRACT: Under natural conditions, excitation of biological molecules, which display
non-unitary open system dynamics, occurs via incoherent processes such as
temperature changes or irradiation by sunlight/moonlight. The dynamics of such
processes is explored analytically in a non-Markovian generic model.
Specifically, a system S in equilibrium with a thermal bath TB is subjected to
an external incoherent perturbation BB (such as sunlight) or another thermal
bath TB', which induces time evolution in (S+TB). Particular focus is on (i)
the extent to which the resultant dynamics is coherent, and (ii) the role of
"stationary coherences", established in the (S+TB) equilibration, in the
response to the second incoherent perturbation. Results for systems with
parameters analogous to those in light harvesting molecules in photosynthesis
show that the resultant dynamical behaviour is incoherent beyond a very short
response to the turn-on of the perturbation.
Physical Review A 10/2012; 87(2). DOI:10.1103/PhysRevA.87.022106 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We consider the general open system problem of a charged quantum oscillator
confined in a harmonic trap, whose frequency can be arbitrarily modulated in
time, that interacts with both an incoherent quantized (blackbody) radiation
field and with an arbitrary coherent laser field. We assume that the oscillator
is initially in thermodynamic equilibrium with its environment, a
non-factorized initial density matrix of the system and the environment, and
that at $t=0$ the modulation of the frequency, the coupling to the incoherent
and the coherent radiation are switched on. The subsequent dynamics, induced by
the presence of the blackbody radiation and the laser field, is studied in the
framework of the influence functional approach. This approach allows
incorporating, in \emph{analytic closed formulae}, the non-Markovian character
of the oscillator-environment interaction at any temperature as well the
non-Markovian character of the blackbody radiation and its zero-point
fluctuations. Expressions for the time evolution of the covariance matrix
elements of the quantum fluctuations and the reduced density-operator are
obtained.
[Show abstract][Hide abstract] ABSTRACT: We examine computational techniques and methodologies currently in use to explore electronic excitation energy transfer in the context of light-harvesting complexes in photosynthetic antenna systems, and comment on some new insights into the underlying physics. Advantages and pitfalls of these methodologies are discussed, as are some physical insights into the photosynthetic dynamics. By combining results from molecular modelling of the complexes (structural description) with an effective non-equilibrium statistical description (time evolution), we identify some general features, regardless of the particular distribution in the protein scaffold, that are central to light-harvesting dynamics and, that could ultimately be related to the high efficiency of the overall process. Based on these general common features, some possible new directions in the field are discussed.
[Show abstract][Hide abstract] ABSTRACT: Whether analytic exact vacuum(electrovacuum) solutions of the
Einstein(Einstein-Maxwell) field equations can accurately describe or not the
exterior spacetime of compact stars remains still an interesting open question
in Relativistic Astrophysics. As an attempt to establish their level of
accuracy, the radii of the Innermost Stable Circular Orbits (ISCOs) of test
particles given by analytic exterior spacetime geometries have been compared
with the ones given by numerical solutions for neutron stars (NSs) obeying a
realistic equation of state (EoS). It has been so shown that the six-parametric
solution of Pach\'on, Rueda, and Sanabria (2006) (hereafter PRS) is more
accurate to describe the NS ISCO radii than other analytic models. We propose
here an additional test of accuracy for analytic exterior geometries based on
the comparison of orbital frequencies of neutral test particles. We compute the
Keplerian, frame-dragging, as well as the precession and oscillation
frequencies of the radial and vertical motions of neutral test particles for
the Kerr and PRS geometries; then we compare them with the numerical values
obtained by Morsink and Stella (1999) for realistic NSs. We identify the role
of high-order multipole moments such as the mass quadrupole and current
octupole in the determination of the orbital frequencies especially in the
rapid rotation regime. The results of this work are relevant to cast a
separatrix between black hole (BH) and NS signatures as well as probe the
nuclear matter EoS and NS parameters from the Quasi-Periodic Oscillations
(QPOs) observed in Low Mass X-Ray Binaries.
The Astrophysical Journal 12/2011; 756(1). DOI:10.1088/0004-637X/756/1/82 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The physical basis for observed long-lived electronic coherence in
photosynthetic light-harvesting systems is identified using an analytically
soluble model. Three physical features are found to be responsible for their
long coherence lifetimes: i) the small energy gap between excitonic states, ii)
the small ratio of the energy gap to the coupling between excitonic states, and
iii) the fact that the molecular characteristics place the system in an
effective low temperature regime, even at ambient conditions. Using this
approach, we obtain decoherence times for a dimer model with FMO parameters of
$\approx$ 160 fs at 77 K and $\approx$ 80 fs at 277 K. As such, significant
oscillations are found to persist for 600 fs and 300 fs, respectively, in
accord with the experiment and with previous computations. Similar good
agreement is found for PC645 at room temperature, with oscillations persisting
for 400 fs. The analytic expressions obtained provide direct insight into the
parameter dependence of the decoherence time scales.
[Show abstract][Hide abstract] ABSTRACT: Decoherence due to contact with a hot environment typically restricts quantum phenomena to the low temperature limit, k_{B}T/ℏω≪1 (ℏω is the typical energy of the system). Here we report the existence of a nonequilibrium state for two coupled, parametrically driven, dissipative harmonic oscillators which, contrary to generalized intuition, has stationary entanglement at high temperatures. This clarifies the role of temperature and could lighten the burden on quantum experiments requiring delicate precooling setups.