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Roncaglia A.J.,FCEyN | Roncaglia A.J.,CONICET | Cerisola F.,FCEyN | Paz J.P.,FCEyN | Paz J.P.,CONICET
Physical Review Letters | Year: 2014

We present a new method to measure the work w performed on a driven quantum system and to sample its probability distribution P(w). The method is based on a simple fact that remained unnoticed until now: Work on a quantum system can be measured by performing a generalized quantum measurement at a single time. Such measurement, which technically speaking is denoted as a positive operator valued measure reduces to an ordinary projective measurement on an enlarged system. This observation not only demystifies work measurement but also suggests a new quantum algorithm to efficiently sample the distribution P(w). This can be used, in combination with fluctuation theorems, to estimate free energies of quantum states on a quantum computer. © 2014 American Physical Society.

Wisniacki D.A.,FCEyN | Ares N.,FCEyN | Vergini E.G.,Comision Nacional de la Energia Atomica | Vergini E.G.,Technical University of Madrid
Physical Review Letters | Year: 2010

The prediction of the response of a closed system to external perturbations is one of the central problems in quantum mechanics, and in this respect, the local density of states (LDOS) provides an in-depth description of such a response. The LDOS is the distribution of the overlaps squared connecting the set of eigenfunctions with the perturbed one. Here, we show that in the case of closed systems with classically chaotic dynamics, the LDOS is a Breit-Wigner distribution under very general perturbations of arbitrary high intensity. Consequently, we derive a semiclassical expression for the width of the LDOS which is shown to be very accurate for paradigmatic systems of quantum chaos. This Letter demonstrates the universal response of quantum systems with classically chaotic dynamics. © 2010 The American Physical Society.

Peralta-Ramos J.,FCEyN | Calzetta E.,FCEyN
Physical Review C - Nuclear Physics | Year: 2010

We apply divergence-type theory (DTT) dissipative hydrodynamics to study the 2+1 space-time evolution of the fireball created in Au+Au relativistic heavy-ion collisions at √sNN=200 GeV. DTTs are exact hydrodynamic theories that do not rely on velocity gradient expansions and therefore go beyond second-order theories. We numerically solve the equations of motion of the DTT for Glauber initial conditions and compare the results with those of second-order theory based on conformal invariance (Baier-Romatschke- Son-Starinets model) and with data. We find that the charged-hadron minimum-bias elliptic flow reaches its maximum value at lower pT in the DTT, and that the DTT allows for a value of η/s slightly larger than that of the BRSS. Our results show that the differences between viscous hydrodynamic formalisms are a significant source of uncertainty in the precise extraction of η/s from experiments. © 2010 The American Physical Society.

McCutcheon D.P.S.,FCEyN | McCutcheon D.P.S.,Imperial College London | Nazir A.,Imperial College London
Physical Review Letters | Year: 2013

We study the crucial role played by the solid-state environment in determining the photon emission characteristics of a driven quantum dot. For resonant driving, we predict a phonon enhancement of the coherently emitted radiation field with increasing driving strength, in stark contrast to the conventional expectation of a rapidly decreasing fraction of coherent emission with stronger driving. This surprising behavior results from thermalization of the dot with respect to the phonon bath and leads to a nonstandard regime of resonance fluorescence in which significant coherent scattering and the Mollow triplet coexist. Off resonance, we show that despite the phonon influence, narrowing of dot spectral sideband widths can occur in certain regimes, consistent with an experimental trend. © 2013 American Physical Society.

Freitas N.,FCEyN | Paz J.P.,CONICET
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2014

We present an analytic expression for the heat current through a general harmonic network coupled with Ohmic reservoirs. We use a method that enables us to express the stationary state of the network in terms of the eigenvectors and eigenvalues of a generalized cubic eigenvalue problem. In this way, we obtain exact formulas for the heat current and the local temperature inside the network. Our method does not rely on the usual assumptions of weak coupling to the environments or on the existence of an infinite cutoff in the environmental spectral densities. We use this method to study nonequilibrium processes without the weak coupling and Markovian approximations. As a first application of our method, we revisit the problem of heat conduction in two- and three-dimensional crystals with binary mass disorder. We complement previous results showing that for small systems the scaling of the heat current with the system size greatly depends on the strength of the interaction between system and reservoirs. This somewhat counterintuitive result seems not to have been noticed before. © 2014 American Physical Society.

Wisniacki D.A.,FCEyN | Roncaglia A.J.,FCEyN
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

The local density of states or its Fourier transform, usually called fidelity amplitude, are important measures of quantum irreversibility due to imperfect evolution. In this Rapid Communication we study both quantities in a paradigmatic many body system, the Dicke Hamiltonian, where a single-mode bosonic field interacts with an ensemble of N two-level atoms. This model exhibits a quantum phase transition in the thermodynamic limit, while for finite instances the system undergoes a transition from quasi-integrability to quantum chaotic. We show that the width of the local density of states clearly points out the imprints of the transition from integrability to chaos but no trace remains of the quantum phase transition. The connection with the decay of the fidelity amplitude is also established. © 2013 American Physical Society.

Alvarez E.,National University of San Luis | Leskow E.C.,FCEyN | Zurita J.,University of Zürich
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We study the constraints on the Lee-Wick Higgs sector arising from direct collider searches. We work in an effective-field theory framework, where all of the Lee-Wick partners are integrated out, with the sole exception of the Lee-Wick Higgs bosons. The resulting theory is a two-Higgs doublet model where the second doublet has wrong-sign kinetic and mass terms. We include the bounds coming from direct Higgs searches at both LEP and Tevatron using the code HiggsBounds, and show the currently excluded parameter space. We also analyze the prospects of LHC Run-I, finding that with a total integrated luminosity of 5fb-1 and a center-of-mass energy of 7 TeV, most of the parameter space for the standard model-like CP-even Higgs will be probed. © 2011 American Physical Society.

Freitas J.N.,FCEyN | Paz J.P.,FCEyN | Paz J.P.,Institute Fisica Of Buenos Aires
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

We analyze the evolution of the Gaussian discord between two resonant harmonic oscillators coupled to a common environment. For this, we use the same tools we applied before to fully characterize the evolution of the entanglement in this system. The asymptotic value of Gaussian discord is obtained as a function of parameters characterizing the environment (temperature, couplings, etc.) and the initial state of the system (initial squeezing, initial purity, etc.) The type of Gaussian measurement optimizing the extraction of information between the oscillators is fully characterized by means of a phase diagram. Such a diagram (with phases corresponding to homodyne or heterodyne measurements) has similar topology to the one describing dynamical phases for the entanglement. We present evidence pointing to the fact that Gaussian discord is not always a good approximation of true discord as the asymptotic value of the former is shown to be a nondecreasing function of temperature (in the high-temperature regime), reaching an asymptotic value of ln(2) for a pure initial state (and lower values for mixed initial states). © 2012 American Physical Society.

Caso A.,FCEyN | Arrachea L.,FCEyN | Lozano G.S.,FCEyN
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We introduce thermometers to define the local temperature of an electronic system driven out of equilibrium by local ac fields. We also define the effective temperature in terms of a local fluctuation-dissipation relation. We show that within the weak driving regime these two temperatures coincide. We also discuss the behavior of the local temperature along the sample. We show that it exhibits spatial fluctuations following an oscillatory pattern. For weak driving, regions of the sample become heated, while others become cooled as a consequence of the driving. © 2010 The American Physical Society.

Cormick C.,FCEyN | Paz J.P.,FCEyN
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

The evolution of the entanglement between two oscillators coupled to a common thermal environment is nontrivial. The long time limit has three qualitatively different behaviors (phases) depending on parameters such as the temperature of the bath. The phases include cases with nonvanishing long-term entanglement, others with a final disentangled state, and situations displaying an infinite sequence of events of disappearance and revival of entanglement. We describe an experiment to realize these different scenarios in an ion trap. The motional degrees of freedom of two ions are used to simulate the system while the coupling to an extra (central) ion, which is continuously laser cooled, is the gateway to a decohering reservoir. The scheme proposed allows for the observation and control of motional entanglement dynamics and is an example of a class of simulations of quantum open systems in the non-Markovian regime. © 2010 The American Physical Society.

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