arXiv subjects

T. Werlang

Publications and source records attributed to T. Werlang.

At least 19 recordsLinked to original sources

arXiv2018

Quantum power boost in a nonstationary cavity-QED quantum heat engine

We show a quantum boost in the output power of a heat engine formed by a two-level system coupled to a single-mode cavity. The key ingredient here is the nonstationary regime achieved when some system parameter (atomic transition frequency, in our case) is subjected to a time-dependent perturbative modulation that is precisely tuned at certain frequencies. We discuss how the extracted power can lead to amplification of the external driving field. Quantum power boost is found both in the nonstationary Jaynes-Cummings and Rabi models, indicating that our predictions can be experimentally tested in circuit quantum electrodynamics setups.

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arXiv2017

Dynamic Stark shift induced by a single photon packet

The dynamic Stark shift results from the interaction of an atom with the electromagnetic field. We show how a propagating single-photon wave packet can induce a time-dependent dynamical Stark shift on a two-level system (TLS). A non-perturbative fully-quantum treatment is employed, where the quantum dynamics of both the field and the TLS are analyzed. We also provide the means to experimentally access such time-dependent frequency by measuring the interference pattern in the electromagnetic field inside a one-dimensional (1D) waveguide. The effect we evidence here may find applications in the autonomous quantum control of quantum systems without classical external fields, that can be useful for quantum information processing as well as for quantum thermodynamical tasks.

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arXiv2017

Anti-Dynamical Casimir Effect as a Resource for Work Extraction

We consider the quantum Rabi model with external time modulation of the atomic frequency, which can be employed to create excitations from the vacuum state of the electromagnetic field as a consequence of the dynamical Casimir effect. Excitations can also be systematically subtracted from the atom-field system by suitably adjusting the modulation frequency, in the so-called anti-dynamical Casimir effect (ADCE). We evaluate the quantum thermodynamical work and show that a realistic out-of-equilibrium finite-time protocol harnessing ADCE allows for work extraction from the system, whose amount can be much bigger then the modulation amplitude, $| W_{\mathrm{ADCE}}| \gg \hbar ε_Ω$, in contrast to the case of very slow adiabatic modulations. We provide means to control work extraction in state-of-the-art experimental scenarios, where precise frequency adjustments or complete system isolation may be difficult to attain.

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arXiv2016

The most elementary heat to work conversion: work extraction from a completely passive state by a single photon

The concepts of work and heat in the quantum domain, as well as their interconversion principles, are still an open debate. We have found theoretical evidence that a single photon packet is capable of extracting work from a single two-level system (TLS). More importantly, this effect is found for a photon as it is spontaneously emitted, therefore carrying only heat away from the emitter. This is the most elementary process in which heat is converted into work, requiring not more than two off-resonance atoms for that purpose. From a more practical point of view it is found that, surprisingly, work can be extracted from a TLS in a completely passive initial state. The process is cyclic in the sense that the TLS initial and final states are equal. The state of the TLS remains passive throughout the interaction time with the single-photon packet. The physical meaning of the work performed by the TLS is found to be a dynamical change in the color of the photon during the reemission process. All our predictions are, in principle, measurable within state-of-the-art experiments.

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arXiv2015

Non-Markovianity induced by a single-photon packet in a one-dimensional waveguide

The concept of non-Markovianity (NM) in quantum dynamics is still an open debate. Understanding how to generate and measure NM in specific models may aid in this quest. In quantum optics, an engineered electromagnetic environment coupled to a single atom can induce NM. The most common scenario of structured electromagnetic environment is an optical cavity, composed by a pair of mirrors. Here, we show how to generate and measure NM on a two-level system coupled to a one-dimensional waveguide with no mirrors required. The origin of the non-Markovian behavior lies in the initial state of the field, prepared as a single-photon packet. We analyze how NM depends on two experimentally controllable parameters, namely, the linewidth of the packet and its central frequency. We relate the presence of NM to a $π$-phase shift between incoming and emitted fields. We also show how the two output channels of the waveguide provide distinct signatures of NM, both experimentally accessible.

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arXiv2014

Energy transport between two pure-dephasing reservoirs

A pure-dephasing reservoir acting on an individual quantum system induces loss of coherence without energy exchange. When acting on composite quantum systems, dephasing reservoirs can lead to a radically different behavior. Transport of energy between two pure-dephasing markovian reservoirs is predicted in this work. They are connected through a chain of coupled sites. The baths are kept in thermal equilibrium at distinct temperatures. Quantum coherence between sites is generated in the steady-state regime and results in the underlying mechanism sustaining the effect. A quantum model for the reservoirs is a necessary condition for the existence of stationary energy transport. A microscopic derivation of the non-unitary system-bath interaction is employed, valid in the ultrastrong inter-site coupling regime. The model assumes that each site-reservoir coupling is local.

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arXiv2014

Optimal rectification by strongly coupled spins

We study heat transport in a pair of strongly coupled spins. In particular, we present a condition for optimal rectification, i.e., flow of heat in one direction and complete isolation in the opposite direction. We show that the strong-coupling formalism is necessary for correctly describing heat flow in a wide range of parameters, including moderate to low couplings. We present a situation in which the strong-coupling formalism predicts optimal rectification whereas the phenomenological approach predicts no heat flow in any direction, for the same parameter values.

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arXiv2012

Interplay between quantum phase transitions and the behavior of quantum correlations at finite temperatures

We review the main results and ideas showing that quantum correlations at finite temperatures (T), in particular quantum discord, are useful tools in characterizing quantum phase transitions that only occur, in principle, at the unattainable absolute zero temperature. We first review some interesting results about the behavior of thermal quantum discord for small spin-1/2 chains and show that they already give us important hints of the infinite chain behavior. We then study in detail and in the thermodynamic limit (infinite chains) the thermal quantum correlations for the XXZ and XY models, where one can clearly appreciate that the behavior of thermal quantum discord at finite T is a useful tool to spotlight the critical point of a quantum phase transition.

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arXiv2012

Cooling by heating in the quantum optics domain

A class of Hamiltonians that are experimentally feasible in several contexts within quantum optics and lead to so-called cooling by heating for fermionic as well as for bosonic systems has been analyzed numerically. We have found a large range of parameters for which cooling by heating can be observed either for the fermionic system alone or for the combined fermionic and bosonic systems. Analyzing the experimental requirements, we conclude that cooling by heating is achievable with present-day technology, especially in the context of trapped-ion and cavity QED, thus contributing to the understanding of this interesting and counterintuitive effect.

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arXiv2012

Purity as a witness for initial system-environment correlations in open-system dynamics

We study the dynamics of a two-level atom interacting with a Lorentzian structured reservoir considering initial system-environment correlations. It is shown that under strong system-reservoir coupling the dynamics of purity can determine whether there are initial correlations between system and environment. Moreover, we investigate the interaction of two two-level atoms with the same reservoir. In this case, we show that besides determining if there are initial system-environment correlations, the dynamics of the purity of the atomic system allows the identification of the distinct correlated initial states. In addition, the dynamics of quantum and classical correlations is analyzed.

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arXiv2011

Spotlighting quantum critical points via quantum correlations at finite temperatures

We extend the program initiated in [T. Werlang et al., Phys. Rev. Lett. 105, 095702 (2010)] in several directions. Firstly, we investigate how useful quantum correlations, such as entanglement and quantum discord, are in the detection of critical points of quantum phase transitions when the system is at finite temperatures. For that purpose we study several thermalized spin models in the thermodynamic limit, namely, the XXZ model, the XY model, and the Ising model, all of which with an external magnetic field. We compare the ability of quantum discord, entanglement, and some thermodynamic quantities to spotlight the quantum critical points for several different temperatures. Secondly, for some models we go beyond nearest-neighbors and also study the behavior of entanglement and quantum discord for second nearest-neighbors around the critical point at finite temperature. Finally, we furnish a more quantitative description of how good all these quantities are in spotlighting critical points of quantum phase transitions at finite T, bridging the gap between experimental data and those theoretical descriptions solely based on the unattainable absolute zero assumption.

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arXiv2011

Quantum behavior of a many photons cavity field revealed by quantum discord

We investigate the quantum-to-classical crossover of a dissipative optical cavity mode based on measurement of the correlations between two atoms which do not interact with each other, but interact with the cavity mode. Firstly, we note that there is a time window where the mode has a classical behavior, which depends on the cavity decay rate, the atom-field coupling strength and the number of atoms. Then, considering only two atoms inside the cavity and working in the steady state of the system, we note that the entanglement between the atoms disappears with the increasing the mean number of photons of the cavity field (n). However, the quantum discord reaches an asymptotic non-zero value, even in the limit of n\rightarrow \infty. This happens either by increasing n coherently (applying a coherent driving field) or incoherently (raising the temperature of the reservoir coupled to the cavity mode). Therefore, the cavity mode, which is quantum by construction, always preserves its quantum behavior in the asymptotic limit and this is revealed only by the quantum discord.

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arXiv2010

Quantum correlations in spin chains at finite temperatures and quantum phase transitions

We compute the quantum correlation (quantum discord (QD)) and the entanglement (EoF) between nearest neighbor qubits (spin-1/2) in an infinite chain described by the Heisenberg model (XXZ Hamiltonian) at finite temperatures. The chain is in the thermodynamic limit and thermalized with a reservoir at temperature T (canonical ensemble). We show that QD, in contrast to EoF and other thermodynamic quantities, spotlight the critical points associated to quantum phase transitions (QPT) for this model even at finite T. This remarkable property of QD may have important implications for experimental characterization of QPTs when one is unable to reach temperatures below which a QPT can be seen.

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arXiv2010

Non-Markovian Dynamics of Quantum Discord

We evaluate the quantum discord dynamics of two qubits in independent and common non-Markovian environments. We compare the dynamics of entanglement with that of quantum discord. For independent reservoirs the quantum discord vanishes only at discrete instants whereas the entanglement can disappear during a finite time interval. For a common reservoir, quantum discord and entanglement can behave very differently with sudden birth of the former but not of the latter. Furthermore, in this case the quantum discord dynamics presents sudden changes in the derivative of its time evolution which is evidenced by the presence of kinks in its behavior at discrete instants of time.

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arXiv2010

Thermal and Magnetic Quantum Discord in Heisenberg models

We investigate how the quantum correlations (quantum discord) of a two-qubit one dimensional XYZ Heisenberg chain in thermal equilibrium depends on the temperature (T) of the bath and also on an external magnetic field B. We show that the behavior of the thermal quantum discord (QD) differs in many unexpected ways from the thermal entanglement. For example, we show situations where QD increases with T when entanglement decreases, cases where QD increases with T even in regions with zero entanglement, and that QD signals a quantum phase transition even at finite T. We also show that by properly tuning B or the interaction between the qubits we get non-zero QD for any T and we present a new effect not seen for entanglement, the regrowth of thermal QD.

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arXiv2010

System-reservoir dynamics of quantum and classical correlations

We address the system-reservoir dynamics of classical and quantum correlations in the decoherence phenomenon, regarding a two qubit composite system interacting with two independent environments. The most common noise channels (amplitude damping, phase damping, bit flip, bit-phase flip, and phase flip) was studied. By analytical and numerical analysis we found that, contrary to what is usually stated in the literature, decoherence may occurs without entanglement between the system and the environment. We also found that, in some cases, the bipartite quantum correlation initially presented in the system is completely evaporated, it is not transferred to the environments.

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arXiv2009

Robustness of quantum discord to sudden death

We calculate the dissipative dynamics of two-qubit quantum discord under Markovian environments. We analyze various dissipative channels such as dephasing, depolarizing, and generalized amplitude damping, assuming independent perturbation, in which each qubit is coupled to its own channel. Choosing initial conditions that manifest the so-called sudden death of entanglement, we compare the dynamics of entanglement with that of quantum discord. We show that in all cases where entanglement suddenly disappears, quantum discord vanishes only in the asymptotic limit, behaving similarly to individual decoherence of the qubits, even at finite temperatures. Hence, quantum discord is more robust than the entanglement against to decoherence so that quantum algorithms based only on quantum discord correlations may be more robust than those based on entanglement.

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arXiv2008

Photon generation through decoherence in cavity QED: analytical analysis

The Rabi Hamiltonian describes the interaction between a two-level atom and a single mode of the quantized electromagnetic field. When the system is subject to the Markovian atomic dephasing reservoir, the anti-rotating term leads to the photon generation from vacuum. In the presence of Markovian damping reservoirs, the asymptotic mean photon number is higher than the thermal photon number expected in the absence of the anti-rotating term. We obtain approximate analytical expressions in the asymptotic regime for the photon creation rate in the pure dephasing case and the mean photon number and the atomic population inversion in the general case. Our analytical results are valid in the small mean photon number limit and they were tested by numerical analyses.

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