Thiago Werlang

The standard Caldeira-Leggett model addresses the problem of Brownian motion in a thermal equilibrium environment. Here, we look for generalizations of the Caldeira-Leggett model to account for thermal gradients in the environment. We devise two types of models, and discuss the advantages and limitations of each one. From both models, we find signatures of thermophoresis, i.e., particle transport due to a thermal gradient. In principle, our models can be employed to describe thermophoresis in quantum Brownian particles, an open problem so far.

Thermophoresis is the migration of a particle due to a thermal gradient. Here, we theoretically uncover a quantum version of thermophoresis, where both the bosonic nature of the thermal baths and quantum tunneling play significant roles. First, we find a thermophoretic force on a trapped quantum particle in a double-well potential. Then, we show that thermophoresis also takes place in an N-site model, for which we analytically derive a diffusion equation in the continuum limit (N→∞). We evidence additional quantum thermophoretic signatures in the transient particle-transport regime, which appear in the particle's average position and variance, as well as in the particle currents. Beyond establishing that single quantum particles can undergo thermally induced transport, our results suggest a route for quantum-state preparation with thermal gradients.

Classical dissipative adaptation is a hypothetical non-equilibrium thermodynamic principle of self-organization in driven matter, and it relates transition probabilities with the non-equilibrium work performed by an external drive on dissipative matter. Recently, the dissipative adaptation hypothesis was extended to a quantum regime with a theoretical model where only one single-photon pulse drives each atom of an ensemble. Here, we further generalize that quantum model by analytically showing that N cascaded single-photon pulses driving each atom still fulfill a quantum dissipative adaptation. Interestingly, we find that the level of self-organization achieved with two pulses can be matched with a single effective pulse only up to a threshold, above which the presence of more photons provides unparalleled degrees of self-organization.

A longstanding challenge in nonequilibrium thermodynamics is to predict the emergence of self-organized behaviors and functionalities typical of living matter. Despite the progress with classical complex systems, it remains far from obvious how to extrapolate these results down to the quantum scale. Here, we employ the paradigmatic master equation framework to establish that some lifelike behaviors and functionalities can indeed emerge in elementary dissipative quantum systems driven out of equilibrium. Specifically, we find both energy-avoiding (low steady dissipation) and energy-seeking behaviors (high steady dissipation), as well as self-adaptive shifts between these modes, in generic few-level systems. We also find emergent functionalities, namely, a self-organized thermal gradient in the system’s environment (in the energy-seeking mode) and an active equilibration against thermal gradients (in the energy-avoiding mode). Finally, we discuss the possibility that our results could be related to the concept of dissipative adaptation.

Frustration can contribute to very slow relaxation times in large open chains, as in spin glasses and in biopolymers. However, frustration may not be sufficient to produce broken ergodicity in finite systems. Here we employ a system-plus-reservoir approach to investigate how strongly inhomogeneous environments and frustration compete in the relaxation of finite open chains. We find a sufficient condition for our inhomogeneous environments to break ergodicity. We use the microscopic model to derive a Markovian quantum master equation for a generic chain with ultrastrong intrachain couplings. We show that this microscopic model avoids a spurious broken ergodicity we find in the phenomenological model. We work out an explicit example of broken ergodicity due to the inhomogeneous environment of an unfrustrated spin chain as far as simulating a recent experiment on protein denaturation (where environment inhomogeneity is especially relevant). We finally show that an inhomogeneous environment can mitigate the effects of frustration-induced degeneracies.

The quantum beamsplitter—a two-level system (TLS) coupled to a one-dimensional continuum of electromagnetic modes—is the most elementary version of a beamsplitter that may not only refract and reflect, but also absorb and then reemit a photon. This raises the question of whether a single-photon pulse, once split by a first quantum beamsplitter, could be made to interfere with a second one. Here, we propose and theoretically analyze a quantum Mach–Zehnder interferometer (QMZ) as formed by two concatenated quantum beamsplitters. The distinctive feature of our QMZ is its considerable saturability for a single photon, arising from the broadband nature of the pulse. We show that (i) off-resonant monochromatic photons produce classical interference patterns, whereas (ii) resonant broadband pulses erase these patterns, and that (iii) off-resonant broadband pulses always preserve some degree of interference if the two TLSs are oppositely detuned.

Proteins are aminoacid chains that diffusively fold or unfold depending on the thermal and chemical environmental conditions. While sophisticated models account for detailed aspects of real proteins, finding traits that unify protein dynamics to general open chains relaxation is still challenging. The principle of minimal frustration represents a key step towards this goal, revealing a fundamental link between proteins and spin glasses. Here, we search for the emergence of protein-like relaxation in open spin chains by going beyond the validity domain of the minimal frustration principle as to focus on the role of system-environment interactions rather than on frustration in the system's Hamiltonian. We find that strong asymmetries between the couplings of each spin to its immediate surroundings imply close similarities to protein folding and unfolding dynamics. Namely, an asymmetric bath can (i) block the system from finding its minimum energy state, even in the complete absence of energetic frustration in the system's Hamiltonian, and (ii) excite the system resembling the well-known distinction between thermal and chemical denaturations.

We show a quantum boost in the output power of a heat engine formed by a two-level system coherently coupled to a single-mode cavity. The key ingredient here is the use of nonstationary cavity-QED regimes (red-sideband transition or antidynamical Casimir effect), 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.

Energy transfer from a quantized field to a quantized dipole is investigated. We find that a single photon can transfer energy to a two-level dipole by inducing a dynamic Stark shift, going beyond the well-known absorption and emission processes. A quantum thermodynamical perspective allows us to unravel these two energy transfer mechanisms and to identify the former as a generalized work and the latter as a generalized heat. We show two necessary conditions for the generalized work transfer by a single photon to occur, namely, off-resonance and finite linewidth of the pulse. We also show that the generalized work performed by a single-photon pulse equals the reactive (dispersive) contribution of the work performed by a semiclassical pulse in the low-excitation regime.