Collective effects on the performance and stability of quantum heat engines.

Recent predictions for quantum-mechanical enhancements in the operation of small heat engines have raised renewed interest in their study both from a fundamental perspective and in view of applications. One essential question is whether collective effects may help to carry enhancements over larger scales, when increasing the number of systems composing the working substance of the engine. Such enhancements may consider not only power and efficiency, that is, its performance, but, additionally, its constancy, that is, the stability of the engine with respect to unavoidable environmental fluctuations. We explore this issue by introducing a many-body quantum heat engine model composed by spin pairs working in continuous operation. We study how power, efficiency, and constancy scale with the number of spins composing the engine and introduce a well-defined macroscopic limit where analytical expressions are obtained. Our results predict power enhancements, in both finite-size and macroscopic cases, for a broad range of system parameters and temperatures, without compromising the engine efficiency, accompanied by coherence-enhanced constancy for finite sizes. We discuss these quantities in connection to thermodynamic uncertainty relations.

Survival and extreme statistics of work, heat, and entropy production in steady-state heat engines.

We derive universal bounds for the finite-time survival probability of the stochastic work extracted in steady-state heat engines and the stochastic heat dissipated to the environment. We also find estimates for the time-dependent thresholds that these quantities do not surpass with a prescribed probability. At long times, the tightest thresholds are proportional to the large deviation functions of stochastic entropy production. Our results entail an extension of martingale theory for entropy production, for which we derive universal inequalities involving its maximum and minimum statistics that are valid for generic Markovian dynamics in nonequilibrium stationary states. We test our main results with numerical simulations of a stochastic photoelectric device.

SARS-CoV-2 Gastrointestinal Shedding in Hospitalized Children.

BACKGROUND: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a respiratory virus that can cause gastrointestinal (GI) symptoms, with studies demonstrating detection of stool viral RNA weeks after respiratory tract clearance. It is unknown if children who test negative for SARS-CoV-2 on a nasopharyngeal (NP) swab may be shedding the virus in their stool. OBJECTIVE: To measure the prevalence of SARS-CoV-2 stool shedding in children with positive and negative SARS-CoV-2 NP polymerase chain reactions (PCR) tests, and to determine clinical factors associated with GI shedding. METHODS: In this cross-sectional study, we enrolled hospitalized patients 0 to 21 years old with a positive or a negative SARS-CoV-2 NP PCR test who had respiratory and/or GI symptoms. Participants were surveyed, and stool samples were sent for viral PCR testing. Fisher's exact test was used to evaluate bivariate associations of stool PCR test positivity with categorical variables. RESULTS: Sixty-seven patients were consented; 34 patients did not provide stool samples so 33 patients were included: 17 NP-positive and 16 NP-negative for SARS-CoV-2. Eight of the 17 NP-positive patients had a positive stool PCR test for SARS-CoV-2, while none of the 16 SARS-CoV-2 NP-negative patients had a positive result (P < .01). For the 17 SARS-CoV-2 NP-positive patients, GI symptoms were associated with a positive stool PCR test (P = .05) for SARS-CoV-2, but this association was not found for all 33 patients (P = .11). No associations were found with patients in an immunocompromised state or those with a comorbid condition, fever and/or chills, respiratory symptoms, headache and/or myalgias, or anosmia and/or ageusia. CONCLUSIONS: SARS-CoV-2 GI shedding is common and associated with GI symptoms in NP-positive children, with 47% having positive stool PCRs for SARS-CoV-2. GI shedding was not demonstrated in SARS-CoV-2 NP-negative children.

Thermodynamics of Gambling Demons.

We introduce and realize demons that follow a customary gambling strategy to stop a nonequilibrium process at stochastic times. We derive second-law-like inequalities for the average work done in the presence of gambling, and universal stopping-time fluctuation relations for classical and quantum nonstationary stochastic processes. We test experimentally our results in a single-electron box, where an electrostatic potential drives the dynamics of individual electrons tunneling into a metallic island. We also discuss the role of coherence in gambling demons measuring quantum jump trajectories.

Quantum Martingale Theory and Entropy Production.

We employ martingale theory to describe fluctuations of entropy production for open quantum systems in nonequilbrium steady states. Using the formalism of quantum jump trajectories, we identify a decomposition of entropy production into an exponential martingale and a purely quantum term, both obeying integral fluctuation theorems. An important consequence of this approach is the derivation of a set of genuine universal results for stopping-time and infimum statistics of stochastic entropy production. Finally, we complement the general formalism with numerical simulations of a qubit system.

Autonomous thermal machine for amplification and control of energetic coherence.

We present a model for an autonomous quantum thermal machine composed of two qubits capable of manipulating and even amplifying the local coherence in a nondegenerate external system. The machine uses only thermal resources, namely, contact with two heat baths at different temperatures, and the external system has a nonzero initial amount of coherence. The method we propose allows for an interconversion between energy, both work and heat, and coherence in an autonomous configuration working in out-of-equilibrium conditions. This model raises interesting questions about the role of fundamental limitations on transformations involving coherence and opens up new possibilities in the manipulation of coherence by autonomous thermal machines.

Optimal Work Extraction and Thermodynamics of Quantum Measurements and Correlations.

We analyze the role of indirect quantum measurements in work extraction from quantum systems in nonequilibrium states. In particular, we focus on the work that can be obtained by exploiting the correlations shared between the system of interest and an additional ancilla, where measurement backaction introduces a nontrivial thermodynamic tradeoff. We present optimal state-dependent protocols for extracting work from both classical and quantum correlations, the latter being measured by discord. Our quantitative analysis establishes that, while the work content of classical correlations can be fully extracted by performing local operations on the system of interest, accessing work related to quantum discord requires a specific driving protocol that includes interaction between system and ancilla.

Performance of autonomous quantum thermal machines: Hilbert space dimension as a thermodynamical resource.

Multilevel autonomous quantum thermal machines are discussed. In particular, we explore the relationship between the size of the machine (captured by Hilbert space dimension) and the performance of the machine. Using the concepts of virtual qubits and virtual temperatures, we show that higher dimensional machines can outperform smaller ones. For instance, by considering refrigerators with more levels, lower temperatures can be achieved, as well as higher power. We discuss the optimal design for refrigerators of a given dimension. As a consequence we obtain a statement of the third law in terms of Hilbert space dimension: Reaching absolute zero temperature requires infinite dimension. These results demonstrate that Hilbert space dimension should be considered a thermodynamic resource.

Entropy production and thermodynamic power of the squeezed thermal reservoir.

We analyze the entropy production and the maximal extractable work from a squeezed thermal reservoir. The nonequilibrium quantum nature of the reservoir induces an entropy transfer with a coherent contribution while modifying its thermal part, allowing work extraction from a single reservoir, as well as great improvements in power and efficiency for quantum heat engines. Introducing a modified quantum Otto cycle, our approach fully characterizes operational regimes forbidden in the standard case, such as refrigeration and work extraction at the same time, accompanied by efficiencies equal to unity.

Nonequilibrium potential and fluctuation theorems for quantum maps.

We derive a general fluctuation theorem for quantum maps. The theorem applies to a broad class of quantum dynamics, such as unitary evolution, decoherence, thermalization, and other types of evolution for quantum open systems. The theorem reproduces well-known fluctuation theorems in a single and simplified framework and extends the Hatano-Sasa theorem to quantum nonequilibrium processes. Moreover, it helps to elucidate the physical nature of the environment that induces a given dynamics in an open quantum system.

Synchronization, quantum correlations and entanglement in oscillator networks.

Synchronization is one of the paradigmatic phenomena in the study of complex systems. It has been explored theoretically and experimentally mostly to understand natural phenomena, but also in view of technological applications. Although several mechanisms and conditions for synchronous behavior in spatially extended systems and networks have been identified, the emergence of this phenomenon has been largely unexplored in quantum systems until very recently. Here we discuss synchronization in quantum networks of different harmonic oscillators relaxing towards a stationary state, being essential the form of dissipation. By local tuning of one of the oscillators, we establish the conditions for synchronous dynamics, in the whole network or in a motif. Beyond the classical regime we show that synchronization between (even unlinked) nodes witnesses the presence of quantum correlations and entanglement. Furthermore, synchronization and entanglement can be induced between two different oscillators if properly linked to a random network.