Coherence and measurement in quantum thermodynamics

Thermodynamics is a highly successful macroscopic theory widely used across the natural sciences and for the construction of everyday devices, from car engines and fridges to power plants and solar cells. With thermodynamics predating quantum theory, research now aims to uncover the thermodynamic laws that govern finite size systems which may in addition host quantum effects. Here we identify information processing tasks, the so-called "projections", that can only be formulated within the framework of quantum mechanics. We show that the physical realisation of such projections can come with a non-trivial thermodynamic work only for quantum states with coherences. This contrasts with information erasure, first investigated by Landauer, for which a thermodynamic work cost applies for classical and quantum erasure alike. Implications are far-reaching, adding a thermodynamic dimension to measurements performed in quantum thermodynamics experiments, and providing key input for the construction of a future quantum thermodynamic framework. Repercussions are discussed for quantum work fluctuation relations and thermodynamic single-shot approaches.Comment: 6 pages + appendix, 4 figures, v2: changed presentation, critically discuss interpretation as measurement, added new conclusions; previous title: "Quantum measurement and its role in thermodynamics

Semiconductor quantum tubes: dielectric modulation and excitonic response

We study theoretically the optical properties of quantum tubes, one-dimensional semiconductor nanostructures where electrons and holes are confined to a cylindrical shell. In these structures, which bridge between 2D and 1D systems, the electron-hole interaction may be modulated by a dielectric substance outside the quantum tube and possibly inside its core. We use the exact Green's function for the appropriate dielectric configuration and exact diagonalization of the electron-hole interaction within an effective mass description to predict the evolution of the exciton binding energy and oscillator strength. Contrary to the homogeneous case, in dielectrically modulated tubes the exciton binding is a function of the tube diameter and can be tuned to a large extent by structure design and proper choice of the dielectric media.Comment: 9 pages, 6 figures, in print for Phys. Rev.

Optimization of the ionization time of an atom with tailored laser pulses: a theoretical study

How fast can a laser pulse ionize an atom? We address this question by considering pulses that carry a fixed time-integrated energy per-area, and finding those that achieve the double requirement of maximizing the ionization that they induce, while having the shortest duration. We formulate this double-objective quantum optimal control problem by making use of the Pareto approach to multi-objetive optimization, and the differential evolution genetic algorithm. The goal is to find out how much a precise time-profiling of ultra-fast, large-bandwidth pulses may speed up the ionization process with respect to simple-shape pulses. We work on a simple one-dimensional model of hydrogen-like atoms (the P\"oschl-Teller potential), that allows to tune the number of bound states that play a role in the ionization dynamics. We show how the detailed shape of the pulse accelerates the ionization process, and how the presence or absence of bound states influences the velocity of the process

Imperfect Thermalizations Allow for Optimal Thermodynamic Processes

Optimal (reversible) processes in thermodynamics can be modelled as step-by-step processes, where the system is successively thermalized with respect to different Hamiltonians by an external thermal bath. However, in practice interactions between system and thermal bath will take finite time, and precise control of their interaction is usually out of reach. Motivated by this observation, we consider finite-time and uncontrolled operations between system and bath, which result in thermalizations that are only partial in each step. We show that optimal processes can still be achieved for any non-trivial partial thermalizations at the price of increasing the number of operations, and characterise the corresponding tradeoff. We focus on work extraction protocols and show our results in two different frameworks: A collision model and a model where the Hamiltonian of the working system is controlled over time and the system can be brought into contact with a heat bath. Our results show that optimal processes are robust to noise and imperfections in small quantum systems, and can be achieved by a large set of interactions between system and bath.Comment: 12 pages + appendix; extended results; accepted in Quantu

Speeding up the solution of the Bethe-Salpeter equation by a double-grid method and Wannier interpolation

The Bethe-Salpeter equation is a widely used approach to describe optical excitations in bulk semiconductors. It leads to spectra that are in very good agreement with experiment, but the price to pay for such accuracy is a very high computational burden. One of the main bottlenecks is the large number of k-points required to obtain converged spectra. In order to circumvent this problem we propose a strategy to solve the Bethe-Salpeter equation based on a double-grid technique coupled to a Wannier interpolation of the Kohn-Sham band structure. This strategy is then benchmarked for a particularly difficult case, the calculation of the absorption spectrum of GaAs, and for the well studied case of Si. The considerable gains observed in these cases fully validate our approach, and open the way for the application of the Bethe-Salpeter equation to large and complex systems.Comment: 5 pages, 3 figures. Accepted for Phys. Rev.

An attention-based view of family firm adaptation to discontinuous technologies:Exploring the role of family CEOs’ non-economic goals

Recent studies show that managerial attention is a particularly important precursor of established firms’ responses to discontinuous technological change. However, little is known about the factors that shape managerial attention-response patterns. In our qualitative study, we investigate how the attention of family firm CEOs to discontinuous technological shifts, the interpretation and decision-making processes associated with these changes, and ultimately organizations’ responses are affected by CEOs’ non-economic goals. Based on seven longitudinal case studies in the German consumer goods industry, we induce a process model that extends the findings of the literature on the attention-based view and helps to explain heterogeneity in family firms’ adaptation to discontinuous technological change. We show that the family CEO’s specific non-economic goals—such as power and control, transgenerational value, the maintenance of family reputation, the continuance of personal ties, or personal affect associated with the family business—determine whether the CEO assesses an emerging technology as relevant enough to warrant a reaction from the firm. Moreover, the family CEO’s non-economic goals constrain the set of considered responses. The outcome of this sensemaking process determines the organization’s response. For instance, in the specific context of this study, the goal of “family power and control” entailed an immediate interpretation of the focal trend as important for maintaining influence, and resulted in an unconstrained set of responses and, ultimately, high innovation in the new domain. Over time, family CEOs might re-evaluate the emerging trend based on their goals and adapt organizational moves accordingly. We identify and discuss how ambiguities and dilemmas may arise during this process. Our findings contribute to the literature on adaptation to discontinuous technological change and to family firm research

Why do incumbents respond heterogeneously to disruptive innovations? The interplay of domain identity and role identity.

We adopt a multifaceted view of organizational identity to contribute to research on organizational identity and incumbent adaptations to disruptive innovations. Based on a qualitative, multi-case study on the responses of German publishing houses to the emergence of digitalization, we distill a novel and thus far disregarded facet of organizational identity: organizational role identity. We show how organizational role identity and organizational domain identity—the facet that has so far dominated research on identity and innovation—interactively determine how organizations interpret and respond to a disruptive innovation. In contrast to previous studies, we show that incumbents experience dysfunctional identity-driven struggles when one of the two identity facets is challenged by the disruptive innovation while the other is enhanced. We also induce that domain and role identities can jointly determine how quickly incumbents react to a disruption, whether they adopt that disruption, and the innovativeness of their responses

Biexciton stability in carbon nanotubes

We have applied the quantum Monte Carlo method and tight-binding modelling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.Comment: revtex4, final, twocolumn. to be published in Phys.Rev.Let. 5 pages 3 figure