Archive ouverte UNIGE | last documents for author 'Ralph Silva'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Ralph Silva'engHybrid Thermal Machines: Generalized Thermodynamic Resources for Multitaskinghttps://archive-ouverte.unige.ch/unige:150405https://archive-ouverte.unige.ch/unige:150405Thermal machines perform useful tasks--such as producing work, cooling, or heating--by exchanging energy, and possibly additional conserved quantities such as particles, with reservoirs. Here we consider thermal machines that perform more than one useful task simultaneously, terming these "hybrid thermal machines". We outline their restrictions imposed by the laws of thermodynamics and we quantify their performance in terms of efficiencies. To illustrate their full potential, reservoirs that feature multiple conserved quantities, described by generalized Gibbs ensembles, are considered. A minimal model for a hybrid thermal machine is introduced, featuring three reservoirs and two conserved quantities, e.g., energy and particle number. This model can be readily implemented in a thermoelectric setup based on quantum dots, and hybrid regimes are accessible considering realistic parameters.Mon, 15 Mar 2021 11:20:33 +0100Unbounded sequence of observers exhibiting Einstein-Podolsky-Rosen steeringhttps://archive-ouverte.unige.ch/unige:127284https://archive-ouverte.unige.ch/unige:127284A sequential steering scenario is investigated, where multiple Bobs aim at demonstrating steering using successively the same half of an entangled quantum state. With isotropic entangled states of local dimension d, the number of Bobs that can steer Alice is found to be NBob∼d/logd, thus leading to an arbitrary large number of successive instances of steering with independently chosen and unbiased inputs. This scaling is achieved when considering a general class of measurements along orthonormal bases, as well as complete sets of mutually unbiased bases. Finally, we show that similar results can be obtained in an anonymous sequential scenario, where none of the Bobs know their position in the sequence.Tue, 03 Dec 2019 14:42:05 +0100Unifying Paradigms of Quantum Refrigeration: A Universal and Attainable Bound on Coolinghttps://archive-ouverte.unige.ch/unige:127281https://archive-ouverte.unige.ch/unige:127281Cooling quantum systems is arguably one of the most important thermodynamic tasks connected to modern quantum technologies and an interesting question from a foundational perspective. It is thus of no surprise that many different theoretical cooling schemes have been proposed, differing in the assumed control paradigm and complexity, and operating either in a single cycle or in steady state limits. Working out bounds on quantum cooling has since been a highly context dependent task with multiple answers, with no general result that holds independent of assumptions. In this letter we derive a universal bound for cooling quantum systems in the limit of infinite cycles (or steady state regimes) that is valid for any control paradigm and machine size. The bound only depends on a single parameter of the refrigerator and is theoretically attainable in all control paradigms. For qubit targets we prove that this bound is achievable in a single cycle and by autonomous machines.Tue, 03 Dec 2019 14:35:05 +0100Anomalous Weak Values Without Post-Selectionhttps://archive-ouverte.unige.ch/unige:127277https://archive-ouverte.unige.ch/unige:127277A weak measurement performed on a pre- and post-selected quantum system can result in an average value that lies outside of the observable's spectrum. This effect, usually referred to as an "anomalous weak value", is generally believed to be possible only when a non-trivial post-selection is performed, i.e., when only a particular subset of the data is considered. Here we show, however, that this is not the case in general: in scenarios in which several weak measurements are sequentially performed, an anomalous weak value can be obtained without post-selection, i.e., without discarding any data. We discuss several questions that this raises about the subtle relation between weak values and pointer positions for sequential weak measurements. Finally, we consider some implications of our results for the problem of distinguishing different causal structures.Tue, 03 Dec 2019 14:29:21 +0100Unifying paradigms of quantum refrigeration: Fundamental limits of cooling and associated work costshttps://archive-ouverte.unige.ch/unige:127271https://archive-ouverte.unige.ch/unige:127271In classical thermodynamics the work cost of control can typically be neglected. On the contrary, in quantum thermodynamics the cost of control constitutes a fundamental contribution to the total work cost. Here, focusing on quantum refrigeration, we investigate how the level of control determines the fundamental limits to cooling and how much work is expended in the corresponding process. jona{We compare two extremal levels of control. First coherent operations, where the entropy of the resource is left unchanged, and second incoherent operations, where only energy at maximum entropy (i.e. heat) is extracted from the resource. For minimal machines, we find that the lowest achievable temperature and associated work cost depend strongly on the type of control, in both single-cycle and asymptotic regimes. We also extend our analysis to general machines.} Our work provides a unified picture of the different approaches to quantum refrigeration developed in the literature, including algorithmic cooling, autonomous quantum refrigerators, and the resource theory of quantum thermodynamics.Tue, 03 Dec 2019 14:02:54 +0100Performance of autonomous quantum thermal machines: Hilbert space dimension as a thermodynamical resourcehttps://archive-ouverte.unige.ch/unige:90699https://archive-ouverte.unige.ch/unige:90699Multilevel 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.Wed, 21 Dec 2016 15:13:46 +0100A small quantum absorption refrigerator with reversed couplingshttps://archive-ouverte.unige.ch/unige:84455https://archive-ouverte.unige.ch/unige:84455Small quantum absorption refrigerators have recently attracted renewed attention. Here we present a missing design of a two-qubit fridge, the main feature of which is that one of the two machine qubits is itself maintained at a temperature colder than the cold bath. This is achieved by 'reversing' the couplings to the baths compared to previous designs, where only a transition is maintained cold. We characterize the working regime and the efficiency of the fridge. We demonstrate the soundness of the model by deriving and solving a master equation. Finally, we discuss the performance of the fridge, in particular the heat current extracted from the cold bath. We show that our model performs comparably to the standard three-level quantum fridge, and thus appears appealing for possible implementations of nano thermal machines.Mon, 13 Jun 2016 11:01:59 +0200Pre- and postselected quantum states: Density matrices, tomography, and Kraus operatorshttps://archive-ouverte.unige.ch/unige:36411https://archive-ouverte.unige.ch/unige:36411We present a general formalism for characterizing 2-time quantum states, describing pre- and postselected quantum systems. The most general 2-time state is characterized by a “density vector” that is independent of measurements performed between the preparation and postselection. We provide a method for performing tomography of an unknown 2-time density vector. This procedure, which cannot be implemented by weak or projective measurements, brings new insight to the fundamental role played by Kraus operators in quantum measurements. Finally, after showing that general states and measurements are isomorphic, we show that any measurement on a 2-time state can be mapped to a measurement on a preselected bipartite state.Tue, 06 May 2014 16:02:07 +0200Entanglement enhances cooling in microscopic quantum refrigeratorshttps://archive-ouverte.unige.ch/unige:36408https://archive-ouverte.unige.ch/unige:36408Small self-contained quantum thermal machines function without external source of work or control but using only incoherent interactions with thermal baths. Here we investigate the role of entanglement in a small self-contained quantum refrigerator. We first show that entanglement is detrimental as far as efficiency is concerned—fridges operating at efficiencies close to the Carnot limit do not feature any entanglement. Moving away from the Carnot regime, we show that entanglement can enhance cooling and energy transport. Hence, a truly quantum refrigerator can outperform a classical one. Furthermore, the amount of entanglement alone quantifies the enhancement in cooling.Tue, 06 May 2014 16:00:58 +0200