### Browsing by Author "Menczel, Paul"

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Item Coherent Thermal Machines: Fluctuations and Performance(Aalto University, 2020) Menczel, Paul; Teknillisen fysiikan laitos; Department of Applied Physics; Quantum Transport Group; Perustieteiden korkeakoulu; School of Science; Flindt, Christian Prof., Aalto University, Department of Applied Physics, FinlandQuantum Engineering is a rapidly evolving discipline, promising the development of groundbreaking new technologies. To overcome the challenges posed by the nanoscopic scales and ultra-cold temperatures that are required for the implementation of quantum devices, understanding the thermodynamic interaction of these devices with their environment is important. Improving our knowledge of the laws of quantum thermodynamics enables engineers to exert better control over quantum circuits and it paves the way for more effective thermal management solutions on the nanoscale. In this dissertation, we focus on the study of thermal machines such as heat engines or refrigerators. These devices have recently been miniaturized in labs to the point where quantum effects can influence their performance. Due to the well-known advantages of quantum over conventional computers, it is natural to wonder whether coherence effects might be able to give a similar performance enhancement to thermal machines. Here, we approach this question on three different levels. First, we study the theory of periodically controlled open quantum systems in order to better understand the mathematical foundations of reciprocating thermal devices. Second, we investigate transport phenomena in open quantum systems, enabling us to discuss the fluctuations of thermodynamic currents and how they affect the performance of thermal machines. Third, and finally, we examine whether the performance of quantum heat engines is subject to universal bounds and how these bounds change in different operation regimes. The results of this thesis shed light on the optimal design of quantum devices, which will soon be ready for experimental implementation and investigation. Overall, we find that coherence leads to quantum friction, that is, to an increase in thermodynamic irreversibility with associated performance losses. In the adiabatic weak-coupling regime, these losses outweigh the benefits stemming from the increased number of degrees of freedom that are accessible in quantum setups. In other words, we find that coherence should generally be kept as small as possible in order to optimize the performance of nanoscale thermal machines in this regime. A promising direction for future investigations is to explore the effects of strong coupling and fast driving, where quantum advantages may be achievable.Item Cooper-Pair Box Coupled to Two Resonators: An Architecture for a Quantum Refrigerator(American Physical Society, 2022-06) Guthrie, Andrew; Satrya, Christoforus Dimas; Chang, Yu Cheng; Menczel, Paul; Nori, Franco; Pekola, Jukka P.; Department of Applied Physics; Quantum Phenomena and Devices; Centre of Excellence in Quantum Technology, QTF; RIKENSuperconducting circuits present a promising platform with which to realize a quantum refrigerator. Motivated by this, we fabricate and perform spectroscopy of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar-waveguide resonators with different frequencies. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at gigahertz frequencies. We go on to discuss how the measured device could be modified to operate as a cyclic quantum refrigerator by terminating the resonators with normal-metal resistors acting as heat baths.Item Optimal Control of a Quantum Refrigerator(2018-04-24) Pyhäranta, Tuomas; Menczel, Paul; Perustieteiden korkeakoulu; Flindt, ChristianItem Photon counting statistics of a microwave cavity(American Physical Society, 2019-02-14) Brange, Fredrik; Menczel, Paul; Flindt, Christian; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum TransportThe development of microwave photon detectors is paving the way for a wide range of quantum technologies and fundamental discoveries involving single photons. Here, we investigate the photon emission from a microwave cavity and find that distribution of photon waiting times contains information about few-photon processes, which cannot easily be extracted from standard correlation measurements. The factorial cumulants of the photon counting statistics are positive at all times, which may be intimately linked with the bosonic quantum nature of the photons. We obtain a simple expression for the rare fluctuations of the photon current, which is helpful in understanding earlier results on heat-transport statistics and measurements of work distributions. Under nonequilibrium conditions, where a small temperature gradient drives a heat current through the cavity, we formulate a fluctuation-dissipation relation for the heat noise spectra. Our work suggests a number of experiments for the near future and it offers theoretical questions for further investigation.Item Quantum jump approach to microscopic heat engines(American Physical Society, 2020-09-21) Menczel, Paul; Flindt, Christian; Brandner, Kay; Centre of Excellence in Quantum Technology, QTF; University of Nottingham; Department of Applied PhysicsModern technologies could soon make it possible to investigate the operation cycles of quantum heat engines by counting the photons that are emitted and absorbed by their working systems. Using the quantum jump approach to open-system dynamics, we show that such experiments would give access to a set of observables that determine the trade-off between power and efficiency in finite-time engine cycles. By analyzing the single-jump statistics of thermodynamic fluxes such as heat and entropy production, we obtain a family of general bounds on the power of microscopic heat engines. Our new bounds unify two earlier results and admit a transparent physical interpretation in terms of single-photon measurements. In addition, these bounds confirm that driving-induced coherence leads to an increase in dissipation that suppresses the efficiency of slowly driven quantum engines in the weak-coupling regime. A nanoscale heat engine based on a superconducting qubit serves as an experimentally relevant example and a guiding paradigm for the development of our theory.Item Thermodynamics of cyclic quantum amplifiers(American Physical Society, 2020-05-11) Menczel, Paul; Flindt, Christian; Brandner, Kay; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe develop a generic model for a cyclic quantum heat engine that makes it possible to coherently amplify a periodically modulated input signal without the need to couple the working medium to multiple reservoirs at the same time. Instead, we suggest an operation principle that is based on the spontaneous creation of population inversion in incomplete relaxation processes induced by periodic temperature variations. Focusing on Lindblad dynamics and systems with equally spaced energy levels, e.g., qubits or quantum harmonic oscillators, we derive a general working criterion for such cyclic quantum amplifiers. This criterion defines a class of candidates for suitable working media and applies to arbitrary control protocols. For the minimal case of a cyclic three-level amplifier, we show that our criterion is tight and explore the conditions for optimal performance.Item Two-stroke optimization scheme for mesoscopic refrigerators(American Physical Society, 2019-06-21) Menczel, Paul; Pyhäranta, Tuomas; Flindt, Christian; Brandner, Kay; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum TransportRefrigerators use a thermodynamic cycle to move thermal energy from a cold reservoir to a hot one. Implementing this operation principle with mesoscopic components has recently emerged as a promising strategy to control heat currents in micro and nanosystems for quantum technological applications. Here we combine concepts from stochastic and quantum thermodynamics with advanced methods of optimal control theory to develop a universal optimization scheme for such small-scale refrigerators. Covering both the classical and the quantum regime, our theoretical framework provides a rigorous procedure to determine the periodic driving protocols that maximize either cooling power or efficiency. As a main technical tool, we decompose the cooling cycle into two strokes, which can be optimized one by one. In the regimes of slow or fast driving, we show how this procedure can be simplified significantly by invoking suitable approximations. To demonstrate the practical viability of our scheme, we determine the exact optimal driving protocols for a quantum microcooler, which can be realized experimentally with current technology. Our work provides a powerful tool to develop optimal design strategies for engineered cooling devices and it creates a versatile framework for theoretical investigations exploring the fundamental performance limits of mesoscopic thermal machines.Item Universal First-Passage-Time Distribution of Non-Gaussian Currents(American Physical Society, 2019-06-13) Singh, Shilpi; Menczel, Paul; Golubev, Dmitry S.; Khaymovich, Ivan M.; Peltonen, Joonas T.; Flindt, Christian; Saito, Keiji; Roldán, Édgar; Pekola, Jukka P.; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Phenomena and Devices; Quantum Transport; Keio University; Abdus Salam International Centre for Theoretical Physics; Max Planck InstituteWe investigate the fluctuations of the time elapsed until the electric charge transferred through a conductor reaches a given threshold value. For this purpose, we measure the distribution of the first-passage times for the net number of electrons transferred between two metallic islands in the Coulomb blockade regime. Our experimental results are in excellent agreement with numerical calculations based on a recent theory describing the exact first-passage-time distributions for any nonequilibrium stationary Markov process. We also derive a simple analytical approximation for the first-passage-time distribution, which takes into account the non-Gaussian statistics of the electron transport, and show that it describes the experimental distributions with high accuracy. This universal approximation describes a wide class of stochastic processes, and can be used beyond the context of mesoscopic charge transport. In addition, we verify experimentally a fluctuation relation between the first-passage-time distributions for positive and negative thresholds.