The emerging quantum technological apparatuses [1,2], such as the quantum computer [3-5], call for extreme performance in thermal engineering at the nanoscale [6]. Importantly, quantum mechanics sets a fundamental upper limit for the flow of information and heat, which is quantified by the quantum of thermal conductance [7,8]. The physics of this kind of quantum-limited heat conduction has been experimentally studied for lattice vibrations, or phonons [9], for electromagnetic interactions [10], and for electrons [11]. However, the short distance between the heat-exchanging bodies in the previous experiments hinders the applicability of these systems in quantum technology. Here, we present experimental observations of quantum-limited heat conduction over macroscopic distances extending to a metre. We achieved this striking improvement of four orders of magnitude in the distance by utilizing microwave photons travelling in superconducting transmission lines. Thus it seems that quantum-limited heat conduction has no fundamental restriction in its distance. This work lays the foundation for the integration of normal-metal components into superconducting transmission lines, and hence provides an important tool for circuit quantum electrodynamics [12-14], which is the basis of the emerging superconducting quantum computer [15]. In particular, our results demonstrate that cooling of nanoelectronic devices can be carried out remotely with the help of a far-away engineered heat sink. In addition, quantum-limited heat conduction plays an important role in the contemporary studies of thermodynamics such as fluctuation relations and Maxwell's demon [16,17]. Here, the long distance provided by our results may, for example, lead to an ultimate efficiency of mesoscopic heat engines with promising practical applications [18].
The emerging quantum technological apparatuses [1,2], such as the quantum computer [3-5], call for extreme performance in thermal engineering at the nanoscale [6]. Importantly, quantum mechanics sets a fundamental upper limit for the flow of information and heat, which is quantified by the quantum of thermal conductance [7,8]. The physics of this kind of quantum-limited heat conduction has been experimentally studied for lattice vibrations, or phonons [9], for electromagnetic interactions [10], and for electrons [11]. However, the short distance between the heat-exchanging bodies in the previous experiments hinders the applicability of these systems in quantum technology. Here, we present experimental observations of quantum-limited heat conduction over macroscopic distances extending to a metre. We achieved this striking improvement of four orders of magnitude in the distance by utilizing microwave photons travelling in superconducting transmission lines. Thus it seems that quantum-limited heat conduction has no fundamental restriction in its distance. This work lays the foundation for the integration of normal-metal components into superconducting transmission lines, and hence provides an important tool for circuit quantum electrodynamics [12-14], which is the basis of the emerging superconducting quantum computer [15]. In particular, our results demonstrate that cooling of nanoelectronic devices can be carried out remotely with the help of a far-away engineered heat sink. In addition, quantum-limited heat conduction plays an important role in the contemporary studies of thermodynamics such as fluctuation relations and Maxwell's demon [16,17]. Here, the long distance provided by our results may, for example, lead to an ultimate efficiency of mesoscopic heat engines with promising practical applications [18].
Superconducting quantum bits are one of the leading frontrunners in the race to build a solid-state quantum computer. In circuit quantum electrodynamics (cQED), electronic quantum circuits, playing the role of quantum bits, qubits, are placed into superconducting cavities with incredibly weak dissipation. The resulting system exhibits both photon and qubit characteristics, and it is possible to perform complex operations by driving the system with microwaves. Progress has been rapid, with vast improvements in cavities, qubits, and in controlling the interaction between the two. As a result, the first simple quantum algorithms to utilize this architecture have recently been implemented. \nAt the same time, there have been significant developments in improving our understanding of the heat flow in nanoelectronics. In particular the development of tunnel junctions has reached such a level, that it is now possible to control and measure the temperature of small metal islands with high precision. Such techniques have enabled the first observations of quantum-limited heat conduction through the photonic modes of a circuit. \nThis thesis introduces some of the benefits that come with the unification of these two fields. Theoretical models for experiments in a superconducting cavity are presented, with heat conduction due to individual cavity photons discussed in detail, along with a novel method to vary the lifetime of a qubit over many orders of magnitude. The realisation of these experiments would be the first steps on the road to integrating normal-metal components with cQED.
Circuit quantum electrodynamics offers versatile opportunities for various discoveries infundamental physics, as well as for different applications such as sensitive detectors and,ultimately, even a large-scale quantum computer. Since these devices work at cryogenictemperatures and excess heat is a substantial source of errors, it is of great importance to controlthe heat. In this thesis, several techniques for heat control are experimentally investigated. We study heattransfer carried by photons between two heat baths, as well as heat flow between superconductingresonators and their environment. In particular, the focus is on using normal-metal componentsto enhance the operation of superconducting microwave circuits operating at the level of singleenergy quanta. First, we investigate long-distance single-channel heat conduction near the quantum limit, i.e.,the quantum of thermal conductance. As compared to earlier experiments, the distance is increasedby a factor of 10,000. In addition, we demonstrate emission and absorption of photons in amicrowave resonator by photon-assisted tunneling in normal-metal–insulator–superconductorjunctions. Another approach for absorbing photons is to use a normal-metal resistor in asuperconducting resonator with a tunable resonance frequency. Finally, we realize a circuitconsisting of two resonators with a singularity in the parameter space, known as an exceptionalpoint. In our circuit, the exceptional point enables the fastest possible heat transfer between thetwo resonators without back and forth oscillation. In the future, these methods may find applications in different cryogenic devices, including a fastinitialization of qubits. Furthermore, the methods may be utilized in the experimental studies offundamental physics, including further investigations of exceptional points in differentconfigurations.
Circuit quantum electrodynamics offers versatile opportunities for various discoveries infundamental physics, as well as for different applications such as sensitive detectors and,ultimately, even a large-scale quantum computer. Since these devices work at cryogenictemperatures and excess heat is a substantial source of errors, it is of great importance to controlthe heat.
In this thesis, several techniques for heat control are experimentally investigated. We study heattransfer carried by photons between two heat baths, as well as heat flow between superconductingresonators and their environment. In particular, the focus is on using normal-metal componentsto enhance the operation of superconducting microwave circuits operating at the level of singleenergy quanta.
First, we investigate long-distance single-channel heat conduction near the quantum limit, i.e.,the quantum of thermal conductance. As compared to earlier experiments, the distance is increasedby a factor of 10,000. In addition, we demonstrate emission and absorption of photons in amicrowave resonator by photon-assisted tunneling in normal-metal–insulator–superconductorjunctions. Another approach for absorbing photons is to use a normal-metal resistor in asuperconducting resonator with a tunable resonance frequency. Finally, we realize a circuitconsisting of two resonators with a singularity in the parameter space, known as an exceptionalpoint. In our circuit, the exceptional point enables the fastest possible heat transfer between thetwo resonators without back and forth oscillation.
In the future, these methods may find applications in different cryogenic devices, including a fastinitialization of qubits. Furthermore, the methods may be utilized in the experimental studies offundamental physics, including further investigations of exceptional points in differentconfigurations.; Piirikvanttisahkodynamiikka tarjoaa monipuolisia mahdollisuuksia seka monenlaisilleperusfysiikkaan liittyville havainnoille etta erilaisille sovelluksille, kuten herkille ilmaisimille jalopulta jopa suuren mittakaavan kvanttitietokoneelle. Lammonsaately on erityisen tarkeaa, sillatallaiset laitteet toimivat erittain matalissa lampotiloissa ja ylimaarainen lampo aiheuttaamerkittavissa maarin toimintavirheita.
Vaitoskirjassa tutkitaan kokeellisesti useita menetelmia lammonsaatelyyn. Tutkimuskohteena onsateilyhiukkasten eli fotonien lammonkuljetus kahden lampokylvyn valilla seka suprajohtavienvarahtelijoiden lammonsiirto ymparistonsa kanssa. Erityisesti tutkitaan normaalimetallistavalmistettujen osien kayttoa suprajohtavissa mikroaaltopiireissa, jotka toimivat yksittaistenenergiakvanttien tasolla ja joiden toimintaa normaalimetalliosilla pyritaan parantamaan.
Aluksi vaitoskirjassa tutkitaan pitkilla etaisyyksilla yksittaista kanavaa pitkin tapahtuvaalammonsiirtoa kvanttimekaniikan maaraamalla rajalla, joka on ns. lammonjohtavuuskvantti.Aikaisempiin tutkimuksiin verrattuna etaisyytta on kasvatettu tekijalla 10 000. Lisaksivaitoskirjassa osoitetaan fotonien kaappaus eli…
Superconducting microwave circuits are a ubiquitous and important tool for devices that exploit the phenomena of superconductivity and cryogenic temperatures as an environment for achieving the generation, manipulation, and detection of quantum states - required for the ongoing development of quantum technologies. In particular, superconducting circuits are a promising platform for the universal quantum computer, which will require an unprecedented density of quantum-coherent elements to perform large-scale quantum-enhanced calculations and simulations, using the framework of cavity quantum lectrodynamics.Dissipation and heat in these superconducting circuits is a key source of error and inefficiency, however the thermodynamics in this regime is poorly understood despite its increasing relevancy.This thesis describes the integration of superconducting resonators and artificial atoms derived from superconducting quantum circuits with ultra-sensitive bolometry, for looking at heat transport through superconducting circuits. We will describe the physics and operation of each of these elements, before combining and utilising them to perform heat transport measurements through a superconducting artificial atom coupled to two resonators, each terminated by a normal-metal mesoscopic resistor, with the resistor temperature measured and temperature gradients across the circuit induced by superconducting tunnel-probes. We will present tunable heat transport through this system, firstly when the resonators are symmetric, allowing us to observe the role of dissipation-limited coupling of the resonators to the artificial atom on the locality of the heat transport, then on an asymmetric system, demonstrating a directional rectification of the heat transport. Additionally, we will discuss how each element of the system can be individually characterised, in particular the quality factor of superconducting resonators in the highly-dissipative limit, by exploiting the superconducting transition to perform a background reference. It is suggested that this hybrid quantum system, and these initial experiments provide a promising platform in the emergent field of circuit quantum thermodynamics. We believe that the techniques and tools developed during this thesis present key steps towards the understanding of the thermodynamics of quantum circuits, towards the realisation of devices that can explore heat transport in the quantum limit, such as a quantum heat engine.
Superconducting circuits provide a controllable and versatile platform for studying quantum phenomena and advancing quantum technologies. Josephson junctions and resonators are the central circuit elements, as they enable the realization of macroscopic quantum systems—such as qubits—that can be conveniently controlled, probed, and integrated with non-superconducting components. The implementation of a thermal reservoir through the integration of a dissipative element, such as a normal-metal resistor, offers an experimental platform for investigating quantum thermodynamics, heat management in circuits, on-chip thermal detectors, and the realization of quantum thermal machines. In this thesis, we integrate a mesoscopic normal metal and its associated thermometers as an on-chip bolometer within circuit quantum electrodynamics devices for thermal detection and quantum thermodynamics experiments. The on-chip bolometer is utilized as a sensitive and broadband thermal spectrometer to characterize the properties of a superconducting resonator and to investigate its dominant loss channels. We demonstrate the capability of our thermal detector to resolve the resonator modes over a wide frequency range extending above 20 GHz. When two thermal baths are connected through a qubit–resonator system, a quantum heat valve can be realized. This device has been demonstrated using both transmon and flux qubits. Photonic heat transport between the two baths is controlled by tuning the qubit energy via an applied static magnetic field. To model these experiments, we propose an approach based on electromagnetic simulation and microwave circuit theory, restricted to the linear-response regime. Despite this limitation, the method successfully captures the experimental observations and provides a practical tool for analysing and designing circuits for future experiments. A periodically driven qubit coupled to two unequal resonators, each terminated by a heat reservoir, enables the realization of a quantum Otto refrigerator. The use of a transmon qubit for this purpose is limited by its weak anharmonicity. We therefore explore alternative platforms for the experimental implementation of a quantum refrigerator based on charge and flux qubits, which exhibit strong anharmonicity. As an important step, we perform experiments on a driven flux qubit galvanically coupled to a resonator that is, in turn, weakly capacitively coupled to a heat reservoir. We observe interference patterns in the heat current arising from driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath appear at driving frequencies that are integer fractions of the resonator frequency. These quantum features are governed by the relative phase accumulated by the qubit states under driving.
As superconducting quantized electric circuits continue to emerge as an exceptionally versatile and powerful platform for quantum computing and other quantum technologies, they also provide a rich foundation for exploring quantum thermodynamics. The study of heat transport in these circuits offers invaluable synergy as understanding the impact of thermal fluctuations becomes increasingly important for their operation. Heat engines represent one of the most fundamental classes of thermal devices, whether regarding classical or quantum applications. Although heat transport in superconducting circuits has been extensively studied, quantum heat engines (QHE) operating at the limit of a few excitation quanta had not been experimentally realized prior to this work. These engines offer valuable insight into the quantum thermodynamics of superconducting circuits. Beyond fundamental research, the beseeching goal is to develop integrated QHEs which generate useful work in conjunction with other quantum technologies. In this context, autonomous QHEs, capable of operating without coherent external control, are particularly compelling. In this thesis, we explore thermodynamics and quantum heat engines, with a particular focus on engineering quantum heat engines using superconducting circuits as our experimental platform. The presented research includes both experimental and theoretical advancements, focusing on driven and autonomous QHEs, respectively. First, we demonstrate that engineered environments in superconducting circuits can serve as effective thermal reservoirs for externally driven quantum heat engines. Experimentally, we show that a quantum-circuit refrigerator (QCR) can be used not only for cooling but also for heating the coupled circuit. This discovery imminently leads to the experimental realization of a quantum heat engine, where a quantum Otto cycle is driven in a transmon qubit coupled to the QCR. Second, regarding autonomous quantum heat engines, we theoretically show that non-linear optomechanical coupling can eliminate the need for external driving while providing a method for directly observing the work output. These promising theoretical results suggest that the experimental realization of an autonomous QHE is ultimately a matter of careful circuit engineering. However, achieving this experimentally remains a tantalising challenge for future research, beyond the scope of the present work.