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…
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…
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.
Superconducting microwave circuits are promising candidates for a variety of different applications, ranging from extremely sensitive photon detectors to large-scale quantum computer. Thus, they may revolutionize our everyday lives in the future through substantial technological development. Superconducting quantum circuits typically operate at low power levels and consist of linear and nonlinear resonators. In order to enhance their operation fidelity, they need to be cooled efficiently, and the dissipation of the circuit needs to be well engineered. In this chapter, we describe a refrigerator for electric quantum circuits based on photon-assisted electron tunneling in hybrid superconductor–insulator–normal-metal junctions. Furthermore, such junctions can also serve as incoherent microwave sources. Importantly, the photon absorption and emission rates can be tuned over several orders of magnitude. In addition, these junctions are used for refrigeration and heating of the normal-metal electrons as well as for probing the electron temperature. Here, we present their operation principle and provide insight to their potential applications as well as briefly describe fabrication methods.
Superconducting circuits are one of the most promising and rapidly developing platforms for implementing quantum bits and their control operations for the purpose of large-scale quantum computing. A quantum algorithm is run by executing a series of precise operations on the quantum bits, or qubits. The most important of these operations are the quantum gates along with the measurement and initialization of the states of the qubits and auxiliary components.
In this dissertation, we develop techniques to enhance each of these three operations. The work employs systems of coupled qubits and resonators in the framework of circuit quantum electrodynamics.
Firstly, we derive a theoretical lower bound for the error of a single-qubit gate implemented with a linear oscillator mode and show how to reach this bound. Secondly, we experimentally demonstrate a technique for measuring the state of a qubit by simultaneously driving the qubit and the accompanying readout resonator. Compared to our method, the standard technique involving only the resonator drive yields up to 100% larger error. Moreover, we experimentally and theoretically develop quantum devices and protocols that may be used to initialize or reset auxiliary quantum circuits such as resonators. By coupling a quantum circuit to a strongly dissipative environment through a coupler, the decay rate of the circuit may be tuned on demand by several orders of magnitude. Finally, we propose a general method to analytically solve the dynamics of a complex, open bosonic system that may be used to describe, for example, the experimental systems of this dissertation.
The experimental techniques developed in this dissertation may potentially be utilized in future implementations of a quantum computer, and the proposed theoretical protocols pave the way for future experiments aiming to improve qubit control and heat management in large-scale quantum processors.; Suprajohtavat kvanttipiirit ovat yksi lupaavimmista ja nopeimmin kehittyvista alustoista, joilla voi mahdollisesti toteuttaa suuren kokoluokan kvanttilaskentaa. Kvanttilaskennassa keskeisessa asemassa ovat kvanttibitit eli kubitit seka niilla tehtavat tarkat operaatiot, joista kvanttialgoritmit muodostuvat. Operaatioista tarkeimmat ovat loogiset kvanttiportit seka kubittien ja muiden tarvittavien komponenttien tilojen mittaus ja alustus.Tassa vaitoskirjassa kehitetaan tapoja naiden kolmen operaation parantamiseksi. Tyossa kasitellaan kvanttielektrodynamiikkaan perustuvia, kubittien ja resonaattorien muodostamia kytkettyja systeemeja.
Ensimmaiseksi johdamme teoreettisen alarajan lineaarisella varahtelymoodilla toteutetulle yhden kubitin kvanttiportin virheelle seka naytamme, miten alaraja on mahdollista saavuttaa. Toiseksi toteutamme kokeellisesti menetelman, jolla voi mitata kubitin tilan ajamalla kubittia ja siihen kytkettya lukuresonaattoria samanaikaisesti. Yleisesti kaytetty mittaustapa, jossa ajetaan ainoastaan resonaattoria, tuottaa jopa 100% suuremman virheen kuin toteuttamamme menetelma.…
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.
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.
The development of single photon detectors (SPD's) in the microwave regime is a fundamental challenge for obtaining a better understanding of the phenomena and components employed in quantum information technologies. Particularly, the field of quantum thermodynamics has until now been mainly theory oriented due to the lack of suitable detectors. In this thesis I present the first steps in developing a high fidelity SPD suitable for detecting itinerant microwave photons in a transmission line.
The device is of mesoscopic size and directly integrable into superconducting quantum circuits. Out of the two main approaches in microwave SPD's, atom-like systems and thermal detectors, we pursue the latter. In a thermal detector, the energy of the photon is transferred into the absorber of the detector, causing an abrupt increase in its temperature. This leads to the destruction of the photon, which can be avoided with an atom-like detector. However, an advantage of the thermal approach is that a broadband microwave detector, operated in a continuous manner, can be realized. This is essential for several experiments for example in quantum thermodynamics.
The absorber of our micro calorimeter is a normal metal nanowire. The electron gas in the wire is used for the photon absorption, whereas the phonons work as a heat sink. The absorber is connected to a superconducting electrode via a thin tunnel barrier and grounded through a direct normal metal-superconductor contact, which acts as a heat mirror. The temperature of the electron gas is probed via the differential conductance of a tunnel-junction. For realising a sensitive SPD, it is essential to minimize the heat capacity of the absorber. Thermal properties of small metal structures can deviate significantly from the properties of bulk materials due to the large surface-to-volume ratio. We have measured thermal relaxation in Cu and Ag thin-film nanowires at sub-kelvin temperatures and observed an anomalously long relaxation time in the Cu wires. In addition to a large specific heat, these results may also originate from the slow thermalisation in the granular structure observed in the evaporated Cu wires.; Saavuttaaksemme paremman ymmarryksen kvantti-informaatioteknologiassa hyodynnettavista ilmioista ja komponenteista, on yksittaisten fotonien resoluutiolla toimivien mikroaaltofotoni-ilmaisinten kehittaminen olennainen haaste. Sopivien havaintolaitteiden puutteen vuoksi on erityisesti kvanttitermodynamiikka pysynyt vahvasti teoriaan keskittyneena alana.
Tassa vaitoskirjassa esitetaan ensimmaiset askeleet herkan, yksittaisia siirtolinjassa kiertavia mikroaaltofotoneja havaitsevan anturin kehittamisessa. Laite on kooltaan mesoskooppinen, ja se voidaan kytkea suprajohtaviin piireihin. Termiset detektorit - joita kehitamme tassa tyossa - ja atomin kaltaiset systeemit ovat yleisimmin kaytettyja, toisiaan taydentavia ilmaisimia. Termisessa anturissa havaitun fotonin energia siirtyy laitteen absorbaattoriin, aiheuttaen siina nopean, havaittavan suuruisen lampotilan nousun.…