Advancements in the field of cavity quantum electrodynamics (c-QED) have deepened understanding of quantum phenomena, enabled reliable Josephson junction-based devices, and facilitated their integration with other quantum technologies to explore new physics and applications. Motivated by these developments, this thesis aims to construct a testbed for studying quantum thermodynamics using a superconducting flux qubit. The flux qubit is particularly suited for thermodynamic experiments due to its strong anharmonicity and its galvanic coupling architectures that facilitate efficient energy transport. In this thesis, the on-chip components include harmonic oscillators realized as superconducting resonators, a strongly nonlinear superconducting flux qubit, two copper thermal reservoirs modelled as Johnson–Nyquist noise sources and NIS thermometers, integrated on a highly resistive silicon platform that acts as a phonon bath. These components enable the study and optimization of heat transport between the metallic reservoirs, mediated by the quantum circuit, with the flux qubit as the central, flux-tunable working substance. Heat transport through the qubit occurs via sequential excitation and relaxation cycles between its ground and excited states, where excitation extracts heat equal to the qubit’s energy gap from the hot reservoir, while relaxation releases the same energy into the cold reservoir. Enhancing the thermal device’s performance critically depends on the coupling strength between on-chip components, particularly between the qubit and resonator. This thesis commences with the investigation and demonstration of a robust qubit–resonator coupling design that achieves the ultrastrong regime experimentally. Building on this result, the developed architecture is extended to connect the qubit with two identical resonators, symmetrically positioned on the two sides. These resonators are, in turn, coupled to metallic reservoirs, enabling the first ever demonstration of flux qubit-controlled heat transport between the two reservoirs with excellent device performance. Added geometric asymmetry in the coupler, combined with the flux qubit’s strong anharmonicity, enables the successful demonstration--presented in this thesis--of a non-reciprocal device such as the Microwave Quantum Diode, which, together with the reservoirs, could potentially form an efficient thermal diode. Finally, the thesis concludes with a brief outlook on future directions and ongoing experiments that build on the flux qubit–based testbed developed here for exploring quantum thermodynamic phenomena and device applications.
Advancements in the field of cavity quantum electrodynamics (c-QED) have deepened understanding of quantum phenomena, enabled reliable Josephson junction-based devices, and facilitated their integration with other quantum technologies to explore new physics and applications. Motivated by these developments, this thesis aims to construct a testbed for studying quantum thermodynamics using a superconducting flux qubit. The flux qubit is particularly suited for thermodynamic experiments due to its strong anharmonicity and its galvanic coupling architectures that facilitate efficient energy transport. In this thesis, the on-chip components include harmonic oscillators realized as superconducting resonators, a strongly nonlinear superconducting flux qubit, two copper thermal reservoirs modelled as Johnson–Nyquist noise sources and NIS thermometers, integrated on a highly resistive silicon platform that acts as a phonon bath. These components enable the study and optimization of heat transport between the metallic reservoirs, mediated by the quantum circuit, with the flux qubit as the central, flux-tunable working substance. Heat transport through the qubit occurs via sequential excitation and relaxation cycles between its ground and excited states, where excitation extracts heat equal to the qubit’s energy gap from the hot reservoir, while relaxation releases the same energy into the cold reservoir. Enhancing the thermal device’s performance critically depends on the coupling strength between on-chip components, particularly between the qubit and resonator. This thesis commences with the investigation and demonstration of a robust qubit–resonator coupling design that achieves the ultrastrong regime experimentally. Building on this result, the developed architecture is extended to connect the qubit with two identical resonators, symmetrically positioned on the two sides. These resonators are, in turn, coupled to metallic reservoirs, enabling the first ever demonstration of flux qubit-controlled heat transport between the two reservoirs with excellent device performance. Added geometric asymmetry in the coupler, combined with the flux qubit’s strong anharmonicity, enables the successful demonstration--presented in this thesis--of a non-reciprocal device such as the Microwave Quantum Diode, which, together with the reservoirs, could potentially form an efficient thermal diode. Finally, the thesis concludes with a brief outlook on future directions and ongoing experiments that build on the flux qubit–based testbed developed here for exploring quantum thermodynamic phenomena and device applications.
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.
Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and point out limitations in quantum information processing due to the coupling of the system to its environment. An important aspect in achieving some of these goals is the regime of strong coupling, which has remained until now a domain of theoretical works only. Our aim is to demonstrate strong coupling features in heat transport using a superconducting flux qubit, which is capable of reaching strong- to deep-ultrastrong-coupling regimes, as shown in previous studies. Here, we show experimental evidence of strong coupling by observing a hybridized state of the qubit with two cavities coupled to it, leading to a triplet-like thermal transport via this combined system around the minimum energy of the qubit at power levels of tens of femtowatts, exceeding by an order of magnitude those in earlier experiments. We also demonstrate a nearly 100% on-off switching ratio of heat current mediated by photons by applying magnetic flux to the qubit. Our experiment opens a way towards testing debated questions in strong-coupling thermodynamics, such as what heat is in this regime. We also present a theoretical model that aligns with our experimental findings and explains the mechanism behind heat transport in our device. Furthermore, our experiment opens possibilities for quantum thermodynamics, aiming to realize true quantum heat engines and refrigerators with enhanced power and efficiency, by leveraging ultrastrong coupling between the system and its environment in future experiments.
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.
This thesis presents circuit quantum electrodynamic (cQED) experiments done with artificial atoms coupled to a high quality factor superconducting microwave resonator. This work discusses the theoretical framework, design, fabrication, implementation and characterization of a multi-qubit system using superconducting persistent current qubits, often referred as flux qubits. \n \nWe developed a new type of cryogenic aid for fast turnaround microwave experiments. The minimum turnaround time to perform microwave experiments in dilution refrigerators is typically a few days, as required by cooling down and warming up the entire refrigerator. The cryogenic microwave payload and wiring needed for each qubit and resonator add a significant heat load to \nthe system which typically only has a few µW s of cooling power at millikelvin temperatures. We present a new experimental approach we developed in which a suitable sample holder is attached to a cold-insertable probe and brought in contact with transmission lines permanently mounted inside the cryostat. The total turnaround time is reduced to 8 hours if the target temperature is 80 mK. This new measurement method increased our ability to successfully identify good devices with fast design-characterization iterations, while allowing months-long measurements at 30 mK. \n \nOur device architecture uses several qubits, each of which is controlled via on-chip fast flux bias lines. Readout of the qubit’s quantum state is performed by homodyne detection of the resonator field at large driving power. We observe long coherence times at the optimal bias point of the flux qubits and present decoherence measurements around this optimal point, where the \nflux qubits couple mostly to magnetic flux noise. Dynamical decoupling techniques employing fast microwave pulses are used to protect the qubit’s coherence and allow calculating the magnetic flux noise power spectral density of the qubit’s environment. Photon-mediated interaction between distant qubits in the resonator allows the implementation of a two-qubit controlled-NOT gate \nusing the selective darkening technique. \n \nThis multi-qubit superconducting design is an interesting platform to study the decoherence properties of flux qubits, the properties of strong interaction of electromagnetic radiation with macroscopic quantum systems. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.
Quantum thermodynamics is emerging, both as a topic of fundamental research, but also as means to understand and potentially improve the performance of quantum devices. Superconducting circuit QED (Quantum Electro-Dynamics) presents a prominent platform for the necessary manipulation of quantum states. Thermalization of a quantum system, a subject of intense current interest, can be achieved by interfacing the circuit QED subsystem with a thermal reservoir of appropriate Hilbert dimensionality. In this context, we consider an assembly consisting of a superconducting qubit capacitively coupled between two nominally identical coplanar waveguide (CPW) resonators, each terminated by a normal-metal mesoscopic resistor. We report observation of tunable photonic heat transport through the resonator-qubit-resonator assembly. With the aid of a theoretical model presented here, we are able to reproduce experimental data. Importantly, the reservoir-to-reservoir heat flux depends on the interplay between the qubit-resonator and the resonator-reservoir couplings, yielding qualitatively dissimilar results in different coupling regimes. Our Quantum Heat Valve (QHV) is a technological prerequisite for the realisation of quantum heat engines and refrigerators, that can be obtained, e.g., by exploiting the time-domain dynamics and coherence of driven superconducting qubits. This effort would ultimately bridge the gap between the fields of quantum information and thermodynamics of mesoscopic systems.
Hybrid quantum systems play an essential role in efforts towards the advancements of quantum information technology and quantum sensing by bridging distinct technological platforms. In particular, hybrid systems comprising superconducting microwave circuits are being pursued intensely in light of existing achievements in superconducting qubits and precise quantum control in microwave circuits. Their realization relies on the design of superconducting resonators capable of coupling to an oscillatory mode of a second quantum system, facilitating the probing and manipulation of one via the other. In order to accommodate the coupled system, its particular constraints and requirements must be respected, which may be quite extreme from the perspective of established superconducting circuit design and operation. Overcoming this challenge has proved to be a substantial obstacle for the attainment of strongly coupled hybrid systems, despite spectacular advances in superconducting microwave circuits individually. In this work, we examine two approaches tackling this issue for different hybrid quantum systems, focusing on the optimization and characterization of the superconducting circuits. The first device aims at a superconductor-atom hybrid system through the coupling to Rydberg-Rydberg transitions in ultra-cold Rb-87 atoms trapped near the superconducting chip. Using simulations in conjunction with theoretical considerations, we demonstrate a comprehensive geometry parameter optimization procedure for maximum coupling rate. Presenting an experimental implementation of the resulting circuit, we validate its suitability for use under the intended operation conditions, including the option of tuning the electronic transition frequencies using a dc voltage. Secondly, we investigate a device designed for applications in large magnetic in-plane fields used to mediate coupling to integrated micromechanical oscillators; more specifically it is a flux-tunable SQUID resonator comprising Josephson nano-constrictions as nonlinear inductive components. Performing a thorough characterization of its frequency and Kerr anharmonicity tuning behavior at fields up to several hundred mT, we discover a pronounced asymmetry in its flux response at large fields. We show this asymmetry to be caused by the nano-constrictions turning into Josephson diodes, develop an intuitive macroscopic model for its field-induced emergence and reconstruct the diode current-phase relation from the flux tuning data. Several figures of merit informing the suitability of the SQUID resonators for optomechanical systems as well as other applications are improved by the diode effect, highlighting the potential of Josephson diodes in superconducting microwave circuits. Our results provide a promising stepping stone for the advancement of hybrid systems involving superconducting microwave circuits, putting new and exciting operation and coupling regimes within reach.