Coherent manipulation of electronic heat currents in superconducting quantum circuits

Sebastiano BATTISTI

In recent decades, the scientific results of superconducting electronics have been remarkable. Observation of complex quantum phenomena, such as quantum coherence or manipulation of electronic quantum states, is nowadays commonplace. The thermal counterpart of superconductive electronics is the field of coherent caloritronics, which is used to control and manage heat transfer in superconducting circuits. Micro- and nano-structured superconductors are excellent laboratories for investigating and implementing low-temperature thermal manipulation architectures, primarily due to their unique density-of-states and coherence properties. Although this topic has been extensively studied in recent decades and several experimental and theoretical results have been achieved, flexible, scalable architectures for thermal transport management systems and reliable platforms for thermal energy manipulation remain missing. The lack of practical implementation of coherent caloritronic devices is primarily due to the need to utilize magnetic fields in all proposed architectures. This makes the devices less scalable and more complex, as is usually the case with magnetic fields.The purpose of our work is to pave the way for a real and scalable implementation of caloritronic architectures that leverage electrical fields. In fact, given the experience with CMOS transistors, it is evident that electrical fields are easier to generate and more scalable than magnetic fields, which require uncomfortable coils or flux lines. The fundamental structure used throughout this thesis is the top-gate architecture, consisting of a metallic electrode separated from the portion of the device where we want to apply the electric field by an insulating layer.This thesis consists of two main results: the first one refers to the generation of thermoelectricity with superconductors and its manipulation with the application of a gate potential; the second one refers to the photonic heat transfer due to electron-photon coupling in superconducting circuits, which we will show can be manipulated via a top gate. Both of these results had already been achieved using magnetic fields, rendering the proposed devices practically useless for real applications. We will show that to reach our results, we had to find new solutions and implement novel ideas to make the structures suitable for gate tuning.

Jorden Senior, Alberto Ronzani, Bayan Karimi +5 more

Stefan Zeppetzauer

Jorden Senior

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.

Yasunobu Nakamura

Those successful demonstrations of manipulating and detecting the quantum states of microwave photons indicate that superconducting quantum circuits will also be versatile tools in interfacing quantum information between different quantum systems in the microwave domain. Hybrid systems combining superconducting quantum circuits with spin ensembles as well as nanoelectromechanical systems have demonstrated coherent coupling in between [14-18]. Furthermore, quantum interface between microwave and optical photons based on those hybrid constructions are currently investigated [19-21].

Yasunobu Nakamura