The study of light-matter interaction in superconducting quantum circuits has seen remarkable progress over the last $20$ years. By engineering artificial atoms, novel quantum phenomena have been demonstrated, and old ideas have come into a new light. Beyond their application to basic science, the prospect of implementing large-scale quantum information processing with superconducting circuits has fueled a rapid development of surrounding technologies, with ever-increasing control over their behavior as a result. The field's success stems primarily from the flexible design and strong non-linearity of the artificial atom, whose coherent interaction with both electrical and mechanical degrees of freedom has opened many doors for science. This thesis deals with the interaction between artificial atoms and quantum fields in one-dimensional waveguides. The waveguide represents a one-dimensional environment for the atom, which we use to study the properties of open quantum systems. All quantum systems are, in fact, open, and interactions between the system and its environment lead inevitably to a loss of energy and quantum coherence. A continuous loss of information into the environment is called a Markovian process. The work contained in this thesis indicates that deviations from a Markovian process can be detected in the steady state of driven systems. This could simplify the detection of non-Markovianity in open quantum systems, as no information about the system's transient dynamics would be necessary.Specifically, this thesis considers both electromagnetic fields in microwave transmission lines and acoustic fields in the form of surface acoustic waves (SAWs) traveling on the surface of solids. The recent realization of artificial atoms interacting with acoustic waves has opened up a new research field called quantum acoustics. We have built a model of the interaction between atoms and SAWs that predicts the existence of a new regime where the atom forms its own cavity. Additionally, we have considered synthesizing electromagnetically induced transparency, a quantum optics phenomena in opaque media where the absorption of photons is canceled, in waveguides using multiple closely spaced two-level systems. Some of the work in this thesis represents experimental work done in collaboration. In the first experiment, we studied the routing of acoustic waves at the quantum level. In the other experiment, we demonstrated electromagnetically induced transparency by creating an effective $\\Lambda$ system using a giant artificial atom. This thesis reviews the numerical techniques used to model these experiments.
The study of light-matter interaction in superconducting quantum circuits has seen remarkable progress over the last 20 years. By engineering artificial atoms, novel quantum phenomena have been demonstrated, and old ideas have come into a new light. Beyond their application to basic science, the prospect of implementing large-scale quantum information processing with superconducting circuits has fueled a rapid development of surrounding technologies, with ever-increasing control over their behavior as a result. The field's success stems primarily from the flexible design and strong non-linearity of the artificial atom, whose coherent interaction with both electrical and mechanical degrees of freedom has opened many doors for science. This thesis deals with the interaction between artificial atoms and quantum fields in one-dimensional waveguides. The waveguide represents a one-dimensional environment for the atom, which we use to study the properties of open quantum systems. All quantum systems are, in fact, open, and interactions between the system and its environment lead inevitably to a loss of energy and quantum coherence. A continuous loss of information into the environment is called a Markovian process. The work contained in this thesis indicates that deviations from a Markovian process can be detected in the steady state of driven systems. This could simplify the detection of non-Markovianity in open quantum systems, as no information about the system's transient dynamics would be necessary.Specifically, this thesis considers both electromagnetic fields in microwave transmission lines and acoustic fields in the form of surface acoustic waves (SAWs) traveling on the surface of solids. The recent realization of artificial atoms interacting with acoustic waves has opened up a new research field called quantum acoustics. We have built a model of the interaction between atoms and SAWs that predicts the existence of a new regime where the atom forms its own cavity. Additionally, we have considered synthesizing electromagnetically induced transparency, a quantum optics phenomena in opaque media where the absorption of photons is canceled, in waveguides using multiple closely spaced two-level systems. Some of the work in this thesis represents experimental work done in collaboration. In the first experiment, we studied the routing of acoustic waves at the quantum level. In the other experiment, we demonstrated electromagnetically induced transparency by creating an effective Lambda system using a giant artificial atom. This thesis reviews the numerical techniques used to model these experiments.
The past 20 years has seen rapid developments in circuit quantum electrodynamics, where superconducting qubits and resonators are used to control and study quantum light-matter interaction at a fundamental level. The development of this field is strongly influenced by quantum information science and the prospect of realizing quantum computation, but also opens up opportunities for combinations of different physical systems and research areas. Superconducting circuits in the microwave domain offer a versatile platform for interfacing with other quantum systems thanks to strong nonlinearities and zero-point fluctuations, as well as flexibility in design and fabrication. Hybrid quantum systems based on circuit quantum electrodynamics could enable novel functionalities by exploiting the strengths of the individual components.This thesis covers experiments coupling superconducting circuits to surface acoustic waves (SAWs), mechanical waves propagating along the surface of a solid. Strong coupling can be engineered using the piezoelectric properties of GaAs substrates, and our experiments exploit this to investigate phenomena in quantum field-matter interaction. A key property of surface acoustic waves is the slow propagation speed, typically five orders of magnitude slower than light in vacuum, and the associated short wavelength. This enables the giant atom regime where the artificial atom in the form of a superconducting circuit is large compared to the wavelength of interacting SAW radiation, a condition which is difficult to realize in other systems. Experiments described in this thesis use these properties to demonstrate electromagnetically induced transparency for a mechanical mode, as well as non-Markovian interactions between an artificial giant atom and the SAW field. When the SAW field is confined to a resonant cavity, the short wavelength allows multimode spectra suitable for interacting with a frequency comb. We use a multimode SAW resonator to characterize the ensemble of microscopic two-level system defects with a two-tone spectroscopy approach. Finally, we introduce a hybrid superconducting-SAW resonator with applications in quantum information processing in mind. Experiments with this device demonstrate entanglement of SAW modes, and show promising results on the way to engineer cluster states for quantum computation in continuous variables.
Waveguide circuit quantum electrodynamics (waveguide circuit QED) studies light-matter interaction with superconducting circuits in one dimension. In circuit QED, natural atoms are replaced by superconducting qubits consisting of a non-linear Josephson junction, resulting in an anharmonic energy spectrum just like real atoms. With superconducting qubits, it is possible to study quantum optical phenomena and reach new regimes hard to achieve with real atoms due to weak coupling to the electromagnetic field. The reduction to one dimension in waveguide QED increases the electromagnetic field's directionality, which results in reduced losses. In this thesis, we first introduce circuit quantisation, giving the basis for the next part, where we investigate a transmon, a charge-insensitive artificial atom, coupled to a semi-infinite transmission line. An atom coupled to a semi-infinite waveguide is referred to as an atom in front of a mirror and is the subject of all appended papers. We proceed by summarising Paper I and III's main results: in Paper I, we investigate the spontaneous emission of a transmon coupled to a semi-infinite transmission line, where we take time-delay effects into account. We find that the system dynamics strongly depend on the coupling strength to the transmission line and the atom's position with respect to the electromagnetic field, leading to the Purcell effect or the convergence to a dark state with finite excitation probability. In the high-impedance regime, which we investigate in Paper III, the properties of the transmon coupled to the high-impedance transmission line change drastically. It becomes highly reflective and creates its own cavity with the mirror, resulting in the emergence of cavity modes and vacuum Rabi oscillations in the spontaneous emission dynamics. In the next chapter of the thesis, we demonstrate how to quantise an electromagnetic field and derive a light-matter interaction Hamiltonian within dipole approximation. We then give an introduction to open quantum systems and derive the quantum-optical master equation in Lindblad form. Furthermore, we introduce the dressed state picture, where the interaction of light and matter is so strong that the individual energy levels of light and matter are no longer separable. Both the quantum optical master equation and the dressed state picture are relevant in Paper II and IV. In Paper II, an experimental collaboration, we perform several experiments to characterise and discriminate different decay rates of a superconducting qubit coupled to the end of a transmission line. One experiment measured the atomic fluorescence spectral density, which shows an asymmetry for off-resonant driving, resulting from pure dephasing: an effect that we explain in more detail in this thesis and Paper II. In Paper IV, we theoretically investigate amplification mechanisms realised by different set-ups of an atom coupled to a semi-infinite waveguide. In the considered systems, the amplification of a probe field happens either due to population inversion between the pure states or dressed states or multi-photon processes. We find that compared to an open waveguide, we can achieve a higher gain in the amplification with a semi-infinite waveguide.
One of the most fundamental phenomena occurring in nature is the interaction between single atoms and electromagnetic fields. This is studied in quantum optics to probe the quantum nature of the system. In appended Paper I, we demonstrated that an artificial atom can couple to propagating sound, which is the acoustic analogue of quantum optics. This thesis covers experiments where surface acoustic waves (SAWs) are used in superconducting circuits at gigahertz frequencies with the aim for quantum applications.Since SAWs are mechanical vibrations that propagate on the surface of solids and dissipate little power, they can propagate freely over long distances before and after they interact with the artificial atom. Their five order of magnitude slower speed and smaller wavelength than light at gigahertz frequencies, open up for exploring new regimes of quantum physics. In addition, the coupling between the artificial atom and the SAWs is promising for reaching ultrastrong coupling limits, which is hard in other types of systems. Many of the future applications are discussed in appended Paper II, together with a more detailed description of the theory and fabrication of the devices.Although the possibilities are many, a variety of potential experiments would benefit from higher conversion efficiency between electric signals and SAWs. Therefore, improvements of this conversion are studied in appended Paper III, making use of the many advances in classical SAW devices. More specifically, the conversion of unidirectional transducers (UDTs) on lithium niobate is studied and compared to symmetric interdigital transducers (IDTs). The results show that 99.4 % of the acoustic power can be focused in the desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, eliminating the largest contribution to loss of the IDTs. However, there is a trade-off between conversion efficiency and bandwidth. This knowledge allows us to better tailor potential quantum experiments based on SAWs, possibly towards measuring quantum sound.
Surface acoustic waves (SAWs) are mechanical vibrations that propagate on the surface of solids while dissipating little power, consequently enabling them to propagate freely over long distances. The speed and wavelength of SAWs are reduced a five order of magnitude compared to when light is used as a carrier at gigahertz frequencies. The unique properties of SAWs combined with the possibility to let them interact with artificial atoms, discovered and shown for the very first time in appended Paper I of this thesis, open up for exploration of new regimes of quantum physics. The appended Paper II is a book chapter providing an overview of many of the new areas of research, as well as going into depth of the method and significance of the results ofthe appended Paper I.The essential interaction between artificial atoms and SAWs was further investigated by using Autler-Townes splitting to achieve fast control of the interactions. The appended Paper IV, shows a transmitted field extinction of 80 %, and provides proof of concept for a SAW router in the quantum regime. In addition, due to the artificial atom's highly frequency dependent coupling to SAWs, electromagnetically induced transparency (EIT) could be observed in the appended Paper V. Furthermore, the EIT region was distinguished from the Autler-Townes splitting region by a threshold in the applied power. The results produce parallel findings to quantum optics, but are perhaps best described as part of a different field, quantum acoustics.Among the many possible areas of research emerging as an outcome of this work, a variety of potential quantum experiments would benefit greatly from a higher conversion efficiency between electric signals and SAWs. Due to this, focus was put on improving this conversion efficiency by studying superconducting unidirectional transducers (UDTs), making use of advances in classical SAW devices. The appended Paper III shows that 99.4~\\% of the acoustic power can be focused in a desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, thereby eliminating the largest source of loss of symmetric inter-digital transducers. There is, however, a trade-off between conversion efficiency and bandwidth. This finding allows tailoring of quantum experiments based on SAWs that may pave the way towards measuring quantum sound.
Artificially engineered atoms, built using superconducting electrical circuits, have had a broad impact on the field of quantum information and quantum computing. Based on the Josephson effect, superconducting qubits have provided a robust platform for engineering light-matter interactions at the single-photon level. The ability to precisely control and manipulate single photons using superconducting qubits and cavities, a field now popularly known as circuit quantum electrodynamics (circuit QED), has enabled new and novel regimes in quantum physics, which previously remained inaccessible. For instance, the coupling between individual photons and artificial atoms have been shown to reach the ultrastrong and deep-strong regimes, a feat which is difficult to achieve with natural atoms. The superconducting circuit platform is now a promising contender for building large-scale quantum processors, attracting large investments from academic, industry and government players. \n \nThis thesis uses superconducting circuits to engineer photon interactions in two separate studies. The first is aimed at studying the physics of ``giant" artificial-atoms. The second study explores the route towards building a quantum heat engine using two parametrically-coupled, superconducting microwave cavities. We review the theoretical ideas and concepts which motivate our work, along with discussions of the design methodology, simulations, fabrication, measurement setup and the experimental findings. \n \nIn the first study, we explore a giant artificial atom, formed from a transmon qubit, which is coupled to propagating microwaves at multiple points along an open transmission line. The multipoint coupling nature of the transmon allows its radiated field to interfere with itself leading to some striking ``giant" atom effects. For instance, we observe strong frequency dependent couplings of the transmon's transition levels to its electromagnetic environment, a feature which is not observed with ordinary artificial atoms. We measure large on/off ratios, as high as 380, for the coupling rate of the ket0−ket1 transition. Furthermore, we show that we can enhance or suppress the coupling rate of the ket1−ket2 transition relative to the ket0−ket1 transition, by more than a factor of 200. The relative modulation of the coupling rates was exploited to engineer a metastable state in the giant transmon and demonstrate electromagnetically-induced transparency (EIT), a typical signature of a lambda system. Our results show that we can transform the ladder structure of an ordinary transmon into a more interesting lambda system using a giant transmon, thereby paving the way for exploring new possibilities to study three-level physics in a waveguide-QED setting. \n \nExtending giant atom physics to multiple giant atoms, we then explore a device with two giant artificial atoms connected in a braided configuration to a transmission line. The braided topology of the qubits, offers an interesting regime where the qubits can interact with each other in a decoherence-free environment, where the interaction is mediated by virtual photons in the transmission line. We probe the resonant behavior of the qubits at two different frequency bias points, where we observe qualitatively different scattering behavior. Furthermore, when probing for the Autler-Townes Splitting (ATS), multiple resonances are observed for both resonant and off-resonant cases instead of the familiar doublet in the ATS spectroscopy. This comes as a surprise as the frequency-level spacings in both qubits are nominally identical. We believe these features could be an indication of a novel resonant interaction between the qubits facilitated by the braided topology. An effort to understand this theoretically is underway. \n \nFor our second study, we explore a system with two parametrically-coupled superconducting resonators, which implements an optomechanical-like interaction in an all-electrical network. The nonlinear nature of this interaction is mediated by a superconducting quantum interference device (SQUID), where the current in one resonator couples to the photon number in the other resonator. We propose to use this system to build a ``photonic piston" engine in the quantum regime. We motivate the feasibility of the proposal by reviewing key theoretical results which demonstrate an Otto-cycle by appropriately driving the system with noise. Our experimental findings demonstrate the crucial nonlinear coupling that is required for the engine to work. We also show that we can increase the coupling strength between the resonators depending on the chosen flux operating point.
Experiments utilizing quantum optics have progressed rapidly in last few decades, particularly in the context of quantum computation, simulation, and communication. Early work in this field focused on implementations of cavity quantum electrodynamics (QED), where atoms, either natural or artificial, are strongly coupled to the confined photonic modes of cavities. However, in recent years, achieving strong coupling between atoms and itinerant photons has also gained significant interest for its applications in quantum networking. To this end, many atomic platforms are attempting to realize the waveguide QED architecture: atoms that interact with the continua of propagating photonic modes within a waveguide. In this thesis, we realize the strong-coupling regime of a waveguide QED architecture by coupling superconducting artificial atoms, typically operated as qubits, to one-dimensional transmission lines. We first demonstrate that superconducting qubits in a waveguide QED system can be used as high-quality quantum emitters. We then leverage the quantum interference between the simultaneous emission from multiple qubits in order to generate non-classical, spatially entangled, and directional itinerant microwave photons. These types of photons are particularly useful for remote entanglement and quantum communication protocols. Finally, we demonstrate that superconducting qubits can be engineered to enter novel regimes of light-matter interactions that are difficult, or even impossible, to achieve in other atomic platforms. In particular, we realize the giant-atom regime of waveguide QED, where the atom can no longer be treated as a point-like object. We use our giant atoms to implement tunable atom-waveguide couplings, as well as decoherence-free waveguide-mediated interactions between multiple atoms.