Ph.D. theses from the Houck group

Circuit quantum electrodynamics (cQED) serves as a promising platform for scalable quantum computation, where precise microwave control of qubits lays the foundation for achieving high-fidelity quantum gates. Despite recent progress in developing various quantum gates, controlling artificial qubits remains a considerable challenge due to intricate Hamiltonian systems and the fragile nature of quantum states. Therefore, further research is needed to improve qubit gates and protect quantum states from qubit decoherence. This thesis presents two studies: 1) controlling cQED systems through black-box optimization to achieve state-of-the-art gate fidelity, 2) stabilizing an entangled two-qubit state indefinitely via engineering dissipation channels. The first study establishes the feasibility of direct black-box optimization as a method to discover novel qubit gates from simple initial conditions. We develop robust quantum optimization algorithms to efficiently learn novel qubit gates and evaluate these algorithms through simulations and experiments. Our findings show the potential to learn high-fidelity qubit gates without depending on the specifics of the system Hamiltonian. In the second study, our objective is to realize entanglement stabilization through quantum reservoir engineering. By coupling two qubits near resonance with a leaky resonator acting as a reservoir, we induce a strong correlated decay of the qubits. We experimentally demonstrate the subradiant effect of an entangled Bell state and, through simulation, reveal the robustness of this system in stabilizing a high-fidelity Bell state.

The potential for quantum computing to expand the number of solvable problems has driven researchers across academia and industry, in multiple disciplines, to develop a variety of different qubit platforms, algorithms, and scaling strategies. At its core, quantum computation relies on the robustness, or coherence, of its building blocks (qubits"). In current small-scale superconducting qubit processors, the fidelity of operations is often limited by qubit coherence. The coherence time of a single qubit depends on its lifetime $T_1$ and pure dephasing time $T_{\phi}$. In this thesis, we focus on the problem of improving $T_1$. Strategies for improving lifetimes are informed by models for relaxation - specifically Fermi's Golden Rule. Relaxation rates depend on noise properties of the environment and on properties of the qubit states. This dependence suggests two strategies for engineering longer lifetimes: environment engineering involves mitigating or filtering the noise that the qubit sees, and Hamiltonian engineering refers to optimizing the qubit circuit and its resulting eigenstates to optimize $T_1$. Significant enhancements of qubit lifetimes will require paradigm shifts in our approaches to both environment and Hamiltonian engineering. First, I present a side-by-side study of transmon coherence and materials measurements of the constituent Nb films, including synchrotron x-ray spectroscopy and electron microscopy. We found correlations between qubit lifetimes and materials properties such as grain size, grain boundary quality, and surface suboxides. This study expands the scope of superconducting qubit research by presenting a broad set of materials analyses alongside device measurements. Second, I will give an overview of Hamiltonian engineering, including the concepts behind intrinsic protection against relaxation and dephasing processes. I'll describe the soft $\mathrm{0-\pi}$ qubit, which is the first experimentally realized superconducting qubit to show signatures of simultaneous $T_1$ and $T_2$ protection. We improved coherence in the soft $\mathrm{0-\pi}$ through optimized fabrication processes. We have also characterized the effects of non-computational levels on gate fidelity, specifically AC Stark shifts and leakage. From the results in this thesis, we have gained a deeper understanding of what limits qubit coherence, informing future directions on both the materials and Hamiltonian engineering fronts.

A useful quantum computer requires a full stack of components, where each layer in the stack can actually scale. In this thesis we go through each layer of the quantum computing stack, from the bottom to the top. First, we discuss planar tantalum transmon qubit fabrication. We iterate on the design and fabrication of an entangling gate module with two fixed-frequency transmon qubits and a tunable coupler. We share our perspective on making a robust parametric entangling gate architecture for planar superconducting qubits. Next, we introduce the QICK (Quantum Instrumentation Control Kit), which is a standalone open source controller for both superconducting and atomic qubits as well as various detectors. Highly integrated open source firmware and software has been designed to allow the QICK to scale to hundreds of qubits. We develop the QICK for the superconducting qubit platform and use it to conduct the first single and multi-qubit experiments. Finally, we develop two modular simulation frameworks---one for a multinode quantum computer, and one for heterogeneous qubit architectures.

Superconducting quantum circuits are a promising platform for quantum computation. The building block for most quantum processors is a qubit (quantum bit) which can store information in a superposition of two states. Superconducting qubits are lithographically defined from metals, often niobium or aluminum. However, these devices have limited use because the information they store decays before most useful computations can take place. In this thesis we explore the cause of these losses. Specifically, we employ tantalum as the capacitor pad of a two-dimensional transmon qubit and find lifetimes and coherence times with dynamical decoupling over 300 us. We then switch to a resonator geometry to probe tantalum materials properties. We develop a power and temperature dependent measurement to quantify sources of decay. We find our resonators are primarily limited by two-level system loss at materials interfaces. Finally we employ this resonator characterization method to determine the effects of processing treatments and new packages onresonator decay, showing a buffered-oxide etch before measurement reduces two-level system loss.

Over the past decade, quantum circuits have been transitioning from being useful solely in fundamental physics research to having applications in a wide variety of fields. This has been made possible by the advancements in the coherence, coupling and optimal control of various elements of these quantum circuits. The experiments presented in this thesis solve critical challenges for the above mentioned areas. We provide the first experimental realization of a protected qubit having simultaneous robustness to relaxation and dephasing processes. We show a 40-fold improvement in the coherence time in fluxonium qubit by harnessing insights from Floquet engineering. Furthermore, we also demonstrate a coupling architecture for suppressing qubit-qubit crosstalk. The above works unlock new directions for improving the state of quantum systems

In recent years, superconducting circuits have become a promising architecture for quantum computing and quantum simulation. This advancing technology offers excellent scalability, long coherence times, and large photon nonlinearities, making it a versatile platform for studying non-equilibrium condensed matter physics with light. This thesis covers a series of experiments and theoretical developments aimed at probing strongly correlated states of interacting photons. Building upon previous efforts on nonlinear superconducting lattices, this work focuses on establishing new platforms for generating interactions between microwave photons in multi-mode circuits. The first experiment presents a new paradigm in exploiting the nonlinearity of a Josephson junction to tailor the Hilbert space of harmonic oscillators using a dynamical three-wave mixing process. This allows a single microwave resonator to be addressed as a two-level system, offering a promising pathway to long-lived qubits. A theoretical proposal is outlined for building a field-programmable quantum simulator, harnessing this dynamical nonlinearity for stimulating strong photon-photon interactions. The system consists of a lattice of harmonic modes in synthetic dimensions, where particle hopping and on-site interactions can be independently controlled via frequency-selective flux modulation. Numerical studies show that for strong interactions the driven-dissipative steady-state develops a crystalline phase for photons. The second experiment explores the physics of quantum impurities, where a single well-controlled qubit is coupled to the many modes of a photonic crystal waveguide. The light-matter coupling strength is pushed into the ultrastrong coupling regime, where the qubit is simultaneously hybridized with many modes and the total number of excitations is not conserved. Probing transport through the waveguide reveals that the propagation of a single photon becomes a many-body problem as multi-photon bound states participate in the scattering dynamics. Furthermore, the effective photon interactions induced by just this single impurity leads to interesting inelastic emission of photons. Probing correlations in the field emission reveals signatures of multi-mode entanglement. This work presents opportunities for exploring large-scale lattices with strongly interacting photons. These platforms are compatible with well-established techniques for generating artificial magnetic fields and stabilizing many-body states through reservoir engineering, complementing growing efforts in the quest for building synthetic quantum materials with light.

In recent years, superconducting circuits have emerged as a promising platform for quantum computation and quantum simulation. One of the main driving forces behind this progress has been the ability to fabricate relatively low-disorder, low-loss circuits with a high-degree of control over many of the circuit parameters, both in fabrication and in-situ. This coupled with advances in cryogenics and microwave control electronics have significantly improved the rate of progress. The field, which is broadly called circuit quantum electrodynamics (cQED) has become one of the cleanest and most flexible platforms for studying strong interactions between light and matter. This thesis builds upon previous work on small-scale interacting cQED lattices and larger-scale, non-interacting superconducting circuit lattices to investigate nonequilibrium quantum simulation using large-scale, interacting cQED lattices. In the first experiment, construct a 72-site 1D chain of CPW cavities with each cavity coupled to a single transmon qubit. Each transmon imparts a strong nonlinearity on the photons in the cavity lattice. This, coupled with the drive and dissipation in the lattice gives rise to a novel dissipative phase transition, where the system exhibits bistability with time-scales many orders of magnitude slower than any intrinsic decay rates in the system. The next project in this thesis involves the study of non-Euclidean lattices which can be made from CPW lattices. This work relies on the fact that the frequency of the resonators in the lattice is dependent mostly on the total length of the cavities, not the length between the ends of the cavity. This means that we can form lattices from CPW cavities where the edge distance is not the same but they are the same frequency. The first result in this direction is the fabrication and measurement of a finite piece of a hyperbolic graph, formed from a ‘regular’ tiling of heptagons. This lattice had an effective curvature which is quite large and in principle exhibited a gapped flat band. Following up on the hyperbolic lattice work, the last result described in this thesis involves the study of exotic new lattices which have band structures that exhibit exotic features such as gapped flat bands and Dirac cones. This work involves looking at the spectra of graphs and their corresponding line-graphs, which are the graphs which are pertinent to the tight-binding Hamiltonian in CPW lattice devices. Here, we derive the mathematical relationship between the spectrum of a graph and its line graph as well as what is known as the split, or subdivision, graph. These operations allow us to exactly maximize the gap between the flat band and the rest of the spectrum for certain line graph lattices.

Over the past 10 years, improvements to the fundamental components in supercon- ducting qubits and the realization of novel circuit topologies have increased the life- times of qubits and catapulted this architecture to become one of the leading hardware platforms for universal quantum computation. Despite the progress that has been made in increasing the lifetime of the charge qubit by almost six orders of magnitude, further improvements must be made to climb over the threshold for fault tolerant quantum computation. Two complimentary approaches towards achieving this goal are investigating and improving upon existing qubit designs, and looking for new types of superconducting qubits which would offer some intrinsic improvements over existing designs. This thesis will explore both of these directions through a detailed study of new materials, circuit designs, and coupling schemes for superconducting qubits. In the first experiment, we explore the use of disordered superconducting films, specifically Niobium Titanium Nitride, as the inductive element in a fluxonium qubit and measure the loss mechanisms limiting the qubit lifetime. In the second experiment, we work towards the experimental realization of the 0 − π qubit archi- tecture, which offers the promise of intrinsic protection in lifetime and decoherence compared to existing superconducting qubits. In the final experiment, we design and measure a two qubit device where the static σz ⊗ σz crosstalk between the two qubits is eliminated via destructive interference. The use of multiple coupling elements re- moves the σz ⊗ σz crosstalk while maintaining the large σz ⊗ σx interaction needed to perform two qubit gates.

The advent of superconducting quantum circuits as a robust scientific platform and contender for quantum computing applications is the result of decades of research in light-matter interaction, low-temperature physics, and microwave engineering. There is growing interest to use this advancing technology to study domains of light-matter interaction that were previously thought to be beyond experimental reach. Our work is part of an initiative to explore non-equilibrium condensed matter physics using photons instead of atoms. Open questions in this area currently pose significant challenges theoretically due to analytical complexity and system sizes which prohibit complete numerical simulations, thus experiment-based research has the potential to lead to significant advancements in this field. Here we examine phenomena that arise when moving beyond standard single-mode strong coupling towards the realm of many-body physics with light in two distinct directions. First we study multimode strong coupling, where a single artificial atom or qubit is simultaneously strongly coupled to a large, but discrete number of non-degenerate photonic modes of a cavity with coupling strengths comparable to the free spectral range. This domain, which falls in between small, discrete and continuum Hilbert spaces, is experimentally realized by coupling a qubit to a low fundamental frequency coplanar waveguide cavity. In this system we report on resonance fluorescence and narrow linewidth emission directly resulting from complex qubit mediated mode-mode interactions. In the second part we explore qubits strongly coupled to photonic crystals, which give rise to exotic physical scenarios, beginning with single and multi-excitation qubit-photon dressed bound states comprising induced, spatially localized photonic modes, centered around the qubits, and the qubits themselves. The localization of these states changes with qubit detuning from the band-edge, offering an avenue of in situ control of bound state interaction. Due to their localization-dependent interaction, these states offer the ability to create one-dimensional chains of bound states with tunable interactions that preserve the qubits' spatial organization, a key criterion for realization of certain quantum many-body models. The unique domains of light-matter interaction discussed here are a subset of exciting research initiatives growing our general understanding of complex, strongly coupled quantum systems.
Circuit quantum electrodynamics (cQED) uses superconducting circuit elements as its building blocks for controllable quantum systems and has become a promising experimental platform for quantum computation and quantum simulation. The ability to tune the coupling rate between circuit elements extends the controllability and flexibility of cQED devices and can be utilized to improve device performance. This thesis presents the study, implementation and application of tunable coupling devices in cQED. The tunability originates from the basic principles of quantum superposition and interference, and unwanted interactions can be suppressed by destructive interference. Following this principle, we design and conduct two experiments that demonstrate the utility of tunable coupling for better device performances in quantum information processing. The first experiment aims to improve the coherence of qubits against noise. We implement a qubit whose frequency and dispersive coupling to a readout resonator can be tuned independently. When the coupling rate is tuned to near zero, the qubit becomes immune to photon number fluctuations in the resonator and exhibits robust coherence time in the presence of noise. The second experiment extends to a multi-qubit system where crosstalk between qubits causes error in quantum gates. We develop a two-qubit device and suppress crosstalk by tuning the ZZ coupling rate between the qubits. The tunable dispersive coupling can also be parametrically modulated to implement a two-qubit entangling gate in the low crosstalk regime. Those devices provide flexible and promising building blocks for cQED systems.
Superconducting circuits have become an ideal platform to implement prototypi- cal quantum computing ideas and to study nonequilibrium quantum dynamics. This thesis covers research topics conducted in both subfields. Fast and reliable readout of volatile quantum states is one of the key requirements to build a universal quantum computer. In the first part, we utilize a number of techniques, ranging from a low noise amplifier to an on-chip stepped-impedance Purcell filter, to improve superconducting qubits readout fidelity. Interestingly, full quantum theory of SIPF requires the understanding of strong coupling quantum electrodynamics near a photonic band-gap. This problem, intimately tied to quantum impurity problems in condensed matter physics, has never been studied experimentally prior to the development of superconducting circuits. This realization then leads to the second part, the study of atom-light interaction in structured vacuum. The word ‘structured’ means the spectral function of the vacuum is drastically different from that of free space. We directly couple a transmon qubit to a microwave photonic crystal and discuss the concepts of photon bound states and quantum dissipative engineering in such a system. Following this research direction, quantum electrodynamics in a driven multimode cavity, another form of structured vacuum, is also investigated both experimentally and theoretically. The most intriguing phenomenon is the multimode ultranarrow resonance fluorescence, attributed to correlated light emission.
Historically our understanding of the microscopic world has been impeded by limitations in systems that behave classically. Even today, understanding simple problems in quantum mechanics remains a dicult task both computationally and experimentally. As a means of overcoming these classical limitations, the idea of using a controllable quantum system to simulate a less controllable quantum system has been proposed. This concept is known as quantum simulation and is the origin of the ideas behind quantum computing. In this thesis, experiments have been conducted that address the feasibility of using devices with a circuit quantum electrodynamics (cQED) architecture as a quantum simulator. In a cQED device, a superconducting qubit is capacitively coupled to a superconducting resonator resulting in coherent quantum behavior of the qubit when it interacts with photons inside the resonator. It has been shown theoretically that by forming a lattice of cQED elements, dierent quantum phases of photons will exist for dierent system parameters. In order to realize such a quantum simulator, the necessary experimental foundation must rst be developed. Here experimental eorts were focused on addressing two primary issues: 1) designing and fabricating low disorder lattices that are readily available to incorporate superconducting qubits, and 2) developing new measurement tools and techniques that can be used to characterize large lattices, and probe the predicted quantum phases within the lattice. Three experiments addressing these issues were performed. In the rst experiment a Kagome lattice of transmission line resonators was designed and fabricated, and a comprehensive study on the eects of random disorder in the lattice demonstrated that disorder was dependent on the resonator geometry. Subsequently a cryogenic 3-axis scanning stage was developed and the operation of the scanning stage was demonstrated in the nal two experiments. The rst scanning experiment was conducted on a 49 site Kagome lattice, where a sapphire defect was used to locally perturb each lattice site. This perturbative scanning probe microscopy provided a means to measure the distribution of photon modes throughout the entire lattice. The second scanning experiment was performed on a single transmission line resonator where a transmon qubit was fabricated on a separate substrate, mounted to the tip of the scanning stage and coupled to the resonator. Here the coupling strength of the qubit to the resonator was mapped out demonstrating strong coupling over a wide scanning range, thus indicating the potential for a scanning qubit to be used as a local quantum probe.
Superconducting circuits have shown promise for exploring quantum optics and computing. This thesis presents an additional element, the tunable coupling qubit (TCQ), to the toolbox available for exploring such physics. The TCQ is shown to have independently tunable qubit energy and dipole coupling strength. High frequency flux control lines allow the varying of the TCQ's properties on very fast time scales. This enables qubit coherence measurements and the calculation of a lower bound on the maximum range of coupling strength tunability. Finally, an experiment demonstrating the TCQ's applicability in quantum state transfer is discussed.