Fault-tolerant superconducting quantum processors are limited by decoherence, limited connectivity, and the wiring burden at cryogenic temperatures. This review surveys superconducting “metamaterials”: circuits built from many repeating, deeply subwavelength building blocks (unit cells) that tailor dispersion, impedance, and bandgaps. By shaping the electromagnetic (and sometimes acoustic) environment of a qubit, these networks can suppress or enhance radiative decay and frequency shifts (Purcell and Lamb effects) and provide compact multimode structures for multiplexed control and photon-mediated coupling. We emphasize the practical trade-offs—added dielectric participation, fabrication disorder, and leakage through spurious chip/package modes—and the measurements needed to quantify them. The review summarizes design and tuning approaches (including Josephson/Superconducting Quantum Interference Device implementations), materials and fabrication limits, and mode-resolved benchmarks, and it highlights emerging directions such as topological lattices and engineered reservoirs while clearly separating demonstrations from proposals.
Fault-tolerant superconducting quantum processors are limited by decoherence, limited connectivity, and the wiring burden at cryogenic temperatures. This review surveys superconducting “metamaterials”: circuits built from many repeating, deeply subwavelength building blocks (unit cells) that tailor dispersion, impedance, and bandgaps. By shaping the electromagnetic (and sometimes acoustic) environment of a qubit, these networks can suppress or enhance radiative decay and frequency shifts (Purcell and Lamb effects) and provide compact multimode structures for multiplexed control and photon-mediated coupling. We emphasize the practical trade-offs—added dielectric participation, fabrication disorder, and leakage through spurious chip/package modes—and the measurements needed to quantify them. The review summarizes design and tuning approaches (including Josephson/Superconducting Quantum Interference Device implementations), materials and fabrication limits, and mode-resolved benchmarks, and it highlights emerging directions such as topological lattices and engineered reservoirs while clearly separating demonstrations from proposals.
Superconducting qubits are pivotal in advancing quantum computing, poised for scale but limited by the complexity and fidelity of their control and readout systems, relying on RF and signal processing infrastructure. This survey serves as a comprehensive and technically grounded review of control and readout architectures tailored for superconducting qubits. Synthesizing insights from device physics, circuit design, microwave engineering, signal processing, and cryogenic integration, this work details the practicalities of RF pulse generation, signal synthesis, and readout signal analysis for quantum systems. it covers key requirements, parameters, and pulse engineering techniques, including commonly used envelopes like Gaussian and DRAG designs. Moving to the system level, this survey systematically classifies and critically analyses current architectural strategies (covering key areas like frequency conversion, waveform management, and system infrastructure) and technology platforms (including adaptive classical control stacks, cryogenic CMOS circuits, and novel interconnects and interfaces), evaluating their trade-offs in performance. Extensive literature analysis identifies prevailing limitations such as wiring complexity, thermal budget constraints, latency, and power consumption, while highlighting underexplored opportunities for on-chip signal processing and novel interconnects, drawing analogies to advanced communication systems design. By consolidating diverse control paradigms and critically evaluating their trade-offs, this survey provides a unified foundation for designing next-generation quantum control stacks. Finally, a forward-looking roadmap outlines key trends in monolithic integration, cryo-compatible digital architectures, and physics-informed hardware co-design, offering both a retrospective synthesis and a prospective vision for quantum hardware engineering beyond the NISQ era.
Metamaterial resonant structures made from arrays of superconducting lumped-circuit elements can exhibit microwave-mode spectra with left-handed dispersion for the standing-wave resonances, resulting in a high density of modes in the same frequency range where superconducting qubits are typically operated, as well as a bandgap at lower frequencies that extends down to dc. Using this regime for multimode circuit quantum electrodynamics, we perform a series of measurements of such a superconducting metamaterial resonator coupled to a flux-tunable transmon qubit. Through microwave measurements of the metamaterial, we observe the coupling of the qubit to each of the modes that it passes through. Using a separate readout resonator, we probe the qubit dispersively and characterize the qubit energy relaxation as a function of frequency, which is strongly affected by the Purcell effect in the presence of the dense mode spectrum. Additionally, we investigate the ac Stark shift of the qubit as the photon number in the various metamaterial modes is varied. Through numerical simulations, we explore designs based on this scheme with enhanced coupling so that the coupling energy between the qubit and the metamaterial modes can exceed the intermode spacing in future devices with achievable circuit parameters. The ability to tailor the dense mode spectrum through the choice of circuit parameters and manipulate the photonic state of the metamaterial through interactions with qubits makes this a promising platform for analog quantum simulation with microwave photons and quantum memories.
Superconducting qubits are a leading platform for quantum computation, yet decoherence and scalability challenges continue to limit progress toward fault-tolerant systems. This review synthesizes research on architectures, noise sources, and error correction to clarify performance trade-offs and guide development. The transition from Transmon to Fluxonium has dramatically improved qubit performance, achieving millisecond-scale coherence times. However, this success reveals a fundamental limit of coherence now being primarily restricted by the physical properties of the materials, specifically dielectric losses occurring at the Josephson junction leads and within the surrounding materials. As overcoming these material limits becomes increasingly difficult, system-level engineering determines whether high-coherence qubits can scale.Scaling quantum systems is difficult because current cooling and control systems create too much heat and this forces a shift to integrated digital superconducting circuits and cryogenic light-based links. While these hardware challenges restrict scaling, experiments in error correction show that logical qubits can already outperform physical ones. For example, a distance-7 surface code improved logical qubit lifetime by 2.4×. Achieving practical fault tolerance, however, requires moving beyond simple error models. Future systems must combine techniques like Dynamical Decoupling and Machine Learning-based Optimal Control with dynamic classical control capable of handling complex, correlated faults such as Multi-Bit Burst Errors. Progress in materials, cryogenic system integration for control, and adaptive error management must advance together to realize scalable, high-fidelity quantum hardware.
Abstract Embedding tunable quantum emitters in a photonic bandgap structure enables control of dissipative and dispersive interactions between emitters and their photonic bath. Operation in the transmission band, outside the gap, allows for studying waveguide quantum electrodynamics in the slow-light regime. Alternatively, tuning the emitter into the bandgap results in finite-range emitter–emitter interactions via bound photonic states. Here, we couple a transmon qubit to a superconducting metamaterial with a deep sub-wavelength lattice constant (λ/60). The metamaterial is formed by periodically loading a transmission line with compact, low-loss, low-disorder lumped-element microwave resonators. Tuning the qubit frequency in the vicinity of a band-edge with a group index of ng = 450, we observe an anomalous Lamb shift of −28 MHz accompanied by a 24-fold enhancement in the qubit lifetime. In addition, we demonstrate selective enhancement and inhibition of spontaneous emission of different transmon transitions, which provide simultaneous access to short-lived radiatively damped and long-lived metastable qubit states.
Superconducting circuits are a powerful platform for studying and controlling the interaction between light and matter at the quantum level. In these systems, superconducting qubits act as artificial atoms (with matter-like degrees of freedom), whereas microwave resonators play the role of oscillators hosting photons (with photonic-like degrees of freedom). A major advantage of this platform is the ability to tailor the electromagnetic environment to which the qubits are coupled. This degree of control makes it possible to realize well-known quantum optics models, like the Jaynes-Cummings model, as well as more complex systems, like quantum electrodynamics close to a bandgap. In this thesis, we engineer structured electromagnetic environments and couple superconducting qubits to them to experimentally study their interaction. By coupling multiple microwave resonators together, we create a metamaterial. When an atom interacts with the metamaterial, an atom-photon bound state emerges. We observe the dynamics of the creation and dissolution of the atom-photon bound state, as well as the spectral content of its photonic component, for the first time to our knowledge.Our metamaterial is intrinsically nonlinear because each resonator is formed by an array of Josephson junctions to serve as an inductor. When the system is pumped, its phase diagram exhibits multimode dissipative phase transitions with timescales on the order of hundred seconds. There is ongoing theoretical debate about whether such phase transitions can occur in one-dimensional systems, a question we address and resolve in this work.Furthermore, we engineer qubit decay using two different approaches. In the first one, we engineer the decay of two coupled atoms to two waveguides by using a symmetric–antisymmetric coupling configuration. This approach enables selective coupling to bright and dark states of the system. In the second one, we suppress the Purcell decay of a qubit using a compact lumped-element Purcell filter. In addition, we characterize the Purcell decay by employing an auxiliary superconducting qubit as a sensitive power detector directly coupled to the feedline waveguide used for the qubit readout.In summary, this thesis presents three different methods for controlling light–matter interactions in structured quantum systems, including decay suppression with both Purcell filters and symmetry-selective couplings. The thesis also introduces new experimental techniques for probing atom-photon bound states and provides direct evidence of collective phenomena in driven-dissipative quantum metamaterials.
Superconducting quantum circuits (SQCs), composed of superconducting capacitors, inductances, Josephson junctions, and transmission lines, exhibit macroscopic quantum effects at ultra-low temperatures. Due to the extremely low dissipation of superconductors, an important application of SQCs is superconducting qubits with long coherence time. Quantum computing based on superconducting qubits is a leading physical realization method of quantum computing, which is theoretically capable of accelerating many computational problems including quantum simulation, database searching, and optimization. In the long run, to use the power of quantum computing to its full extent, universal quantum computers should be constructed with quantum error correction. In the short term, one of the most pressing targets of the superconducting quantum computing community is to demonstrate quantum advantage using noisy intermediate-scale quantum chips. Both goals rely on the increasing of the number of coupled qubits in a quantum circuit and the quality of these qubits, which dictate the breadth and the depth of a problem it can solve, respectively. The fabrication of SQCs has adopted a few manufacturing processes from conventional integrated circuits. Quantum chips containing multiple superconducting qubits can be made and packaged on a large scale. “Quantum supremacy” has been demonstrated on a quantum chip with fifty-three qubits fabricated using the flip-chip bonding technology. However, to demonstrate quantum advantage, the quality and quantity of qubits need to increase simultaneously. Dissipative channels emerge in circuit design, material preparation, chip fabrication, and working environment, limiting the lifetime of superconducting qubits. For quantum chips with tens to hundreds of qubits, the integration will also introduce extra processing steps which may degrade the qubits, and unwanted coupling to loss channels. Most of these channels are microscopically related to materials constructing the quantum chips, chip surfaces, and material interfaces. Thus, it is important to investigate the material growth and device fabrication of SQCs, and to understand the material related loss mechanisms. This review paper investigates the material related topics in superconducting quantum circuits pertinent to the quantum computing application from a comprehensive perspective. The history of SQCs is briefly mentioned, followed by the peculiar characteristics as an engineerable quantum system, and a roadmap to quantum advantage and universal quantum computing in the next a few years. Technically, the superconducting quantum computer is compared to a Schrodinger’s cat, and can be boiled down into three parts, i.e., quantum chips, the environment, and the operating system, corresponding to the cat, the box, and the observer, respectively. The material related decoherence channels of the SQCs can be sorted into four categories; each will be discussed in details. To fabricate SQCs with minimal loss, the processing technologies are elaborated, including qubit design, material selection, surface treatments, film growth, and Josephson junction, etc. Though there are general guidelines that amorphous materials should be avoided and processing induced damage should be minimized, more is to be investigated due to the emerging of new materials and new fabrication methods. The last part presents the potential application of the superconducting quantum computer and its current state of play. SQC for quantum computing is a multi-discipline research topic, demanding joint efforts from different scientific and technical areas. Materials science and device processing will play important roles along the way to quantum practicality.