Scaling up quantum processors to run practical quantum algorithms remains a major challenge due to hardware constraintsand control limitations. This dissertation explores two complementary directions to address the scalability bottlenecksin superconducting quantum architectures: the detrimental impact of strong drives required for fast and high-fidelityoperations, and the integration of quantum memories into hybrid architectures to reduce the hardware overhead.In the first part, we investigate how strong drives can generate detrimental effects in the circuit. In this regime, transmonscan be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from theremainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. Weshow here that these detrimental effects can further propagate to other degrees of freedom, for example to neighbouringqubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise toanother circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noisespectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certainlimit, the interaction with the driven transmon can be modelled as a stochastic diffusive process driving the TLS, therebyrevealing a fundamental source of drive-induced crosstalk in multi-qubit setups.In the second part, we turn to a modular approach for scalable quantum computation, which combines the processingunit with a storage module. We focus on spin-based quantum memories, where itinerant electromagnetic fields arestored in large ensembles of effective two-level systems embedded in a cavity. Using a mean-field framework, we modelthe ensemble as an effective spin communication channel and develop a cascaded quantum model to describe bothabsorption and emission processes. We derive optimal time-dependent modulations of the cavity linewidth that maximizestorage and retrieval efficiency for finite-duration wavepackets. Our analysis yields an upper bound on efficiency, whichcan be met in the narrow bandwidth regime. It also shows the existence of a critical bandwidth above which the efficiencyseverely decreases.
Scaling up quantum processors to run practical quantum algorithms remains a major challenge due to hardware constraintsand control limitations. This dissertation explores two complementary directions to address the scalability bottlenecksin superconducting quantum architectures: the detrimental impact of strong drives required for fast and high-fidelityoperations, and the integration of quantum memories into hybrid architectures to reduce the hardware overhead.In the first part, we investigate how strong drives can generate detrimental effects in the circuit. In this regime, transmonscan be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from theremainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. Weshow here that these detrimental effects can further propagate to other degrees of freedom, for example to neighbouringqubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise toanother circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noisespectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certainlimit, the interaction with the driven transmon can be modelled as a stochastic diffusive process driving the TLS, therebyrevealing a fundamental source of drive-induced crosstalk in multi-qubit setups.In the second part, we turn to a modular approach for scalable quantum computation, which combines the processingunit with a storage module. We focus on spin-based quantum memories, where itinerant electromagnetic fields arestored in large ensembles of effective two-level systems embedded in a cavity. Using a mean-field framework, we modelthe ensemble as an effective spin communication channel and develop a cascaded quantum model to describe bothabsorption and emission processes. We derive optimal time-dependent modulations of the cavity linewidth that maximizestorage and retrieval efficiency for finite-duration wavepackets. Our analysis yields an upper bound on efficiency, whichcan be met in the narrow bandwidth regime. It also shows the existence of a critical bandwidth above which the efficiencyseverely decreases.
Under strong drives, which are becoming necessary for fast high-fidelity operations, transmons can be structurally unstable. Due to chaotic effects, the computational manifold is no longer well separated from the remainder of the spectrum, which correlates with enhanced offset-charge sensitivity and destructive effects in readout. We show here that these detrimental effects can further propagate to other degrees of freedom, for example to neighboring qubits in a multi-qubit system. Specifically, a coherently driven transmon can act as a source of incoherent noise to another circuit element coupled to it. By using a full quantum model and a semiclassical analysis, we perform the noise spectroscopy of the driven transmon coupled to a spectator two-level system (TLS), and we show that, in a certain limit, the interaction with the driven transmon can be modeled as a stochastic diffusive process driving the TLS.
This thesis explores novel strategies for advancing superconducting quantum computing through a series of experimental investigations and device innovations. Grounded in the development of circuit quantum electrodynamics, the work addresses the challenges posed by the limitations of coherent control, coherence time and scaling of superconducting qubits. Focusing on transmon qubits, the thesis first reviews the foundational principles of circuit quantum electrodynamics and the critical role of the transmon in achieving robust qubit performance. Three experimental studies form the core of this work. The first study demonstrates a universal quantum gate set for strongly coupled transmons. By exploiting the residual ZZ interaction, the work introduces a novel two-axis gate protocol for single-qubit operations and implements both CZ and CNOT gates with high fidelities, highlighting the potential for fast, low-error quantum operations in the strong coupling regime. The second study investigates a dual-rail qubit architecture using parametrically coupled transmons, showcasing a hardware-efficient approach to quantum error correction. This experiment converts single-photon loss errors into detectable erasures and leverages mid-circuit detection to enhance qubit coherence, thereby advancing fault-tolerant quantum computing schemes in the NISQ era. The final study focuses on the development of merged-element transmons (MET) and their FinMET variants. By integrating the Josephson junction and shunt capacitor into a single trilayer device, these innovations achieve a dramatic reduction in device footprint while addressing loss channels through advanced materials engineering, ultimately paving the way for scalable, high-coherence quantum circuits. Collectively, these contributions demonstrate significant progress in quantum gate design, error mitigation, and qubit architecture, offering promising routes toward the realization of largescale, fault-tolerant quantum computers
Superconducting circuit quantum electrodynamics is the main architecture for implementation of quantum processors. This work presents a proposal to miniaturize quantum memory devices by combining the quantum memory qubits encoded directly in the first two Fock states ${|0\ensuremath{\rangle},|1\ensuremath{\rangle}}$ and the bus channel in the same specially designed multimode cavity. One key feature of the resonant modes used for quantum memory is that the microwave current is localized, which facilitates the storage and retrieval of information in superconducting qubits while preventing crosstalk from other resonant modes. Frequency design and distinct coupling strengths in this kind of device are analyzed with electromagnetic simulations and numerical calculations. The quantum memory properties, e.g., storage speed, fidelity, and stability, are examined in four cases: (1) memory operation of single-qubit state, (2) disturbing effects, (3) coupling operation, and (4) entangled states storage. Decoherence time of the quantum processor can be extended to more than one order of magnitude longer than that without the memory. By integrating transmons with optimally designed coplanar waveguide multimode quantum memories, the present study could provide an optimistic perspective for the development of large-scale superconducting quantum processors.
Quantum error correction imposes stringent requirements on the underlying hardware: all core operations of a quantum processor – single-qubit gates, two-qubit entangling gates, qubit readout, and qubit reset – must operate well below the error thresholds of leading codes. While such thresholds have already been crossed in several platforms, the associated overhead in physical qubits and circuit depth remains extremely sensitive to the physical error rate. This thesis investigates how far a superconducting platform based on transmon qubits can be pushed toward its physical limits, and how architectural and control choices can be optimized to reduce the overhead required for fault-tolerant quantum computation. First, we develop and characterize tunable-coupler architectures that enable fast, high-fidelity controlled-Z (CZ) gates. A long-distance coupler increases the qubit-qubit separation, freeing chip area for additional on-chip components, while still achieving CZ fidelities above 99.8%. Building on this, we optimize the qubit-coupler coupling to simultaneously support high-performance single- and two-qubit gates. We achieve simultaneous single- and two-qubit gate fidelities exceeding 99.9% on the same chip using a CZ calibration experiment based on our phase-averaged leakage error amplification (PALEA) protocol. PALEA coherently amplifies small over- and under-rotations of the diabatic CZ gate, making calibration errors easier to detect and correct. Second, we address single-qubit gates. Using spectrally engineered FAST-DRAG and higherderivative DRAG pulses, we demonstrate sub-8 ns gates with errors of order 10!" and strongly suppressed leakage. Third, we address readout by combining shelving to a higher qubit level with Purcell-filtered resonators and quantum-limited amplification, achieving readout errors below 10!#. Beyond the transmon workhorse, we explore the unimon qubit, which combines large anharmonicity with reduced charge sensitivity, and investigate a quantum circuit refrigerator as an integrated resource for fast, unconditional qubit reset via engineered dissipation. Finally, we demonstrate a cryogenic microwave link between two dilution refrigerators, enabling remote state transfer and entanglement over a five-meter distance and illustrating a path toward modular and distributed superconducting quantum processors. Together, these results show that with careful co-design of device layout, coupling topology, pulse shaping, and calibration protocols, all core operations – except qubit reset – of a superconducting quantum processor can be operated in the 10!#–10!" error regime within a coherent hardware and control stack, substantially lowering the resource overhead required for future fault-tolerant architectures.
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.
Transmon qubits are ubiquitously used in superconducting quantum information processor architectures.Strong drives are required to realize fast, high-fidelity, gates and measurements, including parametrically activated processes.Here, we show that even off-resonant drives, in regimes routinely used in experiments, can cause strong modifications to the structure of the transmon spectrum rendering a large part of it chaotic.Accounting for the full nonlinear dynamics of the transmon in a Floquet-Markov formalism, we find that these chaotic states, often neglected through the hypothesis that the anharmonicity is weak, strongly impact the lifetime of the transmon's computational states.In particular, we observe that chaos-assisted quantum phase slips greatly enhance band dispersions.In the presence of a measurement resonator, we find that approaching chaotic behavior correlates with strong transmon-resonator hybridization, and an average resonator response centered on the bare resonator frequency.These results lead to a photon-number threshold characterizing the appearance of chaos-induced quantum demolition effects during strong-drive operations, such as dispersive qubit readout.The phenomena described here are expected to be present in all circuits based on low-impedance Josephson junctions.