A leading platform for quantum information processing are superconducting circuits, which can now comprise tenths of qubits. One of the key challenges in the design of these systems is balancing the tradeoff between access to the qubits for readout and control versus environment induced decoherence. In other words, the same lines used to access the information on the system, unavoidable induce losses and destroy the quantum information store on it. To mitigate this issue, nonreciprocal devices are included between the superconducting qubits and the control electronics. Commercially available microwave nonreciprocal devices are bulky, and impractical for scaling these systems to the scales necessary for the implementation of useful quantum computing algorithms. For this reason, efforts are being directed to the development of on-chip microwave nonreciprocal devices. However, due to the large scale of these systems, their design and modelling is an ongoing challenge. In this thesis, we show how effective dynamical models of superconducting qubits can be obtained when these are coupled by nonreciprocal linear responses, extending the previous work to include on-chip nonreciprocal devices, and more formally all kinds of effectively linear lossless microwave devices. Notably, our formalism quantitatively captures the tradeoff between control and dissipation induced by coupling to the external electronics. In addition, we provide a modern presentation of the theoretical background underpinning circuit synthesis and modelling. Additionally, while scaling to thousands of qubits is necessary for the implementation of useful quantum computing algorithms, this is not the only route where quantum information processing systems can find applications in other fields. In particular, quantum metrology offers a venue where only a few well-tuned qubits and oscillators are needed for a quantum sensing advantage. Examples where these advantages have been realize include dark-matter searches, satellite-free navigational systems, imaging of biological tissues, among others. Despite their promise, quantum sensors must balance the tradeoff between sensitivity to external perturbations and robustness against noise sources. A pathway to limit the detrimental effects of noise and maximize the achievable sensitivity is given by reservoir-engineering and quantum error-correction. Thus, the development of quantum sensors robust against noise is a promising venue with broad applications. In our work, we show how recent protocols developed for the stabilization of bosonic codes can be adapted for quantum sensing. In particular, we show how stabilizing protocols for the Gottesman-Kitaev-Preskill code can be adapted for the quantum sensing of displacements. Quantum sensing of displacement may find applications in for example electric field sensing, force sensing and gravitational waves sensing. Remarkably, the protocol we study nearly saturates the sensitivity allowed by quantum mechanics.
A leading platform for quantum information processing are superconducting circuits, which can now comprise tenths of qubits. One of the key challenges in the design of these systems is balancing the tradeoff between access to the qubits for readout and control versus environment induced decoherence. In other words, the same lines used to access the information on the system, unavoidable induce losses and destroy the quantum information store on it. To mitigate this issue, nonreciprocal devices are included between the superconducting qubits and the control electronics. Commercially available microwave nonreciprocal devices are bulky, and impractical for scaling these systems to the scales necessary for the implementation of useful quantum computing algorithms. For this reason, efforts are being directed to the development of on-chip microwave nonreciprocal devices. However, due to the large scale of these systems, their design and modelling is an ongoing challenge. In this thesis, we show how effective dynamical models of superconducting qubits can be obtained when these are coupled by nonreciprocal linear responses, extending the previous work to include on-chip nonreciprocal devices, and more formally all kinds of effectively linear lossless microwave devices. Notably, our formalism quantitatively captures the tradeoff between control and dissipation induced by coupling to the external electronics. In addition, we provide a modern presentation of the theoretical background underpinning circuit synthesis and modelling. Additionally, while scaling to thousands of qubits is necessary for the implementation of useful quantum computing algorithms, this is not the only route where quantum information processing systems can find applications in other fields. In particular, quantum metrology offers a venue where only a few well-tuned qubits and oscillators are needed for a quantum sensing advantage. Examples where these advantages have been realize include dark-matter searches, satellite-free navigational systems, imaging of biological tissues, among others. Despite their promise, quantum sensors must balance the tradeoff between sensitivity to external perturbations and robustness against noise sources. A pathway to limit the detrimental effects of noise and maximize the achievable sensitivity is given by reservoir-engineering and quantum error-correction. Thus, the development of quantum sensors robust against noise is a promising venue with broad applications. In our work, we show how recent protocols developed for the stabilization of bosonic codes can be adapted for quantum sensing. In particular, we show how stabilizing protocols for the Gottesman-Kitaev-Preskill code can be adapted for the quantum sensing of displacements. Quantum sensing of displacement may find applications in for example electric field sensing, force sensing and gravitational waves sensing. Remarkably, the protocol we study nearly saturates the sensitivity allowed by quantum mechanics.
One of the key elements in circuit quantum electrodynamics (cQED) is the superconducting microwave resonator, which can enable strong coupling to and between nanoscale quantum systems, and as a result has emerged as an essential tool for both quantum information and quantum sensing. In this thesis, I will discuss new applications of superconducting resonators in both of these fields, which illustrates how quantum information and quantum sensing can be strongly tied together. As the field of quantum information continues to grow, various platforms for realizing a quantum computer have been proposed. One attractive platform is to control and manipulate the spin of an electron to form spin qubits, which have demonstrated a long coherence time, weak coupling to the environment, and high fidelity single- and two-qubit gates. However, scaling beyond two qubits has proven challenging, as has incorporating long-range coupling, although both are crucial for making spin qubits a scalable quantum computing platform. The confinement of electromagnetic fields in superconducting resonators enables strong coupling to small systems, such as the single electrons of a spin qubit. In the first part of this thesis, I will describe how a singlet-triplet spin qubit coupled to a high-impedance superconducting resonator constitutes a promising architecture for mediating long-ranged interactions based on a longitudinal coupling between the resonator and qubit. I will discuss the technical developments needed to successfully implement this novel type of interaction in a hybrid spin qubit-resonator device. This type of interaction has several advantages over the more commonly studied transverse coupling scheme: it does not require careful tuning of the qubit frequency to match that of the resonator, which facilitates scaling to a large number of qubits, and also avoids the loss of coherence, which is inevitable in resonant coupling. This type of interaction therefore opens promising paths for generating high-fidelity long-ranged two-qubit coupling by employing superconducting microwave resonators. By adapting techniques used in quantum information, we have also developed a new technique in quantum sensing, where we employ superconducting resonators as a probe of quantum materials such as mesoscopic systems, two-dimensional materials, and heterostructures, which are otherwise challenging to experimentally access with conventional techniques. In the second part of my thesis, I will discuss how implementing cQED techniques from quantum information allows us to probe the symmetry of the order parameter in unconventional superconductors. Unconventional superconductors are not readily available, and we therefore study an engineered system that comprises of interfacing a ferromagnet and superconductor, in which unconventional pairing can be induced into the ferromagnet. This platform allows us to simultaneously excite spin waves, a collective excitation of spins, in the ferromagnet, which opens new directions for studying interactions between coherently coupled magnons (quanta of spin waves) and unconventional superconducting states. Finally, by applying the same technique to a two-dimensional (2D) superconductor, tungsten ditelluride, we leverage this platform in an attempt to study the order parameter of its intrinsic superconducting phase. The nature of the order parameter in this material remains an open question, as both topological and superconducting phases can exist, possibly simultaneously. This work thus demonstrates a unique use of superconducting resonators beyond quantum information applications.
Circuit quantum electrodynamics (cQED), exemplified by a coupled system of a superconducting qubit and a microwave cavity, is a powerful framework for the quantum control and measurement of macroscopic quantum states in solid-state circuits. Quantum circuits based on superconducting qubits have emerged as one of the leading platforms in the pursuit of fault-tolerant quantum computing. Despite significant progress in extending qubit lifetimes, refining experimental protocols, and scaling up quantum processors, errors arising from the control and measurement of superconducting qubits continue to hinder the realization of a universal quantum computer. To investigate the error mechanisms for state-of-the-art qubits, we realize long-lived superconducting transmon qubits that exhibit fluctuating lifetimes, with an average of 0.2 ms and a maximum exceeding 0.4 ms. We then investigate their dominant error mechanism. By introducing novel time-resolved error measurements that are synchronized with the operation of the pulse tube cooler in a dilution refrigerator, we find that mechanical vibrations from the pulse tube induce nonequilibrium dynamics in highly coherent qubits, leading to their correlated bit-flip errors. Our findings not only deepen our understanding of qubit error mechanisms but also provide valuable insights into potential error-mitigation strategies for achieving fault tolerance by decoupling superconducting qubits from their mechanical environments. We also present a novel scheme for operating a cQED system in a regime where the cavity is largely displaced. We find that a qubit drive at the bare cavity frequency can effectively displace the cavity field while minimizing the cavity-drive-induced dephasing and the ac Stark shift. Furthermore, we demonstrate that standard cQED protocols, such as qubit control, readout, and cavity single-photon generation, remain compatible with largely displaced cavity fields, even beyond the critical photon number. These findings open new pathways for exploring cQED dynamics in large-photon-number regimes, including the study of nonlinear phenomena in microwave cavities, the saturation of two-level system baths for extending the coherence of microwave quantum states, and the acceleration of the dispersive readout. Finally, mechanical resonators have emerged as integral components in quantum computing for storing and transducing quantum information. Low-frequency mechanical resonators, with extremely low energy dissipation rates, are particularly interesting for extending the lifetimes of stored quantum information. However, integrating low-frequency mechanical resonators with qubits is challenging because their small energy gaps make coherent energy exchange and ground-state preparation difficult. In this thesis, we report the first steps towards the characterization of a hybrid quantum device, consisting of a Transmon qubit and a circuit optomechanical system, both coupled to a 3D microwave cavity.
Quantum metrology based on superconducting qubits offers unprecedented precision in measuring fundamental physical quantities, particularly magnetic fields. In this study, we develop a modified quantum phase estimation algorithm utilizing superconducting qubits, enhanced by optimized initial state preparation, significantly improving measurement sensitivity and reducing estimation times compared to conventional techniques. To ensure practical applicability and robustness, we incorporate advanced noise modeling, including intrinsic decoherence effects such as flux-induced noise and Purcell losses. Through detailed analytical modeling and numerical simulations, we systematically evaluate sensitivity and practical implementation constraints across various superconducting qubit architectures—transmon, fluxonium, conventional flux, and C-shunt flux qubits. Our approach quantifies the critical trade-offs between coherence, measurement dynamics, and sensitivity, identifying optimal operational regimes for high-performance quantum sensing. We introduce custom circuit simulation methods enabling the analysis and optimization of arbitrary superconducting quantum devices. Our findings provide a framework for advancing quantum-enhanced magnetometry and promise to facilitate next-generation quantum sensors for fundamental physics research, gravitational field measurements, and quantum-assisted searches for dark matter candidates.
Engineered quantum systems are poised to revolutionize information science in the near future. A persistent challenge in applied quantum technology is creating controllable, quantum interactions while preventing information loss to the environment, decoherence. In this thesis, we realize mesoscopic superconducting circuits whose macroscopic collective degrees of freedom, such as voltages and currents, behave quantum mechanically. We couple these mesoscopic devices to microwave cavities forming a cavity quantum electrodynamics (QED) architecture comprised entirely of circuit elements. This application of cavity QED is dubbed Circuit QED and is an interdisciplinary field seated at the intersection of electrical engineering, superconductivity, quantum optics, and quantum information science. Two popular methods for taming active quantum systems in the presence of decoherence are discrete feedback conditioned on an ancillary system or quantum reservoir engineering. Quantum reservoir engineering maintains a desired subset of a Hilbert space through a combination of drives and designed entropy evacuation. Circuit QED provides a favorable platform for investigating quantum reservoir engineering proposals. A major advancement of this thesis is the development of a quantum reservoir engineering protocol which maintains the quantum state of a microwave cavity in the presence of decoherence. This thesis synthesizes strongly coupled, coherent devices whose solutions to its driven, dissipative Hamiltonian are predicted a priori. This work lays the foundation for future advancements in cavity centered quantum reservoir engineering protocols realizing hardware efficient circuit QED designs.
This thesis explores applications of Josephson circuits for protecting and processing quantum information. The work is structured around three main research directions. First, we investigate quantum error correction for a logical qubit encoded in Gottesman-Kitaev-Preskill (GKP) grid states within a high-quality microwave resonator. We extract error syndromes through stroboscopic interactions with an auxiliary qubit and apply error correction via Markovian feedback. However, we identify a key limitation: noise propagating from the auxiliary qubit introduces an uncorrectable error channel. To mitigate this, we propose mediating the interaction through a buffer resonator, itself prepared in a GKP state. We demonstrate that asymmetric preparation of the buffer quadratures can robustly suppress propagating errors. Second, we introduce a distinct approach to GKP error correction, where the target resonator interacts with an auxiliary mode through modular operators that commute with logical operators. This configuration prevents noise from propagating at the logical level. Third, we propose a robust on-chip implementation of tunable coupling and circulation using a reconfigurable traveling wave parametric converter. This device supports two propagation modes with different wave velocities, allowing phase-matching of conversion processes between counterpropagating signals. This work contributes to advancing error-corrected quantum information processing with Josephson circuits, addressing key challenges in quantum error correction and circuit design.
Introducing new components and functionalities into quantum devices is critical in advancing state-of-the-art hardware. Here, we propose superconducting diodes (SDs) as a coherent nonreciprocal element in circuit quantum electrodynamics (cQED) architectures. In particular, we use an asymmetric SQUID as an SD controlled with a flux bias - nonreciprocal element with single control handle and on-chip modality. We spectroscopically characterize SD and show that flux bias acts cooperatively with the nonlinear diode response to induce direction-dependent resonance shifts in the transmission spectrum. We show that even with modest diode efficiency the isolation isolation ratio is sufficiently high, and scales with multiple SDs. We demonstrate the use of the SD as a coupler to realize coherent nonreciprocal qubit-qubit coupling. With a minimal two qubit system, we demonstrate nonreciprocal half-iSWAP, thereby showcasing the potential of intrinsic nonreciprocity as a tool to perform arbitrary two-qubit gates. Our work enables high-fidelity signal routing and entanglement generation in all-to-all connected microwave quantum networks, where nonreciprocity is embedded at the device level.