On-Chip NbN Superconducting Resonators for Hybrid Magnon-Photon Coupled Systems
Muntasir Mahdi, Sherman E. Peek, Archit Shah, Chase C. Tillman, John A. Sellers, Andrew Christy, Yuzan Xiong, Nga T. Do, Tae Hee Kim, Wei Zhang, Michael C. Hamilton
Inthe application of quantum information transduction with superconducting (SC) qubits, SC resonators play a phenomenal role in magnon-photon coupled systems. We present NbN (T$_{c}$= 10.8 K) superconducting microstrip resonators tailored for hybrid magnon-photon coupled systems. Our resonators exhibit remarkable coupling capability, enabling rigorous probing of magnon-photon interactions. With a fundamental frequency of 1.4 GHz and a second harmonic of 2.8 GHz, these resonators offer extensive frequency coverage, facilitating the identification and characterization of a wide spectral range. It exhibits a strong magnon-microwave photon coupling with a sphere of yttrium iron garnet (YIG). The average gyromagnetic ratio ($\gamma /2\pi$) and coupling strength ($g/2\pi$) achieved are 2.7 MHz/Oe and 11.14 MHz, respectively. The NbN SC resonators developed in this work address challenges associated with probing magnon-photon coupling in strong magnetic fields and are expected to serve as a unique platform for studying magnon-photon coupled hybrid devices.
Inthe application of quantum information transduction with superconducting (SC) qubits, SC resonators play a phenomenal role in magnon-photon coupled systems. We present NbN (Tc= 10.8 K) superconducting microstrip resonators tailored for hybrid magnon-photon coupled systems. Our resonators exhibit remarkable coupling capability, enabling rigorous probing of magnon-photon interactions. With a fundamental frequency of 1.4 GHz and a second harmonic of 2.8 GHz, these resonators offer extensive frequency coverage, facilitating the identification and characterization of a wide spectral range. It exhibits a strong magnon-microwave photon coupling with a sphere of yttrium iron garnet (YIG). The average gyromagnetic ratio (γ/2π) and coupling strength (g/2π) achieved are 2.7 MHz/Oe and 11.14 MHz, respectively. The NbN SC resonators developed in this work address challenges associated with probing magnon-photon coupling in strong magnetic fields and are expected to serve as a unique platform for studying magnon-photon coupled hybrid devices.
Superconducting resonators have previously been used for magnon-photon coupling, which facilitates quantum information transduction. In this work, we demonstrate robust superconducting Nb resonators that show closely spaced resonances with high quality (Q) factors. The fundamental frequencies ranging from 440 MHz to 668 MHz with second harmonics at 870 MHz and 1.3 GHz, offer extensive frequency coverage. They maintain resonance integrity by exhibiting useful Q-factors for many harmonics, even in magnetic fields up to 1 T at 1.7 K. These resonators provide a versatile platform for investigating magnon-photon interactions in hybrid magnonic systems, overcoming limitations of previous designs and offering the potential for broad frequency coverage and application in quantum information studies.
As the field of superconducting quantum technology progresses beyond single-qubit control, scalable and multifunctional quantum architectures become increasingly vital. A central challenge is integrating fast, controllable superconducting circuits with components that offer longer coherence, richer physics, or new functionalities. Hybrid quantum systems, where superconducting microwave resonators interface with semiconducting or magnetic subsystems, have the capabilities to fulfil this need, yet they face significant engineering obstacles. Conventional superconducting resonator designs struggle under magnetic field biasing and electrostatic gating, both of which are essential for coupling to spin-based, semiconductor, or magnonic elements.This thesis explores solutions to these bottlenecks through the development of magnetic-field-resilient coplanar stripline (CPS) resonators based on thin-film NbTiN. These resonators retain a stable resonance frequency in in-plane magnetic fields up to 1T, can have high kinetic inductance and offer broad geometrical tunability, enabling strong and ultrastrong coupling to charge- and spin-based quantum systems. In the superconducting–semiconductor domain, we demonstrate high-impedance CPS resonators inductively coupled to Josephson junctions hosting Andreev bound states (ABSs), revealing both coherent pair transitions and single-quasiparticle excitations near the ultrastrong coupling regime. This opens the door to fast, gate-tunable qubits based on microscopic ABSs, distinct from conventional macroscopic qubit architectures, and lays the groundwork for spin–photon hybridisation, non-perturbative light–matter interactions, and parity-based quantum logic.In parallel, this work investigates rare-earth iron garnet (ReIG) ferrimagnetic insulators as low-loss spin-wave media compatible with superconducting devices. In sputtered thin-film thulium iron garnet (TmIG) with perpendicular magnetic anisotropy, we observe deterministic spin-orbit torque switching and multiple magnetisation polarity reversals. These properties showcase the attractiveness of TmIG both for low-dissipation magnonic information transport and as magnetic interfaces for superconducting hybrid devices. Taken together, the results of this thesis establish materials, device concepts, and measurement strategies for novel classes of hybrid quantum systems that combine robust superconducting circuitry, electrostatically tunable semiconductor excitations, and industry-scalable magnetic elements and pave the way for next-generation hybrid quantum systems. More broadly, they provide concrete routes to overcoming limitations of existing hybrid platforms and advance the development of versatile, low-dissipation quantum technologies.
We demonstrate strong magnon-photon coupling of a thin-film Permalloy device fabricated on a coplanar superconducting resonator. A coupling strength of 0.152 GHz and a cooperativity of 68 are found for a 30-nm-thick Permalloy stripe. The coupling strength is tunable by rotating the biasing magnetic field or changing the volume of Permalloy. We also observe an enhancement of magnon-photon coupling in the nonlinear regime of the superconducting resonator, which is attributed to the nucleation of dynamic flux vortices. Our results demonstrate a critical step towards future integrated hybrid systems for quantum magnonics and on-chip coherent information transfer.
Hybrid quantum systems play an essential role in efforts towards the advancements of quantum information technology and quantum sensing by bridging distinct technological platforms. In particular, hybrid systems comprising superconducting microwave circuits are being pursued intensely in light of existing achievements in superconducting qubits and precise quantum control in microwave circuits. Their realization relies on the design of superconducting resonators capable of coupling to an oscillatory mode of a second quantum system, facilitating the probing and manipulation of one via the other. In order to accommodate the coupled system, its particular constraints and requirements must be respected, which may be quite extreme from the perspective of established superconducting circuit design and operation. Overcoming this challenge has proved to be a substantial obstacle for the attainment of strongly coupled hybrid systems, despite spectacular advances in superconducting microwave circuits individually. In this work, we examine two approaches tackling this issue for different hybrid quantum systems, focusing on the optimization and characterization of the superconducting circuits. The first device aims at a superconductor-atom hybrid system through the coupling to Rydberg-Rydberg transitions in ultra-cold Rb-87 atoms trapped near the superconducting chip. Using simulations in conjunction with theoretical considerations, we demonstrate a comprehensive geometry parameter optimization procedure for maximum coupling rate. Presenting an experimental implementation of the resulting circuit, we validate its suitability for use under the intended operation conditions, including the option of tuning the electronic transition frequencies using a dc voltage. Secondly, we investigate a device designed for applications in large magnetic in-plane fields used to mediate coupling to integrated micromechanical oscillators; more specifically it is a flux-tunable SQUID resonator comprising Josephson nano-constrictions as nonlinear inductive components. Performing a thorough characterization of its frequency and Kerr anharmonicity tuning behavior at fields up to several hundred mT, we discover a pronounced asymmetry in its flux response at large fields. We show this asymmetry to be caused by the nano-constrictions turning into Josephson diodes, develop an intuitive macroscopic model for its field-induced emergence and reconstruct the diode current-phase relation from the flux tuning data. Several figures of merit informing the suitability of the SQUID resonators for optomechanical systems as well as other applications are improved by the diode effect, highlighting the potential of Josephson diodes in superconducting microwave circuits. Our results provide a promising stepping stone for the advancement of hybrid systems involving superconducting microwave circuits, putting new and exciting operation and coupling regimes within reach.
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.