Superinductive Ultrastrong Couplings in Superconducting Quantum Circuits

by Alba Torras (IFAE)

Monday Nov 17, 2025, 10:00 AM → 2:00 PM Europe/Madrid

IFAE Seminar Room (In-Person)

Supervisor: Pol Forn-Díaz

One of the simplest systems to study light-matter interactions in superconducting quantum circuits consists of a superconducting qubit coupled to a resonator. In general, for quantum computing and other qubit-related applications, the qubit-resonator coupling is designed well below the bare frequencies of the individual elements, allowing for a set of approximations which yield a relatively simple framework. As the coupling increases, the approximations begin to fail and the system enters the so-called ultrastrong coupling regime (USC), where the physics of the system have been largely unexplored.
  In this thesis, we introduce superinductor materials as an approach to couple a flux qubit ultrastrongly to a resonator. Usually, achieving the USC regime has led to circuit designs that impaired qubit coherence. Hence, we study a new approach with superinductors to circumvent these complications. The large kinetic inductance provided by these materials allows one to design large shared linear inductors while keeping relatively small qubit loops and low persistent currents. We present a device consisting of a C-shunted 3-Josephson junction flux qubit galvanically coupled to a resonator with a wire of granular Aluminum (grAl). We derive the necessary Hamiltonians and numerical methods to analyze the theoretical spectrum and obtain an estimate of the coupling coefficient of the system. Additionally, we provide the details of the developed multi-step qubit fabrication recipe which allows to adjust each component to guarantee that the final chip is in the USC regime. In terms of design, special attention is put to obtain a qubit-resonator system close to resonance, at a range of measurable frequencies, and coherent enough.
  We report the spectral measurements of a flux-qubit resonator device with low persistent current and a large shared inductance. We observe USC features such as the effect of counter-rotating terms, evidenced as a Bloch-Siegert shift of 23 MHz. The coupling coefficient is large enough to be in the perturbative USC regime with g/ωr ≃ 0.13. The measured circuit serves as a proof-of-concept for the possibility of reaching large qubit-resonator couplings with low persistent current qubits and opens the door to coherent studies in the USC regime.
  The use of superinductors to study USC also motivates the search for new materials with superior properties to grAl. We present the development a novel superconducting material based on Alumium nitridization which can be obtained by sputtering Aluminum in different N2/Ar fractional flows. In this thesis, we analyze the main superconducting properties of nitridized Aluminum (NitrAl) thin films of 100 nm. We report the measurement of enhanced critical temperatures reaching Tc = (3.38±0.01) K and resilience to in-plane magnetic fields well above 1 T. Similarly to granular aluminum, we observe a dome-shape like distribution of the critical temperature versus room-temperature resistivity. Additionally, we estimate a kinetic inductance ranging from 1 pH/□ for the least resistive samples to 400 pH/□ for higher resistivity films. Finally, we present the first steps towards the characterization of losses of NitrAl and the implementation of the material as a superinductor in superconducting quantum circuit technologies.

Zoom link: https://us02web.zoom.us/j/89787514064?pwd=SkRaOElqanZRNFZXM2d2SE9PN1d0Zz09