Superconducting qubits equipped with quantum non-demolishing readout and active feedback can be used as information engines to probe and manipulate microscopic degrees of freedom, whetherintentionally designed or naturally occurring in their environment. In the case of spin systems, the required magnetic field bias presents a challenge for superconductors and Josephson junctions. Here we demonstrate a granular aluminum nanojunction fluxonium qubit (gralmonium) with spectrum and coherence resilient to fields beyond one Tesla. Sweeping the field reveals a paramagnetic spin-1/2 ensemble, which is the dominant gralmonium loss mechanism when the electron spin resonance matches the qubit. We also observe a suppression of fast flux noise in magnetic field, suggesting the freezing of surface spins. Using an active state stabilization sequence, the qubit hyperpolarizes long-lived two-level systems (TLSs) in its environment, previously speculated to be spins. Surprisingly, the coupling to these TLSs is unaffected by magnetic fields, leaving the question of their origin open. The robust operation of gralmoniums in Tesla fields offers new opportunities to explore unresolved questions in spin environment dynamics and facilitates hybrid architectures linking superconducting qubits with spin systems.
Resonator measurements are a simple but powerful tool to characterize a material’s microwave response. The losses of a resonant mode are quantified by its internal quality factorQi, which can be extracted from the scattering coefficient in a microwave reflection or transmission measurement. Here we show that a systematic error on Qi arises from Fano interference of the signal with a background path. Limited knowledge of the interfering paths in a given setup translates into a range of uncertainty for Qi, which increases with the coupling coefficient. We experimentally illustrate the relevance of Fano interference in typical microwave resonator measurements and the associated pitfalls encountered in extracting Qi. On the other hand, we also show how to characterize and utilize the Fano interference to eliminate the systematic error.
Mesoscopic Josephson junctions (JJs), consisting of overlapping superconducting electrodes separated by a nanometer thin oxide layer, provide a precious source of nonlinearity for superconductingquantum circuits and are at the heart of state-of-the-art qubits, such as the transmon and fluxonium. Here, we show that in a fluxonium qubit the role of the JJ can also be played by a lithographically defined, self-structured granular aluminum (grAl) nano-junction: a superconductor-insulator-superconductor (SIS) JJ obtained in a single layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large grAl nano-junction charging energy in the range of 10−100GHz, comparable to its Josephson energy EJ. We measure average energy relaxation times of T1=10μs and Hahn echo coherence times of Techo2=9μs. The exponential sensitivity of the gralmonium to the EJ of the grAl nano-junction provides a highly susceptible detector. Indeed, we observe spontaneous jumps of the value of EJ on timescales from milliseconds to days, which offer a powerful diagnostics tool for microscopic defects in superconducting materials.
High kinetic inductance materials constitute a valuable resource for superconducting quantum circuits and hybrid architectures. Superconducting granular aluminum (grAl) reaches kineticsheet inductances in the nH/□ range, with proven applicability in superconducting quantum bits and microwave detectors. Here we show that the single photon internal quality factor Qi of grAl microwave resonators exceeds 105 in magnetic fields up to 1T, aligned in-plane to the grAl films. Small perpendicular magnetic fields, in the range of 0.5mT, enhance Qi by approximately 15%, possibly due to the introduction of quasiparticle traps in the form of fluxons. Further increasing the perpendicular field deteriorates the resonators‘ quality factor. These results open the door for the use of high kinetic inductance grAl structures in circuit quantum electrodynamics and hybrid architectures with magnetic field requirements.