Interplay Between Kinetic Inductance, Nonlinearity, and Quasiparticle Dynamics in Granular Aluminum Microwave Kinetic Inductance Detectors

Abstract

Microwave kinetic inductance detectors (MKIDs) are thin film, cryogenic, superconducting resonators. Incident Cooper pair-breaking radiation increases their kinetic inductance, thereby measurably lowering their resonant frequency. For a given resonant frequency, the highest MKID responsivity is obtained by maximizing the kinetic inductance fraction α. However, in circuits with α close to unity, the low supercurrent density reduces the maximum number of readout photons before bifurcation due to self-Kerr non-linearity, therefore setting a bound for the maximum α before the noise equivalent power (NEP) starts to increase. By fabricating granular aluminum MKIDs with different resistivities, we effectively sweep their kinetic inductance from tens to several hundreds of pH per square. We find a NEP minimum in the range of 25 aW/ √ Hz at α ≈ 0.9, which results from a trade-off between the onset of non-linearity and a non-monotonic dependence of the noise spectral density vs. resistivity.

Implementation of a Transmon Qubit Using Superconducting Granular Aluminum

“…Following this proof-of-principle demonstration, future developments will focus on replacing all pure Al components with more field resilient materials, such as lowresistivity grAl [33] or Nb compounds [21,22,49]. This will allow us to operate coherent superconducting qubits in magnetic fields beyond 1 T. In the current design, the Al pads play the role of phonon and quasiparticle traps .…”

“…A more detailed analysis of the qubit transition frequency with much higher resolution is presented in Appendix H. (b) Internal quality factor Q i for the shielded (filled triangles) and unshielded (open pentagons and triangles) case versus the in-plane magnetic field B y . [33,50], enhancing the coherence. In future designs, in which Al is substituted by other superconductors with higher gap and critical field, quasiparticle poisoning could be mitigated by adding dedicated phonon traps disconnected from the qubit [51,52].…”

“…The constituent Al grains, about 3-5 nm in diameter [32], are separated by thin oxygen barriers; therefore, grAl structures can be modeled as arrays of JJs [30]. Their kinetic inductance is tunable over orders of magnitude up to nH=□ [33].…”

DEMETRA: Suppression of the Relaxation Induced by Radioactivity in Superconducting Qubits

“…As explained in the previous section, KIDs are an ideal benchmark to investigate the effects of radioactivity in qubits. For the first test, we fabricated KIDs using granular aluminum (GrAl), a promising material for qubits [35,36]. We deposited three 20-nmthick GrAl resonators on a single 15 × 8 × 0.33 mm 3 sapphire crystal.…”

Compact itinerant microwave photonics with superconducting high-kinetic inductance microstrips

“…In addition, the nonlinearity [22,23] of such HKI microstrips provides a route to amplification at the quantum limit [19,20,[24][25][26] due to wave-mixing phenomena. Single HKI superconducting strips have been used to build high-quality microwave resonators [27], superinductors for use in qubit architectures [28], kinetic inductance detectors [23,[29][30][31], galvanometers [32], and more. This can now be extended to a multi-mode network for more complex photonic tasks.…”