Granular superconductors for high kinetic inductance and low loss quantum devices

Abstract

Granular aluminum is a promising material for high kinetic inductance devices such as qubit circuits. It has the advantage over atomically disordered materials such as NbNx, to maintain a high kinetic inductance concomitantly with a high quality factor. We show that high quality nano-scale granular aluminum films having a sharp superconducting transition with normal state resistivity values on the order of 1×105 μΩ cm and kinetic inductance values on the order of 10 nH/◻ can be obtained, surpassing the state-of-the-art values. We argue that this is a result of the different nature of the metal-to-insulator transition, being electronic correlations driven (Mott type) in the former and disorder driven (Anderson type) in the latter.

The Role of the Short Coherence Length in Unconventional Superconductors

“…Both are in a BCS-to-BEC cross-over regime, as attested to by their short coherence length (a few lattice spacings in the cuprates and a few grain sizes in granular Al) and the large value of the strong coupling factor. In both cases, one is close to a Mott insulating state-spins are present as expected [14]. The experimental results suggest that there is a close relationship between a Mott transition and the BCS-to-BEC cross-over.…”

“…They are not observed in atomically disordered superconductors near their transition, where the critical temperature collapses and many sub-gap states appear, because the transition is of the Anderson type. The vicinity of a Mott transition is what allows the successful use of thin film granular superconductors to produce circuit elements that have, at the same time, a large kinetic inductance and low losses [14].…”

Tuning superinductors by quantum coherence effects for enhancing quantum computing

“…Although U = −1 and n = 0.875 are well in the weak coupling region, we cannot further reduce these values because finite size effects become important. As a consequence, our results are not directly applicable to experiments involving granular Al [13]. The physical mechanism leading to the enhancement of Q and L k are still at play for weaker couplings but the role of collective excitations or the specific effects related to the granularity require a separate study.…”

“…Moreover, a simple mean-field prediction for L k shows that further enhancement can be achieved by increasing disorder. For those reasons, in recent years, there have been an explosion of interest [5][6][7][8][9][10][11][12][13][14][15][16][17][18] in different materials and experimental settings involving small samples of low-dimensional, disordered superconductors as candidates for high-inductance devices, commonly called superinductors.…”

“…Weakly coupled superconducting thin films and nano-wires of NbN x and granular [19][20][21][22] and nano-wires of Al based materials [5][6][7]18] have shown [8][9][10][11][12][13][14][15] promising results. These materials have also some potential drawbacks: collective excitations and in-gap states may increase losses dramatically, which would shorten the coherence time of superconducting qubits.…”