Quantum Phase Slips in Granular Aluminum Nanowires

Over the last decades, superconducting nanowires have developed from a playground for fundamental research to a promising key element for various applications, including superinductors, qubits or quantum transistors. The wide range of electrical properties is due to the quantum phase slip (QPS) effect, a process during which the phase of the superconducting order parameter can slip by 2$\pi$. Each phase slip can be associated with a fluxon tunneling across the wire and therefore be seen as the dual to the tunneling of Cooper pairs in Josephson junctions. Depending on the QPS amplitude, the electrical response of a wire can range from a purely inductive to a capacitive one. In particular, the duality between these phase slip junctions and Josephson junctions, which have become a basic building block of modern quantum circuits, has triggered a variety of theoretical and experimental works.

Many aspects of these fluctuations are still not fully understood. Furthermore, the parameter spread of the nanowires' properties turned out to be a limiting factor for experimental implementations, especially when more than one wire is involved.
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In this work, we investigate quantum phase slips in nanowires made from granular aluminum. We demonstrate that the normal state resistance of single wires can be reduced by orders of magnitude, using the newly developed method "intrinsic electromigration". With this new degree of freedom, we are able to study the influence of the phase slip amplitude on the transport behaviour of a wire and compare it with microscopic theories. Special attention is put on the phase slip driven superconductor to insulator transition (SIT). To probe the coherent nature of quantum phase slips and to further investigate the SIT, we developed a quantum phase slip interferometer based on two strongly coupled wires connected in series. The interference pattern is controlled by a gate voltage and manifests as a periodic modulation of the critical Coulomb blockade voltage. For strong, destructive interference of quantum phase slips, a transition from the insulating to a superconducting state is observed. The simple design of the device as well as the large available parameter range make it an interesting device also for applications beyond fundamental research like e.g a transistor for information processing, a particle detector or as a nonlinear capacitor.