Periodic Reporting for period 1 - Super MagneFiQuE (Superconducting magnetic-field compatible quantum electronics)
Reporting period: 2015-10-01 to 2017-09-30
To achieve this goal, I used newly developed semiconducting nanowire Josephson junctions (NW JJs) as the basic non-linear element. These elements are made entirely from magnetic field compatible materials, such as the high-field superconductor Niobium Titanium Nitride (NbTiN). Preliminary work  has shown that these junctions exhibit more dissipation than their aluminium counterparts. A possible cause for this excess dissipation was the presence of sub-gap quasiparticle (QP) states in the superconductor and/or excess QPs present in the circuit. The objective was to understand the role of QP's in super-semi hybrid circuits and the removal of excess QPs as source for dissipation of microwaves and poisoning, allowing the demonstration of macroscopic quantum coherence of a superconducting circuit in a strong magnetic field.
 G.de Lange et al. Physical Review Letters 115, 127002 (2015).
The project is terminated earlier because the fellow has received and accepted an offer to work at the newly established fundamental research lab of Microsoft in Delft, the Netherlands.
The fellow is very grateful that he was given the opportunity by the EU to work as a Marie Curie postdoc. It has been instrumental in acquiring his new position, which allows him to continue high-impact fundamental academic research in a senior position that bridges both academia and industry.
In the second year of the project (2016-2017) we have incorporated our traps in superconducting qubits. We have fabricated devices and further optimized the fabrication procedure as well as designed new experiments to characterize the trapping efficiency of our newly developed traps. We measured the rate at which our circuits recover after injecting them with a large number of quasiparticles using a high-power microwave pulse (a.k.a. wireless QP injection). We have found a significant reduction in the time required for the circuit to recover, indicating excess quasiparticles to be removed from the circuit at a higher rate.
In addition, we used a superconducting transmon qubit and a parametric amplifier as a parity detector to directly measure the rate at which quasiparticles tunnel through the JJ. From the QP tunneling rate we could measure the presence of non-equilibrium background quasiparticles present in the circuit. Surprisingly, preliminary results show that this rate is not lower for qubits with QP traps. Our method also allowed us to correlate QP tunneling events with qubit relaxation and excitation events. Preliminary results reveal the presence of high-energy QPs in our circuit. This could explain why our traps do not seem efficient in removing the background QP density.
In parallel, we have designed new devices to measure quasiparticle dynamics in semiconducting NW JJs by monitoring the many-body configurations of the microscopic Andreev levels in the NW JJ in real time. The devices were made from NbTiN and InAs NWs covered by thin-film aluminum. This technique can be used to directly probe QP dynamics also in circuits that incorporate QP traps. In addition, these measurements provide us with more insights in the processes underpinning the performance of superconducting-semiconducting NW JJs.
All these results have been presented at the 2016 and 2017 APS March meetings and manuscripts are in preparation for publication.
The techniques developed in this project so far are valuable tools for future devices and will help mitigate QPs in superconducting quantum circuits. Improved performance will boost the capability of superconducting circuits for studying topological superconductivity and Majorana Fermions, and in new technologies such as a quantum computer.