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Superconducting magnetic-field compatible quantum electronics

Periodic Reporting for period 1 - Super MagneFiQuE (Superconducting magnetic-field compatible quantum electronics)

Reporting period: 2015-10-01 to 2017-09-30

In superconducting quantum electronics macroscopic degrees of freedom like currents and voltages can exist in a quantum mechanical superposition. This macroscopic quantum coherence has led to the development of circuits behaving as atoms. An exciting new field of research is circuit-based quantum electrodynamics (cQED), in which these artificial atoms are placed in microwave cavities to perform quantum optics in the microwave regime. This cQED architecture is arguably the most promising platform for processing quantum information and realizing a full-scale quantum computer. However, a major draw-back of these circuits, which are made from aluminum films, is that superconductivity is lost upon applying strong magnetic fields. This limitation poses a fundamental obstacle to interfacing superconducting circuits with other systems that require these strong magnetic fields. Forming such hybrid systems, in which the short-comings of one system are compensated by another, can be used to develop new technologies, such as long term quantum memories for superconducting qubits using solid-state spin ensembles, or a topological quantum computer by exploiting the non-Abelian braiding statistics obeyed by Majorana Fermions. The main goal of this proposal was to realize magnetic-field compatible superconducting quantum circuits for the cQED architecture.
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 [1] 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.
[1] 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 first year of the project (2015-2016) mainly focused on developing an understanding of the fundamental physics behind superconducting devices as well as developing new techniques for incorporating superconducting QP traps in our devices by using the inverse proximity effect. The main result of the first year was the development of titanium traps for aluminum circuits. We developed a high-angle evaporation technique from multiple directions to selectively deposit trapping material in locations of our device where we wanted QPs to be trapped (i.e. away from the JJs). A crucial feature of the technique we developed was that all this could be done without breaking vacuum in between depositions, preventing insulating oxides to be formed between layers. We achieved highly transparent interfaces with transparencies T > 0.3 which turned out to be crucial for achieving a strong suppression of the superconducting gap in the trapping regions. We have developed the technique for aluminum circuits, but it is easily extendible to other material systems as well.
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.
So far there has been little progress on finding the origin of excess QP in superconducting circuits, nor have they been successfully mitigated in superconducting qubits. As the quality of the various elements in superconducting circuits improves (dielectric properties, microwave hygiene, etc.) QPs will become more and more a limiting factor in device performance. Especially in devices that crucially depend on long QP lifetimes, such as Majorana qubit devices for topological quantum computation, mitigating excess QPs will be extremely important. Lastly, employing superconducting devices in large magnetic fields will result in the generation of QPs. Keeping these QPs away from JJs will be crucial in order to operate devices in such high magnetic fields.
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.
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