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] 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.
