QDev Masters Defense: Judith Suter

Microwave-Induced Transitions in a Hybrid Double Quantum Dot

The research field of Majorana fermions, exotic particles that are their own antiparticles,has evolved from being a purely theoretical entity to experimental relevance within less than a decade. Following theoretical proposals [1],[2] experimental signatures consistent with Majorana bound states have been observed in hybrid materials [3], [4], [5], [6]. Due to the topologically protected nature of these quasi particle excitations, they constitute a promising platform for intrinsically fault-tolerant quantum computing.

The motivation of this work is to probe the tunnel coupling between two Majorana bound states across a controllable junction. This would extend previous work probing the tunnel coupling between two charge states using microwave spectroscopy on semiconducting quantum dots [7], [8] and a Cooper pair box [9].
This thesis presents measurements of Cooper-pair and single electron tunneling across a Josephson junction in a quasi one-dimensional hybrid system with a mesoscopic double island geometry. Magnetic field evolution demonstrates a change in Coulomb blockade periodicity from 2e to 1e via an even-odd regime. This transition occurs well below the critical field of the system. Under microwave irradiation photon assisted tunneling is observed both in absence and presence of an axial magnetic field. The Josephson coupling energy EJ at B=0 and the 1e coupling strength E1e at B =750mT are determined by extracting the frequency-dependence of the position of microwave-induced transition resonances. Corresponding transport measurements at finite magnetic field show a zero bias conductance peak extended in gate voltage and separated from the quasiparticle continuum by a superconducting gap. Results are consistent with single electron tunneling between two zero energy quantum states residing on each side of the Josephson junction.