This thesis presents measurements on Majorana islands:
semiconductor-superconductor hybrid nanowire quantum dots
in the trivial and the topological superconducting phase.
We fabricate Majorana island devices based on indium arsenide
nanowires with an epitaxially matched aluminum half-shell. Measuring
quasiparticle transport, we observe a gate voltage dependent
even-odd Coulomb blockade pattern, associated with quasiparticle
occupation of bound states, for which we demonstrate state parity
lifetimes exceeding 10 milliseconds.
Using Coulomb-blockade spectroscopy and varying the magnetic
field, we measure oscillating energy splittings of near-zero energy
states, consistent with theoretical predictions for hybridized Majorana
modes. We present a length study of Majorana islands and
demonstrate the exponential suppression of energy splitting with increasing
island length, as expected for Majorana modes, with a characteristic
length of 260 nm. For long devices exceeding one micron,
transport at high magnetic fields shows discrete zero-energy states,
with an energy gap to a higher-energy continuum, and evenly spaced
Coulomb-blockade conductance peaks, a signature of teleportation
via Majorana modes. A preliminary analysis shows that Coulomb
peaks also feature an alternating magnetic field dependent skew, the
subject of future work.
We additionally observe novel transport signatures of quasiparticle
poisoning in a Majorana island strongly coupled to normal metal
leads. Numerical simulations show good agreement with measurements
and allow us to extract a time for poisoning of the island’s
ground state from the leads of 3 microseconds.