PhD Defense: Morten C. Hels

Towards entanglement detection in nanotube Cooper pair splitters with disorder and spin-orbit coupling

I will present results from my thesis on experimental and theoretical investigations of carbon nanotube (CNT) quantum devices at cryogenic temperatures.
Specifically, Cooper pair splitting (CPS) in CNT devices with beam-splitter geometries and a central superconducting electrode is investigated.

First, the four-level model for the CNT spectrum is extended to include the coupling between longitudinal levels (shells) in a CNT quantum dot.
The model is shown to have good correspondence with transport data obtained from a two-terminal CNT quantum dot device.

Second, I will present results from a CNT CPS device which show noncollinear spin-orbit magnetic fields in the two segments.
Noncollinear fields are essential for a proposal for detecting entanglement of splitting Cooper pairs.
A model is developed to describe the effect of $KK'$ disorder on the viability of this proposal.
Additionally, calculations of the $Q$ parameter, which are required in the above proposal, show that the test of entanglement is robust against false positive results.

These results constitute an experimental proof-of-concept that the entanglement detection experiment can be realized with conventional techniques.