Morten Canth Hels

A thesis submitted  October 2017 for the degree of Doctor of Philosophy and defended December, 2017.

The PhD School of Science
Faculty of Science, Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen

Academic advisors:
Jesper Nygård & Kasper Grove-Rasmussen

 

Academic advisors:

Jesper Nygård & Kasper Grove-Rasmussen

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Towards entanglement detectionin nanotube Cooper pair splitterswith disorder and spin-orbit coupling

This thesis presents results from experimental and theoretical investigationsof carbon nanotube (CNT) quantum devices at cryogenic temperatures. Specif-ically, Cooper pair splitting (CPS) in CNT devices with beam-splitter geome-tries and a central superconducting electrode is investigated.Carbon nanotubes are attractive to use in quantum devices because oftheir exotic electronic and mechanical properties. One proposal involvingcarbon nanotubes utilizes their intrinsic spin-orbit interaction as a spin filterto demonstrate the entangled nature of splitting Cooper pairs. Such a devicewould have applications for quantum computing hardware as a source ofentangled electrons.A model for the CNT spectrum is extended to include the coupling be-tween longitudinal levels in a CNT quantum dot. The extension requires ageneralization of the electrostatic potential along the nanotube. The modelis shown to have god correspondence with transport data obtained from atwo-terminal CNT quantum dot device.A CNT CPS device is fabricated which allows identification of non-collinearspin-orbit magnetic fields in the two segments of the device. This is madepossible because the curved nanotube exhibits low disorder as measured byits ratio ofKK0scattering to spin-orbit coupling∆KK0/∆SO. The spin-orbitmagnetic fields obtained in this device were previously considered to be dif-ficult to obtain without using special fabrication techniques. We provide thedetails for fabrication of the device, but note that the yield for this processwas low. Motivated by the results above theory is developed to describe theeffectofKK0scattering on the viability of the proposal mentioned above todemonstrate entanglement of splitting Cooper pairs.In the same CNT CPS device transport data is analyzed to extract thecontributions from Cooper pair splitting and elastic cotunneling processes tothe overall current. The vanishing of the nonlocal conductance signal withmagnetic field indicates that part of the transport mechanism involves thesuperconducting electrode. Additionally, calculations of theQparameter,which are required in the above proposal, show that the test of entanglementis robust against false positive results.Control of the electrode-dot couplings in CPS devices is desirable for CPSdevices since it impacts the CPS efficiency. Transport data from a bottomgated nanowire CPS device is presented showing that a given electrode-dotcoupling can be tuned by its corresponding bottom gate while leaving theother couplings essentially constant. The dependence of the couplings onthe voltage on the bottom gates is found to be exponential consistent withpredictions from basic quantum theory..

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