This thesis presents results from 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.
Carbon nanotubes are attractive to use in quantum devices because of
their exotic electronic and mechanical properties. One proposal involving
carbon nanotubes utilizes their intrinsic spin-orbit interaction as a spin filter
to demonstrate the entangled nature of splitting Cooper pairs. Such a device
would have applications for quantum computing hardware as a source of
entangled electrons.
A model for the CNT spectrum is extended to include the coupling between
longitudinal levels in a CNT quantum dot. The extension requires a
generalization of the electrostatic potential along the nanotube. The model
is shown to have god correspondence with transport data obtained from a
two-terminal CNT quantum dot device.
A CNT CPS device is fabricated which allows identification of non-collinear
spin-orbit magnetic fields in the two segments of the device. This is made
possible because the curved nanotube exhibits low disorder as measured by
its ratio of KK0 scattering to spin-orbit coupling KK0/SO. The spin-orbit
magnetic fields obtained in this device were previously considered to be difficult
to obtain without using special fabrication techniques. We provide the
details for fabrication of the device, but note that the yield for this process
was low. Motivated by the results above theory is developed to describe the
effect of KK0 scattering on the viability of the proposal mentioned above to
demonstrate entanglement of splitting Cooper pairs.
In the same CNT CPS device transport data is analyzed to extract the
contributions from Cooper pair splitting and elastic cotunneling processes to
the overall current. The vanishing of the nonlocal conductance signal with
magnetic field indicates that part of the transport mechanism involves the
superconducting electrode. 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.
Control of the electrode-dot couplings in CPS devices is desirable for CPS
devices since it impacts the CPS efficiency. Transport data from a bottom
gated nanowire CPS device is presented showing that a given electrode-dot
coupling can be tuned by its corresponding bottom gate while leaving the
other couplings essentially constant. The dependence of the couplings on
the voltage on the bottom gates is found to be exponential consistent with
predictions from basic quantum theory.
Overall, the results in this thesis give new insights into using carbon
nanotubes for Cooper pair splitter devices and entanglement detection experiments