Abstract
This thesis investigates the use of single Cooper-pair transistor (SCPT) for fast
and sensitive detection of quasiparticle dynamics. This investigation is motivated
by the possibility of quantum information processing using superconducting
nanoscale circuits, such as the SCPT and the Cooper-pair-box (CPB).
In the SCPT coherent charge transport can be temporarily halted due to
quasiparticle tunnelling, known as quasiparticle poisoning. Quasiparticle poisoning
can be reduced by the use of engineered island and lead gap energies.
The thesis begins by reporting measurements of the superconducting gap in aluminium
- aluminium-oxide - aluminium tunnel junctions, as a function of film
thickness. We have observed an increase in the superconducting energy gap of
aluminium with decreasing film thickness. This method is used to engineer the island
and gap energies in a SCPT and consequently we observe reduced poisoning
and a modification of the thresholds for finite bias transport processes.
Radio-frequency reflectometry is used to perform high-bandwidth measurements
of quasiparticle tunnelling in a gap engineered SCPT. A model for the
radio-frequency (rf) operation of the SCPT is presented and shows close agreement
with experiment. Thermal activation of the quasiparticle dynamics is investigated,
and consequently, we are able to determine energetics of the poisoning
and unpoisoning processes. This enables an effective quasiparticle temperature
to be determined, allowing the poisoning to be parametrised.
An investigation of the use of normal metal quasiparticle traps for suppression
of quasiparticle poisoning in SCPT devices is performed. To date, there has been
little quantitative information about the behaviour of quasiparticle traps even
though they have been used extensively. The work presented serves to clarify the
nature of quasiparticle trap performance.
Finally the single-quasiparticle sensitivity of the SCPT is employed to directly
probe a few quasiparticle gas in a small superconducting volume. The quasiparticle
population is monitored both in the steady-state and under non-equilibrium
conditions of injection. In the non-equilibrium regime the quasiparticle recombination
time is accessed from the response of the SCPT to pulsed injection.
Agreement to previous experimental studies of recombination times in aluminium
is found.