Abstract
Superconducting circuits and devices have unique
properties that make them interesting from both
theoretical and practical perspective. In a
superconductor cooled below its critical temperature,
electrons bound in Cooper pairs have the ability to carry
current without dissipation. A structure where the Cooper
pairs are coherently tunneling across a weak link is
called a Josephson junction (JJ). The dissipationless and
non-linear character of the JJ has found applications,
e.g., in microwave amplifiers and quantum circuits. These
two subjects are closely related since superconducting
quantum bits (qubits) are artificial atoms with a
transition spectrum in the microwave range. Mediated by
microwave photons, qubit readout in circuit quantum
electrodynamics (cQED) architecture requires signal
boosting with a lownoise preamplifier. In this thesis, a
new type of ultrasensitive JJ microwave amplifier was
characterized and its noise performance was found to be
close to a bound set by quantum mechanics. The amplifier
uses the intrinsic negative differential resistance of a
current-biased JJ.
This work also addressed a challenge related to the
scalability of the cQED architecture when the qubits are
weakly anharmonic. In a frequency-crowded multiqubit
system, driving individual qubits may cause leakage into
non-computational levels of the others. Leakage-avoiding
single-qubit Wah-Wah control was implemented. At maximum
gate speed corresponding to the frequency crowding,
microwave control of two transmon qubits on a 2D cQED
quantum processor was decoherence limited. The results
disclose the usefulness of Wah-Wah in a future quantum
computing platform.
Quasiparticles are excitations from the paired
superconducting ground state of conduction electrons. As
the third topic, the generation-recombination dynamics of
quasiparticles was employed in sensing. In
electrodynamical terms, superconducting thin films have
kinetic inductance from the inertia of the Cooper pairs
and resistive dissipation from the quasiparticles. If the
film is a part of an electrical resonator, quasiparticle
density steers its microwave eigenfrequency and quality
factor. In this work, submillimetre-wave radiation and
external magnetic field were first converted into
quasiparticle-generating temperature variations and
screening currents in a superconductor, respectively. In
the two devices called kinetic inductance bolometer and
magnetometer, the corresponding changes in resonator
parameters were read out to extract the encoded signal.
Sensor characterization indicated potential for high
sensitivity and low noise. Future applications of the
bolometer and the magnetometer include security screening
and biomagnetism, respectively. Here, multiplexability in
frequency domain facilitates the scale-up to large sensor
arrays.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 18 Jun 2015 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 978-951-38-8311-9 |
Electronic ISBNs | 978-951-38-8312-6 |
Publication status | Published - 2015 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- superconductivity
- Josephson junction
- superconducting qubit
- singlequbit control
- quantum-limited amplifier
- kinetic inductance
- submillimetre-wave detector
- magnetometer