Abstract
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Supervisors/Advisors 

Award date  18 Jun 2015 
Place of Publication  Espoo 
Publisher  
Print ISBNs  9789513883119 
Electronic ISBNs  9789513883126 
Publication status  Published  2015 
MoE publication type  G5 Doctoral dissertation (article) 
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Keywords
 superconductivity
 Josephson junction
 superconducting qubit
 singlequbit control
 quantumlimited amplifier
 kinetic inductance
 submillimetrewave detector
 magnetometer
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Microwavecoupled superconducting devices for sensing and quantum information processing : Dissertation. / Vesterinen, Visa.
Espoo : VTT Technical Research Centre of Finland, 2015. 148 p.Research output: Thesis › Dissertation
TY  THES
T1  Microwavecoupled superconducting devices for sensing and quantum information processing
T2  Dissertation
AU  Vesterinen, Visa
PY  2015
Y1  2015
N2  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 nonlinear 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 currentbiased JJ. This work also addressed a challenge related to the scalability of the cQED architecture when the qubits are weakly anharmonic. In a frequencycrowded multiqubit system, driving individual qubits may cause leakage into noncomputational levels of the others. Leakageavoiding singlequbit WahWah 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 WahWah in a future quantum computing platform. Quasiparticles are excitations from the paired superconducting ground state of conduction electrons. As the third topic, the generationrecombination 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, submillimetrewave radiation and external magnetic field were first converted into quasiparticlegenerating 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 scaleup to large sensor arrays.
AB  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 nonlinear 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 currentbiased JJ. This work also addressed a challenge related to the scalability of the cQED architecture when the qubits are weakly anharmonic. In a frequencycrowded multiqubit system, driving individual qubits may cause leakage into noncomputational levels of the others. Leakageavoiding singlequbit WahWah 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 WahWah in a future quantum computing platform. Quasiparticles are excitations from the paired superconducting ground state of conduction electrons. As the third topic, the generationrecombination 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, submillimetrewave radiation and external magnetic field were first converted into quasiparticlegenerating 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 scaleup to large sensor arrays.
KW  superconductivity
KW  Josephson junction
KW  superconducting qubit
KW  singlequbit control
KW  quantumlimited amplifier
KW  kinetic inductance
KW  submillimetrewave detector
KW  magnetometer
M3  Dissertation
SN  9789513883119
T3  VTT Science
PB  VTT Technical Research Centre of Finland
CY  Espoo
ER 