Experimental investigation of quantum coherence in Josephson circuits with superconductor-ferromagnet-superconductor pi-junctions
Final Report Abstract
In this project, we had developed a fabrication process for quantum integrated circuits, i.e. the quartic phase qubit, based on two different technologies: conventional SIS Josephson junctions in Nb-AlOx-Nb technology and SFS π-junctions in Nb-CuNi-Nb technology. The adaptation of the available well-established SIS process for the implementation of the quartic qubit integrated with Nb microwave resonator proved to be significantly more difficult than expected. Several iterations were required to analyze, understand and circumvent the issues encountered during fabrication of the quartic qubit circuits. Almost all of these problems could be solved successfully. Still, the remaining problem of surface contamination effectively prevented the successful co-fabrication of SFS π-junctions onto dedicated qubit chips. This is being worked on so that a successful attempt of cofabricating SFS π-junctions is within range and will be undertaken as soon as possible. The developed circuit design allows co-fabricating of SFS π-junctions with minimum risk of damaging the SIS junctions. Preliminary measurements indicated that the designed values of the quartic qubit and the SIS junctions were closely met. Measurements of the quartic qubits without SFS π-junctions are being prepared. Investigation in the frame of this project of the superconducting circuits which operation is based on the Josephson nonlinearity (including anharmonicity of the quartic qubit, phase shifts controlled by the π-junctions, etc.) had an impact on bifurcations circuits enabling detection of small electric signals. In particular, we proposed and made first experiments with Nb circuits demonstrating a bifurcation effect at 4.2 K. Integration of these circuits with the SFS π-junctions should allow investigation of new regimes of bifurcation-based detectors.
Publications
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Period-doubling bifurcation readout for a Josephson qubit, Phys. Rev. B 83, 224506 (2011)
A. B. Zorin and Yu. Makhlin
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Tagung Kryoelektronische Bauelemente 2013, 6-8 Oktober 2013, Bad Herrenalb, Deutschland. “Bifurcations in thin-film Nb superconducting resonators with integrated Josephson junctions“, p. 38
. M. Khabipov, B. Mackrodt, T. Scheller, R. Dolata, A. B. Zorin
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Tagung Kryoelektronische Bauelemente 2013, 6-8 Oktober 2013, Bad Herrenalb, Deutschland. “Fabrication of quartic qubits for the investigation of SFS pi junctions using niobium Josephson junction technology”, p. 54
T. Scheller, M. Weides, A.B. Zorin
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Investigation of nonlinear superconducting microwave resonators including Nb Josephson junctions and SQUID arrays, Journal of Physics: Conference Series 507, 042016 (2014)
M. Khabipov, B. Mackrodt, R. Dolata, T. Scheller, A.B. Zorin
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Period-doubling-bifurcation microwave cavity with Josephson nonlinearity for detecting small signals and qubit readout, Abstract for 27th Int. Conf. on Low Temp. Phys. (6-13 August 2014, Buenos Aires)
A. B. Zorin, M. Khabipov and Yu. Makhlin
