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Bulletin of the American Physical Society

Microbridge junctions for superconducting phase qubits

Martin P. Weides, Radoslaw C. Bialczak, M. Lenander, Erik A. Lucero, M. Mariantoni, M. Neeley, A. D. O’Connell, D. Sank, H. Wang, J. Wenner, Takahiro Yamamoto, Yi Yin, Andrew N. Cleland, John M. Martinis

Published Mar 17, 2010• 0 citations
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OF JOSEPHSON MICROBRIDGES COUPLED BY SUPERCONDUCTING LOOPS

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We have studied the phase locking of two and eight microbridge arrays shunted by superconducting loops in a dc SQUID configuration. The difference in phase between adjacent bridges is controlled by the applied flux, and thus the radiated power is a periodic function of this flux. With adjustment of the external flux, the individual bridge voltages can be made to add in phase. The effect of additional normal shunts on the phase locking is also investigated. MICROBRIDGES COUPLED BY SUPERCONDUCTING LOOPS Experiment sly on two Josephson microbridges have shown that it is possible to obtain nearly complete in phase coherent radiation with narrowed linewidth by coupling the microbridges with external shunts. To obtain greater ouput power and narrower linewidth it is desirable to phase lock larger arrays of bridges. The narrow range of bias currents over which locking occurs, however, imposes strict tolerances on the uniformity of bridge characteristics if a single current bias is used. In this paper we discuss experiments which eliminate this difficulty by using a superconducting loop to couple the junctions, i.e., a dc SQUID configuration, so that the average frequency of each junction is always equal even for non-identical junctions. Also the phase difference between the junctions in the absence Of noise is set by the amount of externally applied flux due to the fluxoid quantization condition. With identical bridges and zero applied flux, the junctions oscillate in phase. In the geometry used in

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The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge matches with a planar antenna and operates as an optical blackbody at frequencies 600-700 GHz. The coplanar terminal is the blackbody output in the 1-2 GHz frequency range. The microbridge temperature can be set in the range of 0.4-9 K and calibrated using the shot noise of the tunnel junction. Temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power ~ 1 μW. Keywords: Superconducting microbridge, superconducting tunnel junction, thermodynamic noise, shot noise, noise thermometry.

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Sub-micron superconducting electronic devices may play a role in the realisation of proposed architectures for quantum computing and metrology. Further miniaturisation of superconducting electronics relies on the ability to fabricate the basic building block, the Josephson junction, on the scale of the superconducting coherence length, with sufficient accuracy that the junction characteristics may be controlled. Making use of focused ion beam technology to fabricate thin-film Nb microbridge devices, we examine the evolution of device characteristics with dimension, focusing on control of the critical current and suppression of superconductivity due to phase-slip behaviour.
We have studied the phase locking of two and eight microbridge arrays shunted by superconducting loops in a dc SQUID configuration. The difference in phase between adjacent bridges is controlled by the applied flux, and thus the radiated power is a periodic function of this flux. With adjustment of the external flux, the individual bridge voltages can be made to add in phase. The effect of additional normal shunts on the phase locking is also investigated. MICROBRIDGES COUPLED BY SUPERCONDUCTING LOOPS Experiment sly on two Josephson microbridges have shown that it is possible to obtain nearly complete in phase coherent radiation with narrowed linewidth by coupling the microbridges with external shunts. To obtain greater ouput power and narrower linewidth it is desirable to phase lock larger arrays of bridges. The narrow range of bias currents over which locking occurs, however, imposes strict tolerances on the uniformity of bridge characteristics if a single current bias is used. In this paper we discuss experiments which eliminate this difficulty by using a superconducting loop to couple the junctions, i.e., a dc SQUID configuration, so that the average frequency of each junction is always equal even for non-identical junctions. Also the phase difference between the junctions in the absence Of noise is set by the amount of externally applied flux due to the fluxoid quantization condition. With identical bridges and zero applied flux, the junctions oscillate in phase. In the geometry used in
The device operates at temperatures <300 mK and comprises a hafnium microbridge and a superconducting aluminum tunnel junction both integrated into a common coplanar waveguide. The microbridge matches with a planar antenna and operates as an optical blackbody at frequencies 600-700 GHz. The coplanar terminal is the blackbody output in the 1-2 GHz frequency range. The microbridge temperature can be set in the range of 0.4-9 K and calibrated using the shot noise of the tunnel junction. Temperature modulation of each of the sources can be performed independently using a direct current which transit them from the superconducting to the normal state with characteristic times <0.1 ms and heating power ~ 1 μW. Keywords: Superconducting microbridge, superconducting tunnel junction, thermodynamic noise, shot noise, noise thermometry.