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Physica C Superconductivity

Effect of charging energy on critical current of dc-SQUID comprising two sub-micron aluminum Josephson junctions

Yoshinao Mizugaki, Kento Kikuchi, Masataka Moriya, Hiroshi Shimada

Published Feb 22, 2012• 2 citations
Publisher
DOI
10.1016/j.physc.2012.02.009

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Two states of a flux qubit with three Josephson junctions were shown in a single measurement with a dc-SQUID. The qubit is an aluminum superconductor loop surrounded by a dc-SQUID for readout. It has two states, which have persistent currents flowing in opposite directions. The readout data for three samples with different junction sizes suggest that the probability distribution for the double-well potential depends on the ratio of E J / E C , where E J is the Josephson energy and E C is the charging energy. The probability distribution was estimated by calculating the wavefunctions and energy levels for the measured samples. We measured the retrapping current of a dc-SQUID without the qubit as a function of the magnetic field. It was found that the relative phase relation for the magnetic field dependence of the retrapping current jumped from 0 to π at E J ≈ E C . This jump can be explained phenomenologically by taking into account the two kinds of dissipation arising from ac currents flowing through the lead and along the SQUID loop. The fact that the jump occurred at E J ≈ E C strongly suggests that the quantum nature of the electromagnetic environment needs to be taken into account in order to understand the origin of the dissipation.

Physica Scripta•Jan 1, 2002•10 citations
In a small Josephson junction, the critical current is often found experimentally to be smaller than the value expected fromthe Ambegaokar-Baratoff theory because of the quantum phase fluctuations. We also observed in our previous study that the quantum interference in a dc-SQUID comprising sub-micron Al junctions deviated from the classical model. In this work, we have fabricated 13devices having different values of maximal critical currents, and evaluated their quantum interference patterns.The quantum interference of 12among 13 devices has deviated from the classical model. It is confirmed that their maximal critical currents(Icsq,max) are reduced from the values (Icsq0) predicted by the Ambegaokar-Baratofftheory. Besides, the quantum interference for Icsq,max / Icsq0< 0.4 agrees with themodified quantum-interference model.
Two states of a flux qubit with three Josephson junctions were shown in a single measurement with a dc-SQUID. The qubit is an aluminum superconductor loop surrounded by a dc-SQUID for readout. It has two states, which have persistent currents flowing in opposite directions. The readout data for three samples with different junction sizes suggest that the probability distribution for the double-well potential depends on the ratio of E J / E C , where E J is the Josephson energy and E C is the charging energy. The probability distribution was estimated by calculating the wavefunctions and energy levels for the measured samples. We measured the retrapping current of a dc-SQUID without the qubit as a function of the magnetic field. It was found that the relative phase relation for the magnetic field dependence of the retrapping current jumped from 0 to π at E J ≈ E C . This jump can be explained phenomenologically by taking into account the two kinds of dissipation arising from ac currents flowing through the lead and along the SQUID loop. The fact that the jump occurred at E J ≈ E C strongly suggests that the quantum nature of the electromagnetic environment needs to be taken into account in order to understand the origin of the dissipation.