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Journal of Physics Conference Series

High-Tc Josephson junction : towards improvement of IcRN product and realization of phase qubits

Atsutaka Maeda, Haruhisa Kitano, Luis B. Gómez, Tetsuro Kubo, K. Ota, T Ohashi

Published Jun 1, 2006• 2 citations
Publisher

Abstract

Josephson junction of high- T c cuprate superconductor is studied in terms of (1) basic characteristics of junction to improve the I c R N product, and (2) a new candidate for quantum bits. Aiming to fabricate phase qubits, the switching current distribution of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O y was investigated. The switching current distribution down to 40 K was consistent with the thermally activated escape process in the single junction, while a more complex feature appeared below 40 K, strongly suggesting the importance of the multi-junction effects.

DOI
10.1088/1742-6596/43/1/280

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Josephson junction of high- T c cuprate superconductor is studied in terms of (1) basic characteristics of junction to improve the I c R N product, and (2) a new candidate for quantum bits. Aiming to fabricate phase qubits, the switching current distribution of intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O y was investigated. The switching current distribution down to 40 K was consistent with the thermally activated escape process in the single junction, while a more complex feature appeared below 40 K, strongly suggesting the importance of the multi-junction effects.
The macroscopic quantum tunneling (MQT) in the current-biased intrinsic Josephson junctions (IJJs) of high- T c cuprates has attracted much attention for decades. Although the MQT for the phase switches from the zero to the first voltage state (1st SW) in the multiple-branched I-V curves is well explained by the conventional theory, the occurrence of MQT for the higher order switches such as the switch from the 1st to 2nd voltage state (2nd SW) has been still debated. Here, we present an experimental study on the phase switches of small IJJs fabricated from underdoped Bi 2 Sr 2 (Ca,Y)Cu 2 O y . We observed the single photon transition between quantized energy levels in the 3rd phase switches at 59.15 GHz and 2 K. The comparison with the previous studies on the nearly optimal-doped Bi 2 Sr 2 CaCu 2 O y clearly suggests a possibility that the MQT rate for the higher-order phase switches is commonly enhanced by the effective suppression of the energy barrier for the higher-order phase escape due to the phase-running state after the 1st SW, in spite of the large difference in a critical current density and T c .
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Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La2−x_{2-x}2−x​Srx_{x}x​CuO4_{4}4​ (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multi-branches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method.
We study the dynamics of multi-junction switching (MJS): several intrinsic Josephson junctions (IJJs) in an array switch to the finite voltage state simultaneously. The number of multi-switching junctions ( N ) was successfully tuned by changing the load resistance serially connected to an Bi 2 Sr 1.6 La 0.4 CuO 6+δ IJJ array. The independence of the escape rates of N in the macroscopic quantum tunneling regime indicates that MJS is a successive switching process rather than a collective process. The origin of MJS is explained by the gradient of a load curve and the relative magnitudes of the switching currents of quasiparticle branches in the current–voltage plane.
In the process of theoretical fitting the switching current distributions of Josephson junction,macroscopic quantum tunneling (MQT) and thermally activated process are usually considered.Analysis of the results for a Bi2Sr2CaCu2O8+δ surface intrinsic Josephson junction (SIJJ), quantum corrections are considered at the intermediate areas between MQT and thermally activated process. It is found that quantum corrections lead to a perfect fit to the experimental data, which may be useful for the future studies of MQT in SIJJs and their possible applications as a superconducting qubit.
Superconducting quantum circuits (SQCs) are being explored as model systems for scalable quantum computing architectures. Josephson junctions are extensively used in superconducting quantum interference devices (SQUIDs) and in persistent-current qubit systems. Noise excitations, however, have a critical influence on their dynamics. Thus, the primary focus of this research was to investigate the effects of thermal ac-tivation on the superconducting properties of Josephson junctions. Specifically, ther-mal noise tends to result in a range of switching currents, values less than the critical current at which a junction switches from the superconducting to the normal state. First, a general review of superconductivity concepts is given, including a treatment of the Josephson phenomena. Next, I describe some of my work on characterizing the current-voltage traces of Josephson junctions tested at 4 K with a Multi-Chip Probe (MCP). Then, I describe thermal activation theory and examine the equations useful for modeling switching current distributions. The Josephson junctions of a SQUID with a ramped bias current were tested for numerous temperatures T < 4.5 K (and