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arXiv:2606.02770

Early Experiments on Macroscopic Quantum Tunneling

Willem den Boer

Published Jun 1, 2026• 0 citations
arXivHTMLPDF

Abstract

Before conclusive evidence of Macroscopic Quantum Tunneling, MQT, was published by Clarke, Devoret and Martinis in 1985, several other groups reported experimental results interpreted as MQT. The first, in chronological order, was in 1980 based on studies done at Leiden University in 1979. This paper looks back at these experiments on low capacitance Niobium point contacts in an rf SQUID, radio-frequency Superconducting Quantum Interference Device, configuration at temperatures between 1 and 4.2 K. The research was inspired by the theoretical predictions by Ivanchenko and Zilberman in 1969 on MQT in current-biased Josephson junctions and by Leggett in 1978 on MQT in closed loops with a superconducting weak link.

DOI
10.48550/arxiv.2606.02770

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Before conclusive evidence of Macroscopic Quantum Tunneling, MQT, was published by Clarke, Devoret and Martinis in 1985, several other groups reported experimental results interpreted as MQT. The first, in chronological order, was in 1980 based on studies done at Leiden University in 1979. This paper looks back at these experiments on low capacitance Niobium point contacts in an rf SQUID, radio-frequency Superconducting Quantum Interference Device, configuration at temperatures between 1 and 4.2 K. The research was inspired by the theoretical predictions by Ivanchenko and Zilberman in 1969 on MQT in current-biased Josephson junctions and by Leggett in 1978 on MQT in closed loops with a superconducting weak link.
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We study macroscopic quantum tunneling (MQT) in ccc-axis twist Josephson junctions made of high-Tc{T}_{c}Tc​ superconductors in order to clarify the influence of the anisotropic order parameter symmetry (OPS) on MQT. The dependence of the MQT rate on the twist angle $\ensuremath{\gamma}$ about the ccc axis is calculated by using the functional integral and the bounce method. Due to the ddd-wave OPS, the $\ensuremath{\gamma}$ dependence of standard deviation of the switching current distribution and the crossover temperature from thermal activation to MQT are found to be given by $\mathrm{cos}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\gamma}$ and $\sqrt{\mathrm{cos}\phantom{\rule{0.2em}{0ex}}2\ensuremath{\gamma}}$, respectively. We also show that a dissipative effect resulting from the nodal quasiparticle excitation on MQT is negligibly small, which is consistent with recent MQT experiments using ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ intrinsic junctions. These results indicate that MQT in ccc-axis twist junctions becomes a useful experimental tool for testing the OPS of high-Tc{T}_{c}Tc​ materials at low temperature, and suggest high potential of such junctions for qubit applications.
The question of whether macroscopic variables obey the laws of quantum mechanics has been debated ever since the earliest days of quantum mechanics.Since early 1980s results from a series of experiments in Josephson junctions and superconducting interference devices(SQUIDs),including macroscopic quantum tunneling,energy level quantization,macroscopic resonant tunneling(MRT),and ac driven coherent dynamics of the phase and flux,strongly support the notion that under proper conditions motions of macroscopic variables are indeed governed by the laws of quantum mechanics.Among many macroscopic quantum phenomena,MRT,which combines two of the most distinctive quantum phenomena-tunneling and energy level-quantization together and does not involve excitation by high frequency fields,is perhaps the strongest evidence demonstrating quantum nature of macroscopic variables.In this article,we review recent development in theory and experiment of MRT.In particular,we show that how dissipation and flux noise affect the rate and line shape of MRT in the double-well of rf SQUID flux qubits and applications of MRT in the ongoing efforts of identifying and understanding mechanisms of decoherence in superconducting qubits for scalable quantum computing.
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We report experiments on energy level quantization in Nb/AlOx/Nb Josephson junctions while measurements on SQUIDs are in progress. Data on the presence of energy level quantization are shown at temperatures above the classical-quantum regime crossover temperature. This has been possible by extending the measurements of the rate of the escape from the zero-voltage state at higher sweep bias rate in order to induce non-stationary conditions in the energy potential describing the junction dynamics. Data are compatible with extremely low "effective" dissipation in our system. Theoretical predictions on quantum effects related to the presence of non-stationary conditions in the potential associated to a rf SQUID are reported in view of new possible experiments.