Quantum Tunnelling of a Macroscopic Variable

A. J. Leggett

D. Massarotti, Francesco Tafuri

Siyuan Han

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.

DR. ZULFIQAR ALI KHAN

The 2025 Nobel Prize in Physics honours the discovery of macroscopic quantum tunnellingand energy quantisation in an electric circuit. This note asks what the word “macroscopic” refersto. We distinguish three readings: the physical size of the device, the thickness of the barrier,and the nature of the tunnelling variable. In current-biased Josephson-junction experiments thebarrier that is crossed is not the nanometre-thick oxide layer, whose pair tunnelling underliesthe Josephson effect recognised in 1973. It is a barrier in the effective potential of a collectivephase variable that describes billions of Cooper pairs. We argue that “macroscopic” is best readas describing this variable, and that this reading is consistent with the temperature-independentescape rate observed at low temperature. We also note that “macroscopic” has no single sharpdefinition. The note is a clarification and reports no new results.

Gravity Deborah C. Good, Lincoln D. Carr