Wednesday, June 24, 2020

Why Josephson matters

Reflecting on macroscopic quantum effects in condensed matter I have come to the view that Brian Josephson is a key figure. But, the observation of magnetic flux quantisation in superconducting cylinders is also a landmark.

The significance of Josephson is nicely laid out in a fascinating article published by in Physics Today in 2001 by Donald G. McDonald
John Bardeen, the leading condensed matter theorist of his day, was quite wrong when he dismissed a startling prediction by the unknown Brian Josephson. 

The article nicely lays out several important precursors to Josephson's work that all occurred after BCS theory in 1957.

1. The experimental (unanticipated) discovery by Ivar Giaever in 1960 of single-particle tunneling in SIS junctions [superconductor-insulator-superconductor sandwiches]. I-V curves clearly showed the structure of the BCS energy gap.
[Aside. This discovery was also laid the foundation for John Rowell's tunneling experiments that allowed a quantitative (strong-coupling BCS) analysis of the electron-phonon interaction responsible for superconductivity.]

2. The discovery by Hans Meissner [not the discoverer of the Meissner effect!] in 1960 of the proximity effect, where superconductivity is induced in a non-superconducting metal, by close proximity to a superconductor.

3. The discovery in 1961 by two independent experimental groups that the magnetic flux inside a cylinder was quantised in units of h/2e where h is Planck's constant and e is the electronic charge. This effect had been predicted by Fritz London in 1948, albeit without the factor of 2.
These experiments provided ``the first direct demonstration of a macroscopic quantum effect.''

The data above is from a 1971 paper, observing flux quantisation within one-half of a per cent.

A nice article on the history of the discovery is

 Aside. These experiments also clearly showed the physical nature of the magnetic vector potential, A, and illustrated the Aharonov-Bohm effect.

4. Josephson's attendance at a series of lectures ``Concepts in Solids" that Phil Anderson gave to graduate students at Cambridge in 1961-1962. In particular, at the end, Anderson introduced the concept of broken symmetry as an organising principle to describe ``condensed systems" such as antiferromagnets, superfluid 4He, ferroelectrics, and superconductors.

Distinctly quantum phenomena are tunneling, superposition (and the associated coherence and interference), and entanglement. Josephson junctions can be used to illustrate all of these at the macroscopic scale.

This is explored in a nice autobiographical article by Tony Leggett.
Because of the strong prejudice in the quantum foundations community that it would never be possible to demonstrate characteristically quantum-mechanical effects at the macroscopic level, this assertion made us [Leggett and Garg] the target of repeated critical comments over the next few years. Fortunately, our experimental colleagues were more open-minded, and several groups started working toward a meaningful experiment along the lines we had suggested, resulting in the first demonstrations (29, 30) of MQC [Macroscopic Quantum Coherence] in rf SQUIDs (by then rechristened flux qubits) at the turn of the century. However, it would not be until 2016 that an experiment along the lines we had suggested (actually using a rather simpler protocol than our original one) was carried out (31) and, to my mind, definitively refuted macrorealism at that level. I find it rather amusing that nowadays the younger generation of experimentalists in the superconducting qubit area blithely writes papers with words like “artificial atom” in their titles, apparently unconscious of how controversial that claim once was.

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