As announced this morning, the 2025 Nobel Prize in Physics has been awarded to John Clarke, Michel Devoret, and John Martinis, for a series of ground-breaking experiments in the 1980s that demonstrated macroscopic quantum tunneling.
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| Fig. 2 from here |
Clarke, Devoret, and Martinis were working not with a single particle, but with electrons in a superconductor (many many electrons in a coherent quantum state). The particular system they chose was a Josephson junction made from an oxide-coated Nb film contacted by a PbIn electrode with a dc current flowing through it. Instead of an x coordinate of a particle, the relevant coordinate in this system is the phase difference \(\delta\) of the superconducting wave function across the junction. There is an effective potential energy for this system called a "washboard" potential, \(U(\delta)\), as in this figure. At the particular DC current, which tilts \(U(\delta)\), the system can transition from one state (\(\delta\) bopping around a constant value, no voltage across the junction) to a state where \(\delta\) is continuously ramping (corresponding to a nonzero voltage across the junction). The system can get thermally kicked from the zero voltage state to the nonzero voltage state (thermal energy doinks it over the barrier), but the really interesting thing is that the system can quantum mechanically tunnel "through" the barrier as well.
This idea, that a macroscopic (in the sense of comprising many many electrons) system could tunnel out of a metastable state like this, had been investigated by Amir Caldeira and Tony Leggett in this important paper, where they worried about the role of dissipation in the environment. People tried hard to demonstrate this, but issues with thermal radiation and other noise in the experiments were extremely challenging. With great care in experimental setup, the three laureates put together a remarkable series of papers (here, here, here) that showed all the hallmarks, including resonantly enhancing tunneling with tuned microwaves (designed to kick the system between the levels shown in panel (d) of the figure above).
This was an impressive demonstration of controllable, macroscopic quantum tunneling, and it also laid the foundation for the devices now used by the whole superconducting quantum computing community.
7 comments:
Curious to know Doug - did they actually start this part of the experimental work inspired by Leggett/Caldeira and ideas of testing QM on increasingly larger scales? Or it was just SC devices and physics which Clarke had been exploring anyway?
Here's what Clarke had to say in an oral history interview conducted in 2016 for IEEE. "Legget is also a Brit, and he is still at the University of Illinois at Urbana-Champaign. He had developed this theory of something called macroscopic quantum tunneling, which once again involves a Josephson junction in a particular way. We decided that we were going to work on it. The single most important thing that we observed and reported was the fact that the energy of this macroscopic energy, macroscopic object, could be quantized."
Thanks, Charles. That's the impression one comes away with in reading the Nobel summary document as well.
It might interest some of the career researchers reading this to note that of the 11 papers that Clarke, Devoret and Martinis published together, the most highly cited is the 1987 full paper in Physical Review B (DOI: https://doi.org/10.1103/PhysRevB.35.4682). I believe this illustrates the value to other workers of a well-written comprehensive paper with full details of important work. All too often these days the details are stuffed into supplementary material that is easily overlooked. I'd also like to recall the related experimental work of the late Richard Webb who made significant contributions to quantum phenomena in mesoscopic systems. He was recognized with the Simon (1989) and Buckley (1992) prizes, but passed away in 2016. Among other things he and Richard Voss published an important PRL on macroscopic quantum tunneling in 1981. (DOI: https://doi.org/10.1103/PhysRevLett.47.265). Note: these comments are made by me in a personal capacity and do not necessarily represent the views of any institutions that I might be affiliated with.
In a 2020 paper on the history, the laureates state,
"In 1980, Anthony Leggett1 emphasized
the importance of distinguishing
macroscopic quantum phenomena
originating in the somewhat trivial largescale accumulation of effects originating
at the level of microscopic variables from
those displayed, hypothetically, by a single
macroscopic, collective degree of freedom.
Although nothing in theory would seem to
prevent such variables from fully obeying
quantum mechanics, we felt challenged to
see if it really was the case in practice.
In 1985,we answered the question with our experimental observation2
of quantized energy levels of a current-biased Josephson
junction"
https://www.nature.com/articles/s41567-020-0829-5.pdf
The paper that I linked to as “tried hard” is in fact that Voss and Webb paper. Webb was one of the major players in examining all matter of mesoscopic effects in metal nanostructures.
This is Richard Webb’s widow. Thank you so much for mentioning his pioneering work. Every year at this time Rich would say “Stockholm calling” when the phone rang. I’m thrilled as he would be that the experimental works were “finally” awarded the Nobel Prize.
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