John Clarke, Michel H. Devoret, and John M. Martinis received the prize “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit.”
The work was published in three papers in PRL in 1984 and 1985. The New York Times has a nice discussion of the award, including comments from Clarke, Martinis, Tony Leggett, and Steve Girvin.
There is some rich, subtle, and beautiful physics here. As a theorist, I comment on the conceptual and theoretical side, but don't want to minimise that doing the experiments was a technical breakthrough.
The experiments were directly stimulated by Tony Leggett, who, beginning in the late 70s, championed the idea that Josephson junctions and SQIDs could be used to test whether quantum mechanics was valid at the macroscopic level. Many in the quantum foundations community were sceptical. Leggett and Amir Caldeira, performed some beautiful, concrete, realistic calculations of the effect of decoherence and dissipation on quantum tunneling in SQUIDs. The results suggested that macroscopic tunneling should be observable.
Aside: Leggett rightly received a Nobel in 2003 for his work on the theory of superfluid 3He. Nevertheless, I believe his work on quantum foundations is even more significant.
Subtle point 1. What do we mean by a macroscopic quantum state?
It is commonly said that superconductors and superfluids are in a macroscopic quantum state. Signatures are the quantisation of magnetic flux in a superconducting cylinder and how the current through a Josephson junction oscillates as a function of the magnetic flux through the junction. I discuss this in the chapter on Quantum Matter in my Very Short Introduction.
Leggett argued that these experiments are explained by the Josephson equations, which treat the phase of the superconducting order parameter as a classical variable. For example, in a SQUID, it satisfies a classical dynamical equation.
If the state is truly quantum, then the phase variable should be quantised.
Aside: a nice microscopic derivation, starting from BCS theory and using path integrals, of the effective action to describe the quantum dynamics was given in 1982 by Vinay Ambegaokar, Ulrich Eckern, Gerd Schön
Subtle point 2. There are different signatures of quantum theory: energy level quantisation, tunnelling, coherence (interference), and entanglement.
In 1984-5, Clarke, DeVoret, and Martinis observed the first two. Macroscopic quantum coherence is harder to detect and was only observed in 2000.
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, 1985] 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.
Two final comments on the sociology side.
Superconductivity and superfluidity have now been the basis for Nobel Prizes in six years and four years, respectively.
The most widely cited of the three PRLs that were the basis of the Prize is the one on quantum tunnelling with about 500 citations on Google Scholar. (In contrast, Devoret has more than 20 other papers that are more widely cited). From 1986 to 1992 it was cited about a dozen times per year. Between 1993 and 2001 is was only cited a total of 30 times. Since, 2001 is has been cited about 20 times per year.
This is just one more example of how citation rates are a poor measure of the significance of work and a predictor of future success.
On the other hand citations can show the actual utility in advancing science. The follow up PRB is cited more than any of the three PRLs... Clearly folks dig into a detailed paper more.
ReplyDeleteSee here
https://x.com/PhysRevB/status/1975960483019497607
Having seen so much of Michel at Yale for over 20 years, it's wonderful to see his work recognized. As the declared centenary of QM goes past, however, keep in mind that the distinction between classical and quantum physics is not only being eroded by 'quantum' creeping up into macroscopic objects.
ReplyDeleteThe distinction is also being eroded because what we have for the last century called 'classical' is a straw man, stuck in 1925: where CM at most uses probabilities in a Liouville-type formalism, it can easily be extended to use generalized probabilities (by using the full power of the Poisson bracket to generate more than just canonical transformations, in a way that can be understood as a classical form of contextuality) and, inspired by properties of QFT, it can further be extended to include a Lorentz invariant noise as well as thermal noise. [At a less algebraic level, we can use Koopman's Hilbert space formalism for CM, already introduced in 1931.]
CM with just those two changes can describe anything that can be described using QM, pushing Copenhagen's insistence that experiments must be described classically to an obvious extreme. Inevitably there are complications. There has been /some/ pushback, because parts of this are published in Physica Scripta 2019, Annals of Physics 2020, and J. Phys. A 2022, but perhaps(?) because there are no easy criticisms, I am not much known, and I am a flawed messenger there has not been much. The criticism I have heard many times is that it's not as useful as QM, which is certainly true for now and it will take decades to change.
Over the last few years I have been developing the ideas in academic seminars, many of which can be found on my YouTube channel, @petermorganQF. I am told that the most recent video there, a colloquium for NSU Dhaka on May 18th, is currently the most accessible, but there is also a video there of a more technical seminar in Yale Physics on May 1st. I know nobody serious is likely to look at any of that, but I post this in the hope of anything that helps the work progress.