Interview with Ulrich Eckern on the 2025 Nobel Prize in Physics for Clarke, Devoret and Martinis
Author: Erich Runge
My accompanying opinion piece as Chair of the Condensed Matter Division emphasizes that this year’s Nobel Prize in Physics is not an early tribute to the quantum computer, but rather honours a spectacular manifestation of macroscopic quantum mechanics. I consulted an expert on this topic: Ulrich Eckern, Professor Emeritus at the University of Augsburg, Germany, has been working on various aspects of theoretical solid-state physics and mesoscopic physics for many years, including the theory of superconductivity and superfluid 3He, Josephson junctions and networks, and low-dimensional model systems. During his PhD at the University of Karlsruhe (now KIT), 1977-79, discussions with three scientists visiting his mentor, Albert Schmid, left a major mark on him, scientifically as well as personally: John Bardeen, Michael Tinkham – and John Clarke.
How did you get involved into the field which is often called “dissipative quantum mechanics”?
During my postdoc at Cornell University, 1980-82, a pioneering paper by Amir Caldeira and Tony Leggett was published that was devoted to quantum tunnelling in dissipative systems. Motivated by this work, Vinay Ambegaokar, Gerd Schön, and I set out to formulate the “quantum dynamics” of a Josephson junction on the basis of the accepted microscopic description, namely two tunnel-coupled superconductors (within the BCS model). We were able to confirm the Caldeira-Leggett results, but were also able to elucidate some important differences, as in ideal junctions the dissipation is related to quasiparticle excitations.
Without going into details, what is or was the basic question at the time?
The general question, discussed for years, was this: “Given a system that is described by the usual classical equation of motion of a particle with damping, i.e. energy dissipation – what is the correct quantum theory?” Since the classical theory can be derived from quantum theory but not vice versa, there is no general answer to this question.
In the case of a Josephson junction specifically, the “particle” is the order parameter phase difference, and the classical equation of motion is called resistive-shunted junction model. The model’s parameters are the capacitance (proportional to the mass), the resistance (inversely proportional to the damping), and the Josephson coupling energy which determines the magnitude of a periodic force term. An external current slightly (as long as the current is well below the critical current) tilts the potential. Thus, classically, the “particle” sits in one of the potential minima, and it can “escape” via thermal fluctuations only. But how does the escape mechanism evolve with decreasing temperature, when the thermal energy becomes small compared to charging and Josephson energy? Will escape happen via quantum tunnelling through the barrier? And how is the latter modified by damping (dissipation)?
Why did John Clarke impress you so much?
When I met him in Karlsruhe during my PhD, John Clarke was still at an early stage of his career. He impressed me not only by his apparent extraordinary skills in designing and carrying through difficult experiments, but also by his strong desire to grasp the relevant theoretical concepts — in other words, to understand the heart of the matter. I believe these two characteristics led him directly to undertaking the two seminal studies* which are now honoured.
What was the significance of the 1985 papers* by the 2025 Nobel Prize winners?
While there was mostly agreement with respect to the theoretical description, namely that dissipation would decrease the tunnelling rate out of a metastable state, the experiments offered the ultimate confirmation: the theoretical concepts worked! The results included a detailed comparison of theory and experiment with respect to the temperature dependence of the tunnelling rate. So in fact, the very concept of “macroscopic quantum tunnelling” was confirmed – and later also the concept of “macroscopic quantum coherence”, which is relevant for some of today’s quantum computer concepts. The notion of “macroscopic”, by the way, refers to the fact that during a tunnel event close to a million electrons tunnel coherently through the barrier.
What were the experimental challenges?
The crossover between classical escape and quantum tunnelling in the experiments occurs around 30 mK, so I can imagine that an excellent temperature control is needed. You have to exclude as many external effects as possible which might lead to dissipation. For a large charging energy, a capacitance of just a few pF is needed, which implies rather small junctions, even though they are macroscopic compared to the atomic scale.
What about macroscopic quantum coherence?
When Josephson junctions are included in a ring geometry and a suitable magnetic field is applied, a double-well potential can be realised, implying a quantum mechanical two-level system at sufficiently low temperatures. As Sudip Chakravarty realised early on, dissipation will have a profound effect at a certain ‘critical’ value, abruptly and completely suppressing quantum coherence. A similar delocalisation-localisation transition has been predicted theoretically for a periodic potential — i.e. a Josephson junction in the zero-current case. This transition is now known as the Schmid–Bulgadaev dissipative phase transition, and is still the subject of ongoing research.
The questions were asked by Erich Runge, Chair of the Condensed Matter Division of EPS, member of the Executive Board of the German Physical Society and professor of theoretical physics at the Technical University of Ilmenau, Germany.
* References
1. Measurements of Macroscopic Quantum Tunneling out of the Zero-Voltage State of a Current-Biased Josephson Junction
Michel H. Devoret, John M. Martinis, and John Clarke, Phys. Rev. Lett. 55, 1908 – Published 28 October, 1985, DOI: https://doi.org/10.1103/PhysRevLett.55.1908
2. Energy-Level Quantization in the Zero-Voltage State of a Current-Biased Josephson Junction
John M. Martinis, Michel H. Devoret, and John Clarke, Phys. Rev. Lett. 55, 1543 – Published 7 October, 1985, DOI: https://doi.org/10.1103/PhysRevLett.55.1543
