Strange things happen in the quantum world. It’s not just the events that are strange: the ways in which they happen are also weird. Take the nucleus, for example. Things were bad enough when Ernest Rutherford proposed that the atomic nucleus was very small, very dense and packed with positive charge. But things got even worse when it was realised (through alpha decay) that parts of the positive charge could break free from the nucleus. This raised a seemingly inescapable question; why don’t atomic nuclei instantly fall apart?
That question can be split into two parts. Firstly, if nuclei contain positive segments that can be separated (we now know they are particles called protons) then what is binding these entities together when electrostatic repulsion must cause them to be repelled? Secondly, given that there is a bonding mechanism (now known as the Strong Nuclear Force) how is it that alpha particles manage to overcome this force to escape from the nucleus?
The co-existence of the Strong Nuclear Force and alpha decay actually makes it harder to explain why unstable nuclei don’t decay instantly. The answer is that we are dealing with the quantum world where behaviours are governed by probabilities rather than certainties. In short, objects in the quantum world can sometimes do “impossible” things with a probability that is determined by the object’s wave function.
The scale at which quantum behaviour dominates is said to be microscopic (but is in fact far below the scale observable with a microscope) whereas the scale at which the certainties of everyday life apply is macroscopic. It was work demonstrating quantum behaviour in the macroscopic world that won this year’s Nobel prize for Physics.
The paper that first reported macroscopic quantum tunnelling (MQT) was published in February 1988 (https://www.science.org/doi/10.1126/science.239.4843.992) and three of its authors, John Clarke, Michel Devoret and John Martinis, have now been awarded the 2025 Nobel Prize for Physics “for the discovery of macroscopic quantum mechanical tunnelling and energy quantization in an electric circuit”.
Unsurprisingly, the work was not easy. Very low temperatures (mK) were used to prevent thermal energy from overpowering the results and the experiment was designed to identify the behaviour of individual particles (in this case electrons) rather than an overall effect. To meet these criteria, Clarke, Devoret and Martinis used superconducting conditions and a Josephson junction. I don’t pretend to fully understand the exact details but, fortunately, they aren’t important to appreciate the final results.
All we need to realise is that a classical particle trapped in an energy well will have zero probability of escape but a quantum particle will have a finite probability of escaping according to its wave function. In their concluding remarks, the authors state; “These experiments show that the particle in the well is not point-like but must be described by a wave packet… (demonstrating) the existence of quantized energy levels in the well, with energies in very good agreement with predictions. Thus, our macroscopic anharmonic oscillator, namely, a Josephson junction, exhibits quantum behavior.”
Echoing the issues raised by Rutherford’s atomic nucleus, the authors add; “Our system behaves very much as a ‘macroscopic nucleus’ with quantized energy levels and a decay process (MQT) that is closely analogous to α-decay in a heavy atomic nucleus: the particle is initially in a metastable bound state and tunnels out into a continuum of states.”
The Nobel Foundation has published a brilliant popular-physics version of the ideas behind this year’s award, which I highly recommend and from which the two diagrams shown here have been taken. It is available at https://www.nobelprize.org/uploads/2025/10/popular-physicsprize2025-2.pdf. There is also a nice five-minute audio explanation, created by the Perimeter Institute, available here.
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