2025诺贝尔物理学奖深度解读：宏观量子隧穿

The 2025 physics Nobel Prize has been awarded to macroscopic quantum tunneling. I don't know anyone who had this on their bingo list. What is this? Why does it matter? And who are the people who won the prize? I have a brief summary. The prize was awarded to John Clark, Michel Devoret, and John Martinis. And it's a case where much of it can be traced back to a single paper that was published in 1985. This paper shows results which demonstrate that macroscopic quantum tunneling is real. Quantum tunneling you might have heard of. It's a case when a quantum particle can go through a barrier even though it doesn't have enough energy. That is if the particle didn't have quantum properties, it would be trapped. But if it has quantum properties, it can leak through the barrier. The probably best known application of quantum tunneling are electron tunneling microscopes. These probe basically the surface of a material atom by atom because electrons can tunnel into the tip of the microscope. However, this is a teeny tiny effect for single electrons. The three who won this year's Nobel Prize figured out that this can also happen for large collections of particles.

In particular, they were looking at currents in superconducting wires. This is maybe not exactly your idea of macroscopic. Personally, when I hear macroscopic, I think of a bus, but a current in such a wire consists of millions of electrons, and that's quite large compared to the single electron level. What they showed is that the collective behavior of the current in the wire can tunnel across a barrier. So, that's literally a gap. The wire needs to be superconducting for this to happen because otherwise the electrons don't behave as one quantum state. This means that these wires must be cooled to temperatures near absolute zero. You can see in this figure from the Nobel Prize winning paper that as the wires being cooled, the current behaves in a way that it be impossible without quantum effects.

The relevance of this effect and this is why I guess they awarded it a Nobel Prize is that it moved quantum physics into the microchip range in the 40 years after their discovery. This insight exploded into a vast number of applications. The best known of which is probably quantum computing because once you have this current that can tunnel, you can also have a current that's both on and off and that makes a cubit. Now the quantum computers with superconducting circuits which are being used by Google and IBM and Amazon and so on don't use exactly the same technology they used in their 1985 experiment but it goes back to what this group did in the 1980s.

The factors also being used in some experiments to look for dark matter particles that might interrupt the currents. Besides that I don't think it has yet a lot of applications. I think that the biggest downstream effect of this discovery has been that it made quantum technology a reality. It's still mostly an academic enterprise, but one that's on the way to practical applications. And this has entirely changed physicists attitude to quantum mechanics. In the 1980s, when these guys did their experiment, quantum physics was a mostly philosophical enterprise. No one thought it would have much practical relevance one way or another. All this stuff with things being in two places and spooky action was just so far removed from anything tangible. But the macroscopic quantum tunneling was a major turning point that moved quantum physics into the tangible range. At least if you have a physics lab with superconducting wires.

Back then they argued all night over the interpretation of quantum mechanics. Now they argue all night over the interpretation of quantum mechanics but in a clean room. So what are we to make of this year's Nobel Prize? It makes sense that rather than giving a Nobel Prize to quantum computing, they would give a Nobel Prize to the technology that made the first quantum computers possible. Though as a theoretical physicist, I find it to be somewhat depressing. I as many others thought that the Nobel Prize would go to the theoretical work on quantum computing to David Deutsch and maybe Peter Shaw. It's tough to be a theorist. Then again, we must keep in mind that the Nobel Prize is not a community award. It's given out by the Royal Swedish Academy of Sciences, which is tasked with interpreting and fulfilling the will of Alfred Nobel.

It's not great that this one person on the committee that selects the prize recipients should have such a huge influence on science overall. Then again, I think of Nobel Prize Week as science celebration week. It's the one week of the year when the entire world wakes up and remembers how much we owe to science. and the one week my inbox tunnels through the spam filter. Where does Sabina get her science news? It's not on YouTube. My most trusted source is Nautilus. If you aren't already subscribed to them, you should have a look. Nautilus keeps you up to date on the most relevant topics in science today. For example, they just had this very interesting article about whether life is a sort of computation, what that means and why it matters. or this one about the new data from the Vera Rubin Observatory. What I particularly like is that they cover science in its full breadth from astronomy to economics, history, neuroscience to philosophy and physics. They'll pick the most relevant topics and give you all the context. I've written several contributions for Nautilus myself. Nautilus has a digital and a print version and it's just a pleasure to read. They really put a lot of effort into writing and the graphic design is amazing. You notice immediately if you open the print version that it's a high quality production. You can join Nautilus as a digital only member or get a print subscription. In addition to full access to all the stories in Nautilus, members receive benefits like priority access to events, exclusive products, and product discounts. And of course, I have a special offer. If you use my custom link, join nautilus.com/sabina, you'll get 15% off your membership subscription. So, go and check this out. Thanks for watching. See you tomorrow.