The biggest 'Schrödinger's cat' yet — physicists put 7,000 atoms in superposition

0:23Welcome back to the Nature Podcast. This week, quantum superposition gets supersized and Trump 2.0 one year in. I'm Benjamin Thompson and I'm Nick Pertridge-Howe.

0:46First up on the show, quantum physics gets big. And by big, I mean about the size of a protein. To explain, we first need to revisit that famous thought experiment of Schrodinger's cat. In 1935, Austrian physicist Erwin Schrodinger highlighted the absurdity of some interpretations of quantum physics. The theory says that, until you look at a quantum system like an electron, it exists in a haze of possible positions. Only when measured does this superposition disappear, replaced with a single definite location. Weird, but kind of fine for an electron. But what about something bigger, like a cat?

1:28The thought experiment involved a setup where the outcome of a quantum process, such as radioactive decay, decides the fate of a cat. If the system stays isolated, the decay is in a superposition of both having happened and not happened. So the cat is both dead and alive. Now, clearly we don't see a lot of zombie cats, but physicists still want to know whether there is an inherent limit to how big you can make things quantum, as what happens when you reach the boundary between the small quantum world and the large classical world we inhabit is a bit of a mystery, with some physicists even speculating that we'll need new physics to explain it.

2:10And now a team report that they've managed to get something about the size of a protein into a superposition, which is pretty big for quantum phenomena. In this case, this nanoscale object acted both like a wave and a particle. Reporter Lizzie Gibney spoke to Sebastian Pedellino at the University of Vienna, one of the team behind this supersized superposition. She started by asking what they were trying to understand with this experiment. I think at the most fundamental level, the motivation for our experiments is does quantum physics that explains the physics of light and atoms also hold for larger and more complex objects, or does something new make the world look classical as things get bigger?

2:52And people often think that quantum mechanics is only associated to tiny things like electrons or atoms. But the theory, as it's stated right now, never states it stops working above a certain mass or a certain size. So it's just natural to ask where does the everyday classical world come from? And of course, one can say quantum mechanics seems to be universal, but in the end, it's a question that has to be settled experimentally. And tell me about this paradox of Schrödinger's cat and how this experiment is kind of a version of that. In our experiment, we use metal-sodium clusters, sodium nanoparticles, which are kind of our metal cats.

3:30And we prepare this Schrödinger cat state as a centre of mass superposition, which means that the centre of mass of these 5 to 8 nanometre nanoparticles is separated by about 100 nanometre and it is at both here and there at the same time. So instead of being alive and dead like the cat, the metal cluster is in effectively two places at once in a way. Well, it's not the metal cluster that is actually here and there. It's the wave function and it's in a state of here plus there. And it cannot be described by here or there like we would do in classical physics, where we have like a tiny billiard ball throwing through one of the paths.

4:07But we rather have to see it as a wave. And it's in a state where it's here plus there. And only when we measure, we can say that it's either here or there. So it's something that is quite big and chunky and metal, and yet it's still describable as a quantum state. It's a wave rather than being the kind of classical particle vision of the world. Exactly. Our sodium nanoparticle is something where someone would intuitively think it should behave classical, like a small ball that flies through the interferometer and just has a single trajectory. And its mass is already comparable to large proteins and to even small varioids.

4:43So we are already at a scale where biological objects have a similar mass range. And the superposition in space is about 20 times the diameter. So it's far more spread out than the actual diameter, which makes this Schrödinger-Cat type of state. And so tell me a bit more about the experiment and how you get these metal clusters to act like a wave and to prove that that's what's happened. So we usually start by generating a beam of the sodium clusters in an ultra-high vacuum chamber because we need to isolate the nanoparticles from any environmental interaction because this would immediately destroy the so-called quantumness.

5:21And we take these nanoparticles and we send them through the MetaWave interferometer, which consists of three lasers that form cratings, and we send the sodium nanoparticles through this interferometer. And as the particles fly through the interferometer and we see interference in the end. This means that the particle did not follow single trajectories, but it rather ran through many possibilities that it could go through these gratings because it was acting like a wave. So the fact that you see an interference pattern at the end means that each cluster must have not just been like a billiard ball passing through one gap in the grating, it must have gone through several and then like a wave, the peaks and troughs interfere with each other and you get this pattern.

6:08Exactly. That's amazing. And physicists have been making bigger and bigger things in quantum superpositions for years now. And it's taken about a decade to go from a few hundred atoms to what in this experiment is something like 7000, I think. And what did it look like when you finally got this quantum picture of the clusters? So what we're looking at is the signal and we expect the modulation that looks like a sinusoidal like interference fringes. and before we had those interference fringes it was basically just flat lines and yes I was basically looking at flat lines for about two years and the problem is that a flat line doesn't tell you much it does not tell you that quantum mechanics is not working it can also tell you that your alignment is off your laser power is not correct you have too many vibrations your vacuum is not good enough so a flat line is not conclusive by itself so yes once you have interference fringes or any kind of fringes, then you really know that you have them because it's so much different than just a flat line.

7:07And so in this experiment, as you've mentioned, we've got these quite big clusters and the superposition is really quite far apart compared to the size of each cluster. The position of the superposition is, you know, they're quite distant. How does this compare to other previous attempts to, you know, make versions of Schrodinger's cats? You know, is it the biggest or the fattest cat we've seen yet? So there are many different ways that an experiment can be bigger as a strottinger cat so this could first of all mean higher mass larger separation or longer coherence time and usually to compare across this very different platforms we use this value of the macroscopicity which is a single metric that combines all these scales like the coherence time mass and separation and by that measure our result advances by about an order of magnitude so we have a macroscopicity of 15.5 on a logarithmic scale and there's also other experiments that are pursuing similar goals by achieving a large Schrodinger cat and some of them have magnitude of orders higher masses for example up to 16 microgram or even large separations on half a meter scale.

8:16But in both experiments they have for example trade-offs that the separation is only very very tiny or that the mass in case of the half meter separation is very very small because it's only a single atom. And in matter with interferometry we have the advantage that we kind of combine all three of them within one interferometer. So it's not necessarily the highest mass, but it's the highest on this macroscopicity scale. It's the most macroscopic in a way. It's the most macroscopic so far, yes. And where would you like to go from here? So first I think it would be nice to have metal interference of genuinely large biological objects because that would move the anti-quantum interference into a new regime that feels qualitatively different than metal nanoparticles.

8:59And second, I would also like to see the interferometer turn into a very useful tool, because we see these very, very sensitive interference fringes, and we can use those to learn something about the molecules or the biological objects that would fly through our interferometer. When we record these interference fringes, we're usually very, very sensitive to any external forces. But if you can control all the parameters, you can actually apply control forces, like electric fields or magnetic fields, and then shift the interference fringes on a nanometer scale. And this gives us access to the molecular properties.

9:33So it would be a kind of exquisitely sensitive detector of lots of other different properties, not just how quantum it is. And is that something that could be done soon, like putting a virus or a protein or something into this same system, into the interferometer? In terms of the mass, we're already in the scale of large proteins and small virals. But the problem is here not the mass, but really a technical challenge, because when it comes to biological objects, they tend to be more complex, more fragile than metal clusters, for example, and they can fragment, denature or just chemically change.

10:06And this is why it's practically so much harder to detect them efficiently. But we're working on it on a different experiment and I think that it's not so far out of reach anymore. So maybe not Schrodinger's cat, but Schrodinger's amoeba or something like that. Yes, exactly. So you're going to keep doing these experiments and I guess keep trying to make bigger, bigger things quantum? At what point might we get a real cat that's Schrodinger's cat? Yeah, if we boldly extrapolate our achievements of the past years into the future, we could end up with a kind of playful claim that we might see interference of an actual cat around the year 2200.

10:46But of course, we do not have anything like a technology that would make a real cat interferometer possible. so this is not really a serious prediction. But it's nice, it shows that we're on a good trajectory, things are getting bigger. Yes, and I also like the story because it makes this point. When I talked to my senior postdoc, he told me that about 15 years ago, when he graduated, that he thought that interference of these massive sodium particles would technically not be possible. And now we're doing it, so I have thought to be cautious with saying this will never work because who knows what will become possible in decades or even centuries.

11:23That was Sebastian Pedalino from the University of Vienna, talking with reporter Lizzie Gibney. For more on that story, check out the show notes for some links. Coming up, we take a look back at what the first year of Donald Trump's second presidency meant for US science. And look at what might be in store for the next 12 months. Right now, though, it's the research highlights with Dan Fox.

11:49The first unambiguous records of written numbers emerged around 3 ,400 BC in what is now Iraq. But a new analysis suggests the people of this region were engaged in mathematical thinking much earlier, in their pottery. Researchers examined more than 5 ,000 fragments of painted pottery made by people who inhabited northern Mesopotamia between around 6300 and 5500 BC. Around 15 % of the fragments featured plant motifs, with many including images of flowers with 4, 8, 16, 32 or 64 petals. The authors say these patterns show a clear understanding of symmetry and spatial division long before written numbers came into use, a skill that probably helped with tasks such as sharing harvests or dividing communal fields.

12:44You can count on finding that research in the Journal of World Prehistory.

12:52If you have a pet dog, they might be paying more attention to your conversation than you realise. Typical family dogs can understand action words such as sit, but not words that describe objects, unless they're a gifted word learner, namely a dog that can remember labels for hundreds of things. Researchers investigating the gifted learner's abilities found these dogs could observe communications about a new toy between human family members and when tasked with retrieving the plaything from among an assortment, could pick the correct one. Only a few animals, like bonobos and African grey parrots, have been taught to recognise objects through unique names.

13:38Learning words indirectly through watching human interactions is even harder because it requires an animal to follow a person's gaze and, to some extent, understand their intentions. This resembles the abilities of infants at about 18 months, who can passively observe and overhear to learn words. You can fetch that research from science.

14:11This week marks a year since Donald Trump's second presidential inauguration, and it was a tumultuous year for science, I think it's fair to say. Joining me to take stock of what happened and maybe do a little bit of crystal ball gazing as to what might happen this year is my colleague Max Kozlov. Max, thanks for stopping by. Happy to be here. So Max, you and some of our colleagues then have been doing a deep dive, looking backwards and forwards. Before we get into any specifics, what was the overall picture of the last year, would you say? It was a year unlike any other year in the US science funding landscape, I think it's fair to say.

14:49Sometimes things would freeze and then unfreeze and then refreeze based on the courts weighing in on some of these policy decisions. there were terminations of grants and that was something that we hadn't seen before and I think there was a lot of confusion and worry that your own research project might be targeted next and so I think people tried to keep their heads low and try to not catch the attention of the administration. Well let's unpick a few of the things that happened over the past 12 months. Of course science requires grant funding and researchers and as you've alluded to there the numbers of both in some cases were drastically reduced.

15:30Shall we start with grants? What are some of the top line numbers that your research threw up? Well, yeah, there were about 8 ,000 research grants that were either terminated or frozen this past year. And that's just something that never happened before. In speaking with NIH officials, they could count on one hand the number of grants that were terminated before this past year. And so our story, we really are documenting those 8 ,000 projects, what's been lost, what themes of research has been targeted, and how much money was lost or was wasted because of all this. And you and I have spoken before that it seems like some of these terminations have affected some areas of science and some institutions more than others.

16:14Yeah, there were a few institutions and topic areas for grants that were especially targeted. So on the institution front, institutions like Harvard, Columbia were disproportionately targeted by the administration because they claimed that they weren't meeting the mark for fighting anti-Semitism. And so there you saw across the board cuts at these institutions on anything from Alzheimer's and cancer research to infectious diseases research. It was all frozen until they resolved those claims. And then more broadly, separate from those institutions, there were certain topics across the board that were targeted.

16:55Those include what the administration calls DEI or diversity, equity and inclusion research. They never really defined this term. So it's really hard for me to say exactly what that means. But this covered research ranging from ways to diversify the scientific workforce to studying maybe why one minority group has a higher rate of this cancer than another one. Those kinds of projects were often terminated. And then one other big one I'll highlight here is infectious disease research broadly. This might be because the administration wants to move away from funding research about COVID-19, because it says that's a pandemic that we've moved on from.

17:39Or it's also because the administration is quite hostile toward vaccine research. So there have been some big projects developing mRNA vaccines and other vaccine platforms that have been cut. And what has your analysis suggested is the kind of dollar value that has been lost then in terms of these grants being terminated or in some cases frozen, but then not unfrozen? So of these 8 ,000 or so grants that were canceled or suspended, courts have reinstated about half of them, about 4 ,000 of them. But it's unclear how many of those scientists actually got that funding. So if it's past the budget period, OK, sure, the funds are unfrozen or reinstated, but researchers can't actually withdraw them anymore.

18:23And it's not just as simple as turning off and on the lights again. You know, these are experiments that were sitting idle for months. And so sometimes researchers maybe moved out of academia or moved on to other projects. So the projects that were not reinstated, they amount to about$1.4 billion of lost funding. And that's obviously a staggering amount of money and a lot of grants that were cut. But your data shows that there are effects on new grants as well. Yes, and that's something that we saw both at the NSF and NIH, both of which issued about 25 % fewer grants in 2025 compared with the average from the past 10 years.

19:05And that's because they use this new budgeting strategy where instead of giving money out one year at a time for five years, they gave all the money to certain awards in that first year as a lump sum. So what that means is there's less money to go around because they've spent it quickly on these grants. And that means that fewer people get funded. And that turns a competitive environment at the NIH and NSF into a really, really competitive environment because fewer people are getting funded. And speaking of people then, science, of course, doesn't work without scientists, many of whom were affected by actions taken over the last 12 months.

19:46Yeah, and we at Nature have conducted some polls of scientists asking how they feel about the current funding landscape in the U.S. with all the policy changes happening. And we found a staggering amount that we're considering leaving the U.S., packing up their lab and trying to find jobs elsewhere. This is particularly true for international students and researchers. and that's kind of a forbearer of things to come, how many students actually want to come to the United States to study and to do their research. And international students are particularly worried for several reasons. One, seeing that there might be less funding to go around.

20:25But another big one is, of course, that the Trump administration has really cracked down on immigration of any kind, including skilled workers such as researchers. And we can already see that in the numbers. Looks like there was about a 17 % drop in the number of international students enrolled this past year compared with the previous academic year. So quite stark across the board then. But here we are in a new year, the second year of Donald Trump's second presidency. And you've been looking forward at what might happen here as well. And there is something on the near horizon that really could set the tone for the next 12 months.

21:05And that's the upcoming federal budget and a lot of backwards and forwards on what science funding might look like. You've asked this question at an interesting time because we have in the United States about two weeks until the government shuts down again. And this is coming just a few months after the government shut down for the longest stretch in history. So it's possible this happens again. And right now, lawmakers on Capitol Hill are arguing and squaring away the last few bills that will hopefully come to a vote before this deadline. And some of the early signs hint that lawmakers are poised to reject some of the big cuts to science that the Trump administration had requested.

21:44I mean, we're talking about a 40 percent budget cut to the NIH. That's without precedent. It would amount to tens of billions of dollars. So we haven't seen the final bill language yet. They have until the end of the month before the government shuts down again. But there are early signs based on previous negotiations and previous kind of markups of the bills to come that the worst case scenario for scientific funding in the US will not come to pass this coming year. But I suppose even if it doesn't come to pass, given all you've spoken about before then and potentially the hollowing out of institutions and folk leaving research or academia, will the fact that the money's come back fill the void of what has been lost?

22:27Look, I mean, funding is really important. Without funding, I mean, you can forget everything else. But what we've seen this past year is that the Trump administration is flexing its power in every single domain such that, OK, even if there's funds appropriated, they can choose who gets funded. And they're doing it in a way that prioritizes what the administration wants much more than in previous administrations, meaning the voice of scientists and career officials who have been at these agencies for decades is reduced compared with some of the political appointees saying, yes, we want this grant.

23:09No, we don't want that grant. So the politicization of science has been kind of cranked up to a maximum this past year. And what else, Max, do you think we should keep our eyes on in the next 12 months? It's something you've been investigating with the rest of the team here. So I mentioned that political appointees will have unprecedented power this coming year compared with any other year in the past. And there's a few ways that this is going to play out. One is a bunch of the institutes within NIH, the director positions are empty because the old directors either retired or were kind of pushed out because they weren't renewed.

23:48And so right now there's this ongoing search for new institute directors and people are already a little worried because this process has happened much faster than previously that perhaps they're not going to be filled by people who have the relevant scientific background. And these jobs are important because no grant can be issued until the institute director. So I'll be looking to see who fills those positions. Another bigger picture trend is they're kind of moving away from only considering peer review scores on grant applications. They're kind of deemphasizing those and they want to consider other things like the agency's priorities, which by extension are the administration's priorities, and other factors like the applicant's geographic location.

24:37So these other factors that kind of diminish scientific merit and open the door to more politicization is kind of the big picture trend I'll be watching out for this year. And you speak to a lot of researchers, Max. Have they given a sense of how they're feeling about this new year? I think some of the dust has settled. Not because I think people are all of a sudden encouraged by what they're seeing. I think people are very worried about the increasing role of political appointees in grant making. But I think compared with those first two months of the Trump administration from like January through May were particularly tumultuous.

25:19I mean, there was just change after change after change after change. And things are still certainly happening. I mean, we just saw an effort to try to cancel a bunch of mental health and substance abuse grants last week and then they reversed that the day after so like this kind of confusion is still happening but i think people are maybe more ready for it and i think the resistance to some of these changes is more well organized now well max kozlov as always thank you so much for joining us to lay everything out so clearly max thanks for being here thank you nature's max kozlov there to read his and the rest of the team's analysis look out for links in the show notes that's it for this week's show don't forget to look out for the briefing show on friday that podcast will be on the same feed as the wednesday show so it should just appear in your podcast player of choice in the meantime you can reach out to us on social media we're at nature podcast i'm benjamin thompson and i'm nick petrie chow thanks for listening

26:29Thank you.